We develop a classical bit-flip correction method to mitigate measurement errors on quantum computers. This method can be applied to any operator, any number of qubits, and any realistic bit-flip probability. We first demonstrate the successful performance of this method by correcting the noisy measurements of the ground-state energy of the longitudinal Ising model. We then generalize our results to arbitrary operators and test our method both numerically and experimentally on IBM quantum hardware. As a result, our correction method reduces the measurement error on the quantum hardware by up to one order of magnitude. We finally discuss how to preprocess the method and extend it to other error sources beyond measurement errors. For local Hamiltonians, the overhead costs are polynomial in the number of qubits, even if multiqubit correlations are included.

Enhancing quantum entanglement is important for many quantum information processing applications. In this paper, we consider a protocol for entanglement enhancing in a two-mode squeezed vacuum state (TMSVS), attained based on photon subtraction, photon catalysis, and photon addition. Central to such an operation is the task of mixing and detecting number states with each mode of TMSVS. We analyze various settings and find an optimal setup for improving the entanglement of the state.

In recent years, quantum annealing has gained the status of being a promising candidate for solving various optimization problems. Using a set of hard 2-satisfiability (2-SAT) problems, consisting of problems of up to 18 variables, we analyze the scaling complexity of the quantum annealing algorithm and study the distributions of the minimum energy gap and the success probability. We extend the analysis of the standard quantum annealing Hamiltonian by introducing an additional term, the trigger Hamiltonian, which can be of two types: ferromagnetic and antiferromagnetic. We use these trigger Hamiltonians to study their influence on the success probability for solving the selected 2-SAT problems. We find that although the scaling of the runtime is exponential for the standard and modified quantum annealing Hamiltonians, the scaling constant in the case of adding the trigger Hamiltonians can be significantly smaller. Furthermore, certain choices for the trigger Hamiltonian and annealing times can result in a better scaling than that for simulated annealing. Finally, we also use the quantum annealers of D-Wave Systems Inc. to study their performance in solving the 2-SAT problems and compare it with the simulation results.

The continuous-variable (CV) quantum state of light allows for the teleportation of a discrete-variable (DV) photonic qubit. Such an operation is useful in the realm of hybrid quantum networks. However, it is known that the teleportation of a DV qubit via a Gaussian CV resource state is severely limited under channel loss, with a teleportation fidelity beyond the classical limit unattainable for losses exceeding a small threshold. In this paper, we present a nondeterministic teleportation protocol that combines a Gaussian CV resource state with a modified form of the Bell state measurement that accommodates significantly improved DV-qubit fidelity over a wide range of channel losses. Beyond this improvement, we also show how the use of non-Gaussian operations on the CV resource state can lead to a DV-qubit fidelity approaching unity for any channel loss.

We propose a unified and deterministic scheme to generate arbitrary single-photon multimode $W$ states in circuit QED. A three-level system (qutrit) is driven by a pump-laser pulse and coupled to $N$ spatially separated resonators. The coupling strength for each spatial mode $g_i$ totally decide the generated single-photon $N$-mode $W$ state $|W_N\rangle =\frac{1}{A}{\sum }_{i=1}^N{g}_i|0_1{0}_2\cdots 1_i{0}_{i+1}\cdots 0_N\rangle$, so arbitrary $|W_N\rangle$ can be generated inside resonators by adjusting $g_i$ and $N$. Moreover, such $W$ states can be fast emitted in symmetric temporal waveform with a probability reaching $98.9\%$. The generation (or emission) time and fidelity can both be the same for arbitrary $|W_N\rangle$, independent of mode numbers. Finally, the time evolution process is easy to control since only the pump pulse is time-dependent.

Dynamic entanglement between two end molecules in a polar-molecule array is investigated in static electric fields. Specific initial states are set to probe the evolution of the molecular system under the influence of intrinsic decoherence. We analyze the degree of entanglement between the end molecules by use of concurrence. When the Bell states are initially shared between the end molecules, the end-to-end concurrence shows either a fragile or a robust behavior. The robust concurrence is less affected by decoherence, reflecting a situation that the initial Bell state can be shared between the end molecules for long periods of time. The stability of the entanglement shared between two arbitrary molecules in the array is further studied. Furthermore, we show how to induce a high end-to-end concurrence via transfer of a separable state. The state is transferred from one end molecule to the other. During the back-and-forth transfer processes, the concurrence cyclically reveals high peaks, corresponding to different Bell-like states. In addition, population inversion is observed to only occur at the end molecules in the array. We further examine the contribution of effective states to the concurrence and discuss the effects of decoherence, dipole-field interaction, and dipole-dipole interaction on its properties.

We present an alternative approach to the study of equilibrium properties in many-body quantum physics. Our method takes inspiration from density-matrix quantum Monte Carlo in dealing with the sign problem and incorporates additional features. First of all, the dynamics is transferred to the Laplace representation where an exact equation can be derived and solved using an evolution step that, unlike most Monte Carlo methods, is not a priori physically bounded. Moreover, the spawning events are formulated in terms of two-process stochastic unravellings of quantum master equations, a formalism that is particularly useful when working with density matrices. Last, our approach is perturbative in nature, with the free part being integrated exactly and, as such, the convergence is greatly accelerated when the interaction parameter is small. We benchmark our method by applying it to two case studies in condensed-matter physics, perform some ground-state calculations, show its accuracy, and further discuss its efficiency.

Employing the nondipole Volkov states, i.e., the solutions of the time-dependent Schrödinger equation for an electron in a laser field whose spatial dependence is accounted for by including the first-order term in a $1/c$ expansion of the vector potential, we present an analytic approach based on the strong-field approximation that is able to describe ionization and detachment in intense midinfrared laser fields. Our approach includes both the direct transition amplitude and the improved strong-field approximation transition amplitude, accounting for the field-induced rescattering of the liberated electron on its parent core. We apply the theory to strong-field detachment from the $F^{-}$ ion. Comparing the results to those obtained invoking the dipole approximation, we identify and analyze the effects caused by including the nondipole corrections for the cases of circular and linear polarization. We demonstrate that the introduced nondipole corrections induce asymmetries in photoelectron momentum distributions along the propagation direction. For the case of linear polarization, we report a shift of the low-energy region of the momentum distribution against the propagation direction, a feature previously studied on neutral targets and related to Coulomb focusing effects.

We study the low-energy excitations of a bosonic lattice gas with cavity-mediated interactions. By performing two successive Hubbard-Stratonovich transformations, we derive an effective field theory to study the strongly coupling regime. Taking into account the quantum fluctuation, we report the unusual effect of the superradiant cavity light-induced density imbalance, which has been shown to have a negligible effect on the low-energy excitation spectrum in previous studies. Instead, we show that such infinitesimal fluctuation of density imbalance dramatically changes the behavior of low-energy excitations and results in a parameter-driven switching between two types of Goldstone modes, i.e., type I and type II with odd and even power energy-momentum dispersion, respectively. Our proposal would open a horizon for manipulating Goldstone modes by bridging the cavity light and strongly interacting quantum matters.

By means of quantum Monte Carlo simulations we study phase diagrams of dipolar bosons in a square optical lattice. The dipoles in the system are parallel to each other and their orientation can be fixed in any direction in the three-dimensional space. Starting from experimentally tunable parameters like scattering length and dipolar interaction strength, we derive the parameters entering the effective Hamiltonian. Depending on the direction of the dipoles, various types of supersolids (e.g., checkerboard, stripe) and solids (checkerboard, stripe, diagonal stripe, and an incompressible phase) can be stabilized. Remarkably, we find a cluster supersolid characterized by the formation of horizontal clusters of particles. These clusters order along a direction at an angle with the horizontal. Moreover, we find what we call a grain-boundary superfluid. In this phase, regions with solid order are separated by extended defects—grain boundaries—which support superfluidity. We also investigate the robustness of the stripe supersolid against thermal fluctuations. Finally, we comment on the experimental realization of the phases found.

The use of atomically sized quantum systems as highly sensitive measuring devices represents an exciting and quickly growing research field. Here we explore the properties of a quasiparticle formed by a mobile impurity interacting with a two-dimensional fermionic superfluid. The energy of the quasiparticle is shown to be lowered by superfluid pairing, as this increases the compressibility of the Fermi gas, thereby making it easier for the impurity to perturb its surroundings. We demonstrate that the fundamentally discontinuous nature of the superfluid-to-normal phase transition of a two-dimensional system leads to a rapid increase in the quasiparticle energy around the critical temperature. The magnitude of this increase exhibits a nonmonotonic behavior as a function of the pairing strength with a sizable maximum in the crossover region, where the spatial extend of the Cooper pairs is comparable to the interparticle spacing. Since the quasiparticle energy is measurable with present experimental techniques, our results illustrate how impurities entangled with their environment can serve as useful probes for nontrivial thermal and quantum correlations.

The disclosure of basic nonlinear optical properties of graphene-like nanostructures with correlated electron-hole nonlinear dynamics over a wide range of frequencies and pump field intensities is of great importance for both graphene fundamental physics and for the expected novel applications of two-dimensional (2D) hexagonal nanostructures in extreme nonlinear optics. In the current paper, the nonlinear interaction of 2D hexagonal nanostructures with the bichromatic infrared driving field taking into account the many-body Coulomb interaction is investigated. Numerical investigation in the scope of the Bloch equations within the Houston basis that take into account $e\text{–}e$ and $e\text{–}h$ interactions in the Hartree-Fock approximation reveals significant excitonic effects in the high-harmonic generation process in 2D hexagonal nanostructures such as graphene and silicene. It is shown that, due to the correlated electron-hole nonlinear dynamics around the van Hove singularity, spectral caustics in the high-harmonic generation spectrum are induced near the saddle-point excitonic resonances.

We developed the theoretical framework based on the coupled-mode theory which describes spectral and scattering properties of the photonic analog of an extended Fano-Anderson model—a waveguide array with two additional side-coupled waveguides. The structure supports a rich spectrum of eigenmodes, including bound state in the continuum (BIC) and other bound and leaky modes, which can be classified according to the relation between the self-coupling coefficients and eigenvalues. We focus on the structures with broken vertical symmetry with their band structures revealing interesting phenomena, such as exceptional points and level repulsion, and offer a lossless platform for $\mathcal{PT}$-symmetry phase transition. We interpreted the resonant features in the scattering spectra through a generalized Weierstrass factorization. The resonance related with quasi-BIC arises from the interference between two leaky modes: one of them representing a continuum spectrum and the other (quasi-BIC) discrete state. The reflectance near the resonance can be rewritten into the form of the Fano formula where the shape parameter $f$ can be expressed in terms of the poles associated with the two modes. Our approach provides a flexible framework which allows to interpret and to engineer the resonant properties of more complex systems.

We propose a method to enhance bipartite and tripartite entanglement in cavity magnomechanics using an optical parametric amplifier (OPA). We analyze this system and identified parametric regimes where different types of entanglement are enhanced. We show that a proper choice of phase of the parametric amplifier leads to the enhancement of the bipartite entanglements. Moreover, the tripartite entanglement is also significantly enhanced in the presence of the OPA. The OPA not only enhances the strength of entanglement but also increases the domain of entanglement over a wider space of detunings as compared to the system when no OPA is present. Similarly, the robustness of entanglement against temperature is also enhanced. Another important consequence of the OPA is the fact that it relaxes the requirement of strong magnon-phonon coupling to generate cavity-magnon entanglement which is necessary for the case when the OPA is not present. We believe that the presented scheme is a step forward to realize robust quantum entanglement using current technology.

Stable sources of entangled photons are important requirements for quantum communications. In recent years, cascaded down-conversion has been demonstrated as an effective method of directly producing three-photon entanglement. However, to produce polarization entanglement these sources have until now relied on intricate active phase stabilization schemes, thus limiting their robustness and usability. In this paper, we present a completely phase-stable source of three-photon entanglement in the polarization degree of freedom. With this source, which is based on a cascade of two pair sources based on Sagnac configurations, we produce states with over 96% fidelity with an ideal Greenberger-Horne-Zeilinger state. Moreover, we demonstrate the stability of the source over several days without any ongoing optimization. We expect this source to be a useful tool for applications requiring multiphoton entanglement, such as quantum secret sharing and producing heralded entangled photon pairs.

In this paper, we theoretically demonstrate the tunability of the Casimir force both in sign and magnitude between parallel plates coated with dispersive materials. We show that this force, existing between uncharged plates, can be tuned by carefully choosing the value of the plasma frequency (i.e., the epsilon-near-zero frequency) of the coating in the neighborhood of the resonance frequency of the cavity. The coating layer enables a continuous variation of the force between four limiting values when a coating is placed on each plate. We explore the consequences of such variation when pairs of electric and magnetic conductors (i.e., low and high impedance surfaces) are used as substrates on either side, showing that this continuous variation results in changes in the sign of the force, leading to both stable and unstable conditions, which could find interesting potential applications in nanomechanics, including nanoparticle tweezing.

We consider a quantum charged particle moving in the $xy$ plane under the action of a uniform perpendicular time-dependent magnetic field, in the presence of a parabolic binding potential. The time-dependent probability distribution of the magnetic moment is calculated analytically and numerically in the case of initial thermodynamic equilibrium state. In the high-temperature regime, the initial distribution is almost symmetric, resulting in a tiny mean diamagnetism. However, if the magnetic field eventually changes its sign, the fragile balance between the diamagnetic and paramagnetic “wings” of the probability distribution becomes broken, resulting in a giant mean diamagnetic moment, exceeding the initial one by several orders of magnitude. The final mean value of the magnetic moment is inversely proportional to the strength of the binding potential, and it does not depend on the Planck constant in the high-temperature regime. Strong fluctuations of the magnetic moment (described in terms of the variance) exist in all temperature regimes.

Entanglement is an important resource for quantum communication tasks. Most results are focused on qubit entanglement. Our goal in this work is to characterize the multipartite high-dimensional entanglement. We first derive an entanglement polygon inequality for the $q$ concurrence, which manifests the relationship among all the “one-to-group” marginal entanglements in any multipartite qudit system. This implies lower and upper bounds for the marginal entanglement of any three-qudit system. We further extend our study to general entanglement distribution inequalities for high-dimensional entanglement in terms of the unified-$(r,s)$ entropy entanglement, including Tsallis entropy, Rényi entropy, and von Neumann entropy entanglement as special cases. These results provide insights into characterizing bipartite high-dimensional entanglement in quantum information processing.

In this paper, we present the results of the triple differential cross section (TDCS) for the (e, 2e) process on ${\mathrm{H}}_2\mathrm{O}$ molecule for the plane wave and the twisted electron beam impact. The formalism is developed in the first Born approximation. We describe the plane or twisted wave, plane wave, the linear combination of atomic orbitals (LCAO) (self-consistent field LCAO method), and Coulomb wave for the incident electron, scattered electron, the molecular state of ${\mathrm{H}}_2\mathrm{O}$, and the ejected electron, respectively. We investigate the angular profiles of the TDCS for the outer orbitals $1{\mathrm{b}}_1$, $3{\mathrm{a}}_1$, $1{\mathrm{b}}_2$, and $2{\mathrm{a}}_1$ of the water molecule. We compare the angular profiles of the TDCS for the different values of orbital angular momentum (OAM) number $m$ of the twisted electron beam with that of the plane wave beam. We also study the TDCS for macroscopic ${\mathrm{H}}_2\mathrm{O}$ target to explore the effect of opening angle ${\theta }_p$ of the twisted electron beam on the TDCS. Our results clearly show the effect of the twisted electron's OAM number $\left(m\right)$ and the opening angle ${\theta }_p$ on the TDCS of the water molecule.