Boson sampling is one of the leading protocols for demonstrating a quantum advantage, but the theory of how this protocol responds to noise is still incomplete. We extend the theory of classical simulation of boson sampling with partial distinguishability to the case where the degree of indistinguishability between photon pairs is different between different pairs.

Graph states are a class of multipartite entangled quantum states that are ubiquitous in quantum information. We study equivalence relations between graph states under local unitaries (LU) to obtain distinguishing methods both in local and in networked settings. Based on the marginal structure of graph states, we introduce a family of easy-to-compute LU invariants. We show that these invariants uniquely identify the entanglement classes of every graph state up to eight qubits and discuss their reliability for larger numbers of qubits. To handle larger graphs, we generalize tools to test for local Clifford (LC) equivalence of graph states that work by condensing large graphs into smaller graphs. In turn, we show that statements on the equivalence of these smaller graphs (which are easier to compute) can be used to infer statements on the equivalence of the original, larger graphs. We analyze LU equivalence in two key settings, with and without allowing for the permutation of qubits. We identify entanglement classes, whose marginal structure does not allow us to distinguish them. As a result, we increase the bound on the number of qubits where the LU-LC conjecture holds from 8 to 10 qubits in the setting where qubit permutations are allowed.

We investigate the generalization of symmetric quantum joint measurements on multiple qubits. We first describe a method for constructing a symmetric joint measurement basis for three qubits by utilizing single-qubit states corresponding to the four vertices of an isohedral tetrahedron on the Bloch sphere. We then demonstrate the expected tetrahedral symmetry of the current measurement basis and discuss its application in a trilocal star-shaped network. This architecture enables us to generalize the two-qubit symmetric joint measurement to an $n$-qubit version, preserving the tetrahedral or hexahedral symmetry.

Creating robust quantum operations is a major challenge in the current noisy intermediate-scale quantum computing era. Recently, the importance of noise-resilient control methods has become more pronounced in the field. Ordinarily, noisy quantum systems are described by the Lindblad equation. However, minimizing noise susceptibility using this equation has proven challenging because of its irreversibility. In this study, we propose a method for creating robust pulses based on the stochastic Schrödinger equation. This equation describes individual noise realizations under any colored noise process, contrary to the Lindblad equation, which describes mean system behavior under white noise. Using stochastic optimal control techniques, our method, fidelity-enhanced variational quantum optimal control, is able to construct higher-fidelity paths than its nonstochastic counterpart. By accounting for both environmental noise sources as well as noise sources inherent to the control system, highly significant increases in fidelity are noted for both single- and multiple-qubit state preparations.

We propose a laser-based method for the preparation of high-energy polarized electrons, from the ionization of isolated spin polarized hydrogen (SPH) atoms. The SPH atoms are prepared from the photodissociation of hydro-halide molecules, using two consecutive UV pulses of ps duration. By appropriately timing and focusing the pulses, we can spatially separate the highly polarized SPH from other unwanted photoproducts, which then act as the target for the acceleration lasers. We show how elastic collisions define number density $n$ and polarization P regimes for the prepolarized targets, and use particle-in-cell simulations to demonstrate the method's feasibility.

Trapped Rydberg atoms are highly promising candidates for quantum science experiments. While several approaches have been put forward to exert (trapping) forces on isolated Rydberg atoms, a widely applicable lossless technique is lacking. This paper proposes a robust versatile alternative technique that avoids lifetime compromising losses. Our proposal leverages the vector polarizability, which is induced by nonlinearly polarized light and is shown to be several orders of magnitude larger than the usual scalar and tensor polarizabilities for commonly used alkali Rydberg series such as the $nS$, $nP$, and $nD$ series with principal quantum number $n$ as low as 30. The resulting force can be used to trap isolated Rydberg atoms over long times, which constitutes a key advance that is expected to impact quantum simulation applications, as well as to generate large light–Rydberg-atom hybrid states, which possess nontrivial position-dependent forces.

In ultracold atomic gases, radio-frequency coupling between two spin states can lead to atoms being in a stable coherent superposition of the two states (dressed states). When the two-body interactions (scattering lengths) are different between the two states, it offers the possibility not only to control the interatomic interaction but also to modify the equation of states with emerging three-body terms with different signs and scalings with the parameters. We derive these terms in a unified perturbation framework and show that they correspond to genuine three-body processes found at different orders in perturbation theory. Our work justifies the introduction of contact three-body interactions in dressed-state systems when the spin degree of freedom is integrated out.

Quantum nondemolition measurements define a noninvasive protocol to extract information from a monitored quantum system. This protocol exploits an additional quantum system that is sequentially coupled to the system and eventually measured to reconstruct temporal correlations developed by the system. In reference to a given measurement observable, the protocol's outcomes are well described by a quasiprobability distribution, which can be negative. In this paper, we prove that the presence of negative regions in this distribution defined at three times is a necessary and sufficient condition for the violation of macrorealism. This approach falls within the category of sequential noninvasive measurability and automatically satisfies all no-signaling-in-time conditions at multiple times by design. We show that reconstructing the negativity is a much stronger condition than the violation of the Leggett-Garg inequalities, commonly used for the same task, as they may be satisfied even if the macrorealism is violated. Quantum nondemolition measurements provide a clear and operational tool to identify violations of macrorealism directly. Thus, our result could also be applied to the study of the quantum-to-classical transition and in all those fields where quantumness certification is desirable.

We investigate the behavior of light-wave packets injected into non-Hermitian microcavity lattices under highly dissipative conditions. While all eigenstates of the lattice exhibit exponential decay, a specifically excited state maintains coherent propagation. In a one-dimensional lattice, this state undergoes a spatial displacement shift away from the injection position, which is a fundamental property of non-Hermitian systems with a point gap when the spectrum encircles a finite region in the complex plane. Extending such a shift to two-dimensional lattices reveals a geometrically anomalous $\mathsf{V}$-shaped wave-packet formation with orientation-tunable arms. Notably, this geometric control mechanism enables all-optical steering of non-Hermitian photonic states without requiring structural modifications.

Bell inequalities are an important tool for studying nonlocality, however, they quickly become computationally intractable as the system size grows. We consider a method for finding an upper bound for the quantum violation of such inequalities by combining the Navascués-Pironio-Acín (NPA) hierarchy, the method of alternating projections, and a standard memory-efficient optimization algorithm. While our method may not give the tightest upper bound possible, it often does so several orders of magnitude faster than state-of-the-art solvers, with minimal memory usage, thus allowing solutions to problems that would otherwise be intractable. We benchmark using the well-studied $I_{3322}$ inequality as well as a more general large-scale randomized inequality $R_{xx22}$. For randomized inequalities with 130 inputs either side (a first-level moment matrix of size $261\times 261$), our method is $\sim 100\times$ faster than both mosek and SCS while giving a bound only $\sim 2\%$ above the optimum.

Quantum entanglement and quantum nonstabilizerness are fundamental resources that characterize distinct aspects of a quantum state: Entanglement reflects nonlocal correlations, while nonstabilizerness quantifies the deviation from stabilizer states. A quantum state becomes a valuable resource for applications like universal quantum computation only when both quantities are present. Here, we propose that quantum nonlocal nonstabilizerness (NN) serves as an effective measure of this combined resource, incorporating both entanglement and nonstabilizerness. We demonstrate that NN can be precisely computed for two-qubit pure states, establishing a direct relationship with the entanglement spectrum. We then extend the definition of NN to mixed states and explore its presence in many-body quantum systems. We reveal that the two-point NN decays according to a power law in critical states, indicating that it can serve as an effective quantum information probe for phase transitions. Furthermore, we explore measurement-induced NN and uncover an intriguing phenomenon termed “nonstabilizerness swapping,” wherein post-measurement NN decays more slowly than any pre-measurement correlations. The occurrence of this phenomenon provides a sufficient condition for both entanglement swapping and the presence of a nontrivial sign structure. Our results thus represent a pivotal step toward accurately quantifying the “quantumness” of a state and bring insights from multiple perspectives to the property of NN.

Reduced density matrices (RDMs) are fundamental in quantum information processing, allowing the computation of local observables, such as energy and correlation functions, without the exponential complexity of fully characterizing quantum states. In the context of near-term quantum computing, RDMs provide sufficient information to effectively design variational quantum algorithms. However, their experimental estimation is challenging, as it involves preparing and measuring quantum states in multiple bases, a resource-intensive process susceptible to producing nonphysical RDMs due to shot noise from limited measurements. To address this, we propose a method to mitigate shot noise by reenforcing certain physicality constraints on RDMs. While verifying RDM compatibility with a global state is quantum Merlin-Arthur complete, we relax this condition by enforcing compatibility constraints up to a certain level using a polynomial-size semidefinite program to reconstruct overlapping RDMs from simulated data. Our approach yields, on average, tighter bounds for the same number of measurements compared to tomography without compatibility constraints. We demonstrate the versatility and efficacy of our method by integrating it into an algorithmic cooling procedure to prepare low-energy states of local Hamiltonians. Simulations on frustrated Hamiltonians reveal notable improvements in accuracy and resource efficiency, highlighting the potential of our approach for practical applications in near-term quantum computing.

Determining the quantum capacity of a noisy quantum channel is an important problem in the field of quantum communication theory. In this work, we consider the Gaussian random-displacement channel ${\mathcal{N}}_{\sigma }$, a type of bosonic Gaussian channels relevant in various bosonic quantum information processing systems. In particular, we attempt to make progress on the problem of determining the quantum capacity of a Gaussian random-displacement channel by analyzing the error-correction performance of several families of multimode Gottesman-Kitaev-Preskill (GKP) codes. In doing so we analyze the surface-square GKP codes using an efficient and exact maximum-likelihood decoder (MLD) up to a large code distance of $d=39$. We find that the error threshold of the surface-square GKP code is remarkably close to $\sigma =1/\sqrt{e}\simeq 0.6065$ at which the best-known lower bound of the quantum capacity of ${\mathcal{N}}_{\sigma }$ vanishes. We also analyze the performance of color-hexagonal GKP codes up to a code distance of $d=13$ using a tensor-network decoder serving as an approximate MLD. By focusing on multimode GKP codes that encode just one logical qubit over multiple bosonic modes, we show that GKP codes can achieve nonzero quantum state transmission rates for a Gaussian random-displacement channel ${\mathcal{N}}_{\sigma }$ at larger values of $\sigma$ than previously demonstrated. Thus, our work reduces the gap between the quantum communication theoretic bounds and the performance of explicit bosonic quantum error-correcting codes in regards to the quantum capacity of a Gaussian random-displacement channel.

Network quantum steering plays a pivotal role in quantum information science, enabling robust certification of quantum correlations in scenarios with asymmetric trust assumptions among network parties. The intricate nature of quantum networks, however, poses significant challenges for the detection and quantification of steering. In this work, we develop a neural network-based method for measuring network quantum steerability, which can be generalized to arbitrary quantum networks and naturally applied to standard steering scenarios. Our method provides an effective framework for steerability analysis, demonstrating remarkable accuracy and efficiency in standard bipartite and multipartite steering scenarios. Numerical simulations involving isotropic states and noisy Greenberger-Horne-Zeilinger states yield results that are consistent with established findings in these respective scenarios. Furthermore, we demonstrate its utility in the bilocal network steering scenario, where an untrusted central party shares two-qubit isotropic states of different visibilities, $\nu$ and $\omega$, with trusted end-point parties and performs a single Bell state measurement. Through explicit construction of a network local hidden state model derived from numerical results and incorporation of the entanglement properties of network assemblages, we analytically demonstrate that the network steering thresholds are determined by the curve $\nu \omega =1/3$ under the corresponding configuration.

Graph states are a fundamental entanglement resource for multipartite quantum applications which are in general challenging to transform efficiently. While fusion operations for merging entangled states are well developed, no direct protocol exists for the reverse process, which we term “fission.” We introduce a simple, yet powerful, protocol that achieves this, allowing a qubit to split while preserving selective connections with minimum entanglement overhead. This tool offers flexible entanglement management with potential applications in secure communication, error correction, and adaptive entanglement distribution.

We have tested the reachability of the plane-wave impulse approximation (PWIA) in terms of the ratio of spectroscopic factors for $(e,$ $2e)$ ionization of the Ne $2s^{-1}$ and $2p^{-1}$ main ionization transitions at large momentum transfer. Experimentally, the ratio was measured, as a function of the ion recoil momentum, in the symmetric noncoplanar geometry and at incident electron energies of $E_0=1.2$, 2.0, 3.0, and 4.0 keV. The accuracy of the PWIA was examined by comparing with a spectroscopic factor ratio predicted by a theoretical electronic structure study as well as those by the distorted-wave Born approximation (DWBA) model and the two Coulomb waves with variable charge $Z\left(r\right)$ (2CWZ) model in which the exchange interaction is not included but the two outgoing electrons are both described by a Coulomb wave with $Z\left(r\right)$. The DWBA result has been found to be in good agreement with the experiment and the theoretical prediction regarding the spectroscopic factor ratio over the covered $E_0$ range. On the other hand, it has been found that the PWIA and 2CWZ results both reveal almost the same, but an unacceptable, deviation from the theoretical prediction. Furthermore, the deviation has been found to become smaller with the increase in $E_0$. These observations clearly not only indicate the need of improvement of the PWIA model, but also state that the spectroscopic factor ratio is suited for stringently testing a theoretical scattering model for the $(e,$ $2e)$ reaction at large momentum transfer.

In tunneling ionization of an atom induced by a strong laser field, the system initially located in its ground state will be excited. The time-dependent wave function of the excited system includes bound- and continuum-eigenstate components of the field-free Hamiltonian. Here, we study the relation between the average kinetic energy and potential energy of the bound wave packet composed of coherent superposition of higher bound eigenstates. Our simulations show that around the peak time of the laser field at which the tunneling effect is strong, the bound wave packet still approximately agrees with the virial theorem, which reflects the Coulomb-related spatial symmetry of the system. Our results indicate that during the laser-induced exciting process, the system tends to keep its symmetry before ionization. This observation also holds for small molecules. It gives suggestions on analytically constructing Coulomb-included strong-field models.

The description of strongly correlated quantum many-body systems far from equilibrium presents a fundamental challenge due to the vast amount of information it requires. We introduce a generalization of the Jastrow ansatz for time-dependent wave functions that offers an efficient and exact description of the time evolution of various strongly correlated systems. Previously known exact solutions are characterized by scale invariance, enforcing self-similar evolution of local correlations, such as the spatial density. However, we demonstrate that a complex-valued time-dependent Jastrow ansatz (TDJA) is not restricted to scale invariance and can describe a broader class of dynamical processes lacking this symmetry. The associated time evolution is equivalent to the implementation of a shortcut to adiabaticity (STA) via counterdiabatic driving along a continuous manifold of quantum states described by a real-valued TDJA, providing a framework for engineering exact STA in strongly correlated many-body quantum systems. We illustrate our findings in systems with inverse-square interactions, such as the Calogero-Sutherland and hyperbolic models, supplemented with pairwise logarithmic interactions, as well as in the long-range Lieb-Liniger model, where bosons experience both contact and Coulomb interactions in one dimension. Our results enable the study of quench dynamics in all these models and serve as a benchmark for numerical and quantum simulations of nonequilibrium strongly correlated systems with continuous variables.

We explore a system composed of scalar bosons and $\mathrm{SU}\left(3\right)$ fermions in one dimension. Considering only local intra- and interspecies interactions, the system is described by the Bose-Fermi-Hubbard Hamiltonian, which is studied using the density-matrix renormalization-group method. In addition to the common gapless and mixed-Mott-insulator phases, we identify unknown gapped states, in which bosons couple with some or all flavors of fermions. We unveil different flavor-selective states characterized by itinerant fermions that coexist with an insulator state in which the bosons connect with only one- or two-flavor fermions. The states reported here could be implemented in cold-atom experiments.

SO(2,1) dynamical symmetry makes a remarkable prediction that the breathing oscillation of a scale-invariant quantum gas in an isotropic harmonic trap is isentropic and can persist indefinitely. In two dimensions, this symmetry is broken due to quantum anomaly in the strongly interacting range, and consequently the lifetime of the breathing mode becomes finite. The persistent breather in a strongly interacting system has so far not been realized. Here, we experimentally achieve the long-lived breathing mode in a three-dimensional unitary Fermi gas, which is protected by the SO(2,1) symmetry. The nearly perfect SO(2,1) symmetry is realized by loading the ultracold Fermi gas in an isotropic trap and tuning the interatomic interaction to resonance. The breathing mode oscillates at twice the trapping frequency even for large excitation amplitudes. The ratio of damping rate to oscillation frequency is as small as 0.002, providing an interacting persistent breather. The oscillation frequency and damping rate are nearly constant for different atomic densities and temperatures, demonstrating the robustness of the SO(2,1) symmetry in three dimensions. The factors that lead to the residual damping have also been clarified. This work opens the way to study many-body nonequilibrium dynamics related to the dynamical symmetry.