The detailed study of the low-energy spectrum for a mobile impurity in the one-dimensional bosonic environment is performed. Particularly we have considered only two analytically accessible limits, namely, the case of an impurity immersed in a dilute Bose gas where one can use many-body perturbative techniques for low-dimensional bosonic systems and the case of the Tonks-Girardeau gas for which the usual fermionic diagrammatic expansion up to the second order is applied.

We demonstrate site-resolved imaging of a strongly correlated quantum system without relying on laser cooling techniques during fluorescence imaging. We observe the formation of Mott shells in the insulating regime and realize thermometry in an atomic cloud. This work proves the feasibility of the noncooled approach and opens the door to extending the detection technology to new atomic species.

In this theoretical study, we demonstrate that entangled states are able to significantly extend the functionality of Hong-Ou-Mandel (HOM) interferometers. By generating a coherent superposition of parametric-down-conversion photons and spatial entanglement in the input channel, the coincidence probability measured at the output changes from a typical HOM-type dip (photon bunching) into much richer patterns, including an antibunching peak and rapid oscillation fringes with twice the optical frequency. The considered system should be realizable on a single chip using current waveguide technology in the ${\mathrm{LiNbO}}_3$ platform.

We examine coherent population trapping in a circuit-QED system consisting of an Al/${\mathrm{AlO}}_x$/Al transmon qubit embedded in an Al three-dimensional cavity. By engineering the dissipation rate of the cavity to be much larger than that of the qubit and continuously pumping a two-photon process to an excited state of the cavity-qubit system, we are able to invert the population into the qubit excited state with a fidelity of 94%. Applying a second continuous drive tone at the dressed cavity frequency forms an effective $\mathrm{\Lambda }$ system and enables us to coherently trap the system in a dark state formed from a superposition of the excited and ground states of the qubit. By performing quantum state tomography we demonstrate that the position of the dark state on the Bloch sphere can be controlled by changing the relative amplitudes and phases of the two drives.

We compute the energies of the interaction between two atoms placed in the middle of a perfectly reflecting planar cavity, in the weak-coupling nondegenerate regime. Both inhibition and enhancement of the interactions can be obtained by varying the size of the cavity. We derive exact expressions for the dyadic Green's function of the cavity field which mediates the interactions and apply time-dependent quantum perturbation theory in the adiabatic approximation. We provide explicit expressions for the van der Waals potentials of two polarizable atomic dipoles and the electrostatic potential of two induced dipoles. We compute the van der Waals potentials in three different scenarios: two atoms in their ground states, two atoms excited, and two dissimilar atoms with one of them excited. In addition, we calculate the phase-shift rate of the two-atom wave function in each case. The effect of the two-dimensional confinement of the electromagnetic field on the dipole-dipole interactions is analyzed. This effect depends on the atomic polarization. For dipole moments oriented parallel to the cavity plates, both the electrostatic and the van der Waals interactions are exponentially suppressed for values of the cavity width much less than the interatomic distance, whereas for values of the width close to the interatomic distance, the strength of both interactions is higher than their values in the absence of cavity. For dipole moments perpendicular to the plates, the strength of the van der Waals interaction decreases for values of the cavity width close to the interatomic distance, while it increases for values of the width much less than the interatomic distance with respect to its strength in the absence of cavity. We illustrate these effects by computing the dipole-dipole interactions between two alkali atoms in circular Rydberg states.

We consider the elastic scattering of electrons by hydrogen atoms in the presence of a two-color circularly polarized laser field in the domain of moderate intensities below ${10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ and high projectile energies. A hybrid approach is used, where for the interaction of the incident and scattered electrons with the laser field we employ the Gordon-Volkov wave functions, while the interaction of the hydrogen atom with the laser field is treated in second-order perturbation theory. Within this formalism, a closed analytical solution is derived for the nonlinear differential cross section, which is valid for circular as well linear polarizations. Simple analytical expressions of differential cross section are derived in the weak field domain for two-color laser field that is a combination of the fundamental and its second or third harmonics. It is shown that the nonlinear differential cross sections depend on the dynamical phase of the scattering process and on the helicities of the two-color circularly polarized laser field. A comparison between the two-photon absorption scattering signal for two-color co- and counterrotating circularly polarized laser fields is made for even ($2\omega$) or odd ($3\omega$) harmonics, and the effect of the intensity ratio of the two-color laser field components is studied. We analyze the origin of the symmetries in the differential cross sections and we show that the modification of the photon helicity implies a change in the symmetries of the scattering signal.

Supersymmetric systems derive their properties from conserved supercharges which form a supersymmetric algebra. These systems naturally factorize into two subsystems, which, when considered as individual systems, have essentially the same eigenenergies, and their eigenstates can be mapped onto each other. We propose a Mach-Zehnder interference experiment to detect supersymmetry in quantum-mechanical systems, which can be realized with current technology. To demonstrate this interferometric technique, we first propose a one-dimensional ultracold-atom setup to realize a pair of supersymmetric systems. Here, a single-atom wave packet evolves in a superposition of the subsystems and gives an interference contrast that is sharply peaked if the subsystems form a supersymmetric pair. Second, we propose a two-dimensional setup that implements supersymmetric dynamics in a synthetic gauge field.

Free-space optical communication links are promising channels for establishing secure quantum communication. Here we study the transmission of nonclassical light through a turbulent atmospheric link under diverse weather conditions, including rain or haze. To include these effects, the theory of light transmission through atmospheric links in the elliptic-beam approximation presented by Vasylyev et al. [D. Vasylyev et al., Phys. Rev. Lett. 117, 090501 (2016)] is further generalized. It is demonstrated, with good agreement between theory and experiment, that low-intensity rain merely contributes additional deterministic losses, whereas haze also introduces additional beam deformations of the transmitted light. Based on these results, we study theoretically the transmission of quadrature squeezing and Gaussian entanglement under these weather conditions.

We report on the localization of excitability induced by disorder in an array of coupled semiconductor lasers with a saturable absorber. Through numerical simulations we show that the exponential localization of excitable waves occurs if a certain critical amount of randomness is present in the coupling coefficients among the lasers. The results presented in this Rapid Communication demonstrate that disorder can induce localization in lattices of excitable nonlinear oscillators, and can be of interest in the study of photonics-based random networks, neuromorphic systems, and, by analogy, in biology, in particular, in the investigation of the collective dynamics of neuronal cell populations.

The tunneling decay event of a metastable state in a fully connected quantum spin model can be simulated efficiently by path-integral quantum Monte Carlo (QMC) [Isakov et al., Phys. Rev. Lett. 117, 180402 (2016)]. This is because the exponential scaling with the number of spins of the thermally assisted quantum tunneling rate and the Kramers escape rate of QMC are identical [Jiang et al., Phys. Rev. A 95, 012322 (2017)], a result of a dominant instantonic tunneling path. In Isakov et al., it was also conjectured that the escape rate in open-boundary QMC is quadratically larger than that of conventional periodic-boundary QMC; therefore, open-boundary QMC might be used as a powerful tool to solve combinatorial optimization problems. The intuition behind this conjecture is that the action of the instanton in open-boundary QMC is a half of that in periodic-boundary QMC. Here, we show that this simple intuition—although very useful in interpreting some numerical results—deviates from the actual situation in several ways. Using a fully connected quantum spin model, we derive a set of conditions on the positions and momenta of the end points of the instanton, which remove the extra degrees of freedom due to open boundaries. In comparison, the half-instanton conjecture incorrectly sets the momenta at the end points to zero. We also found that the instantons in open-boundary QMC correspond to quantum tunneling events in the symmetric subspace (maximum total angular momentum) at all temperatures, whereas the instantons in periodic-boundary QMC typically lie in subspaces with lower total angular momenta at finite temperatures. This leads to a lesser-than-quadratic speedup at finite temperatures. The results provide useful insights in utilizing open-boundary QMC to solve hard optimization problems. We also outline the generalization of the instantonic tunneling method to many-qubit systems without permutation symmetry using spin-coherent-state path integrals.

The term Einstein-Podolsky-Rosen steering refers to a quantum correlation intermediate between entanglement and Bell nonlocality, which has been connected to another fundamental quantum property: measurement incompatibility. In the finite-dimensional case, efficient computational methods to quantify steerability have been developed. In the infinite-dimensional case, however, less theoretical tools are available. Here, we approach the problem of steerability in the continuous variable case via a notion of state-channel correspondence, which generalizes the well-known Choi-Jamiołkowski correspondence. Via our approach we are able to generalize the connection between steering and incompatibility to the continuous variable case and to connect the steerability of a state with the incompatibility breaking property of a quantum channel, with applications to noisy NOON states and amplitude damping channels. Moreover, we apply our methods to the Gaussian steering setting, proving, among other things, that canonical quadratures are sufficient for steering Gaussian states.

We study the impact of finite-size effects on the key rate of continuous-variable (CV) measurement-device-independent (MDI) quantum key distribution (QKD), considering two-mode Gaussian attacks. Inspired by the parameter estimation technique developed in by Ruppert et al. [Phys. Rev. A 90, 062310 (2014)], we adapt it to study CV-MDI-QKD and, assuming realistic experimental conditions, we analyze the impact of finite-size effects on the key rate. We find that the performance of the protocol approaches the ideal one, increasing the block size, and, most importantly, that blocks between ${10}^6$ and ${10}^9$ data points may provide key rates $\sim {10}^{-2}$ bit/use over metropolitan distances.

The rotational Doppler effect occurs when circularly polarized light interacts with a rotating anisotropic material. It is manifested by the appearance of a spectral shift ensuing from the transfer of angular momentum and energy between radiation and matter. Recently, we reported terahertz-range rotational Doppler shifts produced in third-order nonlinear optical conversion [O. Faucher et al., Phys. Rev. A 94, 051402(R) (2016)]. The experiment was performed in an ensemble of coherently spinning molecules prepared by a short laser pulse exhibiting a twisted linear polarization. The present work provides an extensive analysis of the rotational Doppler effect in third-order-harmonic generation from spinning linear molecules. The underlying physics is investigated both experimentally and theoretically. The implication of the rotational Doppler effect in higher-order processes like high-order-harmonic generation is discussed.

We study the odd-integer filled Mott phases of a spin-1 Bose-Hubbard chain and determine their fate in the presence of a Raman induced spin-orbit coupling which has been achieved in ultracold atomic gases; this system is described by a quantum spin-1 chain with a spiral magnetic field. The spiral magnetic field initially induces helical order with either ferromagnetic or dimer order parameters, giving rise to a spiral paramagnet at large field. The spiral ferromagnet-to-paramagnet phase transition is in a universality class with critical exponents associated with the divergence of the correlation length $\nu \approx 2/3$ and the order-parameter susceptibility $\gamma \approx 1/2$. We solve the effective spin model exactly using the density-matrix renormalization group, and compare with both a large-$S$ classical solution and a phenomenological Landau theory. We discuss how these exotic bosonic magnetic phases can be produced and probed in ultracold atomic experiments in optical lattices.

We investigate systems of interacting bosonic particles confined within slablike boxes of size $L^2\times Z$ with $Z\ll L$, at their three-dimensional (3D) Bose-Einstein-condensation (BEC) transition temperature $T_c$, and below $T_c$ where they experience a quasi-two-dimensional (quasi-2D) Berezinskii-Kosterlitz-Thouless (BKT) transition (at $T_{\mathrm{BKT}}<T_c$ depending on the thickness $Z$). The low-temperature phase below $T_{\mathrm{BKT}}$ shows quasi-long-range order: the planar correlations decay algebraically as predicted by the 2D spin-wave theory. This dimensional crossover, from a 3D behavior for $T\gtrsim T_c$ to a quasi-2D critical behavior for $T\lesssim T_{\mathrm{BKT}}$, can be described by a transverse finite-size scaling limit in a slab geometry. Numerical evidence of the $3\mathrm{D}\to 2\mathrm{D}$ dimensional crossover is presented for the Bose-Hubbard model defined in anisotropic $L^2\times Z$ lattices with $Z\ll L$. We extend this scaling analysis to the case the slab geometry of the gas is effectively realized by a transverse (inhomogeneous) harmonic trap. Finally, we discuss off-equilibrium behaviors arising from slow time variations of the temperature across the BEC transition of gases confined within a slab geometry. We argue that the system develops an off-equilibrium transverse finite-size scaling under these time-dependent protocols.

It has recently been shown that optical parametric oscillator (OPO) Ising machines, consisting of coupled optical pulses circulating in a cavity with parametric gain, can be used to probabilistically find low-energy states of Ising spin systems. In this work, we study optical Ising machines that operate under simplified Gaussian dynamics. We show that these dynamics are sufficient for reaching probabilities of success comparable to previous work. Based on this result, we propose modified optical Ising machines with simpler designs that do not use parametric gain yet achieve similar performance, thus suggesting a route to building much larger systems.

The cross-modulation between the gain and chirped phase in chirped-pulse amplification (CPA) is theoretically and experimentally demonstrated. We propose a gain-phase coupled nonlinear Schrödinger equation (GPC-NLSE) for solving chirped-pulse propagation in a nonlinear gain medium involved in the gain-phase modulation (GPM) process. With the GPC-NLSE, the space-time-frequency-dependent gain, chirped phase, pulse, and spectrum evolutions can be precisely calculated. Moreover, a short-length high-gain Yb-doped fiber CPA experiment is presented in which a self-steepening distortion of the seed pulse is automatically compensated after amplification. This phenomenon can be explained by the GPM theory whereas conventional models cannot. The experimental results for the temporal and spectral intensities show excellent agreement with our theory. Our GPM theory paves the way for further investigations of the finer structures of the pulse and spectrum in CPA systems.

Electromagnetic response of a $\mathcal{PT}$ dipole is studied both analytically and numerically. In the analytical approach, the dipole is represented by two point scatterers. Within the first Born approximation, the asymmetry of the scattering field with respect to the orientation of the dipole is proven. In numerical simulations, the dipole is represented by two infinitely long, parallel cylinders with opposite sign of the imaginary part of a refractive index. Numerical data confirm the validity of the Born approximation in the weak scattering limit, while significant deviations from the Born approximation were observed for stronger scatterers and in the near-field range.

In this paper, we propose an approach to accelerate the dissipation dynamics for quantum state generation with Lyapunov control. The strategy is to add target-state-related coherent control fields into the dissipation process to intuitively improve the evolution speed. By applying the current approach, without losing the advantages of dissipation dynamics, the target stationary states can be generated in a much shorter time as compared to that via traditional dissipation dynamics. As a result, the current approach containing the advantages of coherent unitary dynamics and dissipation dynamics allows for a significant improvement in quantum state generation.

We consider one-dimensional chains of two-level quantum systems coupled via tunneling. The chain is driven by the superposition of dc and ac fields in the strong coupling regime. Based on the fundamental principles of electrodynamics and quantum theory, we have developed a generalized model of quantum dynamics for such interactions, free of rotating-wave approximation. The system of equations of motion was studied numerically. We analyzed the dynamics and spectra of the inversion density, dipole current density, and tunneling current density. In the case of resonant interaction with the ac component, the particle dynamics exhibits itself in the oscillatory regime, which may be interpreted as a combination of Rabi and Bloch oscillations with their strong mutual influence. Such scenario for an obliquely incident ac field dramatically differs from the individual picture of both types of oscillations due to the interactions. This effect is counterintuitive because of the existence of markedly different frequency ranges for such two types of oscillations. These dynamics manifest themselves in multiline spectra in different combinations of Rabi and Bloch frequencies. The effect is promising as a framework of a new type of spectroscopy in nanoelectronics and electrical control of nanodevices.