The photonic router is a key device in optical quantum networks. Conventional routers, however, can only transfer a photon from the input port to the desired port probabilistically. Here, we propose to use chiral photon-atom interactions for targeted routing that can transfer a single photon from the input port to an arbitrarily selected output port deterministically, i.e., with $100\%$ probability. The configuration of the proposed router consists of a driven three-level atom which is chirally coupled to two photonic waveguides simultaneously. It is shown that, by properly adjusting the driving field on the atom, a single photon inputting from one port of the first waveguide can be deterministically transferred to one of the selected output ports of the second waveguide. Both resonant and nonresonant photons can be deterministically routed to the desired ports. We also examine how the routing fidelity is influenced by dissipations and imperfections, and investigate the transmission characters of two resonant photons.

The quantum interference of entangled photons forms a key phenomenon underlying various quantum-optical technologies. It is known that the quantum interference patterns of entangled photon pairs can be reconstructed classically by the time-reversal method; however, the time-reversal method has been applied only to time-frequency–entangled two-photon systems in previous experiments. Here, we apply the time-reversal method to the position-wave-vector–entangled two-photon systems: the two-photon Young interferometer and the two-photon beam focusing system. We experimentally demonstrate that the time-reversed systems classically reconstruct the same interference patterns as the position-wave-vector–entangled two-photon systems.

We model the bang-bang optimization protocol as a shortcut to adiabaticity in the ground-state preparation of a trapped-ion quantum simulator. Compared to a locally adiabatic evolution, the bang-bang protocol typically produces a lower ground-state probability, but its implementation is so much simpler than the locally adiabatic approach, that it can become a competitive choice to use for maximizing ground-state preparation in systems that cannot be solved with conventional computers. We describe how one can optimize the shortcut and provide specific details for how it can be implemented with current trapped-ion quantum simulators. However, when frustration is strong enough, no method appears to work well for adiabatic state preparation within the experimental time frames, and one must confront the issue of dealing with diabatic excitations within the simulation.

We study the problem of locally distinguishing pure quantum states using shared entanglement as a resource. For a given set of locally indistinguishable states, we define a resource state to be useful if it can enhance local distinguishability and optimal if it can distinguish the states as well as global measurements and is also minimal with respect to a partial ordering defined by entanglement and dimension. We present examples of useful resources and show that an entangled state need not be useful for distinguishing a given set of states. We obtain optimal resources with explicit local protocols to distinguish multipartite Greenberger-Horne-Zeilinger and graph states and also show that a maximally entangled state is an optimal resource under one-way local operations and classical communication to distinguish any bipartite orthonormal basis which contains at least one entangled state of full Schmidt rank.

With the significant advancement in quantum computation during the past couple of decades, the exploration of machine-learning subroutines using quantum strategies has become increasingly popular. Gaussian process regression is a widely used technique in supervised classical machine learning. Here we introduce an algorithm for Gaussian process regression using continuous-variable quantum systems that can be realized with technology based on photonic quantum computers under certain assumptions regarding distribution of data and availability of efficient quantum access. Our algorithm shows that by using a continuous-variable quantum computer a dramatic speedup in computing Gaussian process regression can be achieved, i.e., the possibility of exponentially reducing the time to compute. Furthermore, our results also include a continuous-variable quantum-assisted singular value decomposition method of nonsparse low rank matrices and forms an important subroutine in our Gaussian process regression algorithm.

For supporting practical quantum key distribution (QKD), it is critical to stabilize the physical parameters of signals, e.g., the intensity, phase, and polarization of the laser signals, so that such QKD systems can achieve better performance and practical security. In this paper, an approach is developed by integrating a support vector regression (SVR) model to optimize the performance and practical security of the QKD system. First, a SVR model is learned to precisely predict the time-along evolutions of the physical parameters of signals. Second, such predicted time-along evolutions are employed as feedback to control the QKD system for achieving the optimal performance and practical security. Finally, our proposed approach is exemplified by using the intensity evolution of laser light and a local oscillator pulse in the Gaussian modulated coherent state QKD system. Our experimental results have demonstrated three significant benefits of our SVR-based approach: (1) it can allow the QKD system to achieve optimal performance and practical security, (2) it does not require any additional resources and any real-time monitoring module to support automatic prediction of the time-along evolutions of the physical parameters of signals, and (3) it is applicable to any measurable physical parameter of signals in the practical QKD system.

Recently it was shown that the growth of entanglement in an initially separable state, as measured by the purity of subsystems, can be characterized by a timescale that takes a universal form for any Hamiltonian. We show that the same timescale governs the growth of entanglement for all Rényi entropies. Since the family of Rényi entropies completely characterizes the entanglement of a pure bipartite state, our timescale is a universal feature of bipartite entanglement. The timescale depends only on the interaction Hamiltonian and the initial state.

We consider a nonrelativistic approximation for the neutral muonic helium atom, a three-body system, which consists of an electron, a muon, and a helium nucleus. We discuss the case with the muon in the ground state and study the low energy levels and the transition frequencies of the electron ($2s-1s$ and $2p-2s$). We treat neutral muonic helium both as a three-body system (with a numerical variational solution) and as a two-body system of an electron bound by a two-body compound nucleus. We compare the results of two approaches and discuss the validity and consistency of fits and of the nonrelativistic approximation.

Fano models—consisting of a Hamiltonian with a discrete-continuous spectrum—are one of the basic toy models in spectroscopy. They have been successful in explaining the line shape of experiments in atomic physics and condensed matter. These models, however, have largely been beyond the scope of dissipative dynamics, with only a handful of works considering the effect of a thermal bath. Yet in nanostructures and condensed-matter systems, dissipation strongly modulates the dynamics. We present an overview of the theory of Fano interferences coupled to a thermal bath and compare them to the scattering formalism. We provide the solution to any discrete-continuous Hamiltonian structure within the wideband approximation coupled to a Markovian bath. In doing so, we update the toy models that have been available for unitary evolution since the 1960s. We find that the Fano line shape is preserved as long as we allow a rescaling of the parameters, and an additional Lorentzian contribution that reflects the destruction of the interference by dephasings. The universality of the line shape can be traced back to specific properties of the effective Liouvillian.

We derive a general and exact equation of motion for a quantized vortex in an inhomogeneous two-dimensional Bose-Einstein condensate. This equation expresses the velocity of a vortex as a sum of local ambient density and phase gradients in the vicinity of the vortex. We perform Gross-Pitaevskii simulations of single-vortex dynamics in both harmonic and hard-walled disk-shaped traps, and find excellent agreement in both cases with our analytical prediction. The simulations reveal that, in a harmonic trap, the main contribution to the vortex velocity is an induced ambient phase gradient, a finding that contradicts the commonly quoted result that the local density gradient is the only relevant effect in this scenario. We use our analytical vortex velocity formula to derive a point-vortex model that accounts for both density and phase contributions to the vortex velocity, suitable for use in inhomogeneous condensates. Although good agreement is obtained between Gross-Pitaevskii and point-vortex simulations for specific few-vortex configurations, the effects of nonuniform condensate density are in general highly nontrivial, and are thus difficult to efficiently and accurately model using a simplified point-vortex description.

One-dimensional superlattices with periodic spatial modulations of onsite potentials or tunneling coefficients can exhibit a variety of properties associated with topology or symmetry. Recent developments of ring-shaped optical lattices allow a systematic study of those properties in superlattices with or without boundaries. While superlattices with additional modulating parameters are shown to have quantized topological invariants in the augmented parameter space, we also found localized or zero-energy states associated with symmetries of the Hamiltonians. Probing those states in ultracold atoms is possible by utilizing recently proposed methods analyzing particle depletion or the local density of states. Moreover, we summarize feasible realizations of configurable optical superlattices using currently available techniques.

Recent experiments have illustrated that long-range two-body interactions can be induced by laser coupling atoms to highly excited Rydberg states. Stimulated by this achievement, we study the supersolidity of lattice bosons in an experimentally relevant situation. In our setup, we consider two-component atoms on a square lattice, where one species is weakly dressed to an electronically high-lying (Rydberg) state, generating a tunable, soft-core shape long-range interaction. Interactions between atoms of the second species and between the two species are characterized by local inter- and intraspecies interactions. Using a dynamical mean-field calculation, we find that interspecies on-site interactions can stabilize a pronounced region of supersolid phases. This is characterized by two distinctive types of supersolids, where the bare species forms supersolid phases that are immersed in strongly correlated quantum phases, i.e., a crystalline solid or supersolid of the dressed atoms. We show that the interspecies interaction leads to a rotonlike instability in the bare species and therefore is crucially important to the supersolid formation. We provide a detailed calculation of the interaction potential to show how our results can be explored under current experimental conditions.

People have been paying attention to the role of atoms' complex internal level structures in the research of electromagnetically induced transparency (EIT) for a long time, where the various degenerate Zeeman levels usually generate complex linkage patterns for the atomic transitions. It turns out, with special choices of the atomic states and the atomic transitions' linkage structure, clear signatures of quantum interference induced by the probe and coupling light's polarizations can emerge from a typical EIT phenomena. We propose to study a four-state system with double-$\mathsf{\text{V}}$ linkage pattern for the transitions and analyze the polarization-induced interference under the EIT condition. We show that such interference arises naturally under mild conditions on the optical field and atom manipulation techniques. Moreover, we construct a variation form of double-M linkage pattern where the polarization-induced interference enables polarization-dependent cross modulation between incident weak lights that can be effective even at the few-photon level. The theme is to gain more insight into the essential question: how can we build a nontrivial optical medium where incident lights experience polarization-dependent nonlinear optical interactions, valid for a wide range of incidence intensities down to the few-photon level?

We consider the nonstationary circuit QED architecture in which a single-mode cavity interacts with $N>1$ identical qubits, and some system parameters undergo a weak external perturbation. It is shown that in the dispersive regime one can engineer the two-photon exchange interaction by adjusting the frequency of harmonic modulation to (approximately) $2|{\mathrm{\Delta }}_{-}|$, where ${\mathrm{\Delta }}_{-}$ is the average atom-field detuning. A closed analytic description is derived for the weak atom-field coupling regime, and the system dynamics under realistic conditions is studied numerically.

An algorithm to extract phases with dislocations from intensity measurements is proposed, and demonstrated on paraxial wave fields. The proposed algorithm makes use of the partial Fourier transformation which does not conserve the longitudinal orbital angular momentum and hence can create or destroy phase dislocations naturally, and can distinguish between their orientation. A method to realize the partial Fourier transformation using thin cylindrical lenses is outlined. Unambiguous phase retrieval is numerically demonstrated on random complex superpositions of Laguerre-Gaussian modes with both integer and fractional orbital angular momentum.

We propose two experimental schemes for producing coherent-state superpositions which approximate different nonclassical states conditionally in traveling optical fields. Although these setups are constructed of a small number of linear optical elements and homodyne measurements, they can be used to generate various photon number superpositions in which the number of constituent states can be higher than the number of measurements in the schemes. We determine numerically the parameters to achieve maximal fidelity of the preparation for a large variety of nonclassical states, such as amplitude squeezed states, binomial states, squeezed cat states, and various photon number superpositions. The proposed setups can generate these states with high fidelities and with success probabilities that can be sufficient for practical applications.

Emerging photonic, sensing, and quantum applications require high fields and tight localization but low power consumption. Spatial, spectral, and magnitude control of electromagnetic fields is of key importance for enabling experiments in atomic, molecular, and optical physics. We introduce the concept of zero-admittance domains as a mechanism for tailoring giant optical fields bound within or on the surface of dielectric media. The described mechanism permits the creation of highly localized fields of extreme amplitudes simultaneously for incident photons of multiple wavelengths and incidence angles but arbitrary polarization states. No material constraints are placed upon the bounding media. Both intrinsic and extrinsic potential practical limitations of the predicted field enhancement are analyzed and applications relevant to optical sensors and microsources are briefly discussed.

We consider two-dimensional waveguide arrays with anisotropic coupling coefficients. We show using numerical and variational calculations that four stationary soliton types exist: site centered, bond centered, hybrid $X$, and hybrid $Y$. For the isotropic case the last two modes become identical and equivalent to the known hybrid soliton. With a variational calculation using a Gaussian trial function and six variational parameters corresponding to the soliton's position, width, and velocity components, the four stationary soliton types are reproduced and their equilibrium widths are accounted for accurately for a wide range of anisotropy ratios. We obtained using the variational calculation the Peierls-Nabarro potential and barrier heights for the four soliton types and different anisotropy ratios. We have also obtained a phase diagram showing regions of soliton stability against collapse and subregions of mobility in terms of the initial kick-in speed and anisotropy ratio. The phase diagram shows that two-dimensional (2D) solitons become highly mobile for anisotropy ratios larger than some critical values that depend on the initial kick-in speed. This fact was then exploited to design tracks within the 2D waveguide array along which the soliton can be accelerated and routed. We have calculated the actual waveguide separations needed to realize the proposed guided trajectories of 2D solitons.