Quantum error correction is a key challenge for the development of practical quantum computers, a direction in which significant experimental progress has been made in recent years. In solid-state qubits, one of the leading information loss mechanisms is dephasing, usually modeled by phase-flip errors. Here, we introduce quasistatic phase damping, a more subtle error model which describes the effect of Larmor frequency fluctuations due to $1/f$ noise. We show how this model is different from a simple phase-flip error model, in terms of multicycle error correction. Considering the surface code, we provide numerical evidence for an error threshold, in the presence of quasistatic phase damping and readout errors. We discuss the implications of our results for spin qubits and superconducting qubits.

Quantum steering is considered one of the most well-known nonlocal phenomena in quantum mechanics. Unlike entanglement and Bell nonlocality, the asymmetry of quantum steering makes it vital for one-sided device-independent quantum information processing. Although there has been much progress on steering detection for bipartite systems, the criterion for Einstein-Podolsky-Rosen steering in tripartite systems remains challenging and inadequate. In this paper we first derive a promising steering criterion for any three-qubit states via correlation matrix. Furthermore, we propose the monogamy relation between the tripartite steering of system and the bipartite steering of subsystems based on the derived criterion. Finally, as illustrations, we demonstrate the performance of the steering criterion and the monogamy relation by means of several representative examples. We believe that the results and methods presented in this work could be beneficial to capture genuine multipartite steering in the near future.

When storing encoded qubits, if single faults can be corrected and double faults can be postselected against, logical errors occur due to at least three faults. At current noise rates, having to restart when two errors are detected prevents very long-term storage, but that should not be an issue for low-depth computations. We consider distance-four efficient encodings of multiple qubits into a modified planar patch of the 16-qubit surface code. We simulate postselected error correction for up to 12 000 rounds of parallel stabilizer measurements and subsequently estimate the cumulative probability of logical error for up to 12 encoded qubits. Our results demonstrate a combination of low logical error rate and low physical overhead. For example, the distance-four surface code, using postselection, accumulates 25 times less error than its distance-five counterpart. For six encoded qubits, a distance-four code using 25 qubits protects as well as the distance-five surface code using 246 qubits. Hence, distance-four codes, using postselection in a planar geometry, are qubit-efficient candidates for fault-tolerant, moderate-depth computations.

The method of quantum Lanczos recursion is extended to solve for multiple excitations on the quantum computer. While quantum Lanczos recursion is, in principle, capable of obtaining excitations, the extension to a block Lanczos routine can resolve degeneracies with better precision and only costs $O\left(d^2\right)$ for $d$ excitations on top of the previously introduced quantum Lanczos recursion method. Extension to non-Hermitian operators is also discussed.

We consider an interferometer based on the concept of induced coherence, where two photons that originate in different second-order nonlinear crystals can interfere. We derive a complementarity relationship that links the first-order coherence between the two interfering photons with a parameter that quantifies the distinguishing information regarding the nonlinear crystal where they originated. We show that the derived relationship goes beyond the single-photon regime and is valid for any photon-flux rate generated. We report experimental results in the low photon-flux regime that confirm the validity of the derived complementarity relationship.

Since Harrow et al. [A. W. Harrow, A. Hassidim, and S. Lloyd, Phys. Rev. Lett. 103, 150502 (2009)] showed that a system of linear equations with $N$ variables and condition number $\kappa$ can be solved on a quantum computer in $poly\left[log\left(N\right),\kappa \right]$ time, exponentially faster than any classical algorithms, its improvements and applications have been extensively investigated. The state-of-the-art quantum algorithm for this problem is due to Costa et al. [P. C. S. Costa, D. An, Y. R. Sanders, Y. Su, R. Babbush, and D. W. Berry, PRX Quantum 3, 040303 (2022)], with optimal query complexity $\mathrm{\Theta }\left(\kappa \right)$. An important question that is left is whether parallelism can bring further optimization. In this paper, we study the limitation of parallel quantum computing on this problem. We show that any quantum algorithm for solving systems of linear equations with time complexity $poly\left[log\left(N\right),\kappa \right]$ has a lower bound of $\mathrm{\Omega }\left(\kappa \right)$ on the depth of queries, which is tight up to a constant factor.

The entanglement of different macroscopic objects can provide crucial resources for various quantum applications and quantum-enabled devices. The generation, manipulation, and enhancement of entanglement are pivotal areas of research. In this study, we introduce an approach for producing enhanced nonreciprocal entanglement in a hybrid spinning optomechanical system. By exploiting the Sagnac effect to break the time-reversal symmetry of the system, we successfully generate and amplify the nonreciprocal bipartite entanglement between the atomic ensemble and mechanical oscillator, exhibiting an approximately twofold enhancement under competitive equilibrium conditions. Furthermore, we investigate and quantify the nonreciprocal tripartite entanglement encompassing the atomic ensemble, optical cavity, and mechanical oscillator by employing the residual contangle measurement. We find that the nonreciprocal bipartite (tripartite) entanglement is not only enlarged almost two (five) times, but also noise resistant. The proposed scheme of enhanced bipartite and tripartite entanglement has the potential to advance a wide range of quantum technologies, spanning from quantum information processing to quantum sensing applications.

Research in quantum information science aims to surpass the scaling limitations of classical information processing. From a physicist's perspective, performance improvement involves a physical speedup in the quantum domain, achieved by dynamically exploiting quantum correlations. In this study, speed limits in interacting quantum systems are derived by comparing the rates of change in actual quantum dynamics with the quasiclassical evolution confined to the manifold of nonentangled separable states. The utility of the resulting bounds on entanglement-assisted speedup is demonstrated on bipartite qubit systems, bipartite qudit systems, as well as a complex multimode systems. Specifically, the proposed speed limits provide a tight bound on quantum speed advantage, including a quantum gain that can scale exponentially with the system's size. Extensions of the results to open systems and measurable witnesses are discussed.

Graph states known as the resource states for measurement-based quantum computation play an important role in quantum information processing. Verifying the correctness of graph states in an efficient way is crucial to scalable quantum computing. In this paper, we propose an efficient method for verifying two-colorable graph states with Pauli measurements, where the number of required measurement settings is a constant that is independent of the size of graph states. In addition, the number of required tests is less than the existing protocols. We also present several examples, such as the brickwork state and the two-dimensional square lattice state, to show how to get the specific verification strategy. Furthermore, we propose a robust verification of graph states for resisting the noise acting on the measurement device. Finally, based on the verification of two-colorable graph states in the adversarial scenario, we propose a verifiable blind quantum computing protocol, which can realize the less resource overhead.

We delve into the use of photonic quantum computing to simulate quantum mechanics and extend its application towards quantum field theory. We develop and prove a method that leverages this form of continuous-variable quantum computing (CVQC) to reproduce the time evolution of quantum-mechanical states under arbitrary Hamiltonians, and we demonstrate the method's remarkable efficacy with various potentials. Our method centers on constructing an evolver state, a specially prepared quantum state that induces the desired time evolution on the target state. This is achieved by introducing a non-Gaussian operation using a measurement-based quantum computing approach, enhanced by machine learning. Furthermore, we propose a framework in which these methods can be extended to encode field theories in CVQC without discretizing the field values, thus preserving the continuous nature of the fields. This opens new avenues for quantum computing applications in quantum field theory.

Fluorescence spectra of the $8{}^{2}P_J\to 6{}^{2}P_{J^{\prime }}$ $(J \text{and} J^{\prime }=3/2, 1/2)$ electric quadrupole transition of cesium atoms have been observed with a heated cesium vapor cell. We determined the ratio of the transition probabilities of $8{}^{2}P_J\to 6{}^{2}P_{J^{\prime }}$ to $8{}^{2}P_J\to 5{}^{2}D_{3/2}$ by comparing their respective photon emission rates. The results are in good agreement with our theoretical calculations. These measurements provide crucial parameters for tests of the coherent amplification method and improve knowledge of cesium properties which are essential to dark matter detection through atomic transitions.

The demand for efficient preparation methods for dual-species ion crystals is rapidly expanding across quantum technology and fundamental physics applications with trapped ions. We present a deterministic and efficient technique to produce such crystals, utilizing the segmented structure of a linear Paul trap. By precisely tailoring the trapping potentials, we can split, move, and discard parts of an ion chain. This process is automated in a sequence that converts a larger ion sample into the desired configuration. A critical component of our approach is the accurate identification of crystal constituents. This is achieved by matching the measured positions of fluorescing ions against theoretical expectations for larger crystals, thus facilitating the detection of nonfluorescing ions and enabling accurate ion counting. We demonstrate that our method reliably produces two-ion crystals within minutes. These results represent a significant advance in the production of two-species ion crystals with applications ranging from quantum logic spectroscopy and optical clocks to quantum computing and simulations with trapped ions.

We introduce a protocol that gives access to out-of-time-ordered correlation functions in many-body quantum systems. Unlike other such protocols, our proposal, which can be applied to arbitrary initial states, neither requires ancilla degrees of freedom to the quantum system of interest, nor has the need for randomized measurements. Nontrivial experimental capabilities required to implement the protocol are single-site measurements, single-site rotations, and backwards time evolution. To exemplify the implementation of the protocol, we put forward a strategy for Hamiltonian sign inversion $H\to -H$ in arrays of Rydberg-dressed atoms. In this way, a complete and practical toolbox is obtained for the measurement of out-of-time-ordered correlations in equilibrium and nonequilibrium situations.

Topological rainbow concentrators (TRCs) have attracted intense attention due to their capability of capturing photons with varying frequencies to distinct positions. However, the design of TRCs is not straightforward, especially for wideband and dual-polarization ones. In this paper, we propose a general and efficient methodology to realize TRCs by employing the glide-reflection symmetry operation. Starting from an ordinary photonic crystal with complete band gaps, and employing the two degrees of freedom of glide-reflection symmetry, i.e., the position of mirror reflection and the gliding length, wideband and dual-polarization TRCs are constructed successfully. Very interestingly, by monitoring the distance between the trapping positions of the two polarizations, the incident light frequency can be derived precisely. This paper introduces an efficient and flexible approach to configure TRCs, and may contribute to the wide applications in spectral analysis and polarization demultiplexing.

An approach is presented for the inversion of simulated and experimental in-plane, copolarized light scattering data in $p$ and $s$ polarization to obtain the normalized surface-height autocorrelation function and the root-mean-square roughness of a two-dimensional randomly rough metal surface. The approach is based on an expression, obtained on the basis of second-order phase perturbation theory, for the contribution to the mean differential reflection coefficient from the light scattered diffusely by the rough surface. The inversion scheme is validated by using several sets of computer-generated scattering data for rough silver surfaces defined by Gaussian surface-height correlation functions. The reconstructions obtained by this approach are found to be rather accurate for weakly rough surfaces illuminated by $p$- and $s$-polarized incident light; this is also true in cases where the contributions to the input data from multiple scattering of surface plasmon polaritons is not insignificant. Finally, the inversion scheme is applied to experimental scattering data obtained for characterized two-dimensional randomly rough gold surfaces, and the results obtained in this way compare favorably to what is obtained by directly analyzing the surface morphology. Such results testify to the attractiveness of the computationally efficient inversion scheme that we propose.

For partially coherent optical fields in which a single binary degree of freedom (DOF) is relevant, such as polarization, entropy uniquely identifies the class of optical fields that can be converted into each other via unitary transformations. However, when multiple DOFs are taken into consideration, entropy no longer serves this purpose. We investigate the structure of the family of isoentropy partially coherent optical fields defined by two binary DOFs (polarization and two spatial modes) and described by a $4\times 4$ coherence matrix $\mathbf{G}$. We find that the rank of $\mathbf{G}$ (the number of its nonzero eigenvalues) plays a critical role in this context: whereby any pair of isoentropy rank-2 fields can be converted into each other unitarily, this is not necessarily the case for a pair of rank-3 or rank-4 fields. Furthermore, unitary transformations between isoentropy fields of different ranks are strictly forbidden. Instead, such conversions require entropy-maintaining nonunitary transformations that potentially combine filtering projections and randomizing operations. We experimentally synthesize partially coherent isoentropy optical fields of all ranks and tomographically reconstruct their coherence matrices. Moreover, we steer the coherence matrix over isoentropy trajectories that maintain a fixed rank (intrarank conversion) or that involve changes in the rank (interrank conversion). These findings offer a different perspective for the potential utility of partially coherent light in optical communications and sensing.

We developed a reconstruction method for the density matrix and Wigner function of electron beams through analysis of the Airy pattern intensity profile. The density matrix in a transmission electron microscope object plane was calculated using the coherence function and the electron wave amplitude and phase distributions. The Wigner function was then reconstructed using the matrix elements. Based on the Wigner function at the origin of the phase space, we derived a formula to calculate the axial brightness and then determined the axial brightness of a Schottky field emission gun, which reflects the emitter performance more accurately and precisely than the conventional mean brightness measurements.

We investigate the generation of optical shock waves in strongly interacting Rydberg atomic gases with a spatially homogeneous dissipative potential. The Rydberg atom interaction induces an optical nonlocal nonlinearity. We focus on local nonlinear $(R_b\ll R_0)$ and nonlocal nonlinear $(R_b\sim R_0)$ regimes, where $R_b$ and $R_0$ are the characteristic length of the Rydberg nonlinearity and beam width, respectively. In the local regime, we show spatial width and contrast of the shock wave change monotonically when increasing strength of the dissipative potential and optical intensity. In the nonlocal regime, the characteristic quantity of the shock wave depends on $R_b/R_0$ and dissipative potential nontrivially and on the intensity monotonically. We find that formation of shock waves dominantly takes place when $R_b$ is smaller than $R_0$, while the propagation dynamics is largely linear when $R_b$ is comparable to or larger than $R_0$. Our results reveal nontrivial roles played by dissipation and nonlocality in the generation of shock waves, and provide a route to manipulate their profiles and stability. Our study furthermore opens new avenues to explore non-Hermitian physics and nonlinear wave generation and propagation by controlling dissipation and nonlocality in the Rydberg media.