Nondestructive detection of photonic qubits will enable important applications in photonic quantum information processing and quantum communications. Here we present an approach based on a solid-state cavity containing an ensemble of rare-earth ions. First a probe pulse containing many photons is stored in the ensemble. Then a single signal photon, which represents a time-bin qubit, imprints a phase on the ensemble that is due to the ac Stark effect. This phase does not depend on the exact timing of the signal photon, which makes the detection insensitive to the time-bin qubit state. Then the probe pulse is retrieved and its phase is detected via homodyne detection. We show that the cavity leads to a dependence of the imprinted phase on the probe photon number, which leads to a spreading of the probe phase, in contrast to the simple shift that occurs in the absence of a cavity. However, we show that this scenario still allows nondestructive detection of the signal. We discuss potential implementations of the scheme, showing that high success probability and low loss should be simultaneously achievable.

In this article, we prove that conformal transformation optics could be used to construct absolute instruments. The Mikaelian lens, whose refractive index profile is $1/cosh\left(\mathrm{x}\right)$ [A. Mikaelian and A. Prokhorov, Progress in Optics 17, 279 (1980)], are of imaging functionalities along lines and will be used as a generic lens. Under a conformal mapping, it can be transformed to another lens, where imaging or self-imaging happens along predesigned curves. Ray tracing simulations are performed to confirm the imaging functionalities.

By coupling a collection of two-level artificial atoms simultaneously to two superconducting resonators, we propose a quantum switch that can tune the resonator-resonator coupling strength from zero to a large value proportional to the qubit number. This process is realized by preparing the qubits in different pairwise subradiant states, where the microwave photons released by different qubits interfere destructively such that the coupling strength can be stabilized in an open environment. The resulting two-step control scheme tunes the coupling strength between different values in nanoseconds without changing the transition frequency of the qubits. We also use the quantum switch to network multiple resonators with programmable couplings, and we demonstrate its potential applications in quantum simulation and quantum information storage and processing.

Vapor-phase atomic clocks and atomic magnetometers are achieving levels of stability and measurement precision today that would have been considered unrealistic in the not too distant past. To make further progress, researchers will need a more detailed understanding of vapor-phase atomic physics, in particular, the manner in which mesoscopic variations of atomic perturbations map onto the observed resonant phenomena: length scales larger than that of homogeneous quantum dynamics, where atoms within some localized region of the vapor all experience the same optical and microwave fields and collisional perturbations; but smaller than that of the vapor as a whole, where the atomic system is described in terms of (for example) vapor pressure, heat content, and optical depth. In this paper, we discuss the issue of mesoscopic physics as it manifests itself in vapor-phase collision-shift gradients affecting the 0-0 hyperfine transition of ${}^{87}\mathrm{Rb}$. We show that these gradients not only produce increased line broadening but lead to lineshape asymmetry and an increased sensitivity of the observed resonance to power-broadening. We develop a statistical theory describing the experimental results, and through that theory we show that mesoscopic atomic physics encompasses a broader class of phenomena than that described by the circumscribed term “inhomogeneous broadening.”

We study the current of Bose particles between two reservoirs connected by a one-dimensional channel. We analyze the problem from first principles by considering a microscopic model of conductivity in the noninteracting limit. Equations for the transient and the stationary current are derived analytically. The asymptotic current has a form similar to the Landauer-Büttiker equation for electronic current in mesoscopic devices.

A one-dimensional discrete lattice of dimers is known to possess topologically protected edge states when interdimer coupling is stronger than intradimer coupling. Here, we address richer topological properties of photonic superlattices having an arbitrary number of elements in each unit cell. It is shown that the superlattice provides a tunable number of topologically protected edge and interface states depending on certain restrictions on intra- and intercell couplings maintaining inversion symmetry of the lattice. Simultaneous and stable propagation of multiple topological interface states, their interference pattern, and stable oscillation are reported. The configuration can be relevant for topologically protected mode-division multiplexing through a narrow route in photonic devices.

Particle distinguishability is a significant challenge for quantum technologies, in particular for photonics, where the Hong-Ou-Mandel (HOM) effect clearly demonstrates it is detrimental to quantum interference. We take a representation theoretic approach in first quantization, separating particles' Hilbert spaces into degrees of freedom that we control and those we do not, yielding a quantum-information-inspired bipartite model where distinguishability can arise as correlation with an environment carried by the particles themselves. This makes it clear that the HOM experiment is an instance of a (mixed) state discrimination protocol, which can be generalized to interferometers that discriminate unambiguously between ideal indistinguishable states and interesting distinguishable states, leading to bounds on the success probability of an arbitrary HOM generalization for multiple particles and modes. After setting out the first quantized formalism in detail, we consider several scenarios and provide a combination of analytical and numerical results for up to nine photons in nine modes. Although the quantum Fourier transform features prominently, we see that it is suboptimal for discriminating completely distinguishable states.

We propose a scheme to realize parity-time ($\mathcal{PT}$) symmetry and nonlocal optical solitons in a cold Rydberg atomic system with electromagnetically induced transparency. We show that a two-dimensional (2D) periodic optical potential with $\mathcal{PT}$ symmetry can be obtained for the propagation of probe laser field by using an incoherent population pumping between two low-lying levels and spatial modulations of control and assistant laser fields. We also show that, based on the giant nonlocal Kerr nonlinearity originated from the strong, long-range atom-atom interaction, the system supports 2D nonlocal gap solitons with very low light intensity. In particular, we find that the degree of the nonlocality of the Kerr nonlinearity, which can be actively tuned in our system, can be used to manipulate the phase transition of the $\mathcal{PT}$ symmetry and the behavior of the nonlocal optical solitons. Our study opens a route for developing non-Hermitian nonlinear optics, especially for realizing and controlling high-dimensional weak-light optical solitons through adjustable $\mathcal{PT}$ symmetry and giant nonlocal optical nonlinearity.

We study a non-Hermitian Rice-Mele model without breaking time-reversal symmetry, with the non-Hermiticity arising from imbalanced hopping rates. The Berry connection, Berry curvature, and Chern number are introduced in the context of biorthonormal inner product. It is shown that for a bulk system, although the Berry connection can be complex numbers, the Chern number is still quantized, as a topological invariant. For an opened chain system, the midgap edge modes are obtained exactly, obeying the bulk-edge correspondence. Furthermore, we also introduce a local current in the context of biorthonormal inner product to measure the pumping charge generated by a cyclic adiabatic evolution. Analytical analysis and numerical simulation of the time evolution of the midgap states show that the pumping charge can be a dynamical topological invariant in correspondence with the Chern number. It indicates that the geometric concepts for Hermitian topological insulator can be extended to the non-Hermitian regime.

A theoretical framework to investigate the time evolution of the quantum entanglement due to the dynamical Lamb effect between two and three superconducting qubits coupled to a coplanar waveguide in the presence of different sources of dissipation is developed. Guidelines on how to proceed in the $N$-qubit case are also given. We quantitatively analyze the case of single switching of the coupling and absence of dissipation within a perturbative approach and show that it is a good approximation to the case of periodic switching of the coupling for high frequencies of switching. The same systems are analyzed for the general case of periodic switching of the coupling at any frequency and in the presence of dissipation via numerical calculations. Different measures of entanglement compatible with mixed states are adopted. It is demonstrated that the different measures show a different level of detail of the latter. The concurrence and the negativity are obtained in the two-qubit case; the three-$\pi$ and the negativity are obtained in the three-qubit case. It is shown that time-dependent Greenberger-Horne-Zeilinger states can be created even in the presence of dissipation. To maximize the quantum entanglement between the qubits, the effects of tuning several parameters of the system are investigated.

We solve the mean-field-like $p$-spin Ising model under a spatiotemporal inhomogeneous transverse field to study the effects of inhomogeneity on the performance of quantum annealing. We previously found that the problematic first-order quantum phase transition that arises under the conventional homogeneous field protocol can be avoided if the temperature is zero and the local field is completely turned off site by site after a finite time. We show in the present paper that, when these ideal conditions are not satisfied, another series of first-order transitions appear, which prevents us from driving the system while avoiding first-order transitions. Nevertheless, under these nonideal conditions, quantitative improvements can be obtained in terms of narrower tunneling barriers in the free-energy landscape. A comparison with classical simulated annealing establishes a limited quantum advantage in the ideal case, since inhomogeneous temperature driving in simulated annealing cannot remove a first-order transition, in contrast to the quantum case. The classical model of spin-vector Monte Carlo is also analyzed, and we find it to have the same thermodynamic phase diagram as the quantum model in the ideal case, with deviations arising at nonzero temperature.

We propose a method for entangling two noninteracting qubits by measuring their parity indirectly through an intermediate mesoscopic system. The protocol is designed to require only global control and course-grained collective measurement of the mesoscopic system along with local interactions between the target qubits and mesoscopic system. A generalization of the method measures the hamming weight of the qubits' state and probabilistically produces an entangled state by post-selecting on hamming weight one. Our technique provides a design element that can be integrated into quantum processor architectures and quantum measurement devices.

Absolute optical oscillator strengths of the Lyman and Werner bands of molecular deuterium are determined by a $\mathrm{dipole}(\gamma ,\gamma$) method with the high energy resolution of 25 meV. A comprehensive comparison shows that an excellent agreement between the present experimental results and the earlier theoretical calculations is achieved for both Lyman and Werner bands. In addition, the electronic transition dipole moments of ${\mathrm{D}}_2$ are derived. The absolute optical oscillator strengths of the Lyman and Werner bands of ${\mathrm{D}}_2$ reported in this work can serve as the benchmark to test calculations with different theoretical models and calculational codes and, also, be used in thermonuclear fusion plasma and other fields.

We present a comprehensive study of the diverse properties of heteronuclear Rydberg molecules, placing a special emphasis on those composed of the light alkali-metal atoms, Li, Na, and K. Electron-atom scattering phase shifts, which determine the strength of the molecular bond, are calculated at very low energy and then used in a spin-dependent theoretical model to calculate accurate Rydberg molecule potential-energy curves. The wide parameter range accessible by combining the various properties of different alkali-metal atoms often leads to hybridized electronic states accessible via one or two photon excitation schemes. This analysis of heteronuclear molecules leads to a prediction that the relative densities and spatial distributions of atoms in an ultracold mixture can be probed at controllable length scales via spectroscopic detection of these molecules.

We study the polarization dynamics of the molecular rotation and the nuclear spin in the presence of magnetic fields, in the hydrogen isotope molecules, ${\mathrm{H}}_2,{\mathrm{D}}_2$, and HD. If only the rotational or the nuclear angular momentum is polarized, polarization is transferred from one to the other and back in a oscillatory fashion, due to hyperfine quantum beats. Application of magnetic fields is shown to increase the frequencies of these oscillations and reduce their amplitude so that the polarization exchange can be strongly suppressed. We derive the time-averaged polarization as a function of the (static) magnetic field. Finally, we show, using continuous lasers, molecular beams, and static magnetic fields, that the molecular rotation polarization can be transferred and maintained in the nuclear spin, providing highly nuclear-spin-polarized molecules.

We consider photoemission from the $4d$ shell of the free Xe and encapsulated Xe@${\mathrm{C}}_{60}$ atoms by ionizing XUV and probing IR fields typical for a RABBITT (reconstruction of attosecond beating by interference of two-photon transitions) measurement. Our theoretical model is based on the numerical solution of the time-dependent Schrödinger equation in the single-active-electron approximation. The fullerene ${\mathrm{C}}_{60}$ cage is represented by a finite-width well potential. We test our model against an analogous set of nonrelativistic [Phys. Rev. A 89, 053424 (2014)] and relativistic [Phys. Rev. A 96, 053407 (2017)] calculations for the $4d$ shell of Xe and Xe@${\mathrm{C}}_{60}$ driven by a continuous XUV field. Based on this verification, we make predictions for the total ionization probability, angular anisotropy $\beta$ parameter, and the angular-dependent atomic time delay ${\tau }_a$ from the threshold to several hundred eV of excess energy.

We propose an interferometric measurement of weak forces using a single ion subjected to designed time-dependent spin-dependent forces. Explicit expressions of the relation between the unknown force and the final populations are found considering different scenarios, such as the weak force acting on two branches, acting on one branch, or having errors in the driving force. The flexibility to design the trap trajectories is used to minimize errors due to anharmonicities in the trap. The advantages of the approach are the use of geometrical phases, which provides stability, the possibility to design faster-than-adiabatic processes with sensitivity control, and the independence of the results on the motional states for the small-oscillations regime in which the effective potentials are purely harmonic.

This paper is a proposal for the generation of a many-body entangled state in atomic and mechanical systems. Here the detailed feasibility study shows that application of a strong Rydberg dressing interaction and a fast bifurcation scheme in a Bose-Einstein condensate of Rb atoms, results in the formation of large cat states. By detailed study of the decoherence effects using the quantum jump Monte Carlo approach and taking into account the obstacles like collective decoherence and level mixing, this proposal predicts the formation of a 700-atom cat state. Subsequent transfer of the generated superposition to far separated mechanical oscillators is proposed, using dipole coupling between Rydberg atoms and charged cantilevers.

Through understanding Maxwell's equations as an effective Dirac equation (the optical Dirac equation), we reexamine the relationship between electromagnetic interface states and topology. We illustrate a simple case where electromagnetic material parameters play the roles of mass and energy in an equivalent Dirac equation. The modes trapped between a gyrotropic medium and a mirror are then the counterpart of those at a domain wall, where the mass of the Dirac particle changes sign. Considering the general case of arbitrary electromagnetic media, we provide an analytical proof relating the integral of the Berry curvature (the Chern number) to the number of interface states. We show that this reduces to the usual result for periodic media and also that the Chern number can be computed without knowledge of how the material parameters depend on frequency.

We investigated a damped two-level system interacting with a circularly polarized light as described by an asymmetric non-Hermitian Hamiltonian. This is a simple enough system to be studied analytically while complicated enough to exhibit a rich variety of behaviors. This system exhibits a ring of exceptional points in the parameter space of the real and imaginary dipole couplings where within the ring the energy eigenvalue of the system does not change. This leads to unstable regions inside the exceptional ring, which is shown using a linear stability analysis. These unstable regions are unique to gain-loss systems and have the surprising property that no matter how small the gain/loss ratio, the gain always prevails at long times. We also report on eigenvalue switching, phase rigidity and dynamics of the system around the exceptional points. We highlight that some of these properties are different from those in the widely studied case of symmetric non-Hermitian Hamiltonians.