Integrated photonic systems have emerged as a powerful platform for quantum simulation, enabling the study of topological phases and localization phenomena in highly controllable on-chip environments. While previous experiments have largely relied on classical light, the incorporation of quantum optical states promises access to physics beyond semiclassical approximations. In this work we numerically demonstrate the generation and propagation of nonclassical light within waveguide arrays designed to emulate key lattice models, including the Aubry-André, Anderson localization, and Su-Schrieffer-Heeger models. By exploiting third-order nonlinearity via spontaneous four-wave mixing, our simulations reveal both pump-induced single-photon localization and, more notably, the formation and evolution of correlated biphoton states within these topological structures. Our approach combines precise waveguide design simulation, supported by numerical solutions of the photonic Schrödinger equation, with quantum optical measurements to reveal how topological boundaries and disorder influence multiphoton quantum transport. This work establishes a scalable path toward exploring quantum correlations and many-body localization in synthetically engineered photonic matter.

The computational power of quantum phases of matter with symmetry can be accessed through local measurements, but what is the most efficient way of doing so? In this work we show that minimizing operational resources in measurement-based quantum computation (MBQC) on subsystem symmetric resource states amounts to solving a sub-Riemannian geodesic problem between the identity and the target logical unitary. This reveals a geometric structure underlying MBQC and offers a principled route to optimize quantum processing in computational phases.

Quantum entanglement is an important resource in quantum information processing. Estimation of quantum entanglement for general mixed states is a challenging problem. We introduce the concept of a “cooperative local unitary (co-LU) set” and provide specific constructions of such co-LU sets for bipartite systems. These co-LU sets give rise to analytical formulas of entanglement for classes of quantum mixed states under any entanglement measures. Detailed examples are given to illustrate the calculation of entanglement for a class of mixed states in terms of the informationally complete entanglement measure, concurrence and $q$-concurrence, showing the superiority of our analytical results to the ones from the lower bounds of these measures. Our approach simplifies the estimation on entanglement for classes of quantum states and provides a new perspective for understanding the entanglement.

We demonstrate a robust scheme for quantum information storage based on bound states in a two-dimensional coupled-cavity array. When a target cavity is tuned to resonance with the array, a bound state in the continuum (BIC) emerges, coexisting with two conventional bound states outside the band. The resulting dynamics reflects a delicate interplay between these bound states, which can be fully captured through exact analytical solutions. In the weak-coupling regime, the BIC dominates, enabling perfect and persistent information storage. At stronger coupling, all bound states contribute, leading to oscillatory behavior and reduced storage fidelity. These results, valid at both zero and finite reservoir temperatures and further supported by a single-particle framework, reveal distinctive non-Markovian features in continuous-variable systems and highlight the potential of photonic lattices for scalable all-optical decoherence-free quantum memory platforms.

A key technical requirement of any future quantum network is the ability to distribute quantum-entangled resources between two spatially separated points at a high rate and high fidelity. Entanglement distribution protocols based on satellite platforms, which transmit and receive quantum resources directly via free-space optical propagation, are therefore excellent candidates for quantum networking, since the geometry and loss characteristics of satellite networks feasibly allow for up to continental-scale ($\approx {10}^3\mathrm{km}$) over-the-horizon communication without the infrastructure, cost, or losses associated with equivalent fiber-optic networks. In this work, we explore two network topologies commonly associated with quantum networks—entanglement distribution between two satellites in low-Earth orbit mediated by a third satellite and entanglement distribution between two ground stations mediated by a satellite in low-Earth orbit—and two entanglement distribution schemes—one where the central satellite is used as a relay, and the other where the central satellite is used to generate and distribute the entangled resource directly. We compute a bound on the rate of distribution of distillable entanglement achieved by each protocol in each network topology as a function of the network channels for both single-rail discrete-variable (DV) and continuous-variable (CV) resources and use or nonuse of probabilistic quantum noiseless linear amplifiers (NLAs). In the case of atmospheric channels we take into account the turbulent and optical properties of the free-space propagation. We determine that for the triple-satellite network configuration, the optimal strategy is to perform a distributed NLA scheme in either CV or DV, and for the ground-satellite-ground network the optimal strategy is to distribute a DV resource via the central satellite.

Quantum gate benchmarking is inevitably affected by state preparation and measurement errors. Randomized benchmarking addresses this issue by employing group twirling to regularize the noise channel, enabling robust characterization against these errors through exponential fittings. In practice, local twirling gates are preferred due to their high fidelity and experimental accessibility. While existing randomized benchmarking methods based on local twirling gates are effective for Clifford gates, they encounter fundamental limitations when applied to non-Clifford gates. In this work, we overcome this challenge by introducing the Pauli-transfer-character benchmarking, a protocol that estimates elements of the Pauli transfer matrix using only local Pauli operations. Building on this protocol, we develop a fidelity benchmarking method for a class of non-Clifford gates $U$ satisfying $U^2=I$. We demonstrate the feasibility and effectiveness of our approach through numerical simulations, using the Toffoli gate as an example.

We show that any $N$-dimensional unitary matrix can be realized using a finite sequence of concatenated identical fixed multiport beam splitters (MBSs) and phase shifters (PSs). Our construction is based on a Lie group theorem applied to existing decompositions. Using the Bell-Walmsley-Clements framework, we prove that any $N$-dimensional unitary requires $N+2$ phase masks, $N-1$ fixed MBSs, and $N-1$ BSs. Our scheme requires only $O\left(N\right)$ fixed, identical components (MBSs and BSs) compared with the $O\left(N^2\right)$ fixed BSs required by conventional schemes (e.g., Clements), all while keeping the same number of PSs. Experimentally, these MBSs can be realized as a monolithic element via femtosecond laser writing, offering superior performance through reduced insertion losses. As an application, we present a reconfigurable linear optical circuit that implements a three-dimensional unitary emerging in the unambiguous discrimination of two nonorthogonal qubit states.

In this work, we investigate the superradiant phase transition in a two-atom Rabi model with dipole-dipole interactions, which occurs at the finite-frequency limit due to spontaneous breaking of the $Z_2$ symmetry. Upon analyzing the energy spectrum, it is observed that these dipole-dipole interactions induce a shift of the phase-transition point. Interestingly, we find that, as the strength of the light-atom interaction increases, the atoms entangled by dipole-dipole interactions tend to be decoupled. Moreover, such dipole-dipole interactions enable the Rabi model to undergo the superradiant phase transition in the presence of the $A^2$ term. Our results implement the superradiant phase transition beyond the no-go theorem in the two-atom Rabi model and provide a feasible way to explore multi-atom Rabi models with more complex interactions.

We present a formalism based on tracking the flow of parity quantum information to implement algorithms on devices with limited connectivity without qubit overhead, SWAP operations, or shuttling. Instead, we leverage the fact that entangling gates not only manipulate quantum states but can also be exploited to transport quantum information. We demonstrate the effectiveness of this method by applying it to the quantum Fourier transform (QFT) and the quantum approximate optimization algorithm (QAOA) with $n$ qubits. This improves upon all state-of-the-art implementations of the QFT on a linear nearest-neighbor architecture, resulting in a total circuit depth of $5n-3$ and requiring $n^2-1$ cnot gates. For the QAOA, our method outperforms SWAP networks, which are currently the most efficient implementation of the QAOA on a linear architecture. We further demonstrate the potential to balance qubit count against circuit depth by implementing the QAOA on twice the number of qubits using bilinear connectivity, which approximately halves the circuit depth.

Quantum transduction converts quantum states between different frequencies. Similarly, quantum teleportation transfers quantum states between different systems. While often appreciated for quantum communication between distant locations, teleportation can also operate between different frequencies. In terms of bosonic modes, quantum teleportation relies on two-mode squeezing, squeezed on the balanced Einstein-Podolsky-Rosen (EPR) quadratures, for entanglement and a balanced beam splitter plus homodyne detection for Bell measurement. Since cross-frequency-band entanglement is challenging to generate, we propose to enable transduction with teleportation while only relying on a conventional transduction device and narrow-band entanglement. Our insight is that a transduction device performs a cross-frequency-band beam splitter, almost what is needed in a Bell measurement, except that it is often not balanced. We resolve the issue by proposing a generalized teleportation protocol with arbitrarily unbalanced EPR quadratures and matched unbalanced beam splitters. The protocol can enhance quantum transduction, using only off-the-shelf components: intraband beam splitters, offline squeezing, homodyne detection, and displacements. The proposed protocol achieves perfect transduction when applied to a transduction device with an arbitrarily low efficiency, at infinite squeezing, and in the absence of extraction loss. A minimum squeezing in the range of [1.195,4.343] dB is needed to achieve quantum capacity enhancement.

Target detection employing quantum entanglement is a promising alternative for low-reflectivity detection in noisy environments. Most existing studies model the target as a beam splitter with a linear interaction. In this study we examine a target characterized by a nonlinear interaction, specifically an SU(1,1) interaction. Our analysis reveals that even weak nonlinearity can yield performance surpassing the well-known 6-dB limit associated with Gaussian quantum radar. This advantage is particularly significant in a weak-noisy environment, enabling the study of data reading, which typically occurs when environmental noise is substantially less than 1. Our results demonstrate that for a given interrogation signal intensity and noisy environment $N_{\mathrm{B}}$, a quantum memory cell with an SU(1,1) interaction provides additional enhancement. These findings indicate that SU(1,1) nonlinear interactions constitute a powerful tool for validating quantum-enhanced approaches in radar applications.

Systematic theoretical calculations on the polarization properties of x-rays emitted in $K$-shell radiative recombination (RR) into bare ions are carried out, with particular emphasis on the role of spin-flip transitions in $\mathrm{Kr}^{36+}, \mathrm{Sn}^{50+}, \mathrm{W}^{74+}$, and $\mathrm{Bi}^{83+}$ ions. The calculated linear polarization of $K$-shell RR x-rays from bare $\mathrm{Kr}^{36+}$ ions with spin-flip contribution shows good agreement with previous measurement performed at the Tokyo electron-beam ion trap [Rev. Sci. Instrum. 92, 063101 (2021)], demonstrating the necessity of accounting for spin-flip contributions to the angle-differential cross sections. This agreement provides clear identification of spin-flip contribution to the polarization of RR x-rays for the lightest bare ions studied so far. In addition, we show that spin-flip contribution to the linear polarization of the $K$-shell RR x-rays increases significantly with both incident electron energy and atomic number $Z$. This trend suggests that further measurements of the linear polarization of $K$-shell RR x-rays by varying the electron energy and atomic number $Z$ may offer enhanced sensitivity to spin-flip transitions.

Ladder-type two-photon excitation of an atom from a ground state $|g\rangle$, to an intermediate excited state $|e\rangle$, and finally, to a Rydberg state $|r\rangle$ has a variety of uses from quantum information to sensing. A common scheme for detecting this transition optically is through electromagnetically induced transparency (EIT). However, in inverted wavelength schemes, where the ground-to-excited transition wavelength is shorter than the excited-to-Rydberg transition wavelength, the strength of the EIT feature on the lower-leg beam is strongly reduced in a Doppler-broadened medium. Here, we report on an alternative two-photon spectroscopic feature, which we term the two-photon Autler-Townes resonance, observed on the upper-leg beam. Compared to the EIT signal, this feature's superior signal-to-noise ratio allows one to resolve Rydberg resonances with principal quantum number as high as $n=80$. We also show that such a feature can be utilized to generate an error signal for stabilizing the frequency of the upper-leg beam.

We consider a traveling supersolid stripe pattern in a spin-orbit-coupled Bose gas. This configuration is associated with an unequal population of the two single-particle energy minima, giving rise to a chemical potential difference that sets the fringe velocity. Unlike stationary stripes, the moving pattern is spin-polarized, with decreasing contrast as the population imbalance increases, eventually leading to stripe melting and transition to the uniform plane-wave phase. The Bogoliubov spectrum of the moving stripes exhibits asymmetry under inversion of the excitation quasimomentum. At high population imbalance, we identify energetic and dynamical instabilities in the spin-phonon mode which transforms to the roton mode of the plane-wave phase as the stripe structure vanishes.

We develop a general theoretical framework for computing the time-resolved magneto-optical Kerr effect in ultrafast pump-probe setups, formulated within the dynamical projective operatorial approach (DPOA) and its application to the generalized linear-response theory for pumped systems. Furthermore, we exploit this formalism to express the postpump optical conductivity—and consequently the Kerr rotation—in terms of the time-evolved single-particle density matrix (SPDM), providing a transparent and computationally efficient description of photoexcited multiband systems. This extension, in addition to its lower computational cost, has the advantage of allowing the inclusion of phenomenological damping. We illustrate the formalism using both (1) a two-band tight-binding model, which captures the essential physics of ultrafast spin-charge dynamics and the Kerr rotation and (2) weakly spin-polarized germanium, as a realistic playground with a complex band structure. The results demonstrate that, by exploiting DPOA and/or its SPDM extension, one can reliably reproduce both the short-time features under the pump-pulse envelope and the long-time dynamics after excitation, offering a versatile framework for analyzing time-resolved magneto-optical Kerr effect experiments in complex materials. Moreover, this analysis clearly shows that the Kerr rotation can be used to deduce experimentally the relevant $n$-photon resonances for a given specific material.

Four-component Schrödinger cat (4C) states are important physical resources for fault-tolerant quantum computing. However, the generation of 4C states in solid-state platforms remains challenging due to stringent nonlinearity requirements. We propose a Floquet-engineered scheme to generate magnonic four-component Schrödinger cat states in a hybrid superconducting-ferromagnetic system. Periodically driving the two superconducting qubits induces the effective conditional-displacement-type interactions between the magnon mode and the selected superconducting qubit sideband, where the coupling is mediated by virtual photon excitations in the microwave cavity. Notably, our scheme does not require higher-order nonlinearities of the magnon mode, and the relative phase symmetry of the 4C state's coherent-state constellation in phase space is fully controlled by the phase difference of the Floquet drives. Numerical simulations show that the high-fidelity 4C states can be generated even if the decoherence of the system is considered. These results provide a scalable route to multicomponent cat-state engineering in solid-state platforms and open additionally avenues for quantum computation.

We present an interferometric method to measure geometric phases produced by the evolution of heralded single-photon polarization states. Our arrangement is based on a polarization interferometer and operates in a fully transmission configuration. This feature allows us to avoid the phase instabilities typical of interferometers operating with reflective components such as a Mach-Zehnder or a Michelson interferometer. In contrast with previous implementations where heralded single-photon geometric phases were measured using a polarimetric method for virtual interferometry, our arrangement uses real interferometry of heralded single-photon states; that is, along the optical setup the heralded single-photon states evolve due to transformations from which we extract the geometric phase. This is achieved by measuring the total phase acquired for a given evolution and compensating the dynamical phase to extract only the geometric contribution to the total phase. The presented method to measure geometric phases is not restricted to great-circle trajectories on the Poincaré sphere but works for any custom trajectory on it. We exhibit the versatility of our arrangement by measuring geometric phases acquired during the evolution of heralded single-photon states along three different trajectories that closely fit the predictions. In addition, we experimentally demonstrate the relationship between the geometric phase and the solid angle enclosed by the state trajectory on the Poincaré sphere. Finally, we test how the phase sign changes due to the change in the sense in which the state trajectory is traveled.

Single-shot error correction outperforms conventional approaches by requiring only one round of stabilizer measurements for decoding, even in the presence of measurement errors. This capability relates to the soundness and confinement property of codes, which provides an energy barrier lower bound. Earlier research established a confinement property for three-dimensional (3D) hypergraph product codes [A. O. Quintavalle et al., PRX Quantum 2, 020340 (2021)], yielding an energy barrier lower bound for these codes. In this work, by analyzing the structure of logical operators, we show an improved energy barrier lower bound for higher-dimensional hypergraph product (HHGP) codes with low-density parity check (LDPC) property. Our bound exceeds results derived from soundness and confinement, and unlike standard hypergraph product codes, these higher-dimensional variants can possess macroscopic energy barriers even when the underlying classical codes lack this property. Specifically, our results show that the energy barrier of LDPC HHGP codes is lower bounded by the distance of the underlying classical codes. This bound is tight if the underlying classical codes exhibit system size-dependent distances but constant energy barriers, like 3D and 4D toric codes.