Publications
2022

Quantum entangled states of a classically radiating macroscopic spin(2022) arXiv.org. Abstract
Entanglement constitutes a main feature that distinguishes quantum from classical physics and is a key resource of quantum technologies. Here we show, however, that entanglement may also serve as the essential ingredient for the emergence of classical behavior in a composite nonlinear radiating system. We consider the radiation from a macroscopic spin emitter, such as the collective radiation from an atomic ensemble. We introduce a new class of macroscopic spin states, the coherently radiating spin states (CRSS), defined as the asymptotic eigenstates of the SU(2) lowering operator. We find that a spin emitter in a CRSS radiates classicallike coherent light, although the CRSS itself is a quantum entangled state exhibiting spin squeezing. We further show that CRSS are naturally produced in Dicke superradiance and underlie the dissipative Dicke phase transition. Our CRSS theory thus provides new concepts for studying the quantum physics of radiation, with applications in current platforms involving collections of atoms or spins, their consideration in quantum technologies such as metrology and lasing, and the manybody theory of spin systems.
2021

Multichannel waveguide QED with atomic arrays in free space(2021) arXiv.org. Abstract
We study light scattering off a twodimensional (2D) array of atoms driven to Rydberg levels. We show that the problem can be mapped to a generalized model of waveguide QED, consisting of multiple 1D photonic channels (transverse modes), each of which directionally coupled to a corresponding Rydberg surface mode of the array. In the Rydberg blockade regime, collective excitations of different surface modes block each other, leading to multichannel correlated photonic states. Using an analytical approach, we characterize interchannel quantum correlations, and elucidate the role of collective twophoton resonances of the array. Our results open new possibilities for multimode manybody physics and quantum information with photons in a freespace platform.

(2021) Physical review letters. 126, 22, 223602. Abstract
We investigate the potential for twodimensional atom arrays to modify the radiation and interaction of individual quantum emitters. Specifically, we demonstrate that control over the emission linewidths, resonant frequency shifts, and local driving field enhancement in impurity atoms is possible due to strong dipoledipole interactions within ordered, subwavelength atom array configurations. We demonstrate that these effects can be used to dramatically enhance coherent dipoledipole interactions between distant impurity atoms within an atom array. Possible experimental realizations and potential applications are discussed.
2020

Cavity quantum optomechanics with an atomarray membrane(2020) arXiv. arXiv:2006. Abstract
We consider a quantum optomechanical scheme wherein an ordered twodimensional array of lasertrapped atoms is used as a movable membrane. The extremely light mass of the atoms yields very strong optomechanical coupling, while their spatial order largely eliminates scattering losses. We show that this combination opens the way for quantum optomechanical nonlinearities, well within the ultimate singlephoton strongcoupling regime. As an example, we analyze the possibility to observe optomechanically induced quantum effects such as photon blockade and timedelayed nonclassical correlations. We discuss novel opportunities opened by the optomechanical backaction on the internal states of the array atoms.

Theory of cavity QED with 2D atomic arrays(2020) arXiv. arXiv:2006. Abstract
We develop a quantum optical formalism to treat a twodimensional array of atoms placed in an optical cavity. Importantly, and in contrast to typical treatments, we account for cooperative dipoledipole effects mediated by the interaction of the atoms with the outside, noncavityconfined modes. Based on the observation that scattering to these modes is largely inhibited due to these cooperative effects, we construct a generic formalism, independent of the specific cavity structure, and apply it to an array of nonsaturated atoms. By further considering the atomic motion, we show that the inhibited damping can lead to a favorable scaling of the optomechanical parameters of an atomarray membrane placed within a cavity. The developed formalism lays the basis for further investigation of manybody QED with atom arrays in transversely confined geometries.

(2020) Nature Physics. 16, 6, p. 676681 Abstract
Metasurfaces mould the flow of classical light waves by engineering subwavelength patterns from dielectric or metallic thin films. We introduce and analyse a method in which quantum operatorvalued reflectivity can be used to control both the spatiotemporal and quantum properties of transmitted and reflected light. Such quantum metasurfaces are realized by entangling the macroscopic response of atomically thin atom arrays to light. We show that such a system allows for parallel quantum operations between atoms and photons as well as for the generation of highly entangled photonic states such as photonic Greenberger–Horne–Zeilinger and threedimensional cluster states suitable for quantum information processing. We analyse the influence of imperfections as well as specific implementations based on atom arrays excited into Rydberg states.

(2020) Physical Review A. 101, 6, 063833. Abstract
We demonstrate that a twodimensional atomic array can be used as a platform for quantum optomechanics. Such arrays feature both nearly perfect reflectivity and ultralight mass, leading to significantly enhanced optomechanical phenomena. Considering the collective atomarray motion under continuous laser illumination, we study the nonlinear optical response of the array. We find that the spectrum of light scattered by the array develops multiple sidebands, corresponding to collective mechanical resonances, and exhibits nearly perfect quantumnoise squeezing. Possible extensions and applications for quantum nonlinear optomechanics are discussed.
2019

(2019) Advances in Atomic, Molecular and Optical Physics. Perrin H., Dimauro L. F. & Yelin S. F.(eds.). p. 138 (1Advances in Atomic, Molecular and Optical Physics). Abstract
We develop a theoretical formalism for the study of lightinduced motion of atoms trapped in a twodimensional (2D) array, considering the effect of multiple scattering of light between the atoms. We find that the atomic motion can be described by a collective diffusion equation, wherein laserinduced dipole–dipole forces couple the motion of different atoms. This coupling leads to the formation of collective mechanical modes of the array atoms, whose spatial structure and stability depend on the parameters of the illuminating laser and the geometry of the 2D array. We demonstrate the application of our formalism for the analysis of lightinduced heating of the 2D array. The presented approach should be useful for treating the optomechanical properties of recently proposed quantum optical platforms made of atomic arrays.
2018

(2018) Physical Review Letters. 121, 12, 123606. Abstract
We show that a nonlinear optical response associated with a resonant, atomically thin material can be dramatically enhanced by placing it in front of a partially reflecting mirror, rendering otherwise weakly nonlinear systems suitable for experiments and applications involving quantum nonlinear optics. Our approach exploits the nonlinear response of longlived polariton resonances that arise at particular distances between the material and the mirror. The scheme is entirely based on freespace optics, eliminating the need for cavities or complex nanophotonic structures. We analyze a specific implementation based on excitonpolariton resonances in twodimensional semiconductors and discuss the role of imperfections and loss.

(2018) Science Advances. 4, 4, eaaq0842. Abstract
One of the most intriguing manifestations of quantum zeropoint fluctuations are the van der Waals and Casimir forces, often associated with vacuum fluctuations of the electromagnetic field. We study generalized fluctuation potentials acting on internal degrees of freedom of components in electrical circuits. These electronic Casimirlike potentials are induced by the zeropoint current fluctuations of any general conductive circuit. For realistic examples of an electromechanical capacitor and a superconducting qubit, our results reveal the possibility of tunable forces between the capacitor plates, or the level shifts of the qubit, respectively. Our analysis suggests an alternative route toward the exploration of Casimirlike fluctuation potentials, namely, by characterizing and measuring them as a function of parameters of the environment. These tunable potentials may be useful for future nanoelectromechanical and quantum technologies.
2017

(2017) Physical Review A. 95, 6, 062504. Abstract
It was recently shown that transmissionline waveguides can mediate longrange fluctuation forces between neutral objects, potentially leading to novel Casimir forces in electric circuits. Here we present two approaches for the general description of these forces between electric components embedded in transmissionline circuits. The first, following ordinary quantum electrodynamics (QED), consists of the quantization and scattering theory of voltage and current waves inside transmission lines. The second approach relies on a simple circuit analysis with additional noisy current sources due to resistors in the circuit, as per the fluctuationdissipation theorem (FDT). We apply the latter approach to derive a general formula for the Casimir force induced by circuit fluctuations between any two impedances. The application of this formula, considering the sign of the resulting force, is discussed. While both QED and FDT approaches are equivalent, we conclude that the latter is simpler to generalize and solve.

(2017) Physical Review Letters. 118, 11, 113601. Abstract
We consider light scattering off a twodimensional (2D) dipolar array and show how it can be tailored by properly choosing the lattice constant of the order of the incident wavelength. In particular, we demonstrate that such arrays can operate as a nearly perfect mirror for a wide range of incident angles and frequencies, and shape the emission pattern from an individual quantum emitter into a welldefined, collimated beam. These results can be understood in terms of the cooperative resonances of the surface modes supported by the 2D array. Experimental realizations are discussed, using ultracold arrays of trapped atoms and excitons in 2D semiconductor materials, as well as potential applications ranging from atomically thin metasurfaces to single photon nonlinear optics and nanomechanics.
2016

(2016) Optica. 3, 7, p. 725733 Abstract
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 nanowaveguide, where longrange interactions between the atoms can be tailored. When the atoms are in an electromagnetically induced transparency configuration, the atomic interactions are translated to longrange interactions between photons and thus to highly nonlocal optical nonlinearities. We derive and analyze the governing nonlinear propagation equation, finding a rotonlike 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 manybody physics with highly nonlocal interactions of optical fields in one dimension. (C) 2016 Optical Society of America
2015

(2015) Physica Scripta. 90, 12, 128002. Abstract
In this article we argue that thermal reservoirs (baths) are potentially useful resources in processes involving atoms interacting with quantized electromagnetic fields and their applications to quantum technologies. One may try to suppress the bath effects by means of dynamical control, but such control does not always yield the desired results. We wish instead to take advantage of bath effects, that do not obliterate `quantumness' in the systembath compound. To this end, three possible approaches have been pursued by us. (i) Control of a quantum system faster than the correlation time of the bath to which it couples: such control allows us to reveal quasireversible/coherent dynamical phenomena of quantum open systems, manifest by the quantum Zeno or antiZeno effects (QZE or AZE, respectively). Dynamical control methods based on the QZE are aimed not only at protecting the quantumness of the system, but also diagnosing the bath spectra or transferring quantum information via noisy media. By contrast, AZEbased control is useful for fast cooling of thermalized quantum systems. (ii) Engineering the coupling of quantum systems to selected bath modes: this approach, based on fieldatom coupling control in cavities, waveguides and photonic band structures, allows one to drastically enhance the strength and range of atomatom coupling through the mediation of the selected bath modes. More dramatically, it allows us to achieve bathinduced entanglement that may appear paradoxical if one takes the conventional view that coupling to baths destroys quantumness. (iii) Engineering baths with appropriate nonflat spectra: this approach is a prerequisite for the construction of the simplest and most efficient quantum heat machines (engines and refrigerators). We may thus conclude that often thermal baths are `more friends than foes' in quantum technologies.


(2015) 24TH INTERNATIONAL CONFERENCE ON OPTICAL FIBRE SENSORS. Kalinowski HJ., Fabris JL. & Bock WJ.(eds.). (1Proceedings of SPIE). Abstract
Spontaneous Brillouin backscattering, which accompanies the operation of Brillouin Dynamic Gratings (BDG) sensors, is experimentally investigated for the antistokes configuration, where the probe wave propagates against the orthogonally polarized high frequency writing pump. Even in the absence of the low frequency writing pump but for a strong enough high frequency writing pump, the observed antiStokes reflection of the probe becomes much stronger than its corresponding value under classical antiStokes backscattering. It is also shown that, eventually, as the probe reaches a critical value, the antiStokes reflection sharply decreases to its classical value.
2014

(2014) Physical review letters. 113, 19, 193002. Abstract
According to quantum electrodynamics, the exchange of virtual photons in a system of identical quantum emitters causes a shift of its energy levels. Such shifts, known as cooperative Lamb shifts, have been studied mostly in the nearfield regime. However, the resonant electromagnetic interaction persists also at large distances, providing coherent coupling between distant atoms. Here, we report a direct spectroscopic observation of the cooperative Lamb shift of an optical electricdipole transition in an array of Sr+ ions suspended in a Paul trap at interion separations much larger than the resonance wavelength. By controlling the precise positions of the ions, we studied the farfield resonant coupling in chains of up to eight ions, extending to a length of 40 mu m. This method provides a novel tool for experimental exploration of cooperative emission phenomena in extended mesoscopic atomic arrays.

(2014) Proceedings of the National Academy of Sciences of the United States of America. 111, 29, p. 1048510490 Abstract
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 longerrange interaction than in free space. This enhancement may have profound implications on manyparticle 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.

(2014) Optics Letters. 39, 12, p. 36743677 Abstract
We show that atoms subject to laser radiation may form a nonadditive manybody 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 laserinduced 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 nonadditive systems. (C) 2014 Optical Society of America

(2014) Physical Review A. 89, 4, 43419. Abstract
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 laserinduced dipoledipole interactions (LIDDIs) in any geometry and for any laser strength are described in terms of the resonant dipoledipole 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.
2013

(2013) Physical Review A. 87, 6, Abstract
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, CasimirPolderlike from quasistatic, van der Waalslike 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 freespace result is obtained. This shortrange 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.

(2013) Physical Review A. 87, 3, Abstract
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 nonMarkovian regimes, supported by numerical simulations, and discuss possible experimental realizations. DOI: 10.1103/PhysRevA.87.033831

(2013) Physical Review A. 87, 1, Abstract
We show that a thermal reservoir can effectively act as a squeezed reservoir on atoms that are subject to energylevel modulation. For sufficiently fast and strong modulation, for which the rotatingwave approximation is broken, the resulting squeezing persists at long times. These effects are analyzed by a master equation that is valid beyond the rotatingwave approximation. As an example we consider a twolevel 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
2011

(2011) Physical Review A. 83, 3, 033806. Abstract
Hollowcore photoniccrystal waveguides filled with cold atoms can support giant optical nonlinearities through nondispersive propagation of light tightly confined in the transverse direction. Here we explore electromagnetically induced transparency is such structures, considering a pair of counterpropagating weak quantum fields in the medium of coherently driven atoms in the ladder configuration. Strong dipoledipole interactions between optically excited, polarized Rydberg states of the atoms translate into a large dispersive interaction between the two fields. This can be used to attain a spatially homogeneous conditional phase shift of pi for two singlephoton pulses, realizing a deterministic photonic phase gate, or to implement a quantum nondemolition measurement of the photon number in the signal pulse by a coherent probe, thereby achieving a heralded source of singleor fewphoton pulses.
2009

(2009) Physical Review A. 80, 3, 033803. Abstract
We study the use of squeezed light for qubit coherent control and compare it with the coherentstate control field case. We calculate the entanglement between a short pulse of resonant squeezed light and a twolevel atom in free space and the resulting operation error. We find that the squeezing phase, the phase of the light field, and the atomic superposition phase all determine whether atompulse mode entanglement and the gate error are enhanced or suppressed. When averaged over all possible qubit initial states, the gate error would not decrease by a practically useful amount and would in fact increase in most cases. However, the enhanced entanglement may be of use in quantum communication schemes. We discuss the possibility of measuring the increased gate error as a signature of the enhancement of entanglement by squeezing.