The hybrid microwave optomechanical-magnetic system has recently emerged as a promising candidate for coherent information processing because of the ultrastrong microwave photon-magnon coupling and the long life of the magnon and phonon. As a quantum information processing device, the realization of single excitation holds special meaning for the hybrid system. In this paper, we introduce a single two-level atom into the optomechanical-magnetic system and show that an unconventional blockade due to destructive interference cannot offer a blockade of both the photon and magnon. Meanwhile under the condition of single excitation resonance, the blockade of the photon, phonon, and magnon can be achieved simultaneously even in a weak optomechanical region, but the phonon blockade still requires the cryogenic temperature condition.

We investigate corner states in a photonic two-dimensional (2D) Su-Schrieffer-Heeger (SSH) model on a square lattice with zero gauge flux. By considering intracelluar next-nearest-neighbor (NNN) hoppings, we discover a broad class of corner states in the 2D SSH model and show that they are robust against certain fabrication disorders. Moreover, these corner states are located around the corners but not at the corner points. We analytically identify that these corner states are induced by the intracelluar NNN hoppings (long-range interactions) and split off from the edge-state bands. Thus, we refer to them as general bounded corner states. Our paper shows a simple way to induce unique corner states by the long-range interactions and offers opportunities for designing novel photonic devices.

We study the hardness of classically simulating boson sampling with superposition and Gaussian input states at nonzero photon indistinguishability. We find that, similar to regular boson sampling, distinguishability causes exponential attenuation of the many-photon interference terms in both these boson sampling variants. For superposition sampling, we find that it is not simulable with out method at zero indistinguishability, which is evidence for the computational hardness of this problem, and we find that it is simulable at any level of particle distinguishability, similar to regular boson sampling. If an efficient classical algorithm to approximate a given sum over permanents is found, this approach also leads to an efficient classical algorithm to simulate Gaussian boson sampling in the presence of distinguishability.

Strong correlations between the signal and idler beams imprinted during their generation dominantly determine the properties of twin beams. They are also responsible for the waves in intensity coherence observed in the wave-vector space of a twin beam propagating in a nonlinear crystal in the regime with pump depletion. These waves start to develop at certain twin-beam intensities and move from the signal- and idler-beam centers towards their tails. They manifest themselves via the change of coherence volume monitored in the far field by the measurement of local modified ${\overline{g}}^{\left(2\right)}$ function, which acts as a sensitive and stable tool for investigating field intensity coherence.

We explore the quantum criticality of a two-site model combining quantum Rabi models with hopping interaction. Through a combination of analytical and numerical approaches, we find that the model allows the appearance of a superradiant quantum phase transition (QPT) even in the presence of strong ${\mathbf{A}}^2$ terms, preventing single-site superradiance. In the two-site model the effect of ${\mathbf{A}}^2$ terms can be surmounted by the photon delocalization from hopping, and a reversed superradiant QPT occurs as a consequence of the competition between ${\mathbf{A}}^2$ terms and the hopping interaction. We characterize the phase diagram and scaling functions and extract the critical exponents in the vicinity of the critical point, thus establishing the universal behavior of the second-order phase transition. Remarkably the effective hopping strength will be enhanced if more cavities are cascaded. We also prove that the multiqubit counterpart of the quantum Rabi dimer, i.e., the Dicke dimer, has the same properties in beating the ${\mathbf{A}}^2$ effect. Our work provides a path to the study of phase transitions in the presence of the ${\mathbf{A}}^2$ terms and offers the prospect of investigating quantum-criticality physics and quantum devices in many-body systems.

Quantifying quantum coherence is a key task in the resource theory of coherence. Here we establish a good coherence monotone in terms of a state conversion process, which automatically endows the coherence monotone with an operational meaning. We show that any state can be produced from some input pure states via the corresponding incoherent channels. It is found that the coherence of a given state can be well characterized by the least coherence of the input pure states, so a coherence monotone is established by only effectively quantifying the input pure states. In particular, we show that our proposed coherence monotone is the supremum of all the coherence monotones that give the same coherence for any given pure state. We also prove that our coherence monotone is continuous. Considering the convexity, we prove that our proposed coherence measure is a subset of the coherence measure based on the convex roof construction. The similarities and differences between our coherence monotone and coherence cost are studied in detail. As applications, we give a concrete expression of our coherence measure by employing the geometric coherence of a pure state. We also give a thorough analysis of the states of the qubit and finally obtain a series of analytic coherence measures. The numerical examples are also given to show the difference between our coherence monotone and that based on the convex roof construction.

Hybrid entanglement between particlelike and wavelike optical qubits has drawn increasing attention because such hybrid entanglement is a key resource in establishing hybrid quantum networks and connecting quantum processors with different encoding qubits. For convenience, we define “particlelike optical qubits” as PO qubits and “wavelike optical qubits” as WO qubits. In this work, we propose a method to create a hybrid Greenberger-Horne-Zeilinger (GHZ) entangled state of $n$ PO qubits and $n$ WO qubits, by using $2n$ microwave cavities coupled to a superconducting flux qutrit. The two logic states of a PO qubit here are represented by the vacuum state and the single-photon state of a cavity (or represented by the rotated states of the vacuum state and the single-photon state), while the two logic states of a WO qubit are indicated by the two coherent states of a cavity. The procedure for preparing the GHZ state consists of only a few basic operations, and the circuit resources are significantly reduced because of using only one flux qutrit as the coupler. The GHZ-state preparation time does not depend on the number of qubits, and the GHZ state is deterministically generated since no measurement is made. In addition, the intermediate higher-energy level of the qutrit during the entire operation is virtually excited and thus decoherence from this level is greatly suppressed. This proposal is quite general and can be extended to create the proposed hybrid GHZ state, by using a $\mathrm{\Lambda }$-type natural or artificial atom coupled to $2n$ microwave or optical cavities. As an example, our numerical simulation demonstrates that within current circuit-QED technology, the hybrid GHZ state of two PO qubits and two WO qubits can be prepared with a high fidelity.

In this paper, we propose a feasible scheme for generating the Schrödinger-cat-like states of a macroscopic mechanical resonator in pulsed cavity optomechanics via photon detection. Starting with cooling the mechanical oscillator to its ground state, a red pulse and a blue pulse with different powers are simultaneously employed to dissipatively generate squeezed mechanical states. Subsequently, a second red pulse is utilized to achieve large-amplitude mechanical catlike states with high fidelities, conditioned on the multiphoton detection of cavity output and assisted by squeezing injection into the cavity. This combination effectively overcomes the difficulty that only a kitten can be produced by single-photon detection. Finally, after being stored in the resonator for a period of time during which thermal effect is analytically revealed, the mechanical quantum superpositions are mapped, with a third red pulse, to the cavity output field for state verification. Our scheme is generic and can also be used to produce other kinds of non-Gaussian mechanical states, such as optical-catalysis nonclassical states.

Photon statistical measurements on a semiconductor microlaser, obtained using single-photon counting techniques, show that a newly discovered spontaneous pulsed emission regime possesses superthermal statistical properties. The observed spike dynamics, typical of small-scale devices, is at the origin of an unexpected discordance between the probability density function and its representation in terms of the first moments, a discordance so far unnoticed in all devices. The impact of this new dynamics is potentially large, since coincidence techniques are presently the sole techniques capable of characterizing light emitted by nanolasers.

We propose an optomechanical dissipation engineering scheme by introducing an ancillary mechanical mode with a large decay rate to control the density of states of the optical mode. The effective linewidth of the optical mode can be reduced or broadened, manifesting the dissipation engineering. To prove the ability of our scheme to improve the performances of the optomechanical system, we studied optomechanical entanglement and phonon cooling. It is demonstrated that the optomechanical entanglement overwhelmed by thermal phonon excitations could be restored via dissipation engineering. For the phonon cooling, an order-of-magnitude improvement could be achieved. Our scheme can be generalized to other systems with multiple bosonic modes, which is experimentally feasible with advances in materials and nanofabrication, including optical Fabry-Perot cavities, superconducting circuits, and nanobeam photonic crystals.

An alternative and powerful Schrödinger-like equation for describing beyond dipole laser-matter interactions and leading relativistic corrections is derived. It is shown that this particular formulation is numerically very efficient with respect to computational effort and convergence rate of the solutions. Furthermore, its nonrelativistic form turns out to be more compatible with relativity than what seems to be the case with the more common formulations of the nonrelativistic light-matter interaction. Moreover, the extension of this interaction form into the relativistic region preserves, to a large extent, the numerical efficiency. In this work, the formulation is applied to study beyond dipole corrections and relativistic corrections in multiphoton ionization of a hydrogen atom in the x-ray regime.

The polaron picture is employed to investigate and elucidate the physics of the two-qubit quantum Rabi model, which describes two identical qubits coupled to a common harmonic oscillator. This approach enables us to obtain the ground-state energy and some other simpler physical observables with high accuracy in all regimes of the coupling strengths $g$, while there is no constraint to the ration of tunneling frequency $\mathrm{\Omega }$ and field frequency $\omega$, which is not simultaneously possible using previous methods. In addition, we also discover a phenomenon that is not present in the one-qubit Rabi model: with the increase of coupling strength $g$, there is a transition of the ground state of the system from a multipolaron state to a bipolaron state. However, the tunneling frequency $\mathrm{\Omega }$ counteracts this process. Specifically, when the tunneling frequency $\mathrm{\Omega }=0$, the system always stays in the bipolaron state.

We study the influence of acceleration on the twin-Fock state which is a class of specific multibody entangled quantum state and was already realized experimentally with high precision and sensitivity. We show that the multibody quantum entanglement can be increased with the acceleration, consistent with the “anti-Unruh effect” in reference to the counterintuitive cooling previously pointed out for an accelerated detector coupled to the vacuum. In particular, this kind of entanglement increase can lead to the improvement of the phase sensitivity, which provides a way to test the anti-Unruh effect in the future experiments.

Exceptional points (EPs) are degeneracies of classical and quantum open systems, which are studied in many areas of physics including optics, optoelectronics, plasmonics, and condensed matter physics. In the semiclassical regime, open systems can be described by phenomenological effective non-Hermitian Hamiltonians (NHHs) capturing the effects of gain and loss in terms of imaginary fields. The EPs that characterize the spectra of such Hamiltonians (HEPs) describe the time evolution of a system without quantum jumps. It is well known that a full quantum treatment describing more generic dynamics must crucially take into account such quantum jumps. In a recent paper [F. Minganti et al., Phys. Rev. A 100, 062131 (2019)], we generalized the notion of EPs to the spectra of Liouvillian superoperators governing open system dynamics described by Lindblad master equations. Intriguingly, we found that in situations where a classical-to-quantum correspondence exists, the two types of dynamics can yield different EPs. In a recent experimental work [M. Naghiloo et al., Nat. Phys. 15, 1232 (2019)], it was shown that one can engineer a non-Hermitian Hamiltonian in the quantum limit by postselecting on certain quantum jump trajectories. This raises an interesting question concerning the relation between Hamiltonian and Lindbladian EPs, and quantum trajectories. We discuss these connections by introducing a hybrid-Liouvillian superoperator, capable of describing the passage from an NHH (when one postselects only those trajectories without quantum jumps) to a true Liouvillian including quantum jumps (without postselection). Beyond its fundamental interest, our approach allows to intuitively relate the effects of postselection and finite-efficiency detectors.

First-order truncated simplex lattices have been used as interesting data structures to explore the properties of quantum search such as the effect of connectivity on search speed [D. A. Meyer and T. G. Wong, Phys. Rev. Lett. 114, 110503 (2015)]. In this paper, we further discuss quantum search algorithms for truncated simplex lattices. We first propose a multistage quantum algorithm for an $r\mathrm{th}$-order truncated simplex lattice with $N$ vertices based on the numerical calculation up to the fifth-order lattice, which requires an $(r+1)$-stage search process and consumes a run time $\mathrm{\Theta }\left(N^{(2r+1)/(2r+2)}\right)$ in general. Furthermore, with edge weights suitably adjusted (which increases the connectivity according to certain definitions), we merge the multistage search process into a single stage and achieve a fast quantum search algorithm with an optimal run time $\mathrm{\Theta }\left(\sqrt{N}\right)$ for first-order truncated simplex lattices, which we conjecture is generally true for any-order lattice. Under small noise of the lattice structure, both our multistage and our single-stage search algorithms are quite robust.

Branching fractions for decays from the $P_{3/2}$ level in ${}^{138}\mathrm{Ba}^{+}$ have been measured with a single laser-cooled ion. Decay probabilities to $S_{1/2}$, $D_{3/2}$, and $D_{5/2}$ are determined to be 0.741716(71), 0.028031(23), and 0.230253(61), respectively, which are an order-of-magnitude improvement over previous results. Our methodology only involves optical pumping and state detection, and is hence relatively free of systematic effects. Measurements are carried out in two different ways to check for consistency. Our analysis also includes a measurement of the $D_{5/2}$ lifetime, for which we obtain 30.14(40) s.