We report on a mechanism to optimize the generation of steady-state entanglement in a system of coupled qubits driven by microwave fields. Due to the interplay between Landau-Zener-Stückelberg-Majorana pumping involving three levels and a subsequent fast relaxation channel, which is activated by tuning the qubits-reservoir couplings, a maximally entangled state can be populated. This mechanism does not require the fine tuning of multiphoton resonances but depends on the sign of the qubit-qubit coupling. In particular, we find that by a proper design of the system parameters and the driving protocol, the two-qubit steady-state concurrence can attain values close to 1 in a wide range of driving amplitudes. Our results may be useful to gain further insight into entanglement control and manipulation in dissipative quantum systems exposed to strong driving.

The quantum battery is one of the most prominent microdevices in the rapid-developing field of quantum thermodynamics. We propose a long-range charging protocol in which both battery and charger consist of a many-spin system. The battery and charger are indirectly connected via a quantum wire supported by the Kittle mode of a magnon system. Counterintuitively, we find that the undesired spin-spin couplings among both battery and charger can be used to promote the charging performance in comparison with the noninteracting condition. And a “sweet” spot of the average coupling strength allows the battery to store energy to the maximum possible value. Moreover, we apply the quantum-state-diffusion equation to estimate the robustness of our magnon-mediated charging protocol to the fluctuation in the magnon frequency.

We report on an investigation on the fragmentation dynamics of $\mathrm{C}{{\mathrm{F}}_4}^{q+}$ $(q=2,3)$ induced by 1-keV electron collisions utilizing an ion momentum imaging spectrometer. From the time-of-flight correlation maps five dominating dissociation channels of $\mathrm{C}{{\mathrm{F}}_4}^{2+}$ as well as one three-body fragmentation channel of $\mathrm{C}{{\mathrm{F}}_4}^{3+}$ are identified. The kinetic energy release (KER) distributions for these channels are obtained and compared with the data available in the literature. The Dalitz-like momentum diagram and the Newton diagram are employed to analyze the breakup mechanism in the three-body fragmentation channel. We found that, for $\mathrm{C}{{\mathrm{F}}_4}^{2+}$ dissociation into ${\mathrm{F}}^{+}+\mathrm{C}{{\mathrm{F}}_2}^{+}+\mathrm{F}$, ${\mathrm{F}}^{+}+\mathrm{C}{\mathrm{F}}^{+}+2\mathrm{F}$, and ${\mathrm{F}}^{+}+{\mathrm{F}}^{+}+{\mathrm{CF}}_2$, the concerted breakup is the dominating process. Channel ${\mathrm{C}}^{+}+{\mathrm{F}}^{+}+3\mathrm{F}$ is dominated by the initial charge separation, i.e., $\mathrm{C}{{\mathrm{F}}_4}^{2+}\to {\mathrm{F}}^{+}+\mathrm{C}{\mathrm{F}}^{+}+2\mathrm{F}\to {\mathrm{C}}^{+}+{\mathrm{F}}^{+}+3\mathrm{F}$. With the help of the native frame method, we assigned one sequential pathway and two concerted pathways for channel $\mathrm{C}{{\mathrm{F}}_4}^{3+}\to {\mathrm{F}}^{+}+{\mathrm{F}}^{+}+\mathrm{C}{{\mathrm{F}}_2}^{+}$. The branching ratios of these pathways are determined. The momentum correlation of the fragments and the deduced KER distribution indicate that different excited states of $\mathrm{C}{{\mathrm{F}}_4}^{3+}$ with different geometries are responsible for these three pathways. The Coulomb explosion model simulation shows that most of the events in this channel are produced by $\mathrm{C}{{\mathrm{F}}_4}^{3+}$ ions that have deformed geometries from the neutral ${\mathrm{CF}}_4$ molecule.

There has been growing interest in investigating properties of elementary particles predicted by the standard model. Examples of such studies include exploring their low-energy analogs in a condensed-matter system, where they arise as collective states or quasiparticles. Here we show that a toolbox for systematically engineering the emergent elementary fermions, i.e., Dirac, Weyl, and Majorana fermions, can be built in a single atomic system composed of a spinless magnetic dipolar Fermi gas in a three-dimensional optical lattice. The designed direction-dependent dipole-dipole interaction leads to both the basic building block, i.e., in-plane $p+ip$ superfluid pairing instability, and the manipulating tool, i.e., out-of-plane Peierls instability. It is shown that the Peierls instability provides a natural way of tuning the topological nature of $p+ip$ superfluids and can transform the fermion's nature between distinct emergent particles. Our scheme should contribute to the search for elementary particles through manipulating the topology.

We present an evaporative cooling technique for atoms trapped in an optical dipole trap that benefits from narrow optical transitions. For an appropriate choice of wavelength and polarization, a single laser beam leads to opposite light shifts in two internal states of the lowest-energy manifold. Radio-frequency coupling between these two states results in evaporative cooling at a constant trap stiffness. The evaporation protocol is well adapted to several atomic species, in particular to the case of Lanthanides such as Er, Dy, and fermionic Yb, but also to alkali-earth metals such as fermionic Sr. We derive the dimensionless expressions that allow us to estimate the evaporation efficiency. As a concrete example, we consider the case of ${}^{162}\mathrm{Dy}$ and present a numerical analysis of the evaporation in a dipole trap near the $J^{\prime }=J$ optical transition at $832\mathrm{n}\mathrm{m}$. We show that this technique can lead to runaway evaporation in a minimalist experimental setup.

We theoretically study the focusing of a quasicontinuous atom laser beam of rubidium-85 $\left({}^{85}\mathrm{Rb}\right)$. A two-state model analysis based on the Gross-Pitaevskii equation is used which comprises the effects of two-body atom-atom interactions and three-body recombination losses. Utilizing optical focusing potentials such as harmonic potentials, the essential factors such as the width, peak density, and atom loss rate of the focused atom laser beam profile are investigated. Our analysis predicts that using an atom laser offers a dramatic improvement in resolutions of up to 8 nm.

Phonon lasers, the mechanical analogs of optical lasers, have long been a subject of interest as they provide coherent sound waves. We numerically study a magnomechanical phonon laser that operates under a general continuous-wave pump. This system consists of a microwave cavity, a sphere of magnetic material (yttrium iron garnet), a microwave drive pump, and a uniform external bias magnetic field. A population inversion between the supermodes of the system leads to phonon laser action. We study the population inversion and the stimulated emission of phonons under various conditions, such as different drive pump powers and detunings. Unlike previous studies, our results show that the population inversion between the supermodes oscillates with time. Despite the oscillations of the population inversion, one can achieve relatively large stimulated phonon emission if the power of the drive pump is beyond a threshold. Moreover, the optimum operation occurs if the frequency of the drive field matches the frequencies of the cavity photons and magnons. Since it is highly tunable with low loss and has an extra degree of freedom provided by the constant magnetic field, it is possible to develop the system into an alternative to the optomechanical phonon lasers.

We formulate a perturbation theory for determining the frequencies of quasinormal modes in dielectric nonspherical core-shell particles. The theory is based on the extended boundary condition method (also known as the null-field method), which is used to represent the internal field in terms of a matrix $Q$. In this framework, the excitation of a morphology-dependent resonance corresponds to a vanishing eigenvalue of the $Q$ matrix, which can be determined using perturbation techniques to yield explicit formulas for the corresponding eigenfrequency. We specialize our results to the case of a core-shell spherical particle that has been deformed into a spheroid and provide a comparison between numerical simulations and the perturbation theory. Excellent agreement is found for the small perturbations considered here. Further, we investigate the eigenfrequencies of core-shell particles for an extensive range of parameters.

We investigate the two-point polarization correlation properties of random, statistically stationary, two-dimensional (2D) and three-dimensional (3D) electromagnetic fields. To this end two specific mutual polarization correlation functions are introduced, one based on a geometric approach using the Poincaré vectors and the other on the energy distribution employing the Jones vectors. Both approaches provide equivalent information about the space-time polarization correlations and enable one to associate with the field a polarization length over which the equal-time polarization states remain essentially unchanged. For fields obeying Gaussian statistics, the polarization correlation functions take on simple forms in terms of measurable quantities representing electromagnetic coherence. The formalism is demonstrated with two examples: blackbody radiation inside a cavity and in the far zone of a cavity wall aperture.

We propose a scheme to realize nonreciprocal photon blockade and directional amplification in a spinning resonator with clockwise and counterclockwise traveling modes coupled to a stationary two-level atom. By manipulating the rotational frequency of the resonator, the two-level atom can be resonant with one of the traveling modes and largely detuned from the other one. Nonreciprocal conventional photon blockade is induced by the difference of the detunings between the two-level atom and the optical modes traveling in different directions. Besides that, nonreciprocal unconventional photon blockade (UPB) based on the quantum interference can also be observed in the system. Based on the mechanism for nonreciprocal UPB, directional amplification of the blockaded photons is predicted by driving the spinning resonator and stationary two-level atom simultaneously. Our proposal will be applicable to achieve highly efficient nonreciprocal single-photon devices for applications in chiral quantum information processing.

We propose an effective approach for creating robust nonreciprocity of high-order sidebands, including the first-, second-, and third-order sidebands, at microwave frequencies. This approach relies on magnon Kerr nonlinearity in a cavity magnonics system composed of two microwave cavities and one yttrium iron garnet (YIG) sphere. By manipulating the driving power applied on YIG and the frequency detuning between the magnon mode in YIG and the driving field, the effective Kerr nonlinearity can be strengthened, thereby inducing strong transmission nonreciprocity. More interestingly, we find the higher the sideband order, the stronger the transmission nonreciprocity marked by the higher isolation ratio in the optimal detuning regime. Such a series of equally spaced high-order sidebands have potential applications in frequency comblike precision measurement, besides structuring high-performance on-chip nonreciprocal devices.

The photonic local density of states (PLDOS) determines the light-matter interaction strength in nanophotonic devices. For standard dielectric devices, the PLDOS is fundamentally limited by diffraction, but its precise dependence on the size parameter $s$ of a device can be nontrivial. Here, we measure the PLDOS dependence on the size parameter in a waveguide using direct detection of cathode luminescence (CL) through the modes of a nanowaveguide. We observe that, depending on the position within the waveguide cross section, the effective diffraction limit of the PLDOS varies and the PLDOS peak shape changes. Our results are of fundamental importance for optimizing coupling to nanophotonic devices and also opening avenues for applications based on evanescently coupled CL photons.

We investigate the performance of a quantum battery exposed to local Markovian and non-Markovian dephasing noises. The battery is initially prepared as the ground state of a one-dimensional transverse $XY$ model with open boundary condition and is charged (discharged) via interactions with local bosonic reservoirs. We show that in the transient regime, the quantum battery (QB) can store energy faster and has a higher maximum extractable work, quantified via ergotropy, when it is affected by local phase-flip or bit-flip Markovian noise compared to the case when there is no noise in the system. In both the charging and discharging processes, we report the enhancement in work output as well as in ergotropy when all the spins are affected by a non-Markovian Ohmic bath in both the transient and the steady-state regimes, thereby showing a counterintuitive advantage of decoherence in the QB. In both Markovian and non-Markovian cases, we identify the system parameters and the corresponding noise models which lead to maximum enhancement of work output and ergotropy. Moreover, we show that the benefit due to noise persists even with the initial state being prepared at a moderate temperature.

Recent practical approaches for the use of current generation noisy quantum devices in the simulation of quantum many-body problems have been dominated by the use of a variational quantum eigensolver (VQE). These coupled quantum-classical algorithms leverage the ability to perform many repeated measurements to avoid the currently prohibitive gate depths often required for exact quantum algorithms, with the restriction of a parametrized circuit to describe the states of interest. In this work, we show how the calculation of zero-temperature dynamic correlation functions defining the linear response characteristics of quantum systems can also be recast into a modified VQE algorithm, which can be incorporated into the current variational quantum infrastructure. This allows for these important physical expectation values describing the dynamics of the system to be directly converged on the frequency axis, and they approach exactness over all frequencies as the flexibility of the parametrization increases. The frequency resolution hence does not explicitly scale with gate depth, which is approximately twice as deep as a ground-state VQE. We apply the method to compute the single-particle Green's function of ab initio dihydrogen and lithium hydride molecules, and demonstrate the use of a practical active space embedding approach to extend to larger systems. While currently limited by the fidelity of two-qubit gates, whose number is increased compared to the ground-state algorithm on current devices, we believe the approach shows potential for the extraction of frequency dynamics of correlated systems on near-term quantum processors.

Quantum key distribution (QKD) has a prospect to realize unconditional secure communication. However, many real system problems should be solved. Transmitters security is an important part of QKD security. And measuring photon number distribution could help improve it. This measurement is also useful for measuring unknown sources or testing newly designed sources. In this article, a method to measure the photon number distribution of pulses using non-photon-number-resolving detectors is proposed. Numerical simulation is conducted to test this scheme and performs well. Experiment on Poisson source is also used to test this method.

We present a Dicke state preparation scheme which uses global control of $N$ spin qubits: our scheme is based on the standard phase estimation algorithm, which estimates the eigenvalue of a unitary operator. The scheme prepares a Dicke state nondeterministically by collectively coupling the spins to an ancilla qubit via a $ZZ$ interaction, using $\lceil {log}_{2}N\rceil +1$ ancilla qubit measurements. The preparation of such Dicke states can be useful if the spins in the ensemble are used for magnetic sensing: we discuss a possible realization using an ensemble of electronic spins located at diamond nitrogen-vacancy centers coupled to a single superconducting flux qubit. We also analyze the effect of noise and limitations in our scheme.

Using a Wigner-function-based approach, we study the Renyi entropy of a subsystem $A$ of a system of bosons interacting with a local repulsive potential. The full system is assumed to be in thermal equilibrium at a temperature $T$ and density $\rho$. For a $\mathcal{U}\left(N\right)$-symmetric model, we show that the Renyi entropy of the system in the large-$N$ limit can be understood in terms of an effective noninteracting system with a spatially varying mean field potential, which has to be determined self-consistently. The Renyi entropy is the sum of two terms: (a) the Renyi entropy of this effective system and (b) the difference in thermal free energy between the effective system and the original translation-invariant system, scaled by $T$. We determine the self-consistent equation for this effective potential within a saddle-point approximation. We use this formalism to look at one- and two-dimensional Bose gases on a lattice. In both cases, the potential profile is that of a square well, taking one value in subsystem $A$ and a different value outside it. The potential varies in space near the boundary of subsystem $A$ on the scale of density-density correlation length. The effect of interaction on the entanglement entropy density is determined by the ratio of the potential barrier to the temperature and peaks at an intermediate temperature, while the high- and low-temperature regimes are dominated by the noninteracting answer.

Energy loss in collisions of charged projectiles with many-electron systems can be dealt with in time-dependent density functional theory by invoking Ehrenfest's theorem for the time evolution of expectation values of observables. We derive an exact expression for the evaluation of energy loss for systems described in a target reference frame, which is a functional of the electron density. Using an approximation scheme, we then apply the expression to antiproton-atom collisions at intermediate and high energies within the framework of the basis generator method. The calculations are performed within the semiclassical approximation for the nuclear motion, and a straight-line trajectory is employed. The energy loss is evaluated from an expectation value of the time derivative of the time-dependent projectile potential, and it avoids the problem of identifying the excited and ionized many-electron contributions in the many-electron wave function. There is also no need to invoke the independent-event model, since the calculations are performed within the framework of the independent-electron mean-field model. Detailed comparisons are provided for net ionization and total energy loss of antiprotons colliding with hydrogen, helium, neon, carbon, nitrogen, and oxygen. Reasonable agreement is found with the results from one-electron and two-electron calculations for atomic hydrogen and helium, and with experiment in the latter case. For the $\overline{p}\text{−}\mathrm{Ne}$ system at intermediate collision energies, we find discrepancies with previous work that included only single-electron transitions. The sequence of results for C, N, O, and Ne allows one to paint a consistent picture that awaits experimental verification.