One-dimensional (1D) superlattices provide one simplified platform for exploring moiré physics from a low-dimensional perspective, with the ratio of lattice constants playing a role analogous to the twist angle in two-dimensional bilayers. Here we propose a 1D $\mathcal{PT}$-symmetric bichromatic optical lattice for a weakly repulsive Bose-Einstein condensate and investigate how the interplay of dissipation and interaction impacts the lowest moiré flat band. Without interaction, we find that the lowest-band flatness induced by commensurate ratios exhibits a parity-dependent response to the $\mathcal{PT}$-symmetric imaginary potential due to the distinct $\mathcal{PT}$ pairing mechanism for the energy spectrum. For ratios with even denominators (i.e., even parities), the level attraction and thus the $\mathcal{PT}$-symmetry breaking occur within the lowest two bands, leading to a monotonic broadening of the lowest flat band, whereas odd denominators (i.e., odd parities) yield a nonmonotonic response due to the $\mathcal{PT}$-symmetry breaking within the second and the third lowest bands instead while the lowest band remains purely real. This parity-dependent phenomenon can be understood by the perturbation theory. Furthermore, by solving the Gross-Pitaevskii equation, we also find that although the weak repulsive interaction can broaden the moiré bands alone, the combined effects of interaction and imaginary potential also lead to parity-dependent behaviors. For even parities, band flattening is consistently diminished, whereas for odd parities, the imaginary potential can either enhance or reduce the degree of flattening. These results pave the way for experimental studies of dissipation and interaction effects on band flatness in moiré systems.

We study the effective $s$-wave scattering of two heteronuclear atoms harmonically confined in quasi-low dimensions, where the atoms have unequal masses and are subject to different confinement frequencies. The resulting effective scattering parameters in low dimensions, including scattering length and effective range, are derived as functions of three-dimensional scattering parameters and confinement strengths. Using realistic Li-K and Li-Cr mixtures as examples, we further compute the binding energies of universal $(1+N)$ clusters in quasi-low dimensions using the effective scattering parameters. Our findings suggest a promising pathway for practically observing universal clusters and their associated many-body phases in low-dimensional ultracold heteronuclear systems.

The three- and four-boson systems with a large scattering length and a short effective range in the two-body sector are studied in the framework of Short-Range Effective Field Theory (SREFT). The starting point (leading order) of the EFT is taken to be the universal unitarity limit, where the two-body sector is parameter-free and only one three-body parameter enters. In this limit, physical systems manifest discrete scale invariance. Deviations from universality arising from finite scattering-length and effective-range corrections, as well as a four-body force required by renormalization, are included perturbatively at next-to-leading order. The three-body ground state and its associated four-body ground and first-excited states are studied using the Faddeev-Yakubovsky (FY) formalism and a complementary diagrammatic approach. By employing techniques to remove contributions from deep trimers in tetramer calculations, we extend our analysis to larger cutoffs than previously accessible within the FY approach to SREFT. Our results for binding energies and radii of ${}^4\mathrm{He}$ three- and four-atom systems converge well to results obtained with sophisticated phenomenological potentials. These successes suggest that the physics of ${}^4\mathrm{He}$ atomic clusters is governed by only small deviations from discrete scale invariance.

We show that the supercurrent properties of a superfluid or superconducting junction are significantly modified by single-particle losses in a conduction channel. In the presence of a spin-independent particle loss, we find regimes where the Josephson current $I_N\left(\varphi \right)$ can develop additional zeros within $0<\varphi <\pi$ and the direction of the supercurrent is reversed. Although the region is narrow, we also find a regime in which the critical current is enhanced by dissipation. Such anomalous behaviors in the Josephson current are attributed to a subtle interplay between the contribution that is present regardless of dissipation and the unconventional one that is absent without dissipation and is associated with the quantum jump term of the Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) master equation. In the presence of a spin-selective particle loss, it is shown that a dissipation-induced spin supercurrent and its reversal occur. The proposed system is analyzed by means of the Keldysh field-theory approach based on the GKSL master equation and may be realized in ultracold atomic gases and solid-state systems.

We theoretically investigate the rare-earth ${}^{163}\mathrm{Dy}{}^{107}\mathrm{Ag}$ molecule as a quantum sensor to amplify weak symmetry-breaking interactions not included in the standard model of physics. We focus on parity-breaking interactions from the octupole-deformed ${}^{163}\mathrm{Dy}$ nucleus. To characterize key properties for this molecule, we have performed relativistic electronic configuration-interaction and rovibrational coupled-channels calculations. We show that near the equilibrium geometry, the bond is ionic with the Dy atom losing an electron from its open, submerged 4f electron shell to Ag. In fact, the ${}^{163}\mathrm{Dy}{}^{107}\mathrm{Ag}$ ground-state configuration corresponds to a Dy ion with a large electronic angular momentum of $j_{{\mathrm{Dy}}^{+}}=15\hslash /2$ and a spinless silver ion. Here, $\hslash$ is the reduced Planck constant. We also observe that the crystal-field splittings among states with projections $\mathrm{\Omega }$ of $j_{{\mathrm{Dy}}^{+}}$ along the internuclear axis are on the order of the vibrational spacings in ${}^{163}\mathrm{Dy}{}^{107}\mathrm{Ag}$ and orders of magnitude larger than the rotational spacings, Coriolis-induced $\mathrm{\Omega }$-type doubling, the hyperfine interactions between the spins of the electrons and the ${}^{163}\mathrm{Dy}$ nucleus, and Zeeman interactions when an external magnetic field is applied. The spin-stretched state with $\left|\mathrm{\Omega }\right|=j_{{\mathrm{Dy}}^{+}}$ is the ground state. The splittings between the $\mathrm{\Omega }$-doublets, a pair of states with opposite parity, dramatically depend on $\mathrm{\Omega }$. We predict values on the order of $h\times 10\mathrm{GHz}, h\times 10\mathrm{MHz}$, and $h\times 1\mathrm{kHz}$ for $\mathrm{\Omega }=\hslash /2, 3\hslash /2$, and $5\hslash /2$, respectively, with Planck constant $h$. For larger values of $\mathrm{\Omega }$, the splittings decrease by additional orders of magnitude. Parity violations and thus the mixing of the $\mathrm{\Omega }$-doublets can be achieved by simply applying external electric fields or by interactions induced by the octupole-deformed ${}^{163}\mathrm{Dy}$ nucleus through coupling of its nuclear spin to molecular rotation. The actual energy shifts from the latter contribution are not known as they originate from nuclear forces.

Efficient measurement of center-of-mass motion is fundamental to force sensing and quantum control in levitated optomechanics. While scattering theories predict the spatial distribution of displacement information, direct experimental mapping of this information remains undemonstrated, leaving uncertainties regarding the impact of real-world optical imperfections. Here, we introduce a spatial-scanning technique to experimentally visualize the three-dimensional displacement information distribution of a levitated nanoparticle. By decomposing the scattered field into independent motional axes, we reconstruct the information radiation patterns, which reveal the angular density of detectable information. The measured patterns agree well with theoretical calculations while capturing minor distortions attributed to experimental imperfections. Furthermore, we demonstrate that the integrated displacement information remains robust against variations in the detection plane position along the optical axis. This work establishes a direct approach to characterizing information flow in scattered light, providing a practical framework for optimizing detection efficiency in complex or nonstandard optomechanical systems.

Intermittent chaos is a particular nonlinear phenomenon representing a transitional state in the route to chaos. It alternates between periodic and chaotic motions over time, different from the conventional period-doubling bifurcation. Here we qualitatively and quantitatively analyze this specific bifurcation process in a high-Q optomechanical whispering-gallery-mode microtoroidal resonator, where the dynamical process transits from a periodic state into a fully chaotic state via intermittent chaos. Specifically, we show the trajectories in phase space, the evolution of the Poincaré section, and the characteristic patterns of the corresponding Poincaré map, to reveal the unique dynamical features of intermittent chaos in contrast to purely periodic or chaotic behaviors. Moreover, by employing the maximal Lyapunov exponent, Kolmogorov entropy, autocorrelation function, and box-counting dimension, which quantify the trajectory divergence, information generation ability, temporal memory decay, and spatial occupancy in the phase diagram, respectively, we demonstrate a continuous increase in the degree of chaos. This phenomenon observed in an optomechanical resonator may promote further research on intermittent chaos in cavity optomechanics and provide support for the design and utilization of chaotic devices based on such systems.

We investigate the steady-state dynamics of two coupled cavities with balanced gain and loss in the presence of parametric driving using the Schwinger-Keldysh functional formalism. This approach allows us to analyze the non-Hermitian dynamics and evaluate steady-state observables associated with nonclassical behavior. We show that a parametric drive breaks the exact $\mathcal{PT}$ symmetry of the system and acts as a control parameter that induces a population imbalance between the cavities. The interplay of coherent tunneling, balanced loss and gain, and two-photon driving gives rise to a pronounced population imbalance between the cavities that tends to a limiting value for strong dissipation. This signals a drive-induced localization and Zeno-like suppression of photon distribution. Intermode quantum correlations are characterized using entanglement negativity, revealing a drive-dependent threshold for entanglement generation and the occurrence of entanglement sudden death at finite dissipation. In addition, near the exceptional points the system becomes sensitive to fluctuations, resulting in the minimum of the steady-state purity. The presence of the parametric drive modifies this behavior by lifting the non-Hermitian spectral degeneracy and altering the steady-state coherence properties of the system. Our results demonstrate how parametric driving provides a versatile mechanism for controlling localization, quantum correlations, and fluctuation properties in driven-dissipative non-Hermitian systems.

We propose a topological transport platform for microwave-to-optical conversion at the single-photon level in a Rydberg atom-cavity setting. This setting leverages a hybrid dual-mode Jaynes-Cummings (JC) configuration, in which the coupling between a microwave resonator and an optical cavity is mediated by a Rydberg atom ensemble. Our scheme enables the formation of Fock-state lattices (FSLs), where photon hopping rates depend on photon numbers in individual sites. We identify an inherent zero-energy mode corresponding to the dark state of the dual-mode JC model. This enables the construction of a high-efficiency single-photon transducer, which realizes topologically protected photon transport between the microwave and optical modes. Crucially, we analytically show that the FSL features continuous variations in the winding number. Our work establishes a robust mechanism for efficient quantum transduction in synthetic dimensions and opens avenues for exploring topological physics with continuous winding numbers in the atom-cavity system.

We study effective spin interactions arising from quantum electrodynamics scattering between localized fermionic spins. By integrating out photon and mediator fields, the dynamics reduce to an effective spin Hamiltonian. For two qubits in the nonrelativistic regime, the resulting interaction takes a tensor dipolar form with an asymptotic decay proportional to $R^{-3}$. We obtain analytical expressions for the entanglement negativity, highlighting its dependence on coupling strength and spatial configuration. We then examine a setup in which two bath spins interact via a sequential exchange with an intermediate fermionic mediator. At the perturbative order considered, the mediator remains unentangled and induces an effective bath-bath interaction with stronger spatial suppression than in the photon-mediated case. Extending the construction to an $N$-spin setting yields an effective interaction network mediated by virtual exchange processes, which can support the generation of multipartite entanglement across the system.

Criticality-based quantum sensing exploits hypersensitive response to system parameters near phase transition points. This work uncovers two metrological advantages offered by topological phase transitions when the probe is prepared as topological edge states. First, the order of topological band touching is found to determine how the metrology sensitivity scales with the system size. Engineering a topological phase transition with higher-order band touching is hence advocated, with the associated quantum Fisher information scaling as ${\mathcal{F}}_Q\sim L^{2p}$, with $L$ the lattice size in one dimension, and $p$ the order of band touching. Second, with a topological lattice accommodating degenerate edge modes (such as multiple zero modes), preparing an $N$-particle entangled state at the edge and then adiabatically tuning the system to the phase transition point grows quantum entanglement to macroscopic sizes, yielding ${\mathcal{F}}_Q\sim N^2{L}^{2p}$. Favorable scalability is still achievable through multicriticality even when state preparation time and the shrinking sensing parameter window are taken into account. This work hence paves a possible topological phase transition-based route to harness entanglement, large lattice size, and high-order band touching for quantum metrology.

The ability to engineer and control quantum states of many-body systems is a central challenge in quantum information science. For a register of $N$ qubits, the full Hilbert space dimension grows exponentially as $2^N$, rendering generic state preparation and control infeasible without exploiting structure or symmetry. A particularly important and physically motivated restriction is to the fully symmetric subspace, spanned by the Dicke states, which are simultaneous eigenstates of collective spin $J=N/2$. Ensembles of Rydberg atoms interacting via electric dipoles in two-dimensional tweezer arrays form a promising platform for achieving such control. However, the finite range of dipole-dipole interactions poses a challenge to generating and controlling the Dicke manifold because the Hamiltonian incurs leakage from the computational subspace. To counteract this leakage, we perform quantum optimal control algorithms on a truncated Hilbert space according to the method of “irrep distillation” (IRD), which captures the process by which the symmetric subspace couples to leakage error-spaces, using only linear-scaling Hilbert dimension. We implement gradient ascent pulse engineering (GrAPE) on control schemes with little or no local addressing, to generate resourceful states like Greenberger-Horne-Zeilinger, Dicke, and extremal quantum states. We benchmark each scheme of IRD-GrAPE for its quantum speed limit, as well as exactly testing pulse fidelities on small system sizes and predicting fidelities using higher-order IRD on larger systems.

Spin ensembles are promising quantum technological platforms, but their utility relies on the ability to perform quantum error correction (QEC) for decoherences in these systems. Typical QEC for ensembles requires addressing individually resolved qubits, but this is practically challenging in most realistic architectures. Here, we propose QEC schemes for unresolvable spin ensembles. By using degenerate superpositions of excited states, which are fundamentally mixed, we find codes that can protect against both individual and collective errors, including dephasing, decay, and pumping. We show how information recovery can be achieved with only collective measurement and control, and illustrate its applications in extending memory lifetime and loss-tolerant sensing.

The evolution of a quantum system under time-dependent driving exhibits phenomena that are absent in its stationary counterpart. However, the high dimensionality and noncommutative nature of quantum dynamics make this a challenging problem. The Magnus expansion provides an analytic framework to approximate the effective dynamics on short timescales, but computing high-order terms with existing methods is computationally expensive. We introduce a scalable approach for Hamiltonians consisting of a time-independent drift term and a controllable term, in which the high-order Magnus expansion is reduced to a polynomial expression in the duration and the parameters of the time-dependent control function. For a fixed Hamiltonian model, the demanding algebraic calculation is performed once and stored in dynamical coefficients, after which the effective Hamiltonian can be evaluated numerically several orders of magnitude faster than previous techniques. We first demonstrate the method as a proof of principle for quantum simulation, verifying the expected high-order error scaling and benchmarking the numerical efficiency of the polynomial representation. We then apply the same analytically differentiable framework to a state-of-the-art quantum optimal-control problem, designing continuous control pulses for multiqubit phase gates on a neutral-atom Rydberg platform. Our results show that this method provides a practical route toward optimal control of challenging driven quantum systems.

Bipartite entanglement purification is the conversion of copies of weakly entangled pairs shared between two separated parties into a smaller number of strongly entangled shared pairs using only local operations and classical communication. Choosing between different entanglement purification protocols generally involves weighing up a trade-off between the ratio of strongly entangled pairs produced to weakly entangled pairs consumed, which we call the rate of the protocol, and the degree of the entanglement of the strongly entangled pairs, typically measured by the fidelity of those pairs to maximally entangled states. By randomly choosing a protocol according to a probability distribution over a list of protocols for each pair we want to produce, we can achieve rates and fidelities not achieved by any of the original protocols. Here, we show how to choose this distribution to maximize the rate at which we produce qubit pairs with a given fidelity to a Bell state or, equivalently, to maximize the fidelity to a Bell state of the qubit pairs produced at a given rate. We investigate both the asymptotic case, where the number of initial pairs goes to infinity, and the finite-size regime, where protocols are restricted to a finite number of weakly entangled pairs.

Quantum digital signatures (QDSs) utilize shared keys generated by quantum key distribution, thereby providing information-theoretic security. Most existing QDS schemes, however, only support single-signer scenarios, which limits their applications in large-scale quantum networks. Based on previous studies of multisigner QDSs, we design and experimentally demonstrate a scalable multisigner QDS system. The results show that the designed system enables multiple signers to jointly sign the same message while supporting receiver verification and arbitrated validation. The single-photon count rate used in the experiment was approximately $2.6\times {10}^4$ counts/s. The measured coincidence-to-accidental ratio (CAR) was greater than 830, and the second-order correlation function $g^{\left(2\right)}\left(0\right)$ was less than 0.01. The two-photon interference visibility measured for the polarization-entangled state $|{\mathrm{\Phi }}^{+}\rangle =\left(\right|HH\rangle +|VV\rangle )/\sqrt{2}$ was approximately 99.01%, corresponding to a quantum bit error rate below 0.495%. This work demonstrates a reliable implementation of multisigner QDSs and may provide a practical approach to secure authentication and collaborative authorization in future large-scale quantum networks.

Quantum metrology exploits quantum resources to achieve measurement precision beyond classical scaling, but this advantage is often compromised by environmental decoherence. While dynamical decoupling is a powerful tool for eliminating non-Markovian noise, it has a fundamental conflict: The decoupling pulses often average out the parameter-dependent system Hamiltonian along with the noise, effectively erasing the signal to be measured. To overcome this trade-off, we propose a periodic Hamiltonian driving strategy in this work. By synchronizing the switching on and off of the system Hamiltonian with the dynamical decoupling pulses, this approach simultaneously preserves parameter encoding and suppresses environmental noise. We theoretically prove that this binary “bang-bang” modulation of the Hamiltonian is the optimal driving pattern, yielding the highest possible estimation precision within the periodic framework. Furthermore, we demonstrate that when integrated with optimal Hamiltonian control, this strategy successfully restores the Heisenberg scaling for estimation precision. The protocol is illustrated through some examples, where the pulse sequences are optimized to reduce the experimental complexity.

Simulating computationally hard fermionic systems is a promising application of quantum computing. However, mapping nonlocal fermionic operators to qubits often produces deep circuits, rendering such simulations impractical on near-term hardware. We introduce two Majorana swap network compilation strategies for variational quantum eigensolvers that reduce circuit depth and two-qubit gate count. First, we develop a cyclic compilation algorithm that localizes all two-particle interaction terms in a general fermionic Hamiltonian containing up to $O\left(M^4\right)$ such terms using only $O\left(M^3\right)$ auxiliary Majorana-swap transpositions, where $M$ is the number of fermionic modes. Here, the cubic scaling refers to auxiliary routing; a complete UCCGSD Ansatz still contains $O\left(M^4\right)$ double-excitation rotations. Second, we design a Majorana-swap network for the $k$-UpCCGSD variational Ansatz, which is already more compact than UCCGSD. In this setting, our network yields constant-factor reductions of approximately 50% in circuit depth and 20% in two-qubit gate count under all-to-all connectivity. For the more restricted $2\times N$ connectivity, the reductions are larger—about 55% in circuit depth and 40% in gate count. These structural improvements are accompanied by improved robustness in numerical noise simulations on the small molecular instances that are tested.

Quantum state preparation is an important subroutine in many quantum algorithms. The goal is to encode classical information directly to the quantum state so that it is possible to leverage quantum algorithms for data processing. However, quantum state preparation of arbitrary states scales exponentially in the number of two-qubit gates, and this makes quantum state preparation a very difficult task on quantum computers, especially on near-term noisy devices. This represents a major challenge in achieving quantum advantage. We present and analyze a novel two-step state preparation method where we first minimize the entanglement entropy of the target quantum state, thus transforming the state to one that is easier to prepare. The state with reduced entanglement entropy is then represented as a matrix product state, resulting in a high accuracy preparation of the target state. Our method is suitable for NISQ devices and we give rigorous lower bounds on the accuracy of the prepared state in terms of the entanglement entropy. We benchmark our method with 2D normal distribution and Ricker wavelet states with 6–20 qubits.