Quantum process tomography (QPT) is a powerful tool to characterize quantum operations, but it requires considerable resources, making it impractical for more than two-qubit systems. This work proposes an alternative approach that requires significantly fewer resources for unitary process characterization with a built-in method for state preparation and measurement error mitigation. By measuring the quantum process as rotated through the $X$ and $Y$ axes on the Bloch sphere, we can acquire enough information to reconstruct the quantum process matrix $\chi$ and measure its fidelity. We test the algorithm's performance against standard QPT using simulated and physical experiments on several IBM quantum processors and compare the resulting process matrices. We demonstrate with numerical experiments that the method can improve gate fidelity via a noise reduction in the imaginary part of the process matrix, along with a stark decrease in the number of experiments needed to perform the characterization.

Entangling gates in trapped-ion quantum computers are most often applied to stationary ions with initial motional distributions that are thermal and close to the ground state, while those demonstrations that involve transport generally use sympathetic cooling to reinitialize the motional state prior to applying a gate. Future systems with more ions, however, will face greater nonthermal excitation due to increased amounts of ion transport and exacerbated by longer operational times and variations over the trap array. In addition, pregate sympathetic cooling may be limited due to time costs and laser access constraints. In this paper, we analyze the impact of such coherent motional excitation on entangling-gate error by performing simulations of Mølmer-Sørenson (MS) gates on a pair of trapped-ion qubits with both thermal and coherent excitation present in a shared motional mode at the start of the gate. We quantify how a small amount of coherent displacement erodes gate performance in the presence of experimental noise, and we demonstrate that adjusting the relative phase between the initial coherent displacement and the displacement induced by the gate or using Walsh modulation can suppress this error. We then use experimental data from transported ions to analyze the impact of coherent displacement on MS-gate error under realistic conditions.

Magic state distillation is a resource intensive subroutine that consumes noisy input states to produce high-fidelity resource states that are used to perform logical operations in practical quantum-computing architectures. The resource cost of magic state distillation can be reduced by improving the fidelity of the raw input states. To this end, we propose an initialization protocol that offers a quadratic improvement in the error rate of the input magic states in architectures with biased noise. This is achieved by preparing an error-detecting code which detects the dominant errors that occur during state preparation. We obtain this advantage by exploiting the native gate operations of an underlying qubit architecture that experiences biases in its noise profile. We perform simulations to analyze the performance of our protocol with the XZZX surface code. Even at modest physical parameters with a two-qubit gate error rate of $0.7\%$ and total probability of dominant errors in the gate $O\left({10}^3\right)$ larger compared to that of nondominant errors, we find that our preparation scheme delivers magic states with logical error rate $O\left({10}^{-8}\right)$ after a single round of the standard 15-to-1 distillation protocol, two orders of magnitude lower than using conventional state preparation. Our approach therefore promises considerable savings in overheads with near-term technology.

The angular orientation of an anisotropic scatterer with cylindrical symmetry in a linearly polarized light field represents an optomechanical librator. Here, we propose and theoretically analyze an optimal measurement scheme for the two angular degrees of freedom of such a librator. The imprecision–back-action product of this scheme reaches the Heisenberg uncertainty limit. Furthermore, we propose and analyze a realistic measurement scheme and show that, in the absence of spinning motion around the symmetry axis, measurement-based ground-state cooling of the rotational degrees of freedom of an anisotropic point scatterer levitated in an optical trap is feasible.

Nonlinear magnonics based on the coupling of magnons and optical photons has become a crucial topic in the quantum magnonic field. Here, we demonstrate the magnon laser, the analog of the optical laser in magnonics, in a parity-time ($\mathcal{PT}$) symmetric cavity optomagnonical system involving active and passive optical whispering gallery modes and the magnon mode. We show that the stimulated emitted magnon number can be coherently amplified above a threshold driving power in the $\mathcal{PT}$-symmetric regime where the splitting of optical supermodes is resonant with the magnon frequency, indicating the occurrence of the magnon laser. And this magnon laser action can work with low threshold when the system approaches the optical gain-loss balance and when increasing the optomagnonic coupling. Furthermore, the frequency of the magnon laser can be continuously tuned by adjusting the applied magnetic field strength, which is a distinct feature compared to the photon or phonon amplification by stimulated emission. The study of the $\mathcal{PT}$-symmetric magnon laser may promote the intersection and merging of various disciplines like non-Hermitian physics and quantum magnonics, and provide the theoretical basis and reference for the research and development of new frequency-tunable laser devices.

Quantum simulation of quantum field theory is a flagship application of quantum computers that promises to deliver capabilities beyond classical computing. The realization of quantum advantage will require methods that can accurately predict error scaling as a function of the resolution and parameters of the model and that can be implemented efficiently on quantum hardware. In this paper, we address the representation of lattice bosonic fields in a discretized field amplitude basis, develop methods to predict error scaling, and present efficient qubit implementation strategies. A low-energy subspace of the bosonic Hilbert space, defined by a boson occupation number cutoff, can be represented with exponentially good accuracy by a low-energy subspace of a finite-size Hilbert space. The finite representation construction and the associated errors are directly related to the accuracy of the Nyquist-Shannon sampling and the finite Fourier transforms of the boson number states in the field and the conjugate-field bases. We analyze the relation between the boson mass, the discretization parameters used for wave function sampling, and the finite representation size. Numerical simulations of small size ${\mathrm{\Phi }}^4$ problems demonstrate that the boson mass optimizing the sampling of the ground state wave function is a good approximation to the optimal boson mass yielding the minimum low-energy subspace size. However, we find that accurate sampling of general wave functions does not necessarily result in accurate representation. We develop methods for validating and adjusting the discretization parameters to achieve more accurate simulations.

We propose an adaptive random quantum algorithm to obtain an optimized eigensolver. Specifically, we introduce a general method to parametrize and optimize the probability density function of a random number generator, which is the core of stochastic algorithms. We follow a bioinspired evolutionary mutation method to introduce changes in the involved matrices. Our optimization is based on two figures of merit: learning speed and learning accuracy. This method provides high fidelities for the searched eigenvectors and faster convergence on the way to quantum advantage with current noisy intermediate-scaled quantum computers.

Frame synchronization is a crucial step in quantum key distribution (QKD) and a prerequisite for secure key distillation. Due to the extremely low signal-to-noise ratio (SNR) and fast phase drift in the continuous-variable QKD system, efficient frame synchronization is highly demanded. In this paper, a frame-synchronization scheme based on weak coherent states is proposed. This scheme uses a train of coherent-state pulses encoded by a Zadoff-Chu sequence to achieve frame synchronization. The synchronization performance is analyzed under different transmission distances, transmit powers, frequency offsets, and phase drifts through Monte Carlo simulation. Its performance is also compared with other synchronization sequences, confirming its noise-tolerance capability. An optical experiment is conducted to verify the feasibility of the synchronization scheme under low SNR (as low as $-20$ dB), and the applicable SNR range can be further reduced by increasing the sequence length. Due to its efficiency and noise tolerance, this scheme can provide technical support for weak-coherent-state-based QKD and future quantum network construction.

A fully relativistic approach is presented for the calculation of the bremsstrahlung emitted by an electron scattered off an ionic target. The ionic target is described as a combination of a nuclear electrostatic potential and a screening potential induced by the electronic cloud of the ion. The approach allows us to investigate the influence of the target electrons on the properties of the emitted radiation. We calculate the double differential cross section and Stokes parameters of the bremsstrahlung of an electron scattered off uranium ions in different charge states, ranging from bare to neutral uranium. Results on the high-energy endpoint of the electron bremsstrahlung from Li-like uranium ions ${\mathrm{U}}^{89+}$ are compared to the recent experimental data. For this process, it is found that taking into account the screening effect leads to a change of the cross section on the level of 14%, which can, in principle, be seen in present-day experiments.

Electric-current transients driven by high-peak-power midinfrared laser pulses are shown to provide a source of broadband current, wide-angle microwave-terahertz radiation, whose spectral, spatial, and polarization properties can be adequately understood from a perspective of impulsively driven antenna radiation. When suitably tailored, such laser-driven antennas are shown to generate bright microwave-terahertz pulses with energies in the range of tens of microjoules and ultrawide-angle radiation patterns extending to obtuse angles well beyond the broadside plane, with a considerable radiation flux detected at angles $\theta >{125}^{\circ }$ relative to the direction of the driver beam. Polarization of microwave radiation from laser-driven plasmas is shown to bear clear signatures of the symmetry of transient plasma currents, providing a sensitive probe for ultrafast laser-plasma interactions.

Any treatment of magnetic interactions between atoms, molecules, and optical media must start at the form of the interaction energy. This forms the base on which predictions about any number of magnetic atom-light properties stands, spontaneous decay rates and forces included. As is well known, the Heaviside-Larmor duality symmetry of Maxwell's equations, where electric and magnetic quantities are exchanged, is broken by the usual form of the magnetic interaction energy. We argue that this symmetry can be restored by including general local-field effects and that local fields should be treated as a necessity for correctly translating between the microscopic world of the dipole and the macroscopic world of the measured fields. This may additionally aid in resolving a long-standing debate over the form of the force on a dipole in a medium. Finally, we compute the magnetic dipole decay rate in a magnetodielectric with local-field effects taken into account and show that macroscopic quantum electrodynamics can be made to be dual symmetric at an operator level, instead of only for expectation values.

The quantum coherence of a multipartite system is investigated when some of the parties are moving with uniform acceleration and the analysis is carried out using the single-mode approximation. Due to acceleration the quantum coherence is divided into two parts as accessible and inaccessible coherence. First we investigate tripartite systems, considering both GHZ and $W$ states. We find that the quantum coherence of these states does not vanish in the limit of infinite acceleration, rather asymptoting to a nonzero value. These results hold for both single- and two-qubit acceleration. In the GHZ and $W$ states the coherence is distributed as correlations between the qubits and is known as global coherence. However, quantum coherence can also exist due to the superposition within a qubit, the local coherence. To study the properties of local coherence we investigate a separable state. The GHZ state, $W$ state, and separable states contain only one type of coherence. Next we consider the $W\overline{W}$ and star states in which both local and global coherences coexist. We find that under uniform acceleration both local and global coherence show similar qualitative behavior. Finally, we derive analytic expressions for the quantum coherence of $N$-partite GHZ and $W$ states for $n<N$ accelerating qubits. We find that the quantum coherence of a multipartite GHZ state falls exponentially with the number of accelerated qubits, whereas for multipartite $W$ states the quantum coherence decreases only polynomially. We conclude that $W$ states are more robust to Unruh decoherence and discuss some potential applications in satellite-based quantum communication and black-hole physics.

We develop a framework for simulating measure-preserving, ergodic dynamical systems on a quantum computer. Our approach provides an operator-theoretic representation of classical dynamics by combining ergodic theory with quantum information science. The resulting quantum embedding of classical dynamics (QECD) enables efficient simulation of spaces of classical observables with exponentially large dimension using a quadratic number of quantum gates. The QECD framework is based on a quantum feature map that we introduce for representing classical states by density operators on a reproducing kernel Hilbert space, $\mathcal{H}$. Furthermore, an embedding of classical observables into self-adjoint operators on $\mathcal{H}$ is established, such that quantum mechanical expectation values are consistent with pointwise function evaluation. In this scheme, quantum states and observables evolve unitarily under the lifted action of Koopman evolution operators of the classical system. Moreover, by virtue of the reproducing property of $\mathcal{H}$, the quantum system is pointwise-consistent with the underlying classical dynamics. To achieve a quantum computational advantage, we project the state of the quantum system onto a finite-rank density operator on a $2^n$-dimensional tensor product Hilbert space associated with $n$ qubits. By employing discrete Fourier-Walsh transforms of spectral functions, the evolution operator of the finite-dimensional quantum system is factorized into tensor product form, enabling implementation through an $n$-channel quantum circuit of size $O\left(n\right)$ and no interchannel communication. Furthermore, the circuit features a state preparation stage, also of size $O\left(n\right)$, and a quantum Fourier transform stage of size $O\left(n^2\right)$, which makes predictions of observables possible by measurement in the standard computational basis. We prove theoretical convergence results for these predictions in the large-qubit limit, $n\to \infty$. In light of these properties, QECD provides a consistent simulator of the evolution of classical observables, realized through projective quantum measurement, which is able to simulate spaces of classical observables of dimension $2^n$ using circuits of size $O\left(n^2\right)$. We demonstrate the consistency of the scheme in prototypical dynamical systems involving periodic and quasiperiodic oscillators on tori. These examples include simulated quantum circuit experiments in Qiskit Aer, as well as actual experiments on the IBM Quantum System One.

Recent experimental progress in magnetic atoms and polar molecules has created the prospect of simulating dipolar Hubbard models with off-site interactions. When applied to real-space cylindrical optical lattices, these anisotropic dipole-dipole interactions acquire a tunable spatially dependent component while they remain translationally invariant in the axial direction, creating a sublattice structure in the azimuthal direction. We numerically study how the coexistence of these classes of interactions affects the ground state of hard-core dipolar bosons at half filling in a finite-size cylindrical optical lattice with octagonal rings. When these two interaction classes cooperate, we find a solid state where the density order is determined by the azimuthal sublattice structure and builds smoothly as the interaction strength increases. For dipole polarizations where the axial interactions are sufficiently repulsive, the repulsion competes with the sublattice structure, significantly increasing entanglement and creating two distinct ordered density patterns. The spatially varying interactions cause the emergence of these ordered states in small lattices as a function of interaction strength to be staggered according to the azimuthal sublattices.

In the present article, we investigate the behavior of a large collection of three-level $\mathrm{\Lambda }$ atoms interacting with two quantized electromagnetic fields inside a second-order nonlinear quantum cavity. The nonlinearity of the cavity is supposed to be activated by the application of two classical pump fields. The total Hamiltonian of the combination, with due attention to the nonlinearity effects, is diagonalized. The corresponding ground-state energy then follows from minimizing the total Hamiltonian. The structure of the ground state indicates that four distinct optical phases can occur in such a system. These possible phases turn out to be trivial, dark, left-arm and right-arm superradiant ones. Conditions under which any of the four optical phases can actually occur are also analyzed and discussed. The analysis of the conditions, accompanied by several figures, then reveals that with a suitable choice of the pump field amplitudes and/or geometrical phases, one can intensify the two superradiant phases drastically. Moreover, we demonstrate that the dark optical phase cannot occur at all, while the trivial and superradiant ones can, in fact, coexist. Our calculations also show that transition from the trivial phase to left-arm (right-arm) superradiant one is continuous (discrete) and second (first) order in nature. Another important result of our investigation is that by adjusting the pump field strength, one can switch from the left-arm superradiant phase to the right-arm one and vice versa. This point, in turn, provides an alternative mechanism for the development of quantum optical switching devices.

The two-dimensional optical topological insulators have exhibited a series of novel optical behaviors, especially the topologically protected edge states. We theoretically locate an initial excited quantum emitter (QE) in the two-dimensional ring resonator array and analyze the evolution of the QE in three kinds structures, i.e., topologically nontrivial, trivial, and graphene structure. We found that the dynamic evolution in the nontrivial topological structure is much different from that in the trivial and the graphene structures. When the QE excites edge states in nontrivial structures, the excited edge states can detour along the boundary and re-interact coherently with the QE, resulting in the recovery of the QE population. However, this repeated energy exchange is incomplete, depending on the coupling $g$ between the QE and the resonator. More importantly, we check the influence of the QE's position and the coupling $g$ (between the QE and the resonator) on dynamic evolution of the whole system. For the nontrivial structure, the effective excitation of edge states by the QE requires that: (1) the QE is located in the resonator in boundary and (2) $g$ is much less than coupling coefficient $J$ (between neighbor resonators). When $g$ is much larger than $J$, the QE cannot excite edge states anymore. We give the critical value $g_{\mathrm{c}}$ and find that it relates to band gap and size. Our paper is conducive to an in-depth understanding of the interaction between light and matter in optical topological insulators and provides a useful reference for quantum communication and quantum computing mediated by topological photonics.

Based on the stochastic decoupling framework and auxiliary function averaging technique, a functional integral equation for the zero-temperature population dynamics of the spin-boson model is established. This equation naturally displays the hierarchical structure of the dissipative dynamics and is amenable to nonperturbative approximations. It is shown that at strong dissipation the spin-boson model displays exponential decay with the rate decreasing from the maximum at the coherence-incoherence crossover to zero at the critical dissipation, a manifestation of quantum phase transition. Besides, the scaled time does not change in the whole regime of strong dissipation.

Two classes of Bell-diagonal indecomposable entanglement witnesses in ${\mathbb{C}}^4\otimes {\mathbb{C}}^4$ are considered. Within the first class, we find a generalization of the well-known Choi witness from ${\mathbb{C}}^3\otimes {\mathbb{C}}^3$, while the second one contains the reduction map. Interestingly, contrary to the ${\mathbb{C}}^3\otimes {\mathbb{C}}^3$ case, the generalized Choi witnesses are no longer optimal. We perform an optimization procedure of finding spanning vectors that eventually gives rise to optimal witnesses. Operators from the second class turn out to be optimal, however, without the spanning property. This analysis sheds light onto the intricate structure of optimal entanglement witnesses.

Few-mode optical quantum circuits can be simulated and optimized using gradient-descent methods, as optical gates can be parametrized by continuous parameters. However, the parameter space as seen by a nonholomorphic cost function is not Euclidean, which means that the Euclidean gradient does not generally point in the direction of steepest ascent. In order to retrieve the true steepest-ascent direction, one must take into account the local metric in what is known as the natural-gradient (NG) method. In this work we implement the NG for the optimization of optical quantum circuits. Specifically, we adapt the NG approach to a complex-valued parameter space, allowing us to apply the NG method directly within the Bargmann formalism, which we rely on for transforming between phase space and Fock space. We then compare the NG approach to vanilla gradient descent and to Adam over two standard state-preparation benchmarks: a single-photon source and a Gottesman-Kitaev-Preskill state source. We observe that the NG approach converges significantly faster (due in part to the possibility of using larger learning rates) and with a smoother decay of the cost function throughout the optimization. This result further improves our ability to design quantum circuits via classical optimization methods.