A key challenge for semiconductor quantum-dot charge qubits is the realization of long-range qubit coupling and performing high-fidelity gates based on it. Here, we describe a different type of charge qubit formed by an electron confined in a triple-quantum-dot system, enabling single and two-qubit gates working in the dipolar and quadrupolar detuning sweet spots. We further present the form for the long-range dipolar coupling between the charge qubit and the superconducting resonator. Based on the hybrid system composed of the qubits and the resonator, we present two types of entangling gates: the dynamical iswap gate and holonomic entangling gate, which are operating in the dispersive and resonant regimes, respectively. We find that the fidelity for the iswap gate can reach a fidelity higher than 99% for the noise level typical in experiments. Meanwhile, the fidelity for the holonomic gate can surpass 98% if the anharmonicity in the resonator is large enough. Our proposal offers an alternative, useful way to build up high-fidelity quantum computation for charge qubits in the semiconductor quantum dot.

We report on the simultaneous sub-Doppler laser cooling of ${}^6\mathrm{Li}$ and ${}^7\mathrm{Li}$ isotopes using gray molasses operating on their respective $D_1$ atomic transitions. For ${}^7\mathrm{Li}$ we show that the sub-Doppler cooling can be achieved with two distinct $\mathrm{\Lambda }$-type transitions, where the upper level can be either of the two $2^2{P}_{1/2}$ hyperfine states. We obtain temperatures of $\sim 85\mu \mathrm{K}$, with atom numbers of $\sim {10}^8$, and phase-space densities in the range of ${10}^{-6}-{10}^{-5}$ for both isotopes. These conditions provide a good starting point for loading the mixture into an optical dipole trap and performing evaporative cooling to quantum degeneracy. Our work provides a valuable simplification for the preparation of ultracold ${}^6\mathrm{Li}-{}^7\mathrm{Li}$ mixtures, which were proven to be a successful system for the study of impurity physics and Bose-Fermi superfluids.

We propose a one-channel, simple model to describe the dynamics of ultracold dipolar molecules around a Förster resonance. Slightly above a specific electric field, a collisional shielding can take place, suppressing the molecular losses in a gas. The overall description of the quantum physical mechanism comes back to the dynamics on a unique energy surface, which depends on the relative distance and angular approach of the molecules. This surface enables us to interpret how the dipole moments of the molecules are induced and interlocked by the electric field and the dipole-dipole interaction during the process, especially when the shielding is triggered. Averaging the relative angular motion over a unique partial wave (the lowest one when the ultracold regime is reached), the model reproduces well the behavior of the rate coefficients observed experimentally and predicted theoretically [K. Matsuda, L. De Marco, J.-R. Li, W. G. Tobias, G. Valtolina, G. Quéméner, and J. Ye, Science 370, 1324 (2020); J.-R. Li, W. G. Tobias, K. Matsuda, C. Miller, G. Valtolina, L. D. Marco, R. R. W. Wang, L. Lassablière, G. Quéméner, J. L. Bohn, and J. Ye, Nat. Phys. 17, 1144 (2021)]. This economic model encapsulates the main physics of the quantum process. Therefore, it can be used as an alternative to a full quantum dynamical treatment and is promising for future studies of collisions involving more bodies.

We study the equilibrium properties of a one-dimensional mixture of two Tonks-Girardeau gases on a ring geometry in the limit of strongly repulsive interspecies interactions. We derive the exact many-body wave function and compare it to the SU(2) solution where intra- and interspecies interactions are also diverging but equal. We focus on the role of the SU(2) symmetry breaking on the behavior of the large- and short-distance correlations by studying the zero-momentum occupation number and the Tan's contact from the asymptotic behavior of the momentum distribution. Although the symmetry is only weakly broken, it has important consequences on spin correlations in the system as the reduction by a factor of two of the zero-momentum occupation number with respect to the SU(2) case in the thermodynamic limit and the decrease of the Tan's contact.

We discuss the emergence and manipulation of generalized Dirac cones in the subradiant collective modes of a subwavelength dipolar array. We consider a collection of single quantum emitters modeled as point dipoles arranged in a honeycomb lattice with subwavelength periodicity. While conventional honeycomb arrays host bound modes that display Dirac cones at the $K$ and $K^{\prime }$ points, we show that introducing uniaxial anisotropy in the lattice results in modified dispersion relations. These include the tilting of Dirac cones arising purely due to long-range, retarded, electromagnetic coupling in the lattice, which changes the local density of states at the Dirac point from vanishing (type I) to diverging (types II and III), the emergence of semi-Dirac points, with linear and quadratic dispersions in orthogonal directions, and the anisotropic movement of Dirac cones away from the $K$ and $K^{\prime }$ points. Such energy dispersions can modify substantially the dynamics of local probes, such as quantum emitters, for which they have been shown to induce anisotropic power-law interactions.

Charge-state conversion has been a very useful tool for improving the performance of nitrogen-vacancy (NV) centers in diamond in many application scenarios. Lasers of different colors are employed to manipulate the NV charge, ranging from blue to infrared. Here, we focus on the study of the charge-state dynamics under illumination of multicolors and their combination with infrared (IR) light, and the results are directly presented in terms of charge-state population. It is shown that each color of light serves some kind of superiority over others in charge-state conversion and a greatly improved conversion rate can be induced by the additional IR illumination. We report that the steady-state ${\mathrm{NV}}^{-}$ population is increased up to 82% by IR light, and it presents basically the same response trend to infrared power as that of fluorescence counts. We also report a fast-oscillating fluorescence time trace by modulating the IR power. We believe that our work offers some straightforward results that might be of interest to researchers and lead to a clearer understanding of the NV charge-state dynamics.

We study the dynamics of a class-B semiconductor microring laser coupled to a neutral microring resonator for common fabrication parameters. For zero detuning between the resonators, we identify five dynamical regions controlled by the laser pump parameter above threshold and the coupling strength between the resonators. These regions show stable lasing with phase locking for either sufficiently small or large coupling strength parameter values. Multistability and chaos arise in between these stable regions where at least one fixed point is attractive. The transition from stable to unstable lasing regions in the parameter space phase diagram is a crossover.

In recent years, frequency-comb-based multidimensional coherent spectroscopy has emerged as a very powerful optical method for studying samples with long dephasing times. It enables the measurement of comb resolution multidimensional coherent spectra in seconds as opposed to hours or even days required by traditional mechanical stage-based spectrometers. However, for some samples (e.g., cold atoms and molecules) the dephasing times could be longer than the repetition periods of the excitation lasers which means that the generated signal will not fully decay between the pulses. Depending on the relative phase or time between the pulses, the signals generated by the subsequent laser pulses can constructively or destructively interfere with each other. In this paper we study this behavior by solving the optical Bloch equations and provide a theoretical description of frequency-comb-based multidimensional coherent spectroscopy.

The exchange of off-resonant photons between quantum optical emitters in cavity QED or quantum nanophotonic setups induces interactions between them which can be harnessed for quantum information and simulation purposes. So far, these interactions have been mostly characterized for two-level emitters, which restrict their application to engineering quantum gates among qubits or simulating spin-1/2 quantum many-body models. Here, we show how to harness multilevel emitters with several optical transitions to engineer a wide class of photon-mediated interactions between effective spin-1 systems. We characterize their performance through analytical and numerical techniques and provide specific implementations based on the atomic level structure of Alkali atoms. Our results expand the quantum simulation toolbox available in such cavity QED and quantum nanophotonic setups and open up different ways of engineering entangling gates among qutrits.

We introduce a nonperturbative approximation scheme for performing scattering calculations in two dimensions that involves neglecting the contribution of the evanescent waves to the scattering amplitude. This corresponds to replacing the interaction potential $v$ with an associated energy-dependent nonlocal potential ${\mathcal{V}}_k$ that does not couple to the evanescent waves. The scattering solutions $\psi \left(\mathbf{r}\right)$ of the Schrödinger equation, $(-{\nabla }^2+{\mathcal{V}}_k)\psi \left(\mathbf{r}\right)=k^2\psi \left(\mathbf{r}\right)$, have the remarkable property that their Fourier transform $\tilde{\psi }\left(\mathbf{p}\right)$ vanishes unless $\mathbf{p}$ corresponds to the momentum of a classical particle whose magnitude equals $k$. We construct a transfer matrix for this class of nonlocal potentials and explore its representation in terms of the evolution operator for an effective nonunitary quantum system. We show that the above approximation reduces to the first Born approximation for weak potentials and, similar to the semiclassical approximation, becomes valid at high energies. Furthermore, we identify an infinite class of complex potentials for which this approximation scheme is exact. We also discuss the appealing practical and mathematical aspects of this scheme.

We elucidate the multiparticle transport of pair tunneling and spin tunneling in strongly correlated interfaces. Not only usual single-particle tunneling but also interaction-induced multiparticle tunneling processes naturally arise from a conventional microscopic model without any empirical parameters, through the overlap of the many-body wave functions around the interface. We demonstrate how anomalous tunneling currents occur in a strongly interacting system due to the pair-tunneling process which we derived microscopically. Our formulation is useful for junction systems in various disciplines, including atomtronics, spintronics, and nuclear reactions.

Induced transparency in a microresonator can result from cross-polarization coupling of two coresonant orthogonally polarized whispering-gallery modes of very different $Q$. The coupling creates supermodes that are superpositions of the two modes. In this paper we show that mode superpositions that result from simultaneous excitation of two orthogonally polarized modes can also show induced transparency, even in the absence of cross-polarization coupling. Induced transparency is accompanied by pulse delay, and it is also possible to observe induced attenuation with pulse advancement or delay. These effects are modeled, simulated numerically, analyzed theoretically, and confirmed experimentally. The results are presented here.

This work discusses a protocol for constructing highly accurate potential energy curves (PECs) for the lowest two states of ${\mathrm{Rb}}_2^{+}$, i.e., $X{}^2\mathrm{\Sigma }{}_g^{+}$ and $\left(1\right){}^2\mathrm{\Sigma }{}_u^{+}$, using an additivity scheme based on coupled-cluster theory. The approach exploits the findings of our previous work [J. Schnabel, L. Cheng, and A. Köhn, J. Chem. Phys. 155, 124101 (2021)] to avoid the unphysical repulsive long-range barrier occurring for symmetric molecular ions when perturbative estimates of higher-order cluster operators are employed. Furthermore, care was taken to reproduce the physically correct exchange splitting of the $X{}^2\mathrm{\Sigma }{}_g^{+}$ and $\left(1\right){}^2\mathrm{\Sigma }{}_u^{+}$ PECs. The accuracy of our computational approach is benchmarked for ionization energies of $\mathrm{Rb}$ and for spectroscopic constants as well as vibrational levels of the $a{}^3\mathrm{\Sigma }{}_u^{+}$ triplet state of ${\mathrm{Rb}}_2$ . We study high-level correlation contributions, high-level relativistic effects, and inner-shell correlation contributions and find very good agreement with experimental reference values for the atomic ionization potential and the binding energy of ${\mathrm{Rb}}_2$ in the $a{}^3\mathrm{\Sigma }{}_u^{+}$ triplet state. Our final best estimate for the binding energy of the ${\mathrm{Rb}}_2^{+} X{}^2\mathrm{\Sigma }{}_g^{+}$ state including zero-point vibrational contributions is $D_0=6179{\mathrm{cm}}^{-1}$ with an estimated error bound of $O(\pm 30{\mathrm{cm}}^{-1})$. This value is smaller than the experimentally inferred lower bond of $D_0\ge 6307.5{\mathrm{cm}}^{-1}$ [M. A. Bellos et al., Phys. Rev. A 87, 012508 (2013)] and will require further investigation. For the $\left(1\right){}^2\mathrm{\Sigma }{}_u^{+}$ state a shallow potential with $D_0=78.4{\mathrm{cm}}^{-1}$ and an error bound of $\pm 9{\mathrm{cm}}^{-1}$ is computed.

We compute the nonconservative electric dipole forces between the atoms of a suddenly excited binary system. These forces derive from the time variation of the longitudinal electromagnetic momentum. In contrast to the conservative van der Waals forces, the nonconservative forces possess components orthogonal to the interatomic axis. Thus, despite being several orders of magnitude smaller than van der Waals', they might be accessible experimentally. For the case of a binary system of identical atoms in Dicke states, these forces are reciprocal. However, when only one of the atoms is initially excited, as with the van der Waals forces, the nonconservative ones are nonreciprocal, which results in a net force on the two-atom system. We offer an estimate of the spatial displacement caused by the nonconservative forces on a binary system of hydrogen atoms.

We develop a model of a quantum field confined within a cavity with a movable wall where the position of the wall is quantized. We obtain a full description of the dynamics of both the quantum field and the confining wall depending on the initial state of the whole system. Both the reaction and back-reaction of the field on the wall, and the wall on the field, can be taken into account, as well as external driving forces on both the cavity and the wall. The model exactly reproduces the resonant cavity mode stimulation due to the periodic motion of the mirror (dynamical Casimir effect), as well as the standard radiation pressure effects on the quantized wall (optomechanics). The model also accounts for the interplay of the two scenarios. Finally, the time evolution of the radiation force shows the interplay between the static and dynamical Casimir effect.

In the wake of recently established quantum theories for bulk and surface plasmons, and for bulk plasmaritons [Jung and Keller, Phys. Rev. A 103, 063501 (2021); Phys. Rev. A 104, 053508 (2021)] wave-mechanical and second-quantized (QED) theories for surface plasmaritons are developed. Our photon wave-mechanical theory is based on the covariant four-potential in the Lorenz gauge, where dynamical equations for surface plasmariton quantum particles consisting of scalar ($S$), longitudinal ($L$), and transverse ($T1$, $T2$) photons and driven by a surface current density sheet are obtained. The second quantized QED theory of surface plasmaritons is formulated on the basis of the Heisenberg equations of motion for the contravariant four-potential annihilation operators. The connection of the surface plasmariton quantum theory to the quasiparticle picture of bulk plasmaritons is elucidated by studying the $T$-photon exponential decay lengths appearing in the description. The shortest decay length (equal on both sides of the surface) characterizes the one-dimensional spatial confinement of the $T$-photon source. Starting from a reexamination (and correction) of the dynamic boundary conditions for a flat jellium-vacuum interface carrying a surface current density, the dispersion relation for surface plasmaritons is obtained. The explicit form of the dispersion relation is derived upon a study of the local field in the selvedge region of the boundary. Fundamental aspects of the dispersion relation are pointed out, modeling the electron dynamics by the microscopic electrodynamics of a quantum well. Remarks on (i) our covariant surface plasmariton quantum theory seen in a broader perspective, (ii) suggestions for applications of the theory, and (iii) a comparison of the key ingredients in our trio of papers on plasmon and plasmariton quantum physics are presented.

We study parity-time-symmetric non-Hermitian quantum systems at finite temperature, where the Boltzmann distribution law fails to hold. To characterize their abnormal physical properties, a quantum statistical theory (the so-called quantum Liouvillian statistical theory) is developed, in which the Boltzmann distribution law is replaced by the Liouvillian-Boltzmann distribution law. Using the theory, we derive analytical results of thermodynamic properties for parity-time-symmetric non-Hermitian quantum systems and find that a “continuous” thermodynamic phase transition occurs at the exceptional point, where a zero-temperature anomaly exists.

Developing numerical methods to simulate efficiently nonlinear fluid dynamics on universal quantum computers is a challenging problem. In this paper, a generalization of the Madelung transform is defined to solve quantum relativistic charged fluid equations interacting with external electromagnetic forces via the Dirac equation. The Dirac equation is discretized into discrete-time quantum walks which can be efficiently implemented on universal quantum computers. A variant of this algorithm is proposed to implement simulations using current noisy intermediate scale quantum (NISQ) devices in the case of homogeneous external forces. High resolution (up to $N=2^{17}$ grid points) numerical simulations of relativistic and nonrelativistic hydrodynamical shocks on current IBM NISQs are performed with this algorithm. This paper demonstrates that fluid dynamics can be simulated on NISQs, and opens the door to simulating other fluids, including plasmas, with more general quantum walks and quantum automata.

In the problem of quantum channel discrimination, one distinguishes between a given number of quantum channels, which is done by sending an input state through a channel and measuring the output state. This work studies applications of variational quantum circuits and machine-learning techniques for discriminating such channels. In particular, we explore (i) the practical implementation of embedding this task into the framework of variational quantum computing, (ii) training a quantum classifier based on variational quantum circuits, and (iii) applying the quantum kernel estimation technique. For testing these three channel discrimination approaches, we considered a pair of entanglement-breaking channels and the depolarizing channel with two different depolarization factors. For the approach (i), we address solving the quantum channel discrimination problem using widely discussed parallel and sequential strategies. We show the advantage of the latter in terms of better convergence with less quantum resources. Quantum channel discrimination with a variational quantum classifier (ii) allows one to operate even with random and mixed input states and simple variational circuits. The kernel-based classification approach (iii) is also found effective as it allows one to discriminate depolarizing channels associated not with just fixed values of the depolarization factor, but with ranges of it. Additionally, we discovered that a simple modification of one of the commonly used kernels significantly increases the efficiency of this approach. Finally, our numerical findings reveal that the performance of variational methods of channel discrimination depends on the trace of the product of the output states. These findings demonstrate that quantum machine learning can be used to discriminate channels, such as those representing physical noise processes.