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.

A double-heralding technique is presented for producing heralded entangled photon pairs from spontaneous parametric down-conversion (SPDC). Compared to the swap-heralded schemes studied in previous cascaded SPDC and zero-added-loss multiplexing (ZALM) proposals, this double-heralding technique is found to yield the most resource-efficient implementation in terms of minimizing the total number of sources and detectors required to achieve a specified rate and fidelity. This is achieved by reducing the number of modes and mode-sorting optics needed on the heralding path. Specifically, by immediately detecting any two signal photons from an array of down-converters, the corresponding idler photons can be projected onto an anticorrelated pair state which is shown to be unitarily equivalent to the state produced by swap-heralded sources, and hence can be used directly for long-range entanglement distribution in a ZALM architecture. Quasideterministic operation through two distinct multiplexing techniques is analyzed. The analysis derives expressions for the heralded pair probability and fidelity assuming realistic detectors with losses, dark counts, and partial photon number resolution, providing a framework for implementation of the source on a photonic integrated circuit.

Many-body quantum systems can exhibit collective effects that enhance the sensitivity of parameter estimation protocols. An example is provided by resonantly driven two-level atoms subject to collective dissipation, which can display a transition between a stationary phase and a time-crystal one. Previous work has shown that the light emitted in the time-crystal phase can be harnessed for parameter estimation using continuous monitoring protocols, such as photon counting or homodyne detection, which under ideal conditions yield a quadratic enhancement of sensitivity with the number of particles. In this work, we explore what is the minimal information about the emission field that needs to be accessed in order to resolve collective effects and exploit them for parameter estimation. We show that, for short probing times, a single-time measurement of the emission field already captures the collective behavior emerging at the nonequilibrium transition. In contrast, within the time-crystal phase, exploiting collective effects requires at least two-time measurements. To this end, we introduce a family of correlated intensity measurements that extract the relevant information and can be implemented using an interferometric setup. While the ultimate sensitivity bound remains size independent, as recently established within the framework of noisy quantum metrology, our analysis shows that these protocols utilize collective effects to yield a transient increase in sensitivity with particle number.

The contribution of weak interaction to the hyperfine splitting of the ground state of muonium is investigated. The amplitudes of the one- and two-quantum exchanges determined by the Z and W bosons are calculated. One-loop corrections in the photon and Z-boson propagators and their contribution to the hyperfine structure of the spectrum are obtained on the basis of the dispersion method in quantum field theory.

Radiative transition data for open-shell lanthanide ions are essential for large-scale opacity calculations in astrophysical plasmas, yet fully converged configuration-interaction (CI) treatments remain computationally impractical for such systems because of the rapid growth of configuration spaces. Consequently, practical calculations rely on truncated CI models, making it important to identify which correlation contributions dominate transition amplitudes. In the present work, systematically controlled CI calculations for Gd v are performed within the multiconfiguration Dirac-Hartree-Fock framework to investigate the correlation dependence of radiative transition properties. Significant and highly selective variations in transition strengths are observed between single- and double-excitation models, particularly for transitions associated with the $5p\text{−}5d$ and $4f\text{−}5d$ pathways. Through selective inclusion and removal of specific configurations, the $5p^2\to 5d^2$ pair-excitation correlation is identified as the dominant higher-order correlation contribution affecting the redistribution of transition strengths between different CI models. The analysis further shows that higher-order correlation effects primarily modify strongly allowed transition pathways, while the resulting changes propagate to weaker transitions through configuration mixing between different pathways. These results demonstrate that CI-dependent transition behavior in open-shell ions is governed not only by the overall size of the CI expansion, but also by the selective contributions of specific correlation effects, providing guidance for constructing efficient CI expansions in large-scale atomic structure calculations.

In the presence of a magnetic field, hyperfine clock qubits can acquire a vector differential light shift that can be tuned via polarization to suppress the total differential light shift of high-power, off-resonant laser light. We experimentally measure this “magic” polarization condition, suppressing differential light shifts in both the ${}^{2}S_{1/2}$ ground and ${}^{2}F_{7/2}^o$ metastable clock qubits of ${}^{171}\mathrm{Yb}^{+}$. We present calculations of the minimum bias magnetic fields required to suppress differential light shifts in the ground-state clock qubits of commonly trapped-ion species, finding that they are below the strengths of fields already typically present in experiments. We further present methods for metastable clock-qubit control in ${}^{171}\mathrm{Yb}^{+}$, demonstrating a state preparation and measurement infidelity of $2.9_{-1.5}^{+3.0}\times {10}^{-4}$ ($-35\pm 4\mathrm{dB}$).

Resonance laser ionization spectroscopy was conducted on stable chromium isotopes ${}^{50,52,53,54}\mathrm{Cr}$ using the RILIS laser ion source and the CRIS experimental apparatus at the ISOLDE/CERN facility. A unique titanium:sapphire laser-based three-step laser ionization scheme was developed to selectively and efficiently ionize and extract the Cr isotopes. Over 350 electronic transitions were observed, including several efficient transitions to autoionizing states. High-resolution spectroscopic measurements were performed with the $3d^5{(}^{6}S)4s a^7{S}_3\to 3d^5{(}^{6}S)4p z^7{P}_3$ atomic ground-state transition to characterize the scheme and benchmark the performance of the CRIS setup. The isotope shifts $\delta {\nu }_{50,52}=-121\left(7\right)\mathrm{MHz}, \delta {\nu }_{50,53}=-137\left(6\right)\mathrm{MHz}$, and $\delta {\nu }_{54,52}=-14\left(6\right)\mathrm{MHz}$ were measured, and the hyperfine structure parameter $A_{S_3}=-82.64\left(16\right)\mathrm{MHz}$ of ${}^{53}\mathrm{Cr}$ was extracted. A King plot analysis allowed reevaluation of the field shift $F=-271\left(11\right)\mathrm{MHz}{\text{fm}}^{-2}$ and mass shift $M=105.3\left(30\right)\mathrm{GHz}\text{u}$ factors of the transition with improved precision and accuracy. High-resolution measurements agree with the existing literature, supporting the accuracy and the reliability of the method for future studies involving radioactive isotopes.

Within the framework of quantum electrodynamics, the interaction between two atoms at large distances is analyzed. Using the S-matrix formalism, an expression for the magnetic interaction potential is derived, which agrees with the well-known result of classical electrodynamics. However, quantum electrodynamics goes beyond this conventional result and allows one to treat a wide range of problems related to the structure of atomic energy levels. In particular, it is shown that the asymptotic behavior of the interaction potential can deviate from the classical prediction, depending on the atomic states involved. As an example, dispersion coefficients are calculated for the $s$-states of hydrogen atoms, where the long-range potential reduces to a spin-spin interaction. The results obtained open up the possibility of a straightforward comparative analysis of long-range interaction potentials between atoms of matter and antimatter. The applicability of this approach is demonstrated for the hydrogen-antihydrogen system.

The detection dead zone is an important systematic limitation in scalar atomic magnetometers, constraining their practical utility. In this work, we demonstrate a sensitive dead-zone-free scalar magnetometer by integrating previously established techniques into a FID magnetometer based on Bell-Bloom optical pumping. The dead zone is eliminated by inserting a reflecting mirror within a multipass-cavity-assisted atomic cell, which folds the optical beam into orthogonal paths. Our analysis reveals that interregion cross-talk is essential for interpreting the resulted experimental signals. The sensor exhibits oscillation signal amplitude variations within a factor of three across all orientations in three-dimensional space, and a field sensitivity better than 80 ${\mathrm{fT}/\mathrm{Hz}}^{1/2}$ over the full space. We further characterize the heading error in the geomagnetic field range, where experimental results agree with theoretical predictions within 0.7 nT. Additionally, the sensor can operate in a closed-loop mode by feeding back the real-time frequency extracted from the FID signal to modulate the pump beam, we characterize the magnetic field slew rate of the sensor. This work pave the way towards a sensitive vector FID magnetometer.