The coupled discrete linear and Kerr nonlinear Schrödinger equations with gain and loss describing transport on dimers with parity-time ($\mathcal{PT}$)-symmetric potentials are considered. The model is relevant among others to experiments in optical couplers and proposals on Bose-Einstein condensates in $\mathcal{PT}$-symmetric double-well potentials. It is known that the models are integrable. Here, the integrability is exploited further to construct the phase portraits of the system. A pendulum equation with a linear potential and a constant force for the phase difference between the fields is obtained, which explains the presence of unbounded solutions above a critical threshold parameter. The behavior of all solutions of the system, including changes in the topological structure of the phase plane, is then discussed.

We ab initio derive quantum nonclassicality and non-Gaussianity criteria for realistic single-photon sources and discuss the robustness of these quantum features in attenuating quantum channel. The derivation based only on assumptions about the structure of the detector is performed for standard autocorrelation measurement with single-photon detectors. Using both the criteria, we compare the robustness of quantum features of realistic single-photon states with the Poissonian and thermal background noise and discuss a difference between the impact of the multimode structure of thermal noise and truly Poissonian noise.

The propagation of an optical beam in dispersive medium is described by a spatial analog of pulse propagation in time domain. In particular, the evolution of transverse beam profiles with transversely complicated wave fronts passing through spatially dispersive media is examined and explained on the basis of net group and reshaping shifts in the $\omega$ and $\vec{k}$ domains. The experimental results are in good agreement with the theory of net group and reshaping delays as described by Peatross et al. [, , and , Phys. Rev. Lett. 84, 2370 (2000)], which successfully described the arrival time for optical pulses through frequency dispersive media.

Bound states of two or three solitons in dispersion-managed fibers (soliton molecules) were experimentally demonstrated recently. We investigate with a modified perturbation analysis whether the binding mechanism creates a unique stable equilibrium of the relative positions of the solitons in the molecule. Indeed, we find a multitude of equilibrium states, alternatingly stable and unstable. This holds for either case: nearest neighbor solitons having the same or the opposite phase. The number of equilibria are limited by the level of the radiation background. The state with the smallest separation and the highest binding energy (“ground state”) always occurs for opposite-phase pulses; the lowest-order state for in-phase pulses is always unstable. Stable long-chain molecules can be built with a mixture of different nearest-neighbor equilibrium separations. Our results agree with our numerical simulations and experimental results, and connect well with certain results in the literature.

Composite bosons made of two bosonic constituents exhibit deviations from ideal bosonic behavior due to their substructure. This deviation is reflected by the normalization ratio of the quantum state of $N$ composites. We find a set of saturable, efficiently evaluable bounds for this indicator, which quantifies the bosonic behavior of composites via the entanglement of their constituents. We predict an abrupt transition between ordinary and exaggerated bosonic behavior in a condensate of two-boson composites.

Inhomogeneously broadened spin ensembles play an important role in present-day implementation of hybrid quantum processing architectures. When coupled to a cavity, such an ensemble may serve as a multimode quantum memory for the cavity field, and by employing spin-refocusing techniques the quantum-memory time can be extended to the coherence time of individual spins in the ensemble. In this paper, we investigate such a memory protocol capable of storing an unknown cavity-field state, and we examine separately the various constituents of the protocol: the storage and readout part, the memory hold time with the spin ensemble and cavity field decoupled, and the parts employing spin-refocusing techniques. Using both analytical and numerical methods, we derive how the obtainable memory performance scales with various physical parameters.

We study the problem of entangling two spins at the distant ends of a spin chain by exploiting the nonequilibrium dynamics of the system after a sudden global quench. As initial states we consider a canted or spiral order product state of the spins and singlets of neighboring pairs of spins. We find that within the class of canted order initial states, no entanglement is generated at any time except for the special case of the Néel state. While an earlier work has shown that the Néel state is indeed an excellent starting resource for the dynamical generation of long-distance entanglement, the curious fact that this is the sole point within a large class of initial product states of the spins was not noted. On the other hand, we find that an initial state which is a series of nearest-neighbor Bell states, and well motivated by some physical realizations, is also a good starting resource for end to end entanglement in a way similar to that of the Néel state. The scheme is shown to be robust to random single spin flip in the initial Néel state as well as to randomness of the couplings.

We consider a two-level quantum system prepared in an arbitrary initial state and relaxing to a steady state due to the action of a Markovian dissipative channel. We study how optimal control can be used for speeding up or slowing down the relaxation towards the fixed point of the dynamics. We analytically derive the optimal relaxation times for different quantum channels in the ideal ansatz of unconstrained quantum control (a magnetic field of infinite strength). We also analyze the situation in which the control Hamiltonian is bounded by a finite threshold. As by-products of our analysis, we find that (i) if the qubit is initially in a thermal state hotter than the environmental bath, quantum control can not speed up its natural cooling rate; (ii) if the qubit is initially in a thermal state colder than the bath, it can reach the fixed point of the dynamics in finite time if a strong control field is applied; (iii) in the presence of unconstrained quantum control, it is possible to keep the evolved state indefinitely and arbitrarily close to special initial states which are far away from the fixed points of the dynamics.

In a previous paper, we pointed out the problem of missing data outside the detector range for continuous-variable entanglement verification and quantum key distribution, and we provided a straightforward solution based on entropic separability criteria (as those work better than variance-based criteria). We apply that solution here to the verification of time-frequency entanglement of photon pairs, particularly to the quantum key distribution scheme proposed by Nunn et al. [Opt. Express 21, 15959 (2013)]. We find that the scheme does lead to verifiable entanglement, but that transmission noise quickly destroys the ability to verify the entanglement.

We introduce high-fidelity passband (PB) composite pulse sequences constructed by concatenation of recently derived arbitrarily large and arbitrarily accurate broadband $\mathcal{B}$ and narrowband $\mathcal{N}$ composite sequences. Our PB sequences allow us to produce flexible and tunable nearly rectangular two-state inversion profiles as a function of the individual pulse area because the width and the rectangularity of these profiles can be adjusted at will. Moreover, these PB sequences suppress excitation around pulse area 0 and $2\pi$, and suppress deviations from complete population inversion around pulse area $\pi$ to arbitrarily high orders. These features make them a valuable tool for high-fidelity qubit operations in the presence of relatively strong amplitude noise. We construct two types of PB pulses: $\mathcal{N}\left(\mathcal{B}\right)$, in which a broadband pulse is nested into a narrowband pulse, and $\mathcal{B}\left(\mathcal{N}\right)$, in which a narrowband pulse is nested into a broadband pulse; the latter sequences deliver narrower profiles. We derive exact analytic formulas for the composite phases of the PB pulses and exact analytic formulas for the inversion profiles. These formulas allow an easy estimation of the experimental resources needed for any desired qubit inversion profile.

We propose a method, molecular polarimetry, applicable to an extended VUV–x-ray range, allowing us to determine the complete state of elliptically polarized light, including the challenging disentanglement of the circular and unpolarized components. It relies on the determination of the molecular frame photoelectron angular distributions derived from electron-ion velocity vector correlations in dissociative photoionization of simple molecules. The high accuracy of the full set of Stokes parameters determination has been established by comparison with data obtained in parallel with a VUV optical polarimeter used as a benchmark.

Within the strong-field approximation, we study the time-resolved ionization process of a hydrogen atom in strong laser fields. At any time during the interaction between an atom and a laser field, the electron may instantaneously absorb all possible energies and build a wide momentum spectrum. As the interaction evolves in time, the coherent superposition of all previous instantaneous ionization events gradually filters out all other energy components except the part with an energy $n\omega -I_p$, where $I_p$ is the ionization potential, $\omega$ is the photon energy, and $n$ is a positive integer. The establishment of the electron energy spectra in above-threshold ionization, rescattering, and streaking can all be directly viewed by tracking the time-dependent momentum spectra.

We have measured the threshold for creating long-lived excitations when a toroidal Bose-Einstein condensate is stirred by a rotating (optical) barrier of variable height. When the barrier height is on the order of or greater than half of the chemical potential, the critical barrier velocity at which we observe a change in the circulation state is much less than the speed for sound to propagate around the ring. In this regime we primarily observe discrete jumps (phase slips) from the noncirculating initial state to a simple, well-defined, persistent current state. For lower barrier heights, the critical barrier velocity at which we observe a change in the circulation state is higher, and approaches the effective sound speed for vanishing barrier height. The response of the condensate in this small-barrier regime is more complex, with vortex cores appearing in the bulk of the condensate. We find that the variation of the excitation threshold with barrier height is in qualitative agreement with the predictions of an effective one-dimensional hydrodynamic model.

We consider the fundamental problem of high-temperature phase transitions in the system of high-density two-level atoms off-resonantly interacting with a pump field in the presence of optical collisions (OCs) and placed in the cavity. OCs are considered in the framework of thermalization of atomic dressed-state (DS) population. For the case of a strong atom-field coupling condition, we analyze the problem of a thermodynamically equilibrium superradiant phase transition for the order parameter representing a real amplitude of cavity mode and taking place as a result of the atomic DSs thermalization process. Such a transition is also connected with condensed (coherent) properties of low branch DS polaritons occurring in the cavity. For describing nonequilibrium phase transitions we derive Maxwell-Bloch–like equations which account for cavity decay rate, collisional decay rate, and spontaneous emission. Various aspects of transitions to laser field formation by using atomic DS levels for both positive and negative detuning of a pump field from atomic transition frequency are studied in detail. It is revealed that for positive atom-light detuning DS lasing can be obtained in the presence of quasiequilibrium DS population that corresponds to a true two-level atomic system with the inversion in a nonresonant limit.

We study the nonequilibrium steady state (NESS) of a driven dissipative one-dimensional system near a critical point, and explore how the quantum correlations compare to the known critical behavior in the ground state. The model we study corresponds to a cavity array driven parametrically at a two photon resonance, equivalent in a rotating frame to a transverse field anisotropic $XY$ model [C.-E. Bardyn and A. Imamoğlu, Phys. Rev. Lett. 109, 253606 (2012)]. Depending on the sign of transverse field, the steady state of the open system can be either related to the ground state or to the maximum energy state. In both cases, many properties of the entanglement are similar to the ground state, although no critical behavior occurs. As one varies from the Ising limit to the isotropic $XY$ limit, entanglement range grows. The isotropic limit of the NESS is, however, singular, with simultaneously diverging range and vanishing magnitude of entanglement. This singular limiting behavior is quite distinct from the ground state behavior; it can, however, be understood analytically within spin-wave theory.

We analyze a general approach for suppressing inhomogeneous broadening of atomic transitions and thereby increasing the strength of the interaction between the atomic system and near-resonant light. The key idea is to compensate for the frequency shift due to the broadening process by using an intense laser to produce an equal and opposite Stark shift.

Squeezed states of various types of quantum oscillators are important nonclassical resources for quantum physics. We derive how much squeezing is universally distillable from multiple copies of non-Gaussian nonclassical states. We extend the approach to more efficient nonuniversal squeezing distillation, transforming hidden and scattered quantum nonclassicality to applicable squeezing. We demonstrate the distillation of squeezing from the mixtures of Gaussian states and also from quantum non-Gaussian states, incompatible with such the mixtures of Gaussian states.

We theoretically investigate the high-order harmonic generation (HHG) in a two-color multicycle inhomogeneous laser field. The results show that the ionization, acceleration, and recombination of the electrons in the HHG process can be simultaneously controlled with the combination of the temporal waveform synthesis and spatial nonhomogeneity of the laser field. Based on the quantum-mechanical and classical models, the physical mechanism of the supercontinuum generation in such a temporally and spatially shaped field is analyzed and discussed. In contrast to the case of the fundamental field alone, the supercontinuum in the two-color inhomogeneous field shows a constant intensity, a higher yield, and a broader bandwidth, which is beneficial for the efficient generation of broadband-isolated attosecond pulses from multicycle laser fields.

The general class of Gaussian Schmidt-number witness operators for bipartite systems is studied. It is shown that any member of this class is reducible to a convex combination of two types of Gaussian operators using local operations and classical communications. This gives rise to a simple operational method, which is solely based on measurable covariance matrices of quantum states. Our method bridges the gap between theory and experiment of entanglement quantification. In particular, we certify lower bounds of the Schmidt number of squeezed thermal and phase-randomized squeezed vacuum states as examples of Gaussian and non-Gaussian quantum states, respectively.

In this work, we consider the dynamics of bosons in bands with nontrivial topological structure. In particular, we focus on the case where bosons are prepared in a higher-energy band and allowed to evolve. The Bogoliubov theory about the initial state can have a dynamical instability, and we show that it is possible to achieve the interesting situation where the topological edge modes are unstable while all bulk modes are stable. Thus, after the initial preparation, the edge modes will become rapidly populated. We illustrate this with the Su-Schrieffer-Heeger model which can be realized with a double-well optical lattice and is perhaps the simplest model with topological edge states. This work provides a direct physical consequence of topological bands whose properties are often not of immediate relevance for the near-equilibrium properties of bosonic systems.