We develop a discrete truncated Wigner method to analyze the real-time evolution of dissipative $\mathrm{SU}\left(\mathcal{N}\right)$ spin systems coupled with a Markovian environment. This semiclassical approach is not only numerically efficient but also particularly capable of accurately capturing local loss processes due to its local linearity in the dynamical equations. We apply the method to a state-of-the-art experiment involving an analog quantum simulator of a three-dimensional dissipative Bose-Hubbard model in a strongly interacting regime. Our numerical results show good agreement with experimental data, specifically capturing the continuous quantum Zeno effect in the dynamics subjected to a gradual change of the ratio between the hopping amplitude and the onsite interaction across the superfluid Mott insulator crossover. Furthermore, we present comparative analyses with the continuous truncated Wigner method, derived as an effective Fokker-Planck equation for $\mathrm{SU}\left(\mathcal{N}\right)$ classical spin variables, showing that the discrete method outperforms the continuous one in simulating the long-time dynamics of SU(2) and SU(3) spin models. The discrete phase-space framework offers a versatile and powerful tool for exploring a wide range of open quantum many-body systems in dimensions higher than one dimension, where numerically exact methods are impractical due to the exponential growth of the Hilbert-space dimension.

Vortex mass, which is the inertia of a quantum vortex, has never been observed in superfluids and is a long-standing problem in low-temperature physics. The impact of the mass is considered negligible in typical experiments with superfluid ${}^4\mathrm{He}$. Recent developments of experimental techniques for manipulating quantum vortices in superfluid atomic gases have enabled us to test this problem more accurately. By introducing the vortex mass time and length as universal scales to many-body problems of massive quantum vortices, the theoretical description is formulated in the simplest manner and is universally applicable to different quantum fluids, including fermionic and multicomponent superfluids. There are two branches, the cyclotron and massless branches, for the circular motion of a pair of like-sign vortices. Finding a stable cyclotron branch for the motion of vortices is a clear evidence of vortex mass and superfluid ${}^3\mathrm{He}\text{−}\mathrm{B}$ is the specific example of a system where this phenomena could be observed. The impact of the mass on the massless branch is small but can be enhanced by taking the difference in the two-body dynamics of point vortices with different initial conditions. Our results imply that the vortex mass is a direct cause of the splitting instability of a doubly quantized vortex at absolute zero and that the vortex mass length characterizes the final state after the instability. It is also demonstrated that a pair of massive vortices with opposite circulations has a critical distance characterized by the vortex mass length, below which they are spontaneously annihilated without thermal fluctuations.

Optical spin and chirality play key roles in engineering photonic emission and light-matter interactions. Here we show that three-dimensional evanescent coupling of guided modes by strongly confined waveguides can extrinsically produce optical spin and chirality. These emerging optical quantities are essentially derived from an intrinsic phase retardation of π/2 between two coupled modes. For a directional coupler with 2 × 2 input ports, the produced optical spin and chirality are inherently characterized by the side-locked and path-locked phenomena, respectively. We numerically demonstrate that these optical spins and chiralities can be ubiquitously produced by higher-order mode coupling and other structural waveguides. These new quantities in optical waveguides may pave the way to exploit near-field photonic spin and on-chip chiral light-matter interactions.

Theoretical tools used in processing continuous measurement records from real experiments to obtain quantum trajectories can easily lead to numerical errors due to a noninfinitesimal time resolution. In this work we propose a systematic assessment of the accuracy of a map. We perform error analyses for diffusive quantum trajectories, based on single-time-step Kraus operators proposed in the literature, and find the orders in time increments $\mathrm{\Delta }t$, to which such operators satisfy the conditions for valid average quantum evolution (completely positive, convex linear, and trace preserving), and the orders to which they match the Lindblad solutions. Given these error analyses, we propose a Kraus operator that satisfies the valid average quantum evolution conditions and agrees with the Lindblad master equation, to second order in $\mathrm{\Delta }t$, thus surpassing all other existing approaches. In order to test how well our proposed operator reproduces exact quantum trajectories, we analyze two examples of qubit measurement, where exact maps can be derived: a qubit subjected to a dispersive ($z$-basis) measurement and a fluorescence (dissipative) measurement. We show analytically that our proposed operator gives the smallest average trace distance to the exact quantum trajectories compared to existing approaches.

We investigate the fate of dissipative phase transitions in quantum many-body systems when the individual constituents are qudits ($d$-level systems) instead of qubits. As an example system, we employ a permutation-invariant $XY$ model of $N$ infinite-range interacting $d$-level spins undergoing individual and collective dissipation. In the mean-field limit, we identify a dissipative phase transition, whose critical point is independent of $d$ after a suitable rescaling of parameters. When the decay rates between all adjacent levels are identical and $d\ge 4$, the critical point expands, in terms of the ratio between dissipation and interaction strengths, to a critical region in which two phases coexist and which increases as $d$ grows. In addition, a larger $d$ leads to a more pronounced change in spin expectation values at the critical point. Numerical investigations for finite $N$ reveal symmetry-breaking signatures in the Liouvillian spectrum at the phase transition. The phase transition is furthermore marked by maximum entanglement negativity and a significant purity change of the steady state, which become more pronounced as $d$ increases. Considering qudits instead of qubits thus opens new perspectives on accessing rich phase diagrams in open many-body systems.

A quantum advantage can be achieved in the unitary charging of quantum batteries if their cells are interacting. Here, we try to clarify with some analytical calculations whether and how this quantum advantage is achieved for sparse Sachdev-Ye-Kitaev (SYK) interactions and in general for fermionic interactions with disorder. To do this, we perform a simple modelization of the interactions. In particular, we find that for $q$-point rescaled sparse SYK interactions, the quantum advantage is $\mathrm{\Gamma }\sim N^{\frac{\alpha -q}{2}+1}$ for $q\ge \alpha \ge q/2$ and $\mathrm{\Gamma }\sim N^{1-\frac{\alpha }{2}}$ for $q/2>\alpha \ge 0$, where $\alpha$ is related to the connectivity and $N$ is the number of cells. This shows how we can get $\mathrm{\Gamma }\sim N$, i.e., an average power that scales as $N^2$ thanks to the disorder.

In quantum theory, sets of operations that cannot be performed simultaneously exist. These sets of operations are referred to as incompatible. While this definition of incompatibility extends to general probabilistic theories (GPTs), the dependence of the set of compatible sets on the definition of composite systems has not been thoroughly investigated. For quantum channels, compatibility is defined using the tensor product of Hilbert spaces based on the conventional composite system. However, in GPTs, composite systems are not uniquely defined, and the set of states can vary from the minimal tensor to the maximal tensor. In this paper, in addition to the usual quantum compatibility, we introduce min-tensor compatibility using the minimal tensor in the composite system of effect spaces and investigate their relationship employing noisy identity channels on qubits. As a result, we find that the set of min-tensor-compatible channel pairs is strictly broader than the set of quantum-compatible channel pairs. Furthermore, we introduce the concept of almost-quantum-compatible pairs of channels from an operational perspective. This concept corresponds to cases where the correlation functions in the verification of compatibility can be realized through a channel and local reinterpretation of effects. We demonstrate that the set of all almost-quantum-compatible channel pairs is strictly narrower than the set of all min-tensor-compatible channel pairs.

Trilobite molecules are ultralong-range Rydberg molecules formed when a high angular momentum Rydberg electron scatters off of a ground-state atom. Their unique electronic structure and highly oscillatory potential energy curves support a rich variety of dynamical effects yet to be explored. We analyze the vibrational motion of these molecules using a framework of adiabatic wave-packet propagation dynamics and observe that for appropriate initial states, the trilobite potential acts as molecular diffraction grating. The quantum dynamic effects observed are explained using a Fourier analysis of the scattering potential and the associated scattered wave packet. Furthermore, vibrational ground states of the low angular momentum ultralong-range Rydberg molecules are found to be particularly suitable to prepare the relevant wave packets. Hence, we propose a time-resolved pump-probe scheme designed for the realization of the effect in question, and advertise the utilization of a single diatomic Rydberg molecule as a test bed for the study of exaggerated quantum dynamical phenomena.

Coherent control of the fragmentation of van der Waals–bound rare-gas dimers is crucial for gaining insights into the properties of weakly bound complexes. In this study, we experimentally investigate the directional dissociative single ionization of argon dimers, employing relative phase-controlled linearly and elliptically polarized two-color femtosecond laser fields. The emission direction of the ionic fragments is manipulated by adjusting the relative phase of the two-color laser fields. We attribute the observed asymmetric pattern to the interference between different dissociation pathways, corresponding to the nuclear wave packet propagating on the gerade and ungerade states, respectively. The relative phase-dependent asymmetries observed in linearly and elliptically polarized two-color laser fields exhibit similarities, suggesting a limited role of electron recollision. Our results highlight the role of nuclear wave-packet interference in the directional fragmentation of rare-gas dimers, and they promote the possibility of coherent control of van der Waals bond cleavage in chemical reactions.

Photon blockade enhancement is an exciting and promising subject that has been well studied for photons in cavities. However, whether photon blockade can be enhanced in the output fields remains largely unexplored. We show that photon blockade can be greatly enhanced in the mixing output field of a nonlinear cavity and an auxiliary (linear) cavity, where no direct coupling between the nonlinear and auxiliary cavities is needed. We uncover a biquadratic scaling relation between the second-order correlation of the photons in the output field and intracavity nonlinear interaction strength, in contrast to a quadratic scaling relation for the photons in a nonlinear cavity. We identify that this scaling enhancement of photon blockade in the output field is induced by the destructive interference between two of the paths for two photons passing through the two cavities. We then extend the theory to the experimentally feasible Jaynes-Cummings model consisting of a two-level system strongly coupled to one of the two uncoupled cavities, and also predict a biquadratic scaling law in the mixing output field. Our proposed scheme is general and can be extended to enhance blockade in other bosonic systems.

Resonant frequency modulation spectroscopy has been previously used as a highly sensitive method for measuring the output of cold atom interferometers. Using a detailed model that accounts for optical saturation, laser intensities and atomic densities that vary spatially, and radiation pressure on the atoms, we theoretically investigate under what parameter regimes the optimum signal-to-noise ratio is found under experimentally realistic conditions. We compare this technique to the standard method of fluorescence imaging and find that it outperforms fluorescence imaging for compact interferometers using condensed atomic sources or where the photon collection efficiency is limited. However, we find that fluorescence imaging is likely to be the preferred method when using squeezed atomic sources due to limited atom number.

Perceptrons, which perform binary classification, are the fundamental building blocks of neural networks. Given a data set of size $N$ and margin $\gamma$ (how well the given data are separated), the query complexity of the best-known quantum training algorithm scales as either $(\sqrt{N}/{\gamma }^2)log(1/{\gamma }^2)$ or $N/\sqrt{\gamma }$, which is achieved by a hybrid of classical and quantum search. In this paper, we improve the version space quantum training method for perceptrons such that the query complexity of our algorithm scales as $\sqrt{N/\gamma }$. This is achieved by constructing an oracle for the perceptrons using quantum counting of the number of data elements that are correctly classified. Once such an oracle is constructed, bounded-error quantum search can be used to search over the hyperplane instances. The optimality of our algorithm is proven by reducing the evaluation of a two-level and-or tree (for which the query complexity lower bound is known) to a multicriterion search. Our quantum training algorithm can be generalized to train more complex machine learning models such as neural networks, which are built on a large number of perceptrons.

Single-shot quantum error correction has the potential to speed up quantum computations by removing the need for multiple rounds of syndrome extraction to be fault-tolerant. By using Quantinuum's H2 trapped-ion quantum computer, we implement the [[33,1,4]] four-dimensional (4D) surface code and perform an experimental demonstration of single-shot quantum error correction with bare ancilla qubits. We conduct memory experiments comparing the two-dimensional (2D) and 4D surface codes and find that, despite differences in qubit use and syndrome extraction circuit depth, the 4D surface code matches or outperforms the 2D surface code in both the fault-tolerant and single-shot regimes.

We study a hardware-efficient ansatz tailored for a trapped-ion quantum simulator (HEA-TI), utilizing single-qubit rotations and global spin-spin interactions to reduce the reliance on resource-intensive multiqubit gates typical of conventional gate-based approaches. This ansatz also allows streamlined experimental control sequences with continuous global interaction and insertions of single-qubit gates. We examine the scaling behavior of quantum resources required by HEA-TI. We also apply it to the ground-state calculation of chemical molecules and examine its performance under realistic experimental noise and statistical fluctuations. Our results demonstrate that HEA-TI achieves competitive efficiency with other commonly used variational ansatzes, such as UCCSD, while offering significant implementation advantages for trapped-ion platforms.

Quantum chemistry is a promising application in the era of quantum computing since the unique effects of quantum mechanics that take exponential growing resources to simulate classically are controllable on quantum computers. Fermionic degrees of freedom can be encoded efficiently onto qubits and allow for algorithms such as the quantum equation-of-motion method to find the entire energy spectrum of a quantum system. In this paper, we propose the reduced quantum equation-of-motion method by reducing the dimensionality of its generalized eigenvalue equation, which results in half the measurements required compared to the quantum equation-of-motion method, leading to speed up the algorithm and less noise accumulation on real devices. In particular, we analyze the performance of our method on two noise models and calculate the excitation energies of a bulk silicon and gallium arsenide using our method on an IBM quantum processor. Our method is fully robust to the uniform depolarizing error, and we demonstrate that the selection of suitable atomic orbital complexity could increase the robustness of our algorithm under real noise. We also find that taking the average of multiple experiments tends toward the correct energies due to the fluctuations around the exact values. Such noise resilience of our approach could be used on current quantum devices to solve quantum chemistry problems.

The quantum gate and quantum memory are building blocks for distributed quantum computation. It is a longstanding goal to implement high-fidelity quantum operations with quantum network modules to process and store quantum information. Here we propose a high-performance heralded squeezing gate with quantum network modules. By distributing one submode of the entangled state to a quantum network module, a heralded squeezing gate is realized, and the fidelity is enhanced by the heralding filter. Furthermore, the heralded squeezing gate can be realized between two network modules, where quantum operation can be manipulated by another quantum network module. Our scheme is scalable for quantum network modules and provides a direct reference for potential practical applications of distributed quantum computation.

We calculated QED corrections to the $E1$ transition amplitudes in neonlike iron and nickel. For the $2p\to 3d$ transitions the dominant effect came from the many-electron mixing or electronic correlations. For the $2p\to 3s$ transitions the correlation and one-electron effects were comparable and tended to compensate each other. Our ab initio calculations showed that vertex corrections were negligible for both types of transitions. Other QED corrections were accurately reproduced by including effective QEDMOD operator in the many-electron relativistic configuration interaction calculation.