Publications

We propose and experimentally demonstrate a general method allowing us to unravel microscopic noise events that affect a continuous quantum variable. Such unraveling is achieved by frequent measurements of a discrete variable coupled to the continuous one. The experimental realization involves photons traversing a noisy channel. There, their polarization, whose coupling to the photons' spatial wave packet is subjected to stochastic noise, is frequently measured in the quantum Zeno regime. The measurements not only preserve the polarization state, but also enable the recording of the full noise statistics from the spatially resolved detection of the photons emerging from the channel. This method proves the possibility of employing photons as quantum noise sensors and robust carriers of information.

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.

We experimentally demonstrate, for the first time, noise diagnostics by repeated quantum measurements, establishing the ability of a single photon subjected to random polarization noise to diagnose non-Markovian temporal correlations of such a noise process. Both the noise spectrum and temporal correlations are diagnosed by probing the photon with frequent (partially) selective polarization measurements. We show that noise with positive temporal correlations corresponds to our single photon undergoing a dynamical regime enabled by the quantum Zeno effect (QZE), whereas noise characterized by negative (anti) correlations corresponds to regimes associated with the anti-Zeno effect (AZE). This is the first step toward a novel noise spectroscopy based on QZE and AZE in single-photon state probing able to extract information on the noise while protecting the probe state, a conceptual paradigm shift with respect to traditional interferometric measurements.

The consensus regarding quantum measurements rests on two statements: (i) von Neumann's standard quantum measurement theory leaves undetermined the basis in which observables are measured, and (ii) the environmental decoherence of the measuring device (the "meter") unambiguously determines the measuring ("pointer") basis. The latter statement means that the environment (measures) observables of the meter and (indirectly) of the system. Equivalently, a measured quantum state must end up in one of the "pointer states" that persist in the presence of the environment. We find that, unless we restrict ourselves to projective measurements, decoherence does not necessarily determine the pointer basis of the meter. Namely, generalized measurements commonly allow the observer to choose from a multitude of alternative pointer bases that provide the same information on the observables, regardless of decoherence. By contrast, the measured observable does not depend on the pointer basis, whether in the presence or in the absence of decoherence. These results grant further support to our notion of Quantum Lamarckism, whereby the observer's choices play an indispensable role in quantum mechanics.

We study the difference between the energy transfer rate and the engine efficiency with a microscopic model, widely used in the theoretical description of solar cells, as well as in light-harvesting systems. We show no violation of the second law of thermodynamics by correctly assessing the useful output work, even with the simple model treating the later work conversion as a simple \u201csink\u201d.

Characterization of microstructures in living tissues is one of the keys to diagnosing early stages of pathology and understanding disease mechanisms. However, the extraction of reliable information on biomarkers based on microstructure details is still a challenge, as the size of features that can be resolved with noninvasive magnetic resonance imaging (MRI) is orders of magnitude larger than the relevant structures. Here we derive from quantum information theory the ultimate precision limits for obtaining such details by MRI probing of water-molecule diffusion. We show that currently available MRI pulse sequences can be optimized to attain the ultimate precision limits by choosing control parameters that are uniquely determined by the expected size, the diffusion coefficient, and the spin relaxation time T-2. By attaining the ultimate precision limit per measurement, the number of measurements and the total acquisition time may be drastically reduced compared to the present state of the art. These results are expected to open alternative avenues towards unraveling diagnostic information by quantitative MRI.

Soliton solutions are studied for paraxial wave propagation with intensity-dependent dispersion. Although the corresponding Lagrangian density has a singularity, analytical solutions, derived by the pseudo-potential method and the corresponding phase diagram, exhibit one- and two-humped solitons with almost perfect agreement to numerical solutions. The results obtained in this work reveal a hitherto unexplored area of soliton physics associated with nonlinear corrections to wave dispersion. (C) 2020 Optical Society of America

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.

We investigate the evolution of a target qubit caused by its multiple random collisions with N-qubit clusters. Depending on the cluster state, the evolution of the target qubit may correspond to its effective interaction with a thermal bath, a coherent (laser) drive, or a squeezed bath. In cases where the target qubit relaxes to a thermal state, its dynamics can exhibit a quantum advantage, whereby the target-qubit temperature can be scaled up proportionally to N-2 and the thermalization time can be shortened by a similar factor, provided the appropriate coherence in the cluster is prepared by nonthermal means. We dub these effects quantum superthermalization because of the analogies to superradiance. Experimental realizations of these effects are suggested.

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.

A universal approach to decoherence control combined with quantum estimation theory reveals a critical behavior, akin to a phase transition, of the information obtainable by a qubit probe concerning the memory time of environmental fluctuations of generalized Ornstein-Uhlenbeck processes. The criticality is intrinsic to the environmental fluctuations but emerges only when the probe is subject to suitable dynamical control aimed at inferring the memory time. A sharp transition is anticipated between two dynamical phases characterized by either a short or long memory time compared to the probing time. This phase transition of the environmental information is a fundamental feature that characterizes open quantum-system dynamics and is important for attaining the highest estimation precision of the environment memory time under experimental limitations.

In this paper, we address the question: To what extent is the quantum state preparation of multiatom clusters (before they are injected into the microwave cavity) instrumental for determining not only the kind of machine we may operate, but also the quantitative bounds of its performance? Figuratively speaking, if the multiatom cluster is the "crude oil", the question is: Which preparation of the cluster is the refining process that can deliver a "gasoline" with a "specific octane"? We classify coherences or quantum correlations among the atoms according to their ability to serve as: (i) fuel for nonthermal machines corresponding to atomic states whose coherences displace or squeeze the cavity field, as well as cause its heating; and (ii) fuel that is purely "combustible", i.e., corresponds to atomic states that only allow for heat and entropy exchange with the field and can energize a proper heat engine. We identify highly promising multiatom states for each kind of fuel and propose viable experimental schemes for their implementation.

Coherence control was originally conceived by Shapiro and Brumer for closed quantum systems. In open quantum systems, the environment hampers or destroys coherence. Nevertheless, the Kurizki-Shapiro-Brumer coherent photocurrent control showed partial resilience to decoherence. Still, decoherence control is an essential prerequisite to our ability to fully exploit quantum coherence for any operational task. Yet decoherence control need not employ coherent fields: it may be incoherent. In operational tasks such as the preparation, transformation, transmission, and detection of quantumstates, environmental (bath) effects can be suppressed by dynamical decoupling, or by the more general incoherent dynamical control by modulation developed by our group. The resulting control dynamics must be Zeno-like in order to yield suppressed coupling to the bath in unitary operational tasks. There are however tasks which cannot be implemented by closed-system (unitary) evolution, in particular those involving a change of the system's entropy. Such tasks necessitate efficient coupling to a bath for their implementation. Examples are the use of measurements to cool (purify) a system, to equilibrate it, or harvest (and convert) energy from the environment. If the underlying dynamics is anti-Zeno like, enhancement of this coupling to the bath will occur and thereby facilitate the task, as discovered by us. A general task may also require state and energy transfer between non-interacting parties via shared modes of the bath. For such tasks, a more subtle interplay of Zeno and anti-Zeno dynamics may be optimal. We have therefore constructed a general framework for optimizing the way a system interacts with its environment to achieve a desired task. This optimization consists in adjusting a given "score" that quantifies the success of the task, such as the targeted fidelity, purity, entropy, energy or state transfer probability by dynamical modification of the system-bath coupling spectrum on demand.

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.

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.

An extensively pursued current direction of research in physics aims at the development of practical technologies that exploit the effects of quantum mechanics. As part of this ongoing effort, devices for quantum information processing, secure communication, and high-precision sensing are being implemented with diverse systems, ranging from photons, atoms, and spins to mesoscopic superconducting and nanomechanical structures. Their physical properties make some of these systems better suited than others for specific tasks; thus, photons are well suited for transmitting quantum information, weakly interacting spins can serve as long-lived quantum memories, and superconducting elements can rapidly process information encoded in their quantum states. A central goal of the envisaged quantum technologies is to develop devices that can simultaneously perform several of these tasks, namely, reliably store, process, and transmit quantum information. Hybrid quantum systems composed of different physical components with complementary functionalities may provide precisely such multitasking capabilities. This article reviews some of the driving theoretical ideas and first experimental realizations of hybrid quantum systems and the opportunities and challenges they present and offers a glance at the near- and long-term perspectives of this fascinating and rapidly expanding field.

We analyze work extraction from an autonomous (self-contained) heat-powered optomechanical setup. The initial state of the quantized mechanical oscillator plays a key role. As the initial mean amplitude of the oscillator decreases, the resulting efficiency increases. In contrast to laser-powered self-induced oscillations, work extraction from a broadband heat bath does not require coherence or phase-locking: an initial phase-averaged coherent state of the oscillator still yields work, as opposed to an initial Fock-state.

Quantum electromagnetic fluctuations induce forces between neutral particles, known as the van der Waals and Casimir interactions. These fundamental forces, mediated by virtual photons from the vacuum, play an important role in basic physics and chemistry and in emerging technologies involving, e.g., microelectromechanical systems or quantum information processing. Here we show that these interactions can be enhanced by many orders of magnitude upon changing the character of the mediating vacuum modes. By considering two polarizable particles in the vicinity of any standard electric transmission line, along which photons can propagate in one dimension, we find a much stronger and longer-range interaction than in free space. This enhancement may have profound implications on many-particle and bulk systems and impact the quantum technologies mentioned above. The predicted giant vacuum force is estimated to be measurable in a coplanar waveguide line.

We propose a method of optimally controlling the tradeoff of speed and fidelity of state transfer through a noisy quantum channel (spin-chain). This process is treated as qubit state-transfer through a fermionic bath. We show that dynamical modulation of the boundary-qubits levels can ensure state transfer with the best tradeoff of speed and fidelity. This is achievable by dynamically optimizing the transmission spectrum of the channel. The resulting optimal control is robust against both static and fluctuating noise in the channels spin-spin couplings. It may also facilitate transfer in the presence of diagonal disorder (on site energy noise) in the channel.

We consider the dispersion energy of a pair of dipoles embedded in a metallic waveguide with transverse dimension a smaller than the characteristic dipolar wavelength. We find that a sets the scale that separates retarded, Casimir-Polder-like from quasistatic, van der Waals-like interactions. Whereas in the retarded regime the energy decays exponentially with interdipolar distance, typical of evanescent waves, in the van der Waals regime, the known free-space result is obtained. This short-range scaling implies that the additivity of the dispersion interactions inside a waveguide extends to denser media, along with modifications to related Casimir effects in such structures.

A pulsed laser scenario, designed to prepare a variety of self-assembled quantum dots with nearly 100% population in a biexciton state, is proposed. We use realistic parameters for typical self-assembled quantum dots and show that two-colour sequential as well as concurrent excitation of the biexciton state can virtually eliminate unwanted exciton population and prepare a quantum dot in the biexciton state with near-deterministic accuracy. The ensuing radiative emission would be a prerequisite for a high rate, on-demand, source of entangled photons.

In traditional thermodynamics the Carnot cycle yields the ideal performance bound of heat engines and refrigerators. We propose and analyze a minimal model of a heat machine that can play a similar role in quantum regimes. The minimal model consists of a single two-level system with periodically modulated energy splitting that is permanently, weakly, coupled to two spectrally separated heat baths at different temperatures. The equation of motion allows us to compute the stationary power and heat currents in the machine consistent with the second law of thermodynamics. This dual-purpose machine can act as either an engine or a refrigerator (heat pump) depending on the modulation rate. In both modes of operation, the maximal Carnot efficiency is reached at zero power. We study the conditions for finite-time optimal performance for several variants of the model. Possible realizations of the model are discussed.

We show that a thermal reservoir can effectively act as a squeezed reservoir on atoms that are subject to energy-level modulation. For sufficiently fast and strong modulation, for which the rotating-wave approximation is broken, the resulting squeezing persists at long times. These effects are analyzed by a master equation that is valid beyond the rotating-wave approximation. As an example we consider a two-level atom in a cavity with Lorentzian linewidth, subject to sinusoidal energy modulation. A possible realization of these effects is discussed for Rydberg atoms. DOI: 10.1103/PhysRevA.87.013841

We propose a method to maximize the fidelity of quantum memory implemented by a spectrally inhomogeneous spin ensemble. The method is based on preselecting the optimal spectral portion of the ensemble by judiciously designed pulses. This leads to significant improvement of the transfer and storage of quantum information encoded in the microwave or optical field.

We show that coupled-spin network manipulations can be made highly effective by repeated projections of the evolving quantum states onto diagonal density-matrix states (populations). As opposed to the intricately crafted pulse trains that are often used to fine-tune a complex network's evolution, the strategy hereby presented derives from the "quantum Zeno effect" and provides a highly robust route to guide the evolution by destroying all unwanted correlations (coherences). We exploit these effects by showing that a relaxationlike behavior is endowed to polarization transfers occurring within a N-spin coupled network. Experimental implementations yield coupling constant determinations for complex spin-coupling topologies, as demonstrated within the field of liquid-state nuclear magnetic resonance.

Existing optimal control methods of open quantum systems rely on extensive numerical simulations of the dynamics in the presence of a bath, or alternatively ignore the exact bath dynamics. If the bath effects are to be treated properly on both Markovian and non-Markovian timescales using numerical simulations, the number of bath modes cannot be large. This may affect the ability to simulate realistic scenarios. Even if realistic, such simulations are hard to interpret physically. An alternative approach advocated here is to resort to a perturbative analysis provided the system-bath coupling is weak. This analysis would allow for the effects of any given bath (finite or infinite, Markovian or non-Markovian) and any control at our disposal. This poses the challenge of constructing a method for the optimization of various operations requiring proper manipulation of the system, based on a general perturbative treatment to second order in the system-bath coupling. This proposed treatment yields a universal tool for optimizing the fidelity of a given operation. It involves a fidelity-control matrix: a construct that allows us to prioritize the use of available control resources so as to maximize the operation fidelity in any given bath. As an analytically solvable example of this general method, we analyse quantum state-transfer optimization, from a 'noisy' (write-in) qubit to its 'quiet' counterpart (storage qubit). Intriguing interplay is revealed between our ability to avoid bath-induced errors that profoundly depend on the bath-memory time and the limitations imposed by leakage out of the operational subspace. Counterintuitively, under no circumstances is the fastest transfer optimal (for a given transfer energy).

As a rule, the coupling of a quantum system to an uncontrollable thermal reservoir (a "bath") gives rise to the system's decoherence, i.e., the destruction of its unitary coherent evolution [1]. Not less common is the rule that the more complex the quantum system, the more detrimental are the bath effects [2]. Here we point out that the coupling of a complex quantum system to a bath may actually induce advantageous coherent dynamics. Namely, an exact solution for a quantum many-body system characterized by large angular momentum (or spin) that is coupled to a thermal bath reveals a hitherto unexplored general effect: bath-induced effectively nonlinear evolution. This evolution can drive the large-spin system into a macroscopic quantum-superposition ("Schroedinger-cat") state [3, 4]. Such counter-intuitive bath-induced effects should be observable in various setups on long (Markovian) time scales. They change our perspective of non-classicality in open many-body quantum systems. Namely, the bath may cause rather than impede the formation of distinctly non-classical Schroedinger-cat states, despite their fragility in the presence of a bath [2, 5].

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.

We investigate the scaling of decoherence rates and their dynamical suppression with the number N of qubits in various entangled states. Remarkably, for sufficiently large N, coherence time is always in the Zeno regime. This changes the scaling to the square root of the Markov-regime scaling. We find that a simple and effective control strategy is to locally modulate the individual qubits and thereby not only suppress the decoherence rate of each qubit, but also reduce the decoherence scaling of entangled states that are particularly fragile, from N2 to N, resulting in a dramatic reduction of the decoherence (by orders of magnitude). Surprisingly, the conditions for the effectiveness of such decoherence control are independent of N.

A unified theory is given of dynamically modified decay and decoherence of field-driven multipartite systems. When this universal framework is applied to two-level systems (TLS) or qubits experiencing either amplitude or phase noise (AN or PN) due to their coupling to a thermal bath, it results in completely analogous formulae for the modified decoherence rates in both cases. The spectral representation of the modified decoherence rates underscores the main insight of this approach, namely, the decoherence rate is the spectral overlap of the noise and modulation spectra. This allows us to come up with general recipes for modulation schemes for the optimal reduction of decoherence under realistic constraints. An extension of the treatment to multilevel and multipartite systems exploits intra-system symmetries to dynamically protect multipartite entangled states. Another corollary of this treatment is that entanglement, which is very susceptible to noise and can die, i.e., vanish at finite times, can be resuscitated by appropriate modulations prescribed by our universal formalism. This dynamical decoherence control is also shown to be advantageous in quantum computation setups, where control fields are applied concurrently with the gate operations to increase the gate fidelity.

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 consider an unexplored regime of open quantum systems that relax via coupling to a bath while being monitored by an energy meter. We show that any such system inevitably reaches an equilibrium (quasi-steady) state controllable by the effective rate of monitoring. In the non-Markovian regime, this approach suggests the possible 'freezing' of states, by choosing monitoring rates that set a non-thermal equilibrium state to be the desired one. For measurement rates high enough to cause the quantum Zeno effect, the only steady state is the fully mixed state, due to the breakdown of the rotating wave approximation. Regardless of the monitoring rate, all the quasi-steady states of an observed open quantum system live only as long as the Born approximation holds, namely the bath entropy does not change. Otherwise, both the system and the bath converge to their fully mixed states.

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.

A threshold condition for amplification without inversion in a free-electron laser without inversion (FELWI) is determined. This condition is found to be too severe for the effect to be observed in an earlier suggested scheme because a threshold intensity of the field to be amplified appears to be too high. This indicates that alternative schemes have to be found for making the creation of an FELWI realistic.

Initialization of quantum logic operations makes it imperative to cool down the information-carrying qubits as much and as fast as possible, so as to purify their state, or at least their ensemble average. Yet, the limit on the speed of existing cooling schemes 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 show that highly-frequent phase-shifts or measurements of the state of thermalized qubits can be designed to affect the qubit-bath entanglement so that the qubits undergo cooling, well before they re-equilibrate with the bath and without resorting to auxiliary states. These processes can be used in principally novel, advantageous, cooling schemes to assist quantum logic operations.

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.

We study the effect of decoherence on dynamical phase diffusion in the two-site Bose-Hubbard model. Starting with an odd parity excited coherent state, the initial loss of single-particle coherence varies from small bound oscillations in the Rabi regime, through hyperbolic depletion in the Josephson regime, to a Gaussian decay in the Fock regime. The inclusion of local-site noise, measuring the relative number difference between the modes, is shown to enhance phase diffusion. In comparison, site-indiscriminate noise measuring the population imbalance between the two quasimomentum modes slows down the loss of single-particle coherence. Decoherence thus either enhances or suppresses phase diffusion, depending on the details of system-bath coupling and the overlap of decoherence pointer states with collisional-entanglement pointer states. The deceleration of phase diffusion due to the coupling with the environment may be viewed as a many-body quantum Zeno effect. The extended effective decay times in the presence of projective measurement are further enhanced with increasing number of particles N by a bosonic factor of N in the Fock regime and N/logN in the Josephson regime.

We study, both experimentally and theoretically, short-time modifications of the decay of excitations in a Bose-Einstein Condensate (BEC) embedded in an optical lattice. Strong enhancement of the decay is observed compared to the Golden Rule results. This enhancement of decay increases with the lattice depth. It indicates that the description of decay modifications of few-body quantum systems also holds for decay of many-body excitations of a BEC.

We study disturbances of thermal equilibrium between two-level systems (TLS) and a bath by frequent and brief quantum measurements of the TLS energy-states. If the measurements induce either the Zeno or the anti-Zeno regime, namely, the slowdown or speedup of the TLS relaxation, then 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 propose a scheme to control the collision-induced entanglement between atoms in the two-site Bose-Hubbard model, by means of site-indiscriminate noise. This decoherence mechanism induces the continuous measurement of the quasi-momentum, protecting single-particle coherence via a Bose-enhanced many-body Quantum-Zeno effect.

A unified theory is given of dynamically modified decay and decoherence of field-driven multipartite systems. When this universal framework is applied to two-level systems (TLS) or qubits experiencing either amplitude or phase noise (AN or PN) due to their coupling to a thermal bath, it results in completely analogous formulae for the modified decoherence rates in both cases. The spectral representation of the modified decoherence rates underscores the main insight of this approach, namely, the decoherence rate is the spectral overlap of the noise and modulation spectra. This allows us to come up with general recipes for modulation schemes for the optimal reduction of decoherence under realistic constraints. An extension of the treatment to multilevel and multipartite systems exploits intra-system symmetries to dynamically protect multipartite entangled states. Another corollary of this treatment is that entanglement, which is very susceptible to noise and can die, i.e., vanish at finite times, can be resuscitated by appropriate modulations prescribed by our universal formalism. This dynamical decoherence control is also shown to be advantageous in quantum computation setups, where control fields are applied concurrently with the gate operations to increase the gate fidelity.

A one-step introduction of functional defects into a photonic crystal is demonstrated. By using a multi-beam phase-controlled holographic lithography, line-defects in a Bragg structure and embedded waveguides in a two-dimensional photonic crystal are fabricated. Intrinsic defect introduction into a 3-dimensional photonic crystal is also proposed. This technique gives rise to a substantial reduction of the fabrication complexity and a significant improvement on the accuracy of the functional defects in photonic crystals.

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.

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.

We show that by dynamically modulating the gate fields so as to minimize the spectral overlap of the decoherence and modulation spectra specifically for each qubit, and concurrently controlling all qubits, one can significantly increase the fidelity of quantum computation. Notably, crossdecoherence, introduced by two-qubit entanglement, can be eliminated by appropriate dynamical control. In this scheme, contrary to traditional schemes, one can increase the gate duration, while, simultaneously, increasing the total gate fidelity. Experimental scenarios involving laser-driven cold-ions and coldatoms are shown to benefit from this counterintuitive scheme.

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.

The aim of this paper is to revisit the implications of complementarity when we inject into a Mach Zehnder interferometer particles with internal structure, prepared in special translational-internal entangled (TIE) intraparticle states. This correlation causes the path distinguishability to be interferometric phasedependent in contrast to the standard case, where distinguishability depends on some external parameters (not interferometric phase). We show that such a TIE state permits us to detect small phase shifts along with almost perfect path distinguishability, beyond the constraints imposed by complementarity on simultaneous which-way and which-phase measurements for cases when distinguishability is uncoupled to interferometric phase.

We propose a new mechanism for tuning an atomic s-wave scattering length. The effect is caused by virtual transitions between different Zeeman sublevels via magnetic dipole-dipole interactions. These transitions give rise to an effective potential, which, in contrast to standard magnetic interactions, has an isotropic component and thus affects s-wave collisions.

We propose a general formalism for analytical description of multiatomic ensembles interacting with a single-mode quantized cavity field under the assumption that most atoms remain unexcited on average. By combining the obtained formalism with the nilpotent technique for the description of multipartite entanglement we are able to overview in a unified fashion different probabilistic control scenarios of entanglement among atoms or examine atomic ensembles. We then apply the proposed control schemes to the creation of multiatom states useful for quantum information.

We explore the feasibility of creating a translationally entangled state for massive particles, and its use for matter wave teleportation by means of cold-molecule Raman dissociation and association in optical traps.

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.

Periodic structures with resonant dopants allow giantly enhanced cross-coupling between ultraweak pulses within photonic band gaps, subject to self-induced or electromagnetically-induced transparency. These features may lead to novel quantum optical applications.

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. 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 pi-phase flips of the single-qubit evolution.

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.

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?

In this paper, we present a universal control technique, the non-holonomic control, which allows us to impose any arbitrarily prescribed unitary evolution to any quantum system through the alternate application of two well-chosen perturbations.

In this paper, we present a coherence protection method based upon a multidimensional generalization of the Quantum Zeno Effect, as well as ideas from the coding theory. The non-holonomic control technique is employed as a physical tool which allows its effective implementation. The two limiting cases of small and large quantum systems are considered.

The protection of the coherence of open quantum systems against the influence of their environment is a very topical issue. A scheme is proposed here which protects a general quantum system from the action of a set of arbitrary uncontrolled unitary evolutions. This method draws its inspiration from ideas of standard error correction (ancilla adding, coding and decoding) and the quantum Zeno effect. A demonstration of our method on a simple atomic system-namely, a rubidium isotope-is proposed.

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.

A scalable multiatom entangled system, capable of high-performance quantum computations, can be realized by resonant dipole-dipole interacting dopants in a solid state host. In one realization, the qubits are represented by ground and subradiant states of effective dimers formed by pairs of closely spaced two-level systems (TLS). Such qubits are highly robust against radiative decay. The two-qubit entanglement in this scheme relies on coherent excitation exchange between the dimers by external laser fields. This scheme is challenging because of the nanosize control and addressability it requires. Another realization involves dipole-dipole interacting TLS whose resonance frequency lies in a photonic band gap of a dielectric photonic crystal. A sequence of abrupt changes of the resonance frequency can produce controlled entanglement (logic gates) with improved protection from radiation decay and decoherence.

The development of a unified theory of dynamically suppressed and decoherence by external fields in qubits coupled to arbitrary thermal baths and dephasing sources was discussed. A driven two-level system (TLS) undergoing decay into a finite-temperature bath and while its resonant frequency and dipolar coupling to the bath were dynamically modulated by external fields, was utilized for the study. A differential mass equation (ME) for the density operator of the system was derived using Zwanizig's projection-operator technique. The results indicate that the ultrafast modulations give rise to antiresonant effects.

We show that electromagnetically-induced isotropic dipole-dipole interactions in a spin-polarized non-degenerate fermionic gas can cause an extremely exothermic phase transition, analogous to the isothermal collapse in gravitationally interacting star clusters. This collapse may result in fragmentation of the gas into a hot "halo" and a highly degenerate "core". Possible realization is envisaged in microwave-illuminated fermionic molecular gases at microkelvin temperatures.

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 present a brief review of our recent results concerning non-mean-field effects of laser-induced dipole-dipole interactions on static and dynamical properties of atomic Bose-Einstein condensates.

The use of off-resonant lasers to induce long-range dipole-dipole interactions whose characteristic length is the laser wavelength was proposed. These interactions can cause laser-induced "self-gravity" in a Bose Einstein condensate (BEC), leading to three-dimensional self-trapping and electrostriction accompanied by unusual excitation spectra, as well as "supersolid" structures. This paper aims to explore how the excitation spectrum and the correlations of a gaseous BEC are changed when the interatomic potential is modified via laser-induced dipole-dipole interactions.

We consider a gaseous atomic BoseEinstein condensate with dipoledipole interactions induced by a far off-resonance laser. The long-range dipolar interactions introduce atomatom 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.

We propose two alternative scheme for highly efficient nonlinear interaction between weak optical fields. The first scheme is based on the attainment of electromagnetically induced transparency simultaneously for two fields via transitions between magnetically split F = 1 atomic sublevels, in the presence of two driving fields. The second scheme relies on simultaneous electromagnetically- and self-induced transparencies of the two fields. Thereby, equal slow group velocities and giant cross-phase modulation of the weak fields over long distances are achieved.

The impediment towards the successful development of the field of quantum information (QI) is decoherence, i.e., the loss of entanglement by the effect of the environment on the systems of interest. An important challenge is that of QI engineering, by entanglement and decoherence control, in complex systems, such as unimolecular and bimolecular systems, that can simultaneously handle large amounts of QI. Progress towards this goal can be achieved by: (a) decay modification and decoherence suppression in molecules, using laser-induced phase and amplitude modulation of rovibrational levels and inter-mode couplings; (b) transfer of internal-translational entanglement and teleportation of wavepackets via molecular dissociation and collisions.

We show that the dipole-dipole interatomic forces induced by an off-resonant running laser beam can lead to a self-bound pencil-shaped Bose condensate, even if the laser beam is a plane wave. For an appropriate laser intensity the ground state has a quasi-one-dimensional density modulationa Bose-Einstein \u201csupersolid\u201d.

The modification of the properties of a Bose-Einstein or a Fermi-Dirac atomic gas due to laser-induced dipole-dipole interactions between the atoms are considered. Nearly-isotropic illumination of the sample by spectrally-fluctuating laser beams averages out the static r(-3) dipole-dipole interaction, leaving the retarded r(-1) "self-gravitating" attraction in the near zone. The analogies of ultracold many-atom systems, self-bound by such laser-induced "gravity", with compact stars ("Bose stars" or "White Dwarfs") are emphasized. Even a single plane-wave laser induces dipole-dipole interactions capable of causing a cigar-shaped Bose condensate to exhibit self binding and density modulations.

The inhibition or acceleration of the decay of an unstable state by its projective measurements was recently analyzed for a broad class of processes. As such, the arsenal of decal control, whether measurement-like or fully coherent was purported to substantially expand. A universal form of the decay rate of unstable states into any reservoir, modified by weak perturbations with arbitrary time dependence was derived.

We investigate the collective excitations of an atomic Bose-Einstein condensate in the self-binding regime produced by electromagnetically induced "gravity" (1/r attraction). Analytical expressions for the frequencies of the monopole and quadrupole modes are obtained at zero temperature, using the sum-rule approach, and compared with the exact results available in the Thomas-Fermi limit. The low-energy dynamics of such condensates is shown to be dominated by the effective "plasma" frequency. An analog of the Jeans gravitational instability is analyzed.

The possibility of realizing photon-photon nonlinear phase shifters and switches with a potentially very high efficiency and without external laser fields in photonic crystals (PCs) doped at a low concentration with four-level atoms is illustrated. These atoms have two transitions tuned to two incident photons and an intermediate transition tuned to a singular feature of the structured density of modes (DOM) spectrum of the PC. These features have unique advantages for quantum information applications.

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.

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.

Summary form only given. The quantum Zeno effect (QZE) is the striking possibility of inhibiting the decay of an unstable quantum state by sufficiently frequent measurements. This effect is currently believed to be universal, even though it has only been observed for oscillations between discrete states. Recently we showed that both the QZE and the inverse (anti-) Zeno effect (AZE) can be observed on spontaneous decay in cavities. We develop a general theory of evolution of quantum systems in the presence of frequent measurements. We consider both the ideal case of instantaneous projections of the wave function to the initial state and some possible measurement schemes. The considerations imply the impossibility of the continuous-observation limit necessary to stop quantum evolution (the quantum Zeno paradox). Indeed, this limit leads to an unbounded increase of the energy fluctuations of the system, which would cause the system to disintegrate. The present theory describes different measurement effects, including the QZE and AZE, and allows us to obtain the general criteria of their validity. In particular, we show that the AZE, rather than the QZE, is achievable for spontaneous decay of atoms in free space.

The existence of multidimensional solitons localized in both space and time are predicted in two- and three-dimensional self-induced-transparency media embedded in a Bragg grating. These light bullets were found to be stable. Such stability suggest new possibilities of signal transmission control and self-trapping of light.

We demonstrate that interference of evolutions (interaction 'histories') is realizable for two entangled systems, such as a two-level atom coupled to a cavity field over many different time intervals in parallel. Such interference is conditional upon the detection of appropriate atomic momentum observables. It allows us to fully control and manipulate any state of both entangled systems. The sense of evolution (towards the 'past' or 'future') is also controlled by this method. In the case of a multi-mode field it allows us to protect any state of the two-level atom (qubit) from decoherence, caused by entanglement with the field. The success probability of this control method can be optimized. Several schemes for realizing this method are suggested.

\u201cLight bullets\u201d are multidimensional solitons which are localized in both space and time. We show that such solitons exist in two- and three-dimensional self-induced transparency media and that they are fully stable. Our approximate analytical calculation, backed and verified by direct numerical simulations, yields the multidimensional generalization of the one-dimensional sine-Gordon soliton.

We show that particular configurations of intense off-resonant laser beams can give rise to an attractive 1/r interatomic potential between atoms located well within the laser wavelength. Such a \u201cgravitational-like\u201d interaction is shown to give stable Bose-Einstein condensates that are self-bound (without an additional trap) with unique scaling properties and measurably distinct signatures.

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 study the use of the Faraday effect as a quantum clock for measuring traversal times of evanescent photons through magnetorefractive structures. The Faraday effect acts both as a phase shifter and as a filter for circular polarizations. Only measurements based on the Faraday phase-shift properties are relevant to the traversal time measurements. The Faraday polarization filtering may cause the loss of nonlocal (Einstein-Podolsky-Rosen) two-photon correlations, but this loss can be avoided without sacrificing the clock accuracy. We show that a mechanism of destructive interference between consecutive paths is responsible for superluminal traversal times measured by the clock.

We show that giant quasibound diatomic complexes, whose size is typically hundreds of nm, can be formed by intracavity cold diatom photoassociation or photodissociation in the strong atom-cavity coupling regime.

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.

The injection of spectrally narrow probe wavepackets into quantized parametrically amplifying media can give rise to transient tachyonic wavepackets. Remarkably, their quantum information, albeit causal, is confined to times corresponding to superluminal velocities, which is advantageous for communications.

We show that the injection of spectrally narrow probe wave packets into quantized parametrically amplifying media can give rise to transient tachyonic wave packets. Remarkably, their quantum information, albeit causal, is confined to times corresponding to superluminal velocities, which is advantageous for communications. These phenomena are explicitly analyzed for stimulated Raman scattering, parametric down-conversion and four-wave mixing.

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 show that conditional measurements on atoms following their interaction with a resonant cavity field mode can be used to effectively counter the decoherence of Fock-state superpositions due to cavity leakage.

We propose a method which can effectively stabilize fixed points in the classical and quantum dynamics of a phase-sensitive chaotic system with feedback. It is based on feeding back a selected quantum subensemble whose phase and amplitude stabilize the otherwise chaotic dynamics. Although the method is rather general, we apply it to realizations of the inherently chaotic Ikeda map. One suggested realization involves the Mach-Zender interferometer with Kerr nonlinearity. Another realization involves a trapped ion interacting with laser fields.

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.

Keywords: Optics; Physics, Applied

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.

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.

A Reply to the Comment by Qianbing Zheng, Takayoshi Kobayashi, and Takashi Sekiguchi.

We introduce a general strategy for preparation of arbitrary quantum states via optimal control of repeated conditional measurements. The effectiveness of this strategy in generating finite Fock-state superpositions with a high level of confidence from experimentally accessible coherent states is demonstrated for the simple and well known Jaynes-Cummings model dynamics.

A unified theory of two-level atom coupling to vacuum field reservoirs with arbitrary mode-density spectra is used to demonstrate that the quantum Zeno effect on excitation decay of the atom (and, correspondingly, inhibition of spontaneous emission) is observable in open cavities and waveguides, using a sequence of evolution-interrupting pulses on a nanosecond scale.

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 find a Bethe-ansatz solution for pairwise interacting quanta within the effective-mass regime of band-gap propagation in nonlinear Bragg reflectors. Our theory predicts a new kind of collective excitation of the electromagnetic field dressed by such media, namely, optical multiexciton (OME) complexes (or condensates), which are quantum states associated with gap solitary waves. 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. OMEs should have advantageous stability properties.

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-e 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 Schrodinger 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 Pock 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 novel scheme wherein lasing is attained by a coherent superposition of two electronic states in interfering interaction regions that can suppress stimulated absorption without hampering stimulated emission have been put forward. In the original scheme, an electron beam with mean momentum interacts with wiggler mode in region 1, changes its momentum and interact with wiggler mode in region 2. This paper put forward another scheme, wherein the wigglers are replace by Cherenkov effects. This scheme does not require population inversion in momentum space. Present analysis of Cherenkov and the two-wiggler schemes incorporates a new essential element. The possible application of the scheme for x-ray lasing is discussed.

We put forward a simple, feasible scheme for the preparation and subsequent detection of macroscopic quantum superposition (MQS) states. It is based on the two-photon model which obtains when a cascade of two atomic transitions is resonant with twice the field frequency. The initial conditions amount to a field in a mixed state characteristic of lasers or masers and an excited atom. The MQS is generated by a conditional measurement of the atomic excitation after an interaction time that determines the relative phase of the MQS components. Remarkably, the MQS is subsequently detected and its phase is inferred by measuring the excitation probability of a second, "probe," atom, as a function of its interaction time. The realization of the scheme in the optical domain, using dielectric microspheres, is discussed.

We put forward a simple, feasible scheme for the preparation and subsequent detection of macroscopic quantum superposition (MQS) states. It is based on the two-photon model which obtains when a cascade of two atomic transitions is resonant with twice the field frequency. The initial conditions amount to a field in a mixed state characteristic of lasers or masers and an excited atom. The MQS is generated by a conditional measurement of the atomic excitation after an interaction time that determines the relative phase of the MQS components. Remarkably, the MQS is subsequently detected and its phase is inferred by measuring the excitation probability of a second, probe, atom, as a function of its interaction time. The realization of the scheme in the optical domain, using dielectric microspheres, is discussed.

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.

Nonlinear optical processes in spherical microdroplets whose studies have been pioneered by Chang and co-workers are intriguing from the point of view of applications because of the extremely low thresholds they exhibit for the generation of stimulated output, but also from the theoretical point of view, because they require a synthesis between the methods of nonlinear optics and Mie scattering theory. The basis for such a synthesis has been given in the semiclassical theory of Kurizki and Nitzan, which implies that the nonlinearly amplified component of the scattered field is spatially orthogonal ('out of phase') with the linearly scattered field component (the ordinary Mie solution). This approach allows the calculation of amplification coefficients in various nonlinear processes, as a function of the Mie resonant denominators and spatial overlap of the input and output spherical waves. Recently, this approach has been extended to quantized solutions for parametric amplification and oscillation via four-wave mixing in microdroplets. The very low thresholds for oscillation at Mie resonances are predicted to correspond to strong squeezing, i.e., suppression of the photocurrent noise associated with the detection of the output waves below the noise level associated with an ideal laser. Even well below threshold, the spatially orthogonal output waves allow for strong squeezing at periodically recurring distances from the droplet.

We introduce a new approach to photodetection, based on absorbers (atoms, molecules, or dopant donors in semiconductors) that are kept in a steady superposition of two bound states having different spatial symmetries (e.g., s and p0). We consider the illumination of such absorbers by a superposition of two fields that are mutually orthogonal by virtue of polarization or frequency, and measurements of the resulting photocurrent along the direction of polarization of one of the fields. In contrast to conventional photon counters, the directional photocurrent generated in the considered scheme measures only the phase-sensitive cross products of the mutually orthogonal field amplitudes, without responding to the photon flux of each mode separately. If the light is a superposition of a one- or two-mode squeezed vacuum field and a nearly coherent (local-oscillator) field orthogonal to the former, then cross correlations between two such photocurrents yield the autocorrelation function of the squeezed-field quadrature without terms related to the noise of the local oscillator (although shot noise still affects the degree of accuracy to which the autocorrelation is measured).

A class of threshold-free process consisting of parametric conversion of coherent light into X-ray emission from channeled projectiles is introduced. The most promising scheme is a four-wave mixing process in which the channeled electrons e interact in the crystal with a counterpropagating idler optical wave I and two optical pump waves P at 90° to the beam. Whereas conventional four-wave mixing involves waves at close frequencies, here a phase-matched signal S is generated at a much higher (X-ray) frequency than I or P because of the Doppler upshift. The nonlinear I-S coupling constant is independent of the Doppler upshift, so that this process does not weaken as the signal wavelength becomes shorter. The most advantageous case is the distributed-feedback Bragg resonance when the signal wavelength is twice the lattice period. The process can produce significant amounts of coherent X-rays for available channeled beam densities and laser powers.

We predict a new type of time-resolved cooperative fluorescence (CF) obtainable when a homonuclear alkali-metal diatom dissociating via a *1 state evolves (by nonadiabatic radial coupling) into a superposition of two excited adiabatic states correlated to different doublet levels. CF intensity then combines ringing oscillations, caused by time variation of the interference phase between the receding emitting atoms, with quantum beats associated with fine-structure splitting. The calculated CF patterns for Li2 are sensitive to the amplitudes and relative phase of the two superposed adiabatic states.