Publications

In this contribution to Abraham Nitzans Festschrift, we present a perspective of theoretical research over the years that has pointed to the potential of molecular processes to act as quantum information resources. Under appropriate control, homonuclear dimer (diatom) dissociation (half-collision) and the inverse process of atom-pair collisions are shown to reveal translational (EPR-like) entanglement that enables molecular wave packet teleportation. When such processes involve electronic-state excitation of the diatom, the fluorescence following dissociation can serve as an entanglement witness that unravels the molecular-state characteristics and evolution. Such entangling processes can also exhibit anomalous quantum thermodynamic features, particularly temperature enhancement of a cavity field that interacts with dissociated entangled diatoms.

We have recently put forth several schemes of unconventional, nonlinearly-enabled thermodynamic (TD) devices that can operate in either the classical or the quantum domain by transforming thermal-state input in multiple uncorrelated modes into non-gaussian state output in selected modes: a four-mode Kerr-nonlinear interferometer that acts as a heat engine; two coupled Kerr-nonlinear Mach-Zehnder interferometers that act as a phase microscope with unprecedented phase resolution; and a noise sensor that can distinguish between unknown nonlinear quantum processes. These schemes reveal the unique merits of nonlinear TD devices: their ability to act in an autonomous, fully coherent, dissipationless fashion, unlike their conventional counterparts. Here we present the opportunities and challenges facing this new paradigm of nonlinear (NL) quantum and classical TD devices along the following lines: (A) Linear versus nonlinear multimode transformations in TD devices: what are the principal distinctions between the two types of transformations? (B) Classical versus quantum effects in NL TD devices: what are their main differences? Is quantumness an advantage or a disadvantage? (C) Deterministic methods of achieving giant nonlinearity at the few-photon level via coherent processes, including multiatom-bath interactions which can paradoxically yield NL Hamiltonian effects: their comparison with probabilistic, measurement-based methods that can achieve similar NL effects in the quantum domain.

Estimation of the phase delay between interferometer arms is the core of transmission phase microscopy. Such phase estimation may exhibit an error below the standard quantum (shot-noise) limit, if the input is an entangled two-mode state, e.g., a N00N state. We show, by contrast, that such supersensitive phase estimation (SSPE) is achievable by incoherent, e.g., thermal, light that is injected into a Mach-Zehnder interferometer via a Kerr-nonlinear two-mode coupler. Phase error is shown to be reduced below 1/n¯, n¯ being the mean photon number, by thermal input in such interferometric setups, even for small nonlinear phase shifts per photon pair or for significant photon loss. Remarkably, the phase accuracy achievable in such setups by thermal input surpasses that of coherent light with the same n¯. Available mode couplers with giant Kerr nonlinearity that stem either from dipole-dipole interactions of Rydberg polaritons in a cold atomic gas, or from cavity-enhanced dispersive atom-field interactions, may exploit such effects to substantially advance interferometric phase microscopy using incoherent, faint light sources.

Atoms falling into a black hole (BH) through a cavity are shown to enable coherent amplification of light quanta powered by the BH-gravitational vacuum energy. This process can harness the BH energy towards useful purposes, such as propelling a spaceship trapped by the BH. The process can occur via transient amplification of a signal field by falling atoms that are partly excited by Hawking radiation reflected by an orbiting mirror. In the steady-state regime of thermally equilibrated atoms that weakly couple to the field, this amplifier constitutes a BH-powered quantum heat engine. The envisaged effects substantiate the thermodynamic approach to BH acceleration radiation.

In recent works, we have put forth and experimentally demonstrated several novel schemes [1-5] for the detection of quantum noise signatures by exploiting frequent photonic measurements, allowing us to reach unprecedented, ultrahigh sensitivity to quantum noise.

Our goal in this article is to elucidate the rapport of work and information in the context of a minimal quantum-mechanical setup: a converter of heat input to work output, the input consisting of a single oscillator mode prepared in a hot thermal state along with a few much colder oscillator modes. The core issues we consider, taking account of the quantum nature of the setup, are as follows: (i) How and to what extent can information act as a work resource or, conversely, be redundant for work extraction? (ii) What is the optimal way of extracting work via information acquired by measurements? (iii) What is the bearing of information on the efficiency-power tradeoff achievable in such setups? We compare the efficiency of work extraction and the limitations of power in our minimal setup by different, generic, measurement strategies of the hot and cold modes. For each strategy, the rapport of work and information extraction is found and the cost of information erasure is allowed for. The possibilities of work extraction without information acquisition, via nonselective measurements, are also analyzed. Overall, we present, by generalizing a method based on optimized homodyning that we have recently proposed, the following insight: extraction of work by observation and feedforward that only measures a small fraction of the input is clearly advantageous to the conceivable alternatives. Our results may become the basis of a practical strategy of converting thermal noise to useful work in optical setups, such as coherent amplifiers of thermal light, as well as in their optomechanical and photovoltaic counterparts.

A minimal model of a quantum thermal machine is analyzed, where a driven two level working medium (WM) is embedded in an environment (reservoir) whose spectrum possesses bandgaps. The transition frequency of the WM is periodically modulated so as to be in alternating spectral overlap with hot or cold reservoirs whose spectra are separated by a bandgap. Approximate and exact treatments supported by analytical considerations yield a complete characterization of this thermal machine in the deep quantum domain. For slow to moderate modulation, the spectral response of the reservoirs is close to equilibrium, exhibiting sideband (Floquet) resonances in the heat currents and power output. In contrast, for faster modulation, strong-coupling and non-Markovian features give rise to correlations between the WM and the reservoirs and between the two reservoirs. Power boost of strictly quantum origin ('quantum advantage') is then found for both continuous and segmental fast modulation that leads to the anti-Zeno effect of enhanced spectral reservoir response. Such features cannot be captured by standard Markovian treatments.

We put forward the concept of work extraction from thermal noise by phase-sensitive (homodyne) measurements of the noisy input followed by (outcome-dependent) unitary manipulations of the postmeasured state. For optimized measurements, noise input with more than one quantum on average is shown to yield heat-to-work conversion with efficiency and power that grow with the mean number of input quanta, the efficiency and the inverse temperature of the detector. This protocol is shown to be advantageous compared to common models of information and heat engines.

We reveal the potentially important role of a general mechanism in quantum heat management schemes, namely, spectral filtering of the coupling between the heat baths in the setup and the quantum system that controls the heat flow. Such filtering is enabled by interfaces between the system and the baths by means of harmonic-oscillator modes whose resonant frequencies and coupling strengths are used as control parameters of the system-bath coupling spectra. We show that this uniquely quantum-electrodynamic mechanism, here dubbed bath spectral filtering, boosts the performance of a minimal quantum heat manager comprised of two interacting qubits or an analogous optomechanical system, allowing this device to attain either perfect heat diode action or strongly enhanced heat transistor action.

In this book, Henry Bar, physicist and the first quantum superhero, guides the reader through the amazing quantum world. His hair-raising adventures in his perilous struggle for quantum coherence are graphically depicted by comics and thoroughly explained to the lay reader. Behind each adventure lies a key concept in quantum physics. These concepts range from the basic quantum coherence and entanglement through tunnelling and the recently discovered quantumdecoherence control, to the principles of the emerging technologies of quantum communication and computing. The explanations of the concepts are accessible, but nonetheless rigorous and detailed. They are followed by an account of the broader context of these concepts, their historic perspective, current status and forthcoming developments. Finally, thought-provoking philosophical and cultural implications of these concepts are discussed. The mathematical appendices of all chapters cover in a straightforward manner the core aspects of quantum physics at the level of a university introductory course. The Quantum Matrix presents an entertaining, popular, yet comprehensive picture of quantum physics . It can be read as a light-hearted illustrated tale, a philosophical treatise, or a textbook. Either way, the book lets the reader delve deeply into the wondrous quantum world from diverse perspectives and obtain glimpses into the quantum technologies that are about to reshape our lives. This book offers the reader an enjoyable and rewarding voyage through the quantumworld.

We propose to implement a quantized thermal machine based on a mixture of two atomic species. One atomic species implements the working medium and the other implements two (cold and hot) baths. We show that such a setup can be employed for the refrigeration of a large bosonic cloud starting above and ending below the condensation threshold. We analyze its operation in a regime conforming to the quantized Otto cycle and discuss the prospects for continuous-cycle operation, addressing the experimental as well as theoretical limitations. Beyond its applicative significance, this setup has a potential for the study of fundamental questions of quantum thermodynamics.

The efficiency of cyclic heat engines is limited by the Carnot bound. This bound follows from the second law of thermodynamics and is attained by engines that operate between two thermal baths under the reversibility condition whereby the total entropy does not increase. By contrast, the efficiency of engines powered by quantum non-thermal baths has been claimed to surpass the thermodynamic Carnot bound. The key to understanding the performance of such engines is a proper division of the energy supplied by the bath to the system into heat and work, depending on the associated change in the system entropy and ergotropy. Due to their hybrid character, the efficiency bound for quantum engines powered by a non-thermal bath does not solely follow from the laws of thermodynamics. Hence, the thermodynamic Carnot bound is inapplicable to such hybrid engines. Yet, they do not violate the principles of thermodynamics. An alternative means of boosting machine performance is the concept of heat-to-work conversion catalysis by quantum non-linear (squeezed) pumping of the piston mode. This enhancement is due to the increased ability of the squeezed piston to store ergotropy. Since the catalyzed machine is fueled by thermal baths, it adheres to the Carnot bound. We conclude by arguing that it is not quantumness per se that improves the machine performance, but rather the properties of the baths, the working fluid and the piston that boost the ergotropy and minimize the wasted heat in both the input and the output.

Interesting effects arise in cyclic machines where both heat and ergotropy transfer take place between the energising bath and the system (the working fluid). Such effects correspond to unconventional decompositions of energy exchange between the bath and the system into heat and work, respectively, resulting in efficiency bounds that may surpass the Carnot efficiency. However, these effects are not directly linked with quantumness, but rather with heat and ergotropy, the likes of which can be realised without resorting to quantum mechanics.

The purpose of this article, which is our contribution to Wolfgang Schleich's Festschrift, is to present an unconventional perspective of two fundamental, interrelated problems: the emergence of classicality and the observer's role in quantum mechanics. In our perspective, the decoherence of quantum states of a system or its measuring device (meter) by the environment cannot replace the role of the observer, nor can it provide the ultimate explanation of the transition of the system from quantumness to classicality. The reason is that decoherence, despite appearances, typically does not irretrievably obliterate the information on the system's state. Even in cases where the environment has the potential of erasing this information, the observer can negate decoherence effects in the meter or the system by dynamical control. We contend that the choice of measuring bases and observables is largely up to the controller-observer (CO) that need not possess unlimited resources in order to steer or freeze (at will) the system-state evolution and its quantumness, notwithstanding the environment effects. An analogy is drawn between the outlined approach and Lamarckism, the theory of evolution that preceded Darwinism. Quantum Lamarckism expresses our view of evolution as functional adaptation of the system to the interplay between environmental and CO effects. In our view, both conceptually and practically, decoherence may be deemed non-essential for determining the evolution of a quantum system embedded in its environment, provided the CO has an adequate, albeit limited, arsenal of resources.

We present a comprehensive theory of heat engines (HE) based on a quantum-mechanical "working fluid" (WF) with periodically modulated energy levels. The theory is valid for any periodicity of driving Hamiltonians that commute with themselves at all times and do not induce coherence in the WF. Continuous and stroke cycles arise in opposite limits of this theory, which encompasses hitherto unfamiliar cycle forms, dubbed here hybrid cycles. The theory allows us to discover the speed, power, and efficiency limits attainable by incoherently operating multilevel HE depending on the cycle form and the dynamical regimes.

Diverse models of engines energised by quantum-coherent, hence non-thermal, baths allow the engine efficiency to transgress the standard thermodynamic Carnot bound. These transgressions call for an elucidation of the underlying mechanisms. Here we show that non-thermal baths may impart not only heat, but also mechanical work to a machine. The Carnot bound is inapplicable to such a hybrid machine. Intriguingly, it may exhibit dual action, concurrently as engine and refrigerator, with up to 100% efficiency. We conclude that even though a machine powered by a quantum bath may exhibit an unconventional performance, it still abides by the traditional principles of thermodynamics.

Nonlinear optical phenomena are typically local. Here, we predict the possibility of highly nonlocal optical nonlinearities for light propagating in atomic media trapped near a nano-waveguide, where long-range interactions between the atoms can be tailored. When the atoms are in an electromagnetically induced transparency configuration, the atomic interactions are translated to long-range interactions between photons and thus to highly nonlocal optical nonlinearities. We derive and analyze the governing nonlinear propagation equation, finding a roton-like excitation spectrum for light and the emergence of order in its output intensity. These predictions open the door to studies of unexplored wave dynamics and many-body physics with highly nonlocal interactions of optical fields in one dimension.

We investigated peculiarities of absorption, emission and photonic density of states of a cholesteric liquid crystal with an isotropic defect layer inside. The influence of the defect layer position on absorption and emission in the system was studied. It was shown that for non-diffracting circularly polarized incident light absorption/emission is maximum if the defect is in the centre of the system; and for diffracting circularly polarized incident light absorption/emission is maximum if the defect is shifted from the centre of the system to its left border from where light is incident. We also investigated influence of the defect layer thickness and those parameters which characterize loss and gain on absorption and emission. The influence of anisotropic absorption in the cholesteric liquid crystal layer on photonic density states was investigated, too.

We revisit the thermodynamic bounds of work extraction in simple quantum heat machines subject to control by frequent modulations that do not comply with adiabatic assumptions. The laws of thermodynamics are obeyed, yet anomalous deviations from the known bounds are revealed.

We explore means of maximizing the power output of a heat engine based on a periodically-driven quantum system that is constantly coupled to hot and cold baths. It is shown that the maximal power output of such a heat engine whose "working fluid" is a degenerate V-type three-level system is that generated by two independent two-level systems. Hence, level degeneracy is a thermodynamic resource that may effectively double the power output. The efficiency, however, is not affected. We find that coherence is not an essential asset in such multilevel-based heat engines. The existence of two thermalization pathways sharing a common ground state suffices for power enhancement.

Nonlinear optical propagation in cholesteric liquid crystals (CLC) with a spatially periodic helical molecular structure is studied experimentally and modeled numerically. This periodic structure can be seen as a Bragg grating with a propagation stopband for circularly polarized light. The CLC nonlinearity can be strengthened by adding absorption dye, thus reducing the nonlinear intensity threshold and the necessary propagation length. As the input power increases, a blue shift of the stopband is induced by the self-defocusing nonlinearity, leading to a substantial enhancement of the transmission and spreading of the beam. With further increase of the input power, the self-defocusing nonlinearity saturates, and the beam propagates as in the linear-diffraction regime. A system of nonlinear couple-mode equations is used to describe the propagation of the beam. Numerical results agree well with the experiment findings, suggesting that modulation of intensity and spatial profile of the beam can be achieved simultaneously under low input intensities in a compact CLC-based micro-device.

We explore, theoretically and experimentally, a method for cooling a broadband heat reservoir, via its laser-assisted collisions with two-level atoms followed by their fluorescence. This method is shown to be advantageous compared to existing laser-cooling methods in terms of its cooling efficiency, the lowest attainable temperature for broadband baths, and its versatility: it can cool down any heat reservoir, provided the laser is red detuned from the atomic resonance. It is applicable to cooling down both dense gaseous and condensed media.

We explore the dependence of the performance bounds of heat engines and refrigerators on the initial quantum state and the subsequent evolution of their piston, modeled by a quantized harmonic oscillator. Our goal is to provide a fully quantized treatment of self-contained (autonomous) heat machines, as opposed to their prevailing semiclassical description that consists of a quantum system alternately coupled to a hot or a cold heat bath and parametrically driven by a classical time-dependent piston or field. Here, by contrast, there is no external time-dependent driving. Instead, the evolution is caused by the stationary simultaneous interaction of two heat baths (having distinct spectra and temperatures) with a single two-level system that is in turn coupled to the quantum piston. The fully quantized treatment we put forward allows us to investigate work extraction and refrigeration by the tools of quantum-optical amplifier and dissipation theory, particularly, by the analysis of amplified or dissipated phase-plane quasiprobability distributions. Our main insight is that quantum states may be thermodynamic resources and can provide a powerful handle, or control, on the efficiency of the heat machine. In particular, a piston initialized in a coherent state can cause the engine to produce work at an efficiency above the Carnot bound in the linear amplification regime. In the refrigeration regime, the coefficient of performance can transgress the Carnot bound if the piston is initialized in a Fock state. The piston may be realized by a vibrational mode, as in nanomechanical setups, or an electromagnetic field mode, as in cavity-based scenarios.

We show that atoms subject to laser radiation may form a non-additive many-body system on account of their longrange forces, when the atoms are trapped in the vicinity of a fiber with a Bragg grating. When the laser frequency is inside the grating's bandgap but very close to its edge, we find that the range and strength of the laser-induced interaction becomes substantially enhanced, due to the large density of states near the edge, while the competing process of scattering to the fiber is inhibited. The dynamics of the atomic positions in this system conforms to a prominent model of statistical physics which exhibits slow relaxation. This suggests the possibility of using laserilluminated atoms to study the characteristics of non-additive systems.

Polarizable dipoles, such as atoms, molecules, or nanoparticles, subject to laser radiation may attract or repel each other. We derive a general formalism in which such laser-induced dipole-dipole interactions (LIDDIs) in any geometry and for any laser strength are described in terms of the resonant dipole-dipole interaction (RDDI) between dipoles dressed by the laser. This approach provides a simple route towards the analysis of LIDDI in a general geometry. Our general results reveal LIDDI effects due to nonlinear dipolar response to the laser, previously unaccounted for. The origin of these nonlinear effects is discussed. Our general formalism is illustrated for LIDDI between atoms in a cavity.

We show that frequent nondemolition measurements of a quantum system immersed in a thermal bath allow the extraction of work in a closed cycle from the system-bath interaction (correlation) energy, a hitherto unexploited work resource. It allows for work even if no information is gathered or the bath is at zero temperature, provided the cycle is within the bath memory time. The predicted work resource may be the basis of quantum engines embedded in a bath with long memory time, such as the electromagnetic bath of a high-Q cavity coupled to two-level systems.

We present an approach to monitoring and controlling a free quantum particle by coupling an internal (discrete) state of the particle to a detector (or probe). We consider a sequence of time-dependent, spatially localized interactions of the particle with the probe that are purely coherent (nondissipative), without mean energy-momentum exchange. We show that a sequence of such force-free interactions can freeze or deflect the particle.

We show that nonradiative interactions between atomic dipoles placed in a waveguide can give rise to deterministic entanglement at ranges much larger than their resonant wavelength. The range increases as the dipole resonance approaches the waveguide's cutoff frequency, caused by the giant density of photon modes near cutoff, a regime where the standard (perturbative) Markov approximation fails. We provide analytical theories for both the Markovian and non-Markovian regimes, supported by numerical simulations, and discuss possible experimental realizations.

For quantum systems under manipulations frequent enough to break the Markov approximation, bounds considered fundamental are revisited: the Szilard-Landauer bound on work traded for information, the Carnot engine efficiency bound or Nernst's third law.

A minimal model of a quantum refrigerator, i.e., a periodically phase-flipped two-level system permanently coupled to a finite-capacity bath (cold bath) and an infinite heat dump (hot bath), is introduced and used to investigate the cooling of the cold bath towards absolute zero (T=0). Remarkably, the temperature scaling of the cold-bath cooling rate reveals that it does not vanish as T→0 for certain realistic quantized baths, e.g., phonons in strongly disordered media (fractons) or quantized spin waves in ferromagnets (magnons). This result challenges Nernst's third-law formulation known as the unattainability principle.

We develop a general optimization strategy for performing a chosen unitary or nonunitary task on an open quantum system. The goal is to design a controlled time-dependent system Hamiltonian by variationally minimizing or maximizing a chosen function of the system state, which quantifies the task success (score), such as fidelity, purity, or entanglement. If the time dependence of the system Hamiltonian is fast enough to be comparable to or shorter than the response time of the bath, then the resulting non-Markovian dynamics is shown to optimize the chosen task score to second order in the coupling to the bath. This strategy can protect a desired unitary system evolution from bath-induced decoherence, but can also take advantage of the system-bath coupling so as to realize a desired nonunitary effect on the system.

Since the pioneering works of Carr-Purcell and Meiboom-Gill [Carr HY, Purcell EM (1954) Phys Rev 94:630; Meiboom S, Gill D (1985) Rev Sci Instrum 29:688], trains of π-pulses have featured amongst the main tools of quantum control. Echo trains find widespread use in nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI), thanks to their ability to free the evolution of a spin-1/2 from several sources of decoherence. Spin echoes have also been researched in dynamic decoupling scenarios, for prolonging the lifetimes of quantum states or coherences. Inspired by this search we introduce a family of spin-echo sequences, which can still detect site-specific interactions like the chemical shift. This is achieved thanks to the presence of weak environmental fluctuations of common occurrence in high-field NMR - such as homonuclear spin-spin couplings or chemical/biochemical exchanges. Both intuitive and rigorous derivations of the resulting "selective dynamical recoupling" sequences are provided. Applications of these novel experiments are given for a variety of NMR scenarios including determinations of shift effects under inhomogeneities overwhelming individual chemical identities, and model-free characterizations of chemically exchanging partners.

Historically, the completeness of quantum theory has been questioned using the concept of bipartite continuous-variable entanglement. The non-classical correlations (entanglement) between the two subsystems imply that the observables of one subsystem are determined by the measurement choice on the other, regardless of the distance between the subsystems. Nowadays, continuous-variable entanglement is regarded as an essential resource, allowing for quantum enhanced measurement resolution, the realization of quantum teleportation and quantum memories, or the demonstration of the Einstein-Podolsky-Rosen paradox. These applications rely on techniques to manipulate and detect coherences of quantum fields, the quadratures. Whereas in optics coherent homodyne detection of quadratures is a standard technique, for massive particles a corresponding method was missing. Here we report the realization of an atomic analogue to homodyne detection for the measurement of matter-wave quadratures. The application of this technique to a quantum state produced by spin-changing collisions in a Bose-Einstein condensate reveals continuous-variable entanglement, as well as the twin-atom character of the state. Our results provide a rare example of continuous-variable entanglement of massive particles. The direct detection of atomic quadratures has applications not only in experimental quantum atom optics, but also for the measurement of fields in many-body systems of massive particles.

Decoherence is a major obstacle to any practical implementation of quantum information processing. One of the leading strategies to reduce decoherence is dynamical decoupling-the use of an external field to average out the effect of the environment. The decoherence rate under any control field can be calculated if the spectrum of the coupling to the environment is known. We present a direct measurement of the bath-coupling spectrum in an ensemble of optically trapped ultra-cold atoms, by applying a spectrally narrow-band control field. The measured spectrum follows a Lorentzian shape at low frequencies but exhibits non-monotonic features at higher frequencies due to the oscillatory motion of the atoms in the trap. These features agree with our analytical models and numerical Monte Carlo simulations of the collisional bath. From the inferred bath-coupling spectrum, we predict the performance of some well-known dynamical decoupling sequences. We then apply these sequences in experiment and compare the results to predictions, finding good agreement in the weak-coupling limit. Thus, our work establishes experimentally the validity of the overlap integral formalism and is an important step towards the implementation of an optimal dynamical decoupling sequence for a given measured bath spectrum.

We show, using an exactly solvable model, that nonlinear dynamics is induced in a double-well Bose-Einstein condensate (BEC) by collisions with a thermal reservoir. This dynamics can facilitate the creation of phase or number squeezing and, at longer times, the creation of macroscopic nonclassical superposition states. Enhancement of these effects is possible by loading the reservoir atoms into an optical lattice.

We demonstrate through exact solutions that a spin bath leads to stronger (faster) dephasing of a qubit than a bosonic bath with an identical bath-coupling spectrum. This difference is due to the spin-bath "dressing" by the coupling. Consequently, the quantum statistics of the bath strongly affects the pulse sequences required to dynamically decouple the qubit from its bath.

We demonstrate that collective continuous variables of two species of trapped ultracold bosonic gases can be Einstein-Podolsky-Rosen-correlated (entangled) via inherent interactions between the species. We propose two different schemes for creating these correlations-a dynamical scheme and a static scheme analogous to two-mode squeezing in quantum optics. We quantify the correlations by using known measures of entanglement and study the effect of finite temperature on these quantum correlations.

Optical microscopy with spatial resolution below the diffraction limit is at present attracting extensive attentions. Further advancement of the near-field scanning optical microscopy (NSOM), a practical super-resolution microscopy, is mainly limited by the low transmission of optical power through the nano-meter apex. This work shows that lightwave can be efficiently delivered to a sub-100 nm apex inside a tapered metallic guiding structure. The enhanced light delivery, about 5-fold, is made possible with an adaptive optimization of the transmission via a spatial light phase-modulator. Numerical simulation shows the mechanism for the efficient light delivery to be the selective excitation of predominantly the lowest-order transverse component of standing wavevector with proper input wavefront modulation, hence favoring the transmission of lightwave in the longitudinal direction. The demonstration of such efficient focusing, to about full-width at half-maximum of a quarter wavelength, has a direct and immediate application in the improvement of the existing NSOMs.

We introduce a model of a two-dimensional (2D) self-attractive medium embedded into a quasi-1D symmetric double-well potential (DWP), whose depth is subject to periodic modulations (management). The model applies to matter waves in Bose-Einstein condensates (BECs), as well as to the nonlinear transmission of light (in spatial and temporal domains alike). It is known that, in the absence of the management, the DWP induces the spontaneous symmetry breaking (SSB) of 2D solitons, when their norm exceeds a critical value. Above the SSB point, symmetric solitons are unstable, while the system supports stable asymmetric ones. The DWP also admits Josephson oscillations of solitons between the two wells. We study effects of periodic modulations of the DWP's depth on the stability of symmetric, asymmetric, and oscillating solitons. Stability areas for the solitons of these three types are produced in the plane of the modulation frequency and amplitude. The shape of the stability borders is strongly affected by the proximity of the modulation frequency to the frequency of free oscillations of the soliton in the static DWP structure, the solitons being destroyed by the modulations with an arbitrarily small amplitude at the point of the exact resonance. Similar results are obtained for an extended 2D model, which combines the transverse DWP and a longitudinal periodic potential (optical lattice, in terms of BEC). The findings are compared with those reported in other recently studied management models for 1D and 2D solitons, which makes it possible to draw general conclusions about the stability limits of solitons under the resonant management.

We propose a control mechanism for the stabilization of symmetry-broken modes in Bose-Einstein condensates and optical beams trapped in double-well potentials in the presence of symmetric or asymmetric noise. The control, based on periodic π shifts of the phase in one well, is demonstrated for solitons and in the two-mode approximation. The latter setting is considered in both the mean-field and quantum (Bose-Hubbard) regimes.

We study deviations from thermal equilibrium between two-level systems (TLS) and a bath by frequent and brief quantum measurements of the TLS energy-states. The resulting entropy and temperature of both the system and the bath are found to be completely determined by the measurement rate, and unrelated to what is expected by standard thermodynamical rules that hold for Markovian baths. These anomalies allow for very fast control heating, cooling and state-purification (entropy reduction) of quantum systems much sooner than their thermal equilibration time.

We put forward a general strategy for dynamic control that ensures bath-optimized fidelity of a desired multidimensional quantum operation in the presence of non-Markovian baths and noises with stationary autocorrelations. It benefits from the vast freedom of arbitrary, not just pulsed, time-dependent control. This allows the dramatic reduction of the invested energy and the corresponding error compared to pulsed control.

Quantum two-state systems, known as quantum bits (qubits), are unavoidably in contact with their uncontrolled thermal environment, also known as a macroscopic 'bath'. The higher the temperature of the qubits, the more impure their quantum state and the less useful they are for coherent control or quantum logic operations, hence the desirability of cooling down the qubits as much and as fast as possible, so as to purify their state prior to the desired operation. Yet, the limit on the speed of existing cooling schemes, which are all based on Markovian principles, is either the duration of the qubit equilibration with its bath or the decay time of an auxiliary state to one of the qubit states. Here we pose the conceptual question: can one bypass this existing Markovian limit? We show that highly frequent phase shifts or measurements of the state of thermalized qubits can lead to their ultrafast cooling, within the non-Markov time domain, well before they re-equilibrate with the bath and without resorting to auxiliary states. Alternatively, such operations may lead to the cooling down of the qubit to arbitrarily low temperatures at longer times. These anomalous nonMarkov cooling processes stem from the hitherto unfamiliar coherent quantum dynamics of the qubit-bath interaction well within the bath memory time.

A general quantum noisy channel is analyzed, wherein the transmitted qubits may experience symmetry-breaking decoherence, along with memory effects. We examine generalized Bell-basis and find that the optimal basis is not the fully entangled one, but a combination of factorized and partially-entangled states in the presence of memory, asymmetry and the state-bias of the noise. Capacity-maximization is shown to be achievable by combining temporal shaping of the transmitted qubits and optimal basis selection.

An analytical expression is received for the effective interaction potential of a fast charged particle with the ionic crystal CsCl near the direction of axis 100 as a function of the temperature of the medium. By numerical analysis it is shown that the effective potential of axial channeling of positrons along the axis 100 of negatively charged Cl^- ions practically doesn't depend on temperature of the media. The wavefunction and the energy spectrum of the localized state are investigated, and the possibility is proved for the formation of metastable, two-dimensional relativistic positron systems or the positron atoms (PA), i.e. the bound state of a positron in the present case with a negatively charged axis of ions. The problem of one-photon decay of PA is investigated. The possibilities of stimulation of resonant transitions between quantum states of PA by the external hypersound are investigated in detail.

We examine the possibility of coherent reversible information transfer between solid-state superconducting qubits and ensembles of ultracold atoms. Strong coupling between these systems is mediated by a microwave transmission line resonator that interacts near resonantly with the atoms via their optically excited Rydberg states. The solid-state qubits can then be used to implement rapid quantum logic gates, while collective metastable states of the atoms can be employed for long-term storage and optical readout of quantum information.

Given a multilevel system coupled to a bath, we use a Feshbach P, Q partitioning technique to derive an exact trace-nonpreserving master equation for a subspace Si of the system. The resultant equation properly treats the leakage effect from Si into the remainder of the system space. Focusing on a second-order approximation, we show that a one-dimensional master equation is sufficient to study problems of quantum state storage and is a good approximation, or exact, for several analytical models. It allows a natural definition of a leakage function and its control and provides a general approach to study and control decoherence and leakage. Numerical calculations on an harmonic oscillator coupled to a room temperature harmonic bath show that the leakage can be suppressed by the pulse control technique without requiring ideal pulses.

We examine two set-ups that reveal different operational implications of path-phase complementarity for single photons in a Mach-Zehnder interferometer (MZI). In both set-ups, the which-way (WW) information is recorded in the polarization state of the photon serving as a 'flying which-way detector'. In the 'predictive' variant, using a fixed initial state, one obtains duality relation between the probability to correctly predict the outcome of either a which-way (WW) or which-phase (WP) measurement (equivalent to the conventional path-distinguishability-visibility). In this set-up, only one or the other (WW or WP) prediction has operational meaning in a single experiment. In the second, 'retrodictive' protocol, the initial state is secretly selected for each photon by one party, Alice, among a set of initial states which may differ in the amplitudes and phases of the photon in each arm of the MZI. The goal of the other party, Bob, is to retrodict the initial state by measurements on the photon. Here, a similar duality relation between WP and WW probabilities governs their simultaneous guesses in each experimental run.

We consider single photons propagating along two paths, with the polarization correlated to the path. Two information related aspects of this translational-internal entanglement (TIE) are analyzed: a) Using the polarization to record the path (a "flying detector" scheme), we characterize the tradeoff between path- and phase-information. b) We investigate the effects of non-Markovian noise on the two-qubit quantum channel consisting of the photon path and polarization (that are both used to encode information), and suggest noise protection schemes.

We propose a novel protocol for the creation of macroscopic quantum superposition (MQS) states based on a measurement of a non-monotonous function of a quantum collective variable. The main advantage of this protocol is that it does not require switching on and off nonlinear interactions in the system. We predict this protocol to allow the creation of multiatom MQS by measuring the number of atoms coherently outcoupled from a two-component (spinor) Bose-Einstein condensate.

Self-pulsed lasing in the form of a coherent solitary wave train from a periodically amplifying Bragg structure is proposed. It is shown both analytically and numerically that the generated pulse train propagates in the periodically amplifying Bragg structure chip with superluminal group velocity. Its stability and coherence are in contrast with the inherent instability and chaoticity of self-pulsing from a uniform gain medium embedded in a Bragg reflector. Bragg-periodic semiconductor quantum-well heterostructures are candidates for realizing the required periodically amplifying Bragg structure.

We study the effect of noise-induced dephasing on collisional phase diffusion in the two-site Bose-Hubbard model. Dephasing of the quasimomentum modes may slow down phase diffusion in the quantum Zeno limit. Remarkably, the degree of suppression is enhanced by a bosonic factor of order N/log N as the particle number N increases.

Heat flow between a large thermal 'bath' and a smaller system brings them progressively closer to thermal equilibrium while increasing their entropy. Fluctuations involving a small fraction of a statistical ensemble of systems interacting with the bath result in deviations from this trend. In this respect, quantum and classical thermodynamics are in agreement. Here we predict a different trend in a purely quantum mechanical setting: disturbances of thermal equilibrium between two-level systems (TLSs) and a bath, caused by frequent, brief quantum non-demolition measurements of the TLS energy states. By making the measurements increasingly frequent, we encounter first the anti-Zeno regime and then the Zeno regime (namely where the TLSs' relaxation respectively speeds up and slows down). The corresponding entropy and temperature of both the system and the bath are then found to either decrease or increase depending only on the rate of observation, contrary to the standard thermodynamical rules that hold for memory-less (Markov) baths. From a practical viewpoint, these anomalies may offer the possibility of very fast control of heat and entropy in quantum systems, allowing cooling and state purification over an interval much shorter than the time needed for thermal equilibration or for a feedback control loop.

A polarized photon with well-defined orbital angular momentum that emerges from a Mach-Zehnder interferometer (MZI) is shown to seemingly circumvent wave-particle duality constraints. For certain phase differences between the MZI arms, this pattern yields both reliable which-path information and high phase sensitivity.

We present and compare stochastic open-loop techniques aimed at controlling quantum coherence in dissipative environments. One approach describes the evolution time as a random non-Gaussian variable. The other implements dynamical control on non-Markovian time-scales via stochastic modulation of the system-bath coupling.

The evolution of nonlinear light fields traveling inside a resonantly absorbing Bragg reflector is studied by use of Maxwell-Bloch equations. Numerical results show that a pulse initially resembling a light bullet may effectively experience negative refraction and anomalous dispersion in the resonantly absorbing Bragg reflector.

We investigate conditions under which multiatom absorption of a single photon leads to cooperative decay. Our analysis reveals the symmetry properties of the multiatom Dicke states underlying the cooperative decay dynamics and their spatio-temporal manifestations, particularly, the forward-directed spontaneous emission investigated by Scully et al (2006 Phys. Rev. Lett. 96 010501).

Using quantum channels to transmit classical information has been proven to be advantageous in several scenarios. These channels have been assumed to be memoryless, meaning that consecutive transmissions of information are uncorrelated. However, as shown experimentally, such correlations do exist, and thereby retain memory of previous information. This memory complicates the protection of entangled-information transmission from decoherence. We have recently addressed these fundamental questions by developing a generalized master equation for multipartite entangled systems coupled to finite-temperature baths and subject to arbitrary external perturbations whose role is to provide dynamical protection from decay and decoherence. Here we explore and extend the foregoing strategy to quantum optical communication schemes wherein polarization-entangled photons traverse a bit-flip channel with temporal and spatial memory, such that the two channels experience cross-decoherence. We introduce a novel approach to the protection of the entangled information from decoherence in such schemes. It is based on selectively modulating the photon polarizations in each channel. We show that by applying selective modulation, one can independently control the symmetry and spatial memory attributes of the. channel. We then explore the effects of these attributes on the channel capacity. Remarkably, we show that there is a nontrivial interplay between the effects of asymmetry and memory on the channel capacity.

We theoretically investigate the free dispersion, scattering and disintegration of an N-particle quantum soliton by Bragg light pulses, comparing the exact quantum result with its quasiclassical (mean-field) limit. Remarkably, we find that the correlation properties of a quantum soliton approach the (mean-field) limit very rapidly as the particle number N grows. A modest discrepancy between the classical and quantum results is observed for N = 3.

A unified theory is given of dynamically modified decay and decoherence of field-driven multilevel multipartite entangled states that are weakly coupled to zero-temperature baths or undergo random phase fluctuations. The theory allows for arbitrary local differences in their coupling to the environment. Due to such differences, the optimal driving-field modulation to ensure maximal fidelity is found to substantially differ from conventional "Bang-Bang" or π-phase flips of the single-qubit evolution.

Here we aim at setting the principles of and quantifying translational entanglement by collisions and half-collisions. In collisions, the resonance width s and the initial phase-space distributions axe shown to determine the degree of post-collisional momentum entanglement. Half-collisions (dissociation) axe shown to yield different types of approximate EPR states. We analyse a feasible realization of translational EPR entanglement and teleportation via cold-molecule Raman dissociation and subsequent collisions, resolving both practical and conceptual difficulties it has faced so far.

We present an electrostatic mechanism that gives rise to dramatic enhancements of tunneling through the inter-nuclear Coulomb-barrier. The enhancement is due to an increase in the effective negative electric charge in the mid region between two protons (deutrons) belonging to an H2+ (D2+) molecule placed inside a molecular cage, such as a fullerene molecule. This charge increase results in a marked reduction of the outer regions of the Coulomb-barrier, thereby dramatically increasing the nuclear tunneling rates. Coupled with compression of the molecular cage this effect may lead to meaningful enhancements in fusion rates.

A scalable, high-performance quantum processor can be implemented using near-resonant dipole-dipole interacting dopants in a transparent solid state host. In this scheme, the qubits are represented by ground and subradiant states of effective dimers formed by pairs of closely spaced two-level systems, while the two-qubit entanglement either relies on the coherent excitation exchange between the dimers or is mediated by external laser fields.

An entangled multipartite system coupled to a zero-temperature bath undergoes rapid disentanglement in many realistic scenarios due to local, symmetry-breaking differences in the particle-bath couplings. We show that locally controlled perturbations, addressing each particle individually, can impose a symmetry allowing the existence of decoherence-free multipartite entangled systems.

We show that the giant Kerr nonlinearity in the regime of electromagnetically induced transparency in vapor can give rise to the formation of Thirring-type spatial solitons, which are supported solely by cross-phase modulation.

To date, the translationally-entangled state originally proposed by Einstein, Podolsky and Rosen (EPR) in 1935 has not been experimentally realized for massive particles. Opatrný and Kurizki [Phys. Rev. Lett. 86, 3180 (2000)] have suggested the creation of a position- and momentum-correlated, i.e., translationally-entangled, pair of particles approximating the EPR state by dissociation of cold diatomic molecules, and further manipulation of the EPR pair effecting matter-wave teleportation. Here we aim at setting the principles of and quantifying translational entanglement by collisions and half-collisions. In collisions, the resonance width s and the initial phase-space distributions are shown to determine the degree of post-collisional momentum entanglement. Half-collisions (dissociation) are shown to yield different types of approximate EPR states. We analyse a feasible realization of translational EPR entanglement and teleportation via cold-molecule Raman dissociation and subsequent collisions, resolving both practical and conceptual difficulties it has faced so far: How to avoid entanglement loss due to the wavepacket spreading of the dissociation fragments? How to measure both position and momentum correlations of the dissociation fragments with sufficient accuracy to verify their EPR correlations? How to reliably perform two-particle (Bell) position and momentum measurements on one of the fragments and the wavepacket to be teleported?

We show that very large nonlocal nonlinear interactions between pairs of colliding slow-light pulses can be realized in atomic vapors in the regime of electromagnetically induced transparency. These nonlinearities are mediated by strong, long-range dipole-dipole interactions between Rydberg states of the multilevel atoms in a ladder configuration. In contrast to previously studied schemes, this mechanism can yield a homogeneous conditional phase shift of π even for weakly focused single-photon pulses, thereby allowing a deterministic realization of the photonic phase gate.

A unified theory is given of dynamically modified decay and decoherence in driven two-level and multilevel quantum systems that are weakly coupled to arbitrary finitetemperature reservoirs and undergo random phase fluctuations. Criteria for the optimization of decoherence suppression and the limitations of this approach are obtained. For a driven qubit that is strongly coupled to the continuum edge of reservoir's spectrum, we demonstrate that only an appropriately ordered sequence of abrupt changes of the resonance frequency, near the continuum edge, can effectively protect the qubit state from decoherence.

In this paper, we show how the non-holonomic control technique can be employed to build completely controlled quantum devices. Examples of such controlled structures are provided.

In this paper, we present a realistic application of the coherence protection method proposed in the previous article. A qubit of information encoded on the two spin states of a Rubidium isotope is protected from the action of electric and magnetic fields.

The fabrication and study of dielectric structures whose refractive index is periodically modulated on a micron or submicron scale, known as photonic crystals (PCs), are attracting considerable interest at present. One-, two-, or three-dimensional (1D-, 2D-, or 3D-) periodic PCs exhibit photonic band gaps (PBGs), analogously to electronic band gaps in ordinary crystals, and can incorporate defects, designed to form localized narrow-linewidth (high-Q) modes at PBG frequencies.

We study collisions mediated by finite-range potentials as a tool for generating translational entanglement between unbound particles or multipartite systems. The general analysis is applied to one-dimensional scattering, where resonances and the initial phase-space distribution are shown to determine the degree of postcollisional entanglement.

We study the giant Kerr nonlinear interaction between two ultraweak optical fields in which the cross-phase-modulation is not accompanied by spectral broadening of the interacting pulses. This regime is realizable in atomic vapors, when a weak probe pulse, upon propagating through the electromagnetically induced transparency (EIT) medium, interacts with a signal pulse that is dynamically trapped in a photonic band gap created by spatially periodic modulation of its EIT resonance. We find that large conditional phase shifts and entanglement between the signal and probe fields can be obtained with this scheme. The attainable π phase shift, accompanied by negligible absorption and quantum noise, is shown to allow a high-fidelity realization of the controlled-phase universal logic gate between two single-photon pulses.

Decay acceleration by frequent measurements (interruptions of the coupling), known as the anti-Zeno effect is argued to be much more ubiquitous than its inhibition in one- or two-level systems coupled to reservoirs (continua). In multilevel systems, frequent measurements cause accelerated decay by destroying the multilevel interference, which tends to inhibit decay in the absence of measurements.

The fabrication and study of dielectric structures whose refractive index is periodically modulated on a micron or submicron scale, known as photonic crystals (PCs), are attracting considerable interest at present. One-, two-, or three-dimensional (1D-, 2D-, or 3D-) periodic PCs exhibit photonic band gaps (PBGs), analogously to electronic band gaps in ordinary crystals, and can incorporate defects, designed to form localized narrow-linewidth (high-Q) modes at PBG frequencies. The advent of such structures opens up new perspectives in atomic physics and quantum optics, since they are expected to allow an unprecedented control over the spectral density of modes (DOM) and the spatial modulation of narrow-linewidth (high-Q) modes, in the microwave, infrared, and optical domains. An interesting situation arises when foreign atoms or ions dopants with transition frequencies within the PBG, are implanted in the PC. Then light near one of these frequencies resonantly interacts with the dopants and is concurrently affected by the PBG dispersion. Consequently, highly nonlinear processes with a rich variety of unusual PC-related features are anticipated. Such nonlinear optical processes, involving near-resonant transitions in PCs, undergo basic modifications as compared to the corresponding processes in free space, which are attributed to the strong suppression of the DOM within PBGs, to sharp bandedges and to intra-gap narrow lines associated with high-Q defect modes. Results are detailed below for spontaneous emission of radiation in PCs and for the resonant interaction of atoms with the field of a single high-Q defect mode. These results stem from the failure of perturbation theory....

Eukaryotic cells produce a variety of specialized actin-rich surface protrusions. These include filopodia - thin, highly dynamic projections that help cells to sense their external environment [1]. Filopodia consist of parallel filaments of actin, bundled by actin crosslinking proteins. The filaments are oriented with their rapidly growing "barbed" ends at the protruding tip and their slowly growing "pointed" ends at the base [2]. Extension occurs by polymerization at the tip [3] and is controlled by regulation of filament capping [4]. The Rho GTPase Cdc42 is a key mediator of filopodia formation, which it regulates through binding CRIB domain-containing effectors [2]. Cdc42 binds and activates the WASP proteins, which in turn activate the actin-nucleating complex Arp2/3 [2]. It also binds and activates IRSp53, which recruits the Ena/WASP family protein Mena [5] to the filopodial tip and protects elongating actin filaments from capping [4]. Previously, we identified another Rho family GTPase, Rif, as a potent stimulator of filopodial protrusion through a mechanism that does not require Cdc42 [6]. Here we characterize the differences between filopodia induced by these two small GTPases and show that the Rif effector in this pathway is the Diaphanous-related formin mDia2. Thus, Rif and Cdc42 represent two distinct routes to the induction of filopodia - producing structures with both shared and unique properties.

The scattering threshold and cavity-enhanced gain in nonlinear spheres was investigated. Pairs of pump-driven idler and signal modes were also considered. The thresholds and gain coefficients of amplitude and stimulated Raman scattering were derived and evaluated under typical conditions. A simplified model of two interacting modes satisfying both the input and output resonant conditions was used.

A method to treat macroscopic quantum tunneling (MQT) by controlling it dynamically was discussed. The methods showed that decay modifications reminiscent of the quantum Zeno effect (QZE) and anti-Zeno effect (AZE) that coupled to the continuum through macroscopic quantum tunneling were realiazable for macroscopic quantum states. The MQT were dynamically controlled at low temperatures. The high-sensitivity of the dynamics and the modification of the decay rate to moderate changes were also elaborated.

We study a gaseous atomic Bose-Einstein condensate with laser-induced dipole-dipole interactions using the Hartree-Fock-Bogoliubov theory within the Popov approximation. The dipolar interactions introduce long-range atom-atom correlations which manifest themselves as increased depletion at momenta similar to that of the laser wavelength, as well as a 'roton' dip in the excitation spectrum. Surprisingly, the roton dip and the corresponding peak in the depletion are enhanced by raising the temperature above absolute zero.

We introduce and discuss two schemes for generation and transfer of photon-photon and atom-atom entanglement. First we propose a method to achieve a large conditional phase shift of a probe field in the presence of a single-photon control field whose carrier frequency is within the photonic band gap created by spatially-periodic modulation of the electromagnetically induced transparency resonance. Then we present the concept of a reversible transfer of the quantum state of two internally-translationally entangled fragments, formed by molecular dissociation, to a photon pair. Our scheme allows, in principle, high-fidelity state transfer from the entangled dissociated fragments to light, thereby producing a highly correlated photon pair. This process can be followed by its reversal at a distant node of a quantum network resulting in the recreation of the original two-fragment entangled state. The proposed schemes may have advantageous applications in quantum teleportation, cryptography, and quantum computation.

A study was performed on inversionless free-electron laser having a drift region consisting of two magnets. The numerical simulations of electron motion inside wigglers and the drift region were performed. It was found that this system has a positive mean gain over the entire energy distribution of the electron beam.

We consider a gaseous atomic Bose-Einstein condensate with dipole-dipole interactions induced by a far off-resonance laser. The long-range dipolar interactions introduce atom-atom correlations on the new scale of the laser wavelength, giving a controllable method of squeezing low energy modes. At high enough laser intensities the correlations lead to a roton minimum in the excitation spectrum.

This paper studies a strong modulation of the phase of a weak pulse subject to electromagnetically-induced transparency (EIT) by a control pulse in the form of an self-induced transparency (SIT) gap soliton (GS) moving at the same slow velocity. The aim of this work is to match the group velocities of the weak probe field subject to EIT and the free-propagating field having the form of a slow SIT GS.

Photonic crystals doped with resonant atoms allow for an efficient cross-coupling between pulses exhibiting electromagnetically induced transparency and self-induced transparency gap solitons. These features may find applications in high-fidelity classical and quantum optical communications.

We predict the existence of a new type of spatiotemporal soliton (so-called light bullets) in twodimensional self-induced-transparency media with refractive-index modulation in the direction transverse to that of pulse propagation. These self-localized guided modes are found in an approximate analytical form. Their existence and stability are confirmed by numerical simulations, and they may have advantageous properties for signal transmission.

The weakness of optical nonlinearities in conventional media preclude the effective interaction of extremely feeble fields containing few photons only. This weakness sets a limit on the performance of ultrasensitive photonic elements, as well as on nonclassical effects. This paper proposes a scheme that can remove this bottleneck, by basically modifying the nonlinear interaction of weak optical fields.

We propose a realization of a scalable, high-performance quantum processor whose qubits are represented by the ground and subradiant states of effective dimers formed by pairs of two-level systems coupled by resonant dipole-dipole interaction. The dimers are implanted in low-temperature solid host material at controllable nanoscale separations. The two-qubit entanglement either relies on the coherent excitation exchange between the dimers or is mediated by external laser fields.

We consider a trapped cigar-shaped atomic Bose-Einstein condensate irradiated by a single far-off resonance laser polarized along the cigar axis. The resulting laser-induced dipole-dipole interactions between the atoms significantly change the size of the condensate, and can even cause its self-trapping.

Interference in free electron laser (FEL) was analyzed to study the broadband optical gain of an optically dispersive interface region in FEL. The dynamics of an electron interacting with laser field in optical wiggler was presented to determine the small-signal gain in two interfering wigglers. The electron drift between the wigglers in dispersive regions of an optical klystron acquired a phase shift relative to the ponderomotive potential with an increase in the deviation of electron velocity. The analysis suggested universal dependence of broadband gain on optical dispersion.

Cold diatomic molecules are analyzed as a source of atom pairs. The dissociation of these molecules are investigated. The atoms pairs have highly correlated positions and momenta. The collision with one of the EPR correlated atoms and manipulation of the other atom in the pair can cause wave packet teleportation.

Novel regimes of probe scattering by atoms trapped in optical lattices in the random-density and multiple-scattering regimes were identified. These regimes were characterized by a universal feature of large density fluctuations.

We consider the influence of refractive index fluctuations (due to randomly distributed inclusions inside a dielectric spherical droplet) on the line shapes of scattering Mie resonances. The significant difference in the spatial distributions of the mode functions participating in the process does not allow one to employ the standard statistical ensembles used in random matrix theory. We propose to model the system by a simplified random ensemble which gives a very good agreement with the available experimental data, and we predict a type of line shape for narrow scattering resonances.

The quantum Zeno effect (QZE) is the striking prediction that the decay of any unstable quantum state can be inhibited by sufficiently frequent observations (measurements). The consensus opinion has upheld the QZE as a general feature of quantum mechanics which should lead to the inhibition of any decay. The claim of QZE generality hinges on the assumption that successive observations can in principle be made at time intervals too short for the system to change appreciably. However, this assumption and the generality of the QZE have scarcely been investigated thus far. We have addressed these issues by showing that (i) the QZE is principally unattainable in radiative or radioactive decay, because the required measurement rates would cause the system to disintegrate; (ii) decay acceleration by frequent measurements (the anti-Zeno effect - AZE) is much more ubiquitous than its inhibition. The AZE is shown to be observable as the enhancement of tunneling rates (e.g., for atoms trapped in ramped-up potentials or in current-swept Josephson junctions), fluorescence rates (e. g., for Rydberg atoms perturbed by noisy optical fields) and photon depolarization rates (in randomly modulated Pockels cells).

A free-electron laser without inversion (FELWI) consisting of two noncollinear wigglers with an intermediate magnetic drift region was studied. Different types of phase shifts, linear in the detuning, which optimize the FELWI gain were analyzed. The realization of a free-electron laser system was possible with the noncollinear geometry and the angular divergence of the electron beam at the exit from the first wriggler. The phase shifts optimizing the gain were found.

We show that the fidelity of teleportation of continuous quantum variables can be improved by conditional photon-number measurement of the entangled state. Further, we propose a teleportation scheme based on photon counting on the output fields of a squeezer that combines the mode whose quantum state is desired to be teleported and one mode of the two-mode squeezed vacuum playing the role of the entangled state.

We show that the recently proposed scheme of teleportation of continuous variables [S.L. Braunstein and H.J. Kimble, Phys. Rev. Lett. 80, 869 (1998)] can be improved by a conditional measurement of the entangled state shared by the sender and the recipient. The conditional measurement subtracts photons from the original entangled two-mode squeezed vacuum, by transmitting each mode through a low-reflectivity beam splitter and performing a joint photon-number measurement on the reflected beams. In this way the degree of entanglement of the shared state is increased and so is the fidelity of the teleported state.

We examine several proposed schemes by Franson et al. for quantum logic gates based on non-local exchange interactions between two photons in a medium. In these schemes the presence of a single photon in a given mode is supposed to induce a large phase shift on another photon propagating in the same medium. We conclude that the schemes proposed so far are not able to produce the required conditional phase shift, even though the proposals contain many stimulating and intriguing ideas.

We address several generic quantum optical processes that undergo basic modifications in photonic crystals: (a) spontaneous formation of atomic coherence; (b) two-photon binding and entanglement; (c) self-induced transparency and gap solitons.

We point out that the quantum Zeno effect, i.e., inhibition of spontaneous decay by frequent measurements, is observable only in spectrally finite reservoirs, i.e., in cavities and waveguides, using a sequence of evolution-interrupting pulses or randomly-modulated CW fields. By contrast, such measurements can only accelerate decay in free space.

We consider an optical medium consisting of a periodic refractive-index grating and a periodic set of thin layers of two-level systems resonantly interacting with the electromagnetic field. Recently, it has been shown that such a system gives rise to a vast variety of stable bright solitons. In this work, we demonstrate that the system has another very unusual property: stable bright solitons can coexist with stable continuous-wave (cw) states and stable dark solitons (DSs). Depending on the parameters values, a DS frequency band coexists (without overlap) with one or two bright-soliton bands. Quiescent (standing) DSs are found in an analytical form, and moving ones are obtained numerically. Simulations show that a considerable part of the DS solutions are completely stable against arbitrary small perturbations. The fact that this system supports both stable bright and dark solitons for the same parameters values may find interesting applications in photonics.

With the help of some remarkable examples, it is shown that conditional measurements performed on two-level atoms just after they have interacted with a resonant cavity field mode are able to recover the coherence of number-state superpositions, which is lost due to dissipation.

Spontaneous decay of excited cold atoms into a cavity can drastically affect their translational dynamics, namely, atomic reflection and transmission through a potential barrier.

We demonstrate the possibility of simply and effectively controlling the inversionless gain between two excited states in a ladder configuration, by injecting atoms (molecules) into a cavity. The control is effected by the cavity-mode decay rate and vacuum Rabi frequency, chosen according to the decay and dephasing rates of the upper and lower states. The analogy with the control of inversionless gain by a classical strong field is discussed. This scheme is expected to be robust under conditions of thermal equilibrium or non-selective pumping of excited states.

Novel methods are discussed for the state control of atoms coupled to multi-mode reservoirs with non-Markovian spectra: 1) Excitation decay control: we point out that the quantum Zeno effect, i.e., inhibition of spontaneous decay by frequent measurements, is observable in open cavities and waveguides using a sequence of evolution-interrupting pulses or randomly-modulated CW fields. 2) Location-dependent interference of decay channels - nonadiabatic (resonant) control: We show that the control of populations and coherences of two metastable states is feasible via resonant single-photon absorption to an intermediate state, by controlled spontaneous emission in a cavity. 3) Decoherence control by conditionally interfering parallel evolutions: We demonstrate that an arbitrary internal atomic state can be completely protected from decoherence by interference of its interactions with the reservoir over many different time intervals in parallel. Such interference is conditional upon the detection of appropriate atomic-momentum observables. Realization in cavities is suggested. The rich arsenal of control methods described above can improve the performance of single-atom devices. It can also advance the state-of-the-art of quantum information encoding and processing.

We theoretically analyze wave packet transmission through a phase-conjugating mirror and show that the transmission of a suitably chosen input pulse is superluminal, i.e. the peak of the pulse emerges from the mirror before the time it takes to travel the same distance in vacuum. This pulse reshaping effect can be attributed directly to the dispersion relation in the nonlinear medium constituting the mirror. Thus, for the first time a connection is laid between optical phase conjugation and superluminal behavior. In view of its additional amplifying ability, a phase-conjugating mirror is a most promising candidate for an experimental observation of tachyonic signatures.

We theoretically investigate the response of optical phase conjugators to incident probe pulses. In the stable (subthreshold) operating regime of an optical phase conjugator it is possible to transmit probe pulses with a superluminally advanced peak, whereas conjugate reflection is always subluminal. In the unstable (above-threshold) regime, superluminal response occurs both in reflection and in transmission, at times preceding the onset of exponential growth due to the instability.

Remarkable quantum effects are predicted for an initially excited two-level wave packet incident on a broadband vacuum-field reservoir with sharp spatial boundaries: (a) reflection of the excited wave packet from the boundary (skin effect); (b) transient three-dimensional binding of the decayed wave packet to the boundary by photons carrying more energy than the incident wave packet. These effects are realizable for cold atoms in cavities where excitation decay is nearly exponential but strongly enhanced.

Quantum theory is applied to interference of emission and absorption in a two-section Čerenkov free-electron laser (FEL). A scheme for absorption cancellation with enhanced gain, which resembles "lasing without inversion" in atomic systems, is proposed. This scheme is based on the adjustment of the phases of electrons and light by appropriate dispersion between the two sections. It enables FEL operation even with a broad electron momentum spread. Such a scheme is interesting in the context of FEL operation in the ultraviolet and X-ray regimes.

We put forward a theory of excitation decay in two-level atoms that tunnel through a square potential barrier while spontaneously emitting photons into an effectively one-dimensional mode continuum. The resulting decoherence can exponentially enhance the total tunneling probability. This enhancement is due to atoms whose final kinetic energy is raised above the barrier by the emission of photons detuned below resonance.

We investigate the resonant interaction of a dipolar j=0⇆j=1 angular-momentum transition with the quantized field in a dielectric sphere. New features arise on account of the degeneracy of atomic levels and field modes with low azimuthal angular momentum, in slightly deformed spheres (oblate spheroids). For TE-mode excitation we obtain the dynamics of a degenerate Λ or V configuration with the usual coherent-state collapse and revivals. For TM-mode excitation new behavior is found: due to interference between σ- and π-polarized transitions, which can be controlled by the atomic position and/or dipole orientation, coherent-state revivals and the corresponding atom-field energy exchange may be suppressed or delayed.

A system of two identical atoms sharing a photon with a high-[Formula Presented] resonator mode is studied by a nonperturbative formalism. As opposed to previously considered models, here the interatom coupling is shown to result from competing effects of vacuum Rabi splitting and photon exchange via off-resonant modes. Strong suppression of interatom excitation transfer is predicted at both near- and far-zone separations.

Electromagnetic fields dressed by inverted two-level atoms become tachyonlike excitations with group velocities which are faster than c, infinite, or negative. Such excitations describe the stable modes of the medium when it is weakly probed off resonance. The launching of these tachyonlike excitations is discussed, along with a proposed experiment to observe them. Their existence does not violate Einstein causality.

We survey our recent results on the modifications of optical processes by field confining structures.

We establish a quantum theory of gap solitons in a one-dimensional periodic dielectric structure [distributed Bragg reflector (DBR)] of a Kerr nonlinearity. The electromagnetic field in the structure is quantized within the two-band approximation. The basic idea for this quantization is that incident photons with frequencies in the band gap of the DBR are scattered into the upper \u201cconduction\u201d and lower \u201cvalence\u201d bands by the nonlinearity. The effective Hamiltonian of quantum gap solitons is derived within the effective-mass regime. The eigenstates of the Hamiltonian are constructed exactly by Bethes ansatz method. We find that in certain band gaps of the DBR these eigenstates can be bound, consisting of one or more photon pairs from the conduction and valence bands. These bound multiphoton states are optical analogs of exciton molecules [optical multiexcitons (OMEs)]. Their existence should be manifested by the discrete spectrum of band-gap transmission as a function of the transmitted photon number and by the multiexponential falloff of intensity-intensity correlations on a 0.1-mm scale. Quantum gap solitons are shown to arise as superpositions of OMEs whenever the nonlinear binding exceeds phonon scattering.

We analyze the near-resonant interaction between a two-level atom with resonance frequency in a photonic band gap and a local-defect field mode with initially thermal or Poissonian photon statistics. Whereas spontaneous decay is forbidden in this case, atomic states dressed by the defect field can decay via coupling to the mode continuum outside the band gap. Remarkably, the resulting non-Markovian cascade of dressed states transitions can lead to the accumulation of nearly all of the initial photon-number distribution in a single Fock state, upon measuring the excitation of the atom emerging from the structure. Under certain conditions, such a measurement can yield a correlated combination of two adjacent Fock states, which reflects the initial coherence in either the atom or the field. Another striking coherence effect is the non-decaying oscillation (as a function of interaction time) exhibited by Fock-state populations prepared by measuring the final atomic state.

We show that pulse transmission through near-resonant media embedded within periodic dielectric structures can produce self-induced transparency (SIT) in the band gap of such structures. This SIT constitutes a principally new type of gap soliton.

We present a comprehensive quantum electrodynamical analysis of the interaction between a continuum with photonic band gaps (PBGs) or frequency cut-off and an excited two-level atom, which can be either 'bare' or 'dressed' by coupling to a near-resonant field mode. A diversity of novel features in the atom and field dynamics is shown to arise from the non-Markovian character of radiative decay into such a continuum of modes. Firstly the excited atom is shown to evolve, by spontaneous decay, into a superposition of nondecaying single-photon dressed states, each having an energy in a different PBG, and a decaying component. This superposition is determined by the atomic resonance shift, induced by the spontaneously emitted photon, into or out of a PBG. The main novel feature exhibited by the decaying excited-state component is the occurrence of beats between the shifted atomic resonance frequency and the PBG cut-off frequencies, corresponding to a non-Lorentzian emission spectrum. Secondly the induced decay of a resonantly driven atom into such a continuum exhibits a cascade of transitions down the ladder of dressed states, which are labelled by decreasing photon numbers of the driving mode. Remarkably, this cascade is terminated at the dressed-state doublet, from which all subsequent transitions to lower doublets are forbidden because they fall within the PBG. This doublet then becomes an attractor state for the populations of higher-lying doublets. As a result, the photon-number distribution of the driving mode becomes strongly sub-Poissonian.

A simple scheme is presented that allows the generation and detection of nonclassical states of the electromagnetic (em) field with controllable (predetermined) photon-number and phase distributions. It is based on the two-photon resonant interaction of a single em field mode in a high-Q cavity with initially excited atoms crossing the cavity sequentially (one at a time). The sequence duration should be much shorter than the cavity-mode lifetime. Nonclassical states of the field are generated conditionally, by selecting only those sequences wherein each atom is measured to be in the excited state after the interaction. The field distribution resulting from a sequence of N such measurements is peaked about 2N positions in the phase plane, which evolve sinusoidally as a function of the atomic transit times and are therefore simply controlled. When these peaks are chosen not to overlap, the field state constitutes a generalized Schrödinger cat. By choosing them to overlap, we can make parts of the field distribution strongly interfere, giving rise to decimation of the photon-number distribution. In particular, this process can prepare Fock states with controlled photon numbers. The generated phase distribution can be detected by monitoring the pattern of revivals in the excitation of a ''probe'' atom.

A structural defect in a two-dimensionally periodic dielectric structure may form a local field mode photonic within a band gap in which no other E-polarized field modes exist. We show that such a defect mode can give rise to new quantum-electrodynamic effects in resonant field-atom interaction, owing to the spatially modu- lated standing-wave character of its field. Atomic-beam motion is considered (a) along an interplanar defect and (b) through a spherical defect. The resonant interaction of a moving two-level atom with the quantized field of these defects can yield hitherto unknown featuresoscillatory patterns of the atomic population inversion, fluorescence spectra, and nonclassical field statesthat are essentially different from their counterparts in the standard Jaynes-Cummings model, which holds for atomic-beam motion in a spatially uniform singlemode field.

We consider the quantum electrodynamics of an atom uniformly moving through a single spatially periodic field mode. The shape and periodicity of the field modulation can be designed by an appropriate choice of a defect in a periodic structure that possesses a forbidden spectral band (a photonic band gap). This design of the periodic modulation can improve our control over the evolution and properties (e.g., photon statistics) of nonclassical Schrödinger-cat states of the field, generated by resonant interaction with the atom.

Keywords: PARTICLES

It is suggested that the quantum uncertainty of cavity radiation is measurable via Aharonov-Bohm effects for electrons in a ring which is threaded by the magnetic cavity flux. When the electrons follow this oscillatory flux nearly adiabatically, and dissipation is negligible, the noise in the ring current should provide a quantum-nondemolition measure of the flux uncertainty. Unlike standard photodetection, such measurements should reveal the scaling of the flux uncertainty with the diamagnetic or dielectric index of the cavity medium, depending on the dimensions of the electron ring.

Radiative coupling between identical atoms sharing an excitation is studied in media where dispersion creates photonic band gaps, i.e., forbidden bands of light propagation for all directions and polarizations. Photonic band gaps can exist in media whose dielectric index exhibits strong three-dimensionally periodic modulations (photonic crystals) or in bands of polaritonic media. It is shown that the resonant dipole-dipole interaction as well as cooperative fluorescence of two-atom systems can be strongly enhanced or suppressed, to the extent of being essentially eliminated, by one of the following mechanisms: (a) dependence on location of the atoms within a unit cell of a photonic crystal, i.e., sensitivity to the spatial variation of the field normal modes; (b) dependence on the density of normal modes in allowed bands and on the density of virtual (evanescent) modes in band gaps. The ability to suppress the dipole-dipole interactions at interatomic separations characterizing quasimolecules would have far-reaching implications on their dissociative or collisional dynamics, spectroscopy, and rates of energy transfer.

A semiclassical theory of stimulated processes in dielectric spherical particles is formulated. The theory applies to the small-signal regime and to isotropic (but radially nonuniform) pumping. Iterative treatment of the pumped-medium susceptibility by scattering theory demonstrates the basic features observed experimentally by Chang and co-workers [H. M. Tzeng, K. F. Wall, M. B. Long, and R. K. Chang, Opt. Lett. 9, 499 (1984); S. X. Qian and R. K. Chang, Phys. Rev. Lett. 56, 926 (1986)], namely, the drastic reduction of the threshold for lasing and multiorder stimulated Raman processes, and the frequency pulling from Mie resonances of the inactive medium.

It is pointed out that the emission of photons in the GeV range produced by the annihilation of channeled positrons can be stimulated by superfluorescent emission of characteristic radiation in the MeV range from a beam of channeled electrons or positrons. The threshold current for the entire process is 1010 times lower than that of stimulated Compton scattering of visible light into the MeV range of frequencies.

The time-resolved cooperative emission from a system of correlated neutral dissociation fragments, or molecular collision products in beams, is investigated. The investigation is focused on emission at large fragment separations (between 1 nm and a few emission wavelengths), exceeding the domain of short-range interactions within the reactive or collisional molecular complex. A master-equation approach is used to obtain a general expression for the cooperative emission rate, which consists of nonexponential decay factors multiplied by temporal ringing patterns. These features result from the time-dependent radiative coupling between the receding fragments; they depend in an essential manner on the initial electronic state of the parent molecular complex and its symmetry which determine the correlations between the fragments. In the model system of a pair of identical two-level fragments two cases are considered separately. (a) A single photon shared by the fragments, where the emission is initially superradiant or subradiant (radiation trapping), depending on the spin and inversion symmetry of the parent molecular system and of the nascent fragments. The ringing pattern depends on the electronic angular momentum state of the parent molecule and on the polarization of the emitted light. (Such a ringing has been observed recently by Grangier, Aspect, and Vigué [Phys. Rev. Lett. 54, 418 (1985)] in the emission of photodissociated Ca2.) (b) Two initially excited fragments, where the ringing pattern is of smaller amplitude, and is weakly dependent on the electronic angular momentum of the parent molecule. All the aforementioned cooperative features generally last until the fragments recede several radiation wavelengths away from each other. The application of this time-resolved analysis to various diagnostic problems is discussed, especially with regard to the identification of excited electronic states of the parent molecular complex, and the stereospecificity of the fragmental orbitals.