arXiv subjects

H. H. Jen

Publications and source records attributed to H. H. Jen.

At least 55 records · Page 3Linked to original sources

arXiv2021

A non-Hermitian optical atomic mirror

Explorations of symmetry and topology have led to important breakthroughs in quantum optics, but much richer behaviors arise from the non-Hermitian nature of light-matter interactions. A high-reflectivity, non-Hermitian optical mirror can be realized by a two-dimensional subwavelength array of neutral atoms near the cooperative resonance associated with the collective dipole modes. Here we show that exceptional points develop from a nondefective degeneracy by lowering the crystal symmetry of a square atomic lattice, and dispersive bulk Fermi arcs that originate from exceptional points are truncated by the light cone. We also find, although the dipole-dipole interaction is reciprocal, the geometry-dependent non-Hermitian skin effect emerges. Furthermore, skin modes localized at a boundary show a scale-free behavior that stems from the long-range interaction and whose mechanism goes beyond the framework of non-Bloch band theory. Our work opens the door to the study of the interplay among non-Hermiticity, topology, and long-range interaction.

quant-ph↗

arXiv2021

Quantum correlations of localized atomic excitations in a disordered atomic chain

Atom-waveguide interface mediates significant and long-range light-matter interactions through the guided modes. In this one-dimensional system, we theoretically investigate the excitation localization of multiple atomic excitations under strong position disorders. Deep in the localization side, we obtain the time evolutions of quantum correlations via Kubo cumulant expansions, which arise initially and become finite and leveled afterward, overtaking the ones without disorders. This indicates two distinct regimes in time: before the onset of excitation localization, the disorders engage the disturbance of quantum correlations, which is followed by disorder-assisted build-up of quantum correlations that maintain at a later stage owing to the absence of excitations diffusion. The crossing of distinct regimes is pushed further in time for longer-range correlations, which indicates a characteristic timescale needed for disorders to sustain them. We also explore the effect of directionality of couplings and resonant dipole-dipole interactions, which can drive the system toward the delocalized side when it is under chiral couplings or large dipole-dipole interaction strengths. The time-evolved quantum correlations can give insights to the studies of few-body localization phenomenon and nonequilibrium dynamics in open quantum systems.

quant-ph↗

arXiv2021

Spectral compression and entanglement reduction in the cascaded biphoton state with cavities

The cascaded biphoton state generated from a cold atomic ensemble presents one of the strongly correlated resources that can preserve and relay quantum information. Under the four-wave mixing condition, the emitted signal and idler photons from the upper and lower excited states become highly correlated in their traveling directions and entangled in continuous frequency spaces. In this system, we theoretically study the spectral compression of the biphoton source using an external cavity and show the reduction in its frequency entanglement entropy. This indicates, respectively, an improved light absorption efficiency for the idler photon as well as an almost pure biphoton source which is useful in optical quantum networks. We further investigate the limit of the spectral compression that can be achieved by using multiple cavities. Our results show the capability and potential of the biphoton source with external cavities, where the performance of atom-based quantum memory can be enhanced and the entanglement property can be manipulated by tailoring the spectral compression.

quant-ph↗

arXiv2021

Bound and Subradiant Multi-Atom Excitations in an Atomic Array with Nonreciprocal Couplings

Collective decays of multiply-excited atoms become subradiant and bound in space when they are strongly coupled to the guided modes in an atom-waveguide interface. In this interface, we analyze their average density-density and modified third-order correlations via Kubo cumulant expansions, which can arise and sustain for long time. The shape-preserving dimers and trimers of atomic excitations emerge in the most subradiant coupling regime of light-induced dipole-dipole interactions. This leads to a potential application of quantum information processing and quantum storage in the encoded nonreciprocal spin diffusion, where its diffusion speed depends on the initial coherence between the excited atoms and is robust to their relative phase fluctuations. The state-dependent photon routing can be viable as well in this interface.

quant-ph↗

arXiv2020

Crossover from a delocalized to localized atomic excitation in an atom-waveguide interface

An atom-waveguide system, which presents one of the quantum interfaces that enable strong couplings between light and atoms, can support tightly-confined guided modes of light. In this distinctive quantum interface, we theoretically investigate the crossover from a delocalized to localized atomic excitation under long-range dipole-dipole interactions and lattice disorders. Both localization lengths of the excitation distributions and power-law scalings of dissipative von Neumann entanglement entropy show signatures of this crossover. We further calculate numerically the level statistics of the underlying non-Hermitian Hamiltonian, from which as the disorder strength increases, the gap ratio decreases and the intrasample variance increases before reaching respective saturated values. The mean gap ratio in the deeply localized regime is close to the one from Poisson statistics along with a relatively large intrasample variance, whereas in the nondisordered regime, a significant level repulsion emerges. Our results provide insights to study the non-ergodic phenomenon in an atom-waveguide interface, which can be potentially applied to photon storage in this interface under dissipations.

quant-ph↗

arXiv2020

Disorder-assisted excitation localization in chirally coupled quantum emitters

One-dimensional quantum emitters with chiral couplings can exhibit nonreciprocal decay channels, along with light-induced dipole-dipole interactions mediated via an atom-waveguide interface. When the position disorders are introduced to such atomic array, we are able to identify the dynamical phase transition from excitation delocalization to localization, with an interplay between the directionality of decay rates and the strength of light-induced dipole-dipole interactions. Deep in the localization phase, its characteristic length decreases and saturates toward a reciprocal coupling regime, leading to a system dynamics whose ergodicity is strongly broken. We also find an interaction-driven re-entrant behavior of the localization phase and a reduction of level repulsion under strong disorder. The former coincides with a drop in the exponent of power-law decaying von Neumann entropy, which gives insights to a close relation between the preservation of entanglement and nonequilibrium dynamics in open quantum systems, while the latter presents a distinct narrow distribution of gap ratios in this particular disordered system.

quant-ph↗

arXiv2020

Quantum-coherence-enhanced subradiance in a chiral-coupled atomic chain

We theoretically study the quantum-coherence-enhanced subradiance in a chiral-coupled atomic chain with nonreciprocal decay channels. The collective radiation in this one-dimensional (1D) nanophotonics system results from the resonant dipole-dipole interactions (RDDI) in 1D reservoirs, which allow infinite-range couplings between atoms. When single photon interacts with part of the atomic chain from a side excitation, the subradiant decay can be further reduced when highly correlated states are initially excited. The excitation plateau in the decay process can emerge due to the ordered population exchanges, which presents one distinctive signature of long-range and light-induced atom-atom correlations. Multiple time scales of the decay behaviors also show up due to multiple scattering of light transmissions and reflections in the chain. We further investigate the effect of atomic position fluctuations, and find that the cascaded scheme with uni-directional coupling is more resilient to the fluctuations, while the overall decay constant can be increased due to large deviations. Our results present a fundamental study on the subradiance and light-induced atom-atom correlations in such 1D nanophotonics platforms, and offer rich opportunities in potential applications of quantum storage of photons.

quant-ph↗

arXiv2020

Subradiance dynamics in a singly-excited chiral-coupled atomic chain

We theoretically investigate the subradiance dynamics in a nonreciprocal chiral-coupled atomic chain, in which infinite-range dipole-dipole interaction emerges in the dissipation. We find that super- and subradiance are both present in the dissipation process following single photon excitation, and the decay dynamics shows burst emissions from uniform initial excitations, which reflects the influence of atomic ordering on the propagation of light-induced atom-atom correlations. By tuning the nonreciprocal couplings in the chiral-coupled atomic system, we show that the subradiance dynamics can be greatly modified. We further study the effect of atomic local disorder, and find occurrence of plateaus on the decay curve dependent on the defect locations, as well as persistent localized excitations induced by disorders. We also discuss the effect of imperfections of systems on the subradiance dynamics. Our results show rich opportunities in the chiral-coupled system toward photon storage and routing.

quant-ph↗

arXiv2019

Steady-state Phase Diagram of a Weakly Driven Chiral-coupled Atomic Chain

A chiral-coupled atomic chain of two-level quantum emitters allows strong resonant dipole-dipole interactions, which enables significant collective couplings between every other emitters. We numerically obtain the steady-state phase diagram of such system under weak excitations, where interaction-driven states of crystalline orders, edge or hole excitations, and dichotomy of chiral flow are identified. We distinguish these phases by participation ratios and structure factors, and find two critical points which relate to decoherence-free subradiant sectors of the system. We further investigate the transport of excitations and emergence of crystalline orders under spatially-varying excitation detunings, and present non-ergodic butterfly-like system dynamics in the phase of extended hole excitations with a signature of persistent subharmonic oscillations. Our results demonstrate the interaction-induced quantum phases of matter with chiral couplings, and pave the way toward simulations of many-body states in nonreciprocal quantum optical systems.

quant-ph↗

arXiv2019

Spectral shaping of the biphoton state from multiplexed thermal atomic ensembles

We theoretically investigate the spectral property of a biphoton state from multiplexed thermal atomic ensembles. This biphoton state originates from the cascade emissions, which can be generated by two weak pump fields under four-wave mixing condition. Under this condition, a signal photon from the upper transition, chosen in a telecommunication bandwidth, can be generated along with a correlated idler photon from the lower infrared transition. We can spectrally shape the biphoton state by multiplexing the atomic ensembles with frequency-shifted emissions, where the entropy of entanglement can be analyzed via Schmidt decompositions. We find that this spectral entanglement increases when more vapor cells are multiplexed with correlated or anti-correlated signal and idler fields. The eigenvalues in Schmidt bases approach degenerate under this multiplexing scheme, and corresponding Schmidt numbers can be larger than the number of the multiplexed vapor cells, showing the enlarged entropy of entanglement and excess correlated modes in continuous frequency spaces. We also investigate the lowest entropy of entanglement allowed in the multiplexing scheme, which is preferential for generating a pure single photon source. This shows the potentiality to spectrally shape the biphoton source, where high-capacity spectral modes can be applied in long-distance quantum communication and multimode quantum information processing.

quant-ph↗

arXiv2019

Super- and Sub-radiance from Two-dimensional Resonant Dipole-dipole Interactions

We theoretically investigate the super- and sub-radiance from the resonant dipole-dipole interactions (RDDI) in a confined two-dimensional (2D) reservoir. The distinctive feature of 2D RDDI shows qualitatively and quantitatively different long-range behavior from RDDI in free space. We investigate the superradiant properties of the singly-excited symmetric state under this 2D RDDI. This state also allows subradiant decays in much longer distances than the transition wavelength, showing long-range atom-atom correlations. We further study the dynamics of the subradiant states which can be accessed by imprinting spatially dependent phases on the atomic arrays. Our results demonstrate rich opportunities in engineering light-matter interactions in a confined 2D reservoir, and hold promise in applications of quantum light storage and single-excitation state manipulations.

quant-ph↗

arXiv2018

Selective transport of atomic excitations in a driven chiral-coupled atomic chain

We theoretically investigate the flow of the atomic excitations in a driven chiral-coupled atomic chain with nonreciprocal decay channels. This one-dimensional system allows infinite-range dipole-dipole interaction, and enables directional guided modes of scattered light. Under a weakly driven condition, we are able to simulate the transport properties of atomic excitations between the left and right parts of the chain. In the steady states, the transport is highly dependent on the equidistant positions of the ordered array, the excitation field detunings, and the directionality of such chiral-coupled system. We discuss the parameter regimes which are resilient or sensitive to position fluctuations, and provide insights to precise and genuine state preparations. Furthermore, we study the effect of position fluctuations on the transport of excitations. Our results can shed light on deterministic state preparations and pave the way for many-body state manipulations in the driven and dissipative chiral-coupled systems.

quant-ph↗

arXiv2018

Spectrally entangled biphoton state of cascade emissions from a Doppler-broadened atomic ensemble

We theoretically investigate the spectral property of biphoton state from the cascade emissions from a Doppler-broadened atomic ensemble. This biphoton state is spontaneously created in the four-wave-mixing process. The upper transition of the emissions lies in telecom bandwidth, which prevails in fiber-based quantum communication for low-loss transmission. We obtain the spectral property in terms of superradiant decay rates of the lower transition, excitation pulse durations, and temperature of the medium. We quantify their frequency entanglement by Schmidt decomposition and find that more entangled source can be generated with longer excitation pulses, enhanced decay rates, and significant Doppler broadening. A minimally entangled biphoton source can also be located at some optimal temperature of the atoms. This allows spectral shaping of continuous frequency entanglement, which is useful in multimode long-distance quantum communication.

quant-ph↗

arXiv2018

Spin-incoherent Luttinger liquid of one-dimensional SU($κ$) fermions

We theoretically investigate one-dimensional (1D) SU($κ$) fermions in the regime of spin-incoherent Luttinger liquid. We specifically focus on the Tonks-Girardeau gas limit where its density is sufficiently low that effective repulsions between atoms become infinite. In such case, spin exchange energy of 1D SU($κ$) fermions vanishes and all spin configurations are degenerate, which automatically puts them into spin-incoherent regime. In this limit, we are able to express the single-particle density matrices in terms of those of anyons. This allows us to numerically simulate the number of particles up to $N=32$. We numerically calculate single-particle density matrices in two cases: (1) equal populations for each spin components (balanced) and (2) all $S_z$ manifolds included. In contrast to noninteracting multi-component fermions, the momentum distributions are broadened due to strong interactions. As $κ$ increases, the momentum distributions are less broadened for fixed $N$, while they are more broadened for fixed number of particle per spin component. We then compare numerically calculated high momentum tails with analytical predictions which are proportional to $1/p^4$, in good agreement. Thus, our theoretical study provides a comparison with the experiments of repulsive multicomponent alkaline-earth fermions with a tunable SU($κ$) spin-symmetry in the spin-incoherent regime.

cond-mat.quant-gas↗

arXiv2018

Directional subradiance from helical-phase-imprinted multiphoton states

We theoretically investigate the far-field scattering properties of multiphoton super- and subradiant states which can be prepared by multiphoton excitations with orbital angular momentum (OAM).\ Due to multiphoton interference, the far-field patterns of the subradiant modes show directional scattering along the excitation direction or transverse scattering with number of peaks equal to the number of atoms.\ When more atoms are involved, we consider structures of stacked and concentric rings, which respectively show enhanced directional scattering and smoothed emission patterns.\ Our scheme gives insights to prepare many-body subradiant states, and is potentially applicable to quantum storage of multiphoton with OAM.\ By designing atomic spatial distributions, these cooperative states can tailor the far-field emission properties, which is useful for light collections and quantum information manipulations.

quant-ph↗

arXiv2018

Cooperative light scattering from helical-phase-imprinted atomic rings

We theoretically investigate the light scattering of the super- and subradiant states which can be prepared by the excitation of a single photon which carries an orbital angular momentum (OAM).\ With this helical phase imprinted on the stacked ring of atomic arrays, the subradiant modes show directional side scattering in the far-field, allowing for light collimation and quantum storage of light with OAM.\ For the excitations with linear polarizations, we find a discrete $C_4$ rotational symmetry in scattering for the number of atoms $N$ $=$ $4n $ with integers $n$, while for circular polarizations with arbitrary $N$, the azimuthal and $C_N$ symmetries emerge for the super- and subradiant modes respectively.\ When the radial and azimuthal polarizations are considered, a mode shift can happen in the scattering pattern.\ The forward scattering of the superradiant modes can be enhanced as we stack up the rings along the excitation direction, and for the subradiant modes, we find the narrowing effects on the scattering in the azimuthal and the polar angles when more concentric rings are added in the radial direction.\ By designing the atomic spatial distributions and excitation polarizations, helical-phase-imprinted subradiant states can tailor and modify the radiation properties, which is detectable in the directional super- and subradiant emissions and is potentially useful in quantum information manipulations.

quant-ph↗

arXiv2017

Phase-imprinted multiphoton subradiant states

We propose to generate the multiphoton subradiant states and investigate their fluorescences in an array of two-level atoms. These multiphoton states are created initially from the timed-Dicke states. Then we can use either a Zeeman or Stark field gradient pulse to imprint linearly increasing phases on the atoms, and this phase-imprinting process unitarily evolves the system to the multiphoton subradiant states. The fluorescence engages a long-range dipole-dipole interaction which originates from a system-reservoir coupling in the dissipation. We locate some of the subradiant multiphoton states from the eigenmodes, and show that an optically thick atomic array is best for the preparation of the state with the most reduced decay rate. This phase-imprinting process enables quantum state engineering of the multiphoton subradiant states, and realizes a potential quantum storage of the photonic qubits in the two-level atoms.

quant-ph↗

arXiv2017

Spin-incoherent Luttinger liquid of one-dimensional spin-1 Tonks-Girardeau Bose gas: Spin-dependent properties

Spin-incoherent Luttinger liquid (SILL) is a different universal class from the Luttinger liquid.\ This difference results from the spin incoherence of the system when the thermal energy of the system is higher than the spin excitation energy.\ We consider one-dimensional spin-$1$ Bose gas in the SILL regime and investigate its spin-dependent many-body properties.\ In Tonks-Girardeau limit, we are able to write down the general wave functions in a harmonic trap.\ We numerically calculate the spin-dependent (spin-plus, minus, and $0$) momentum distributions in the sector of zero magnetization which allows to demonstrate the most significant spin-incoherent feature compared to the spinless or spin-polarized case.\ In contrast to the spinless Bose gas, the momentum distributions are broadened and in the large momentum limit follow the same asymptotic $1/p^4$ dependence but with reduced coefficients.\ While the density matrices and momentum distributions differ between different spin components for small $N$, at large $N$ they approach each other.\ We show these by analytic arguments and numerical calculations up to $N$ $=$ $16$.

cond-mat.quant-gas↗