A major source of error in quantum error correction (QEC) arises from imperfect two-qubit gates, where leakage errors are notoriously difficult to correct within the QEC framework. The iswap gate’s inherently low-leakage nature makes it a compelling approach to address this challenge. However, its implementation in transmon qubits is hindered by parasitic $ZZ$ coupling induced by the required $XY$ interaction, posing a significant challenge for achieving high-fidelity control. In this work, we propose a Graph Surgery framework to theoretically analyze this problem and present a dual-microwave-drive scheme that dynamically cancels $ZZ$ interaction. To address residual transition errors, which we identified as the key bottleneck after $ZZ$ suppression, we further develop the Calibrated Derivative Error Nulling via Correlated Envelopes pulse-shaping protocol. Through this optimized approach, we demonstrate a 46-ns iswap gate with a fidelity of $(99.84\pm 0.06)\%$, suppressing intrinsic leakage to $(3.5\pm 2.6)\times {10}^{-5}$ and conditional phase error to $2.1\times {10}^{-6}$. The scheme features low dependency on specific hardware parameters and requires no auxiliary control architecture. Our results provide a flexible pathway to mitigate leakage challenges, establishing the iswap gate as a high-fidelity primitive for scalable QEC architectures.

Fast and robust entanglement generation is a central requirement for scalable quantum technologies, where limited coherence time, parameter drift, and timing uncertainty can otherwise erase quantum advantage. We investigate entanglement generation in topological open quantum systems governed by an effective non-Hermitian Hamiltonian. For a lossy two-qubit dimer with asymmetric dissipation, tuning the coherent coupling across an exceptional point (EP) produces a PT-like crossover between decay-rate splitting and frequency splitting, which directly reshapes the conditional (no-jump) concurrence dynamics in the single-excitation manifold. Embedding the dimer into a non-Hermitian Su-Schrieffer-Heeger (SSH)-like ladder yields multiple EPs through edge-bulk hybridization. Our key contribution is a practical design rule: among these EPs, the operational one for entanglement is the “edge-mode EP,” defined by the EP whose coalesced eigenvector is maximally localized on the target edge pair; the near-optimal working point lies slightly above this EP, balancing preparation speed, no-jump success probability, timing-jitter tolerance, and robustness to bulk detuning disorder. Using thresholded operational metrics (preparation time, success probability, rate, jitter window, and disorder sensitivity), we map these trade-offs and show that topology-assisted confinement plus EP-enabled mode selection enables fast and robust heralded edge entanglement.

Photonic quantum computing is a strong contender in the race to fault tolerance. Recent proposals using qubits encoded in frequency modes promise a large reduction in hardware footprint and have garnered much attention. In this encoding, linear optics, i.e., beam splitters and phase shifters, is necessarily not energy conserving and is costly to implement. In this work, we present designs of frequency-mode beam splitters based on modulated arrays of coupled resonators. We develop a methodology to construct their effective transfer matrices based on the SLH formalism for quantum input-output networks. Our methodology is flexible and highly composable, allowing us to define $N$ mode beam splitters either natively based on arrays of $N$-resonators of arbitrary connectivity or as networks of interconnected $l$ mode beam splitters with $l<N$. We apply our methodology to analyze a two-resonator device, a frequency-domain phase shifter, and a Mach-Zehnder interferometer obtained from composing these devices, a four-resonator device, and derive a no-go theorem forbidding the native implementation of a subclass of $N$ mode frequency-domain beam splitters with arrays of $N$-resonators.

Efficient loading of single atoms into tightly confined traps is crucial for advancing quantum information processing and exploring atom-photon interactions. However, directly loading atoms from a magneto-optical trap (MOT) into static traps in cavity-based systems and hybrid atom-photon interfaces remains a challenge. Here, we demonstrate atom loading in a tightly confined optical tweezer $0.6\mathrm{mm}$ away from MOT by an optical conveyor belt. By employing real-time feedback control of the atom number in the overlapping region between the conveyor belt and the tweezer, we achieve a single-atom loading probability of 77.6%. Our technique establishes a feedback-controlled loading protocol for deterministic single-atom loading in various experimental settings and provides a foundation for diverse applications based on hybrid photonic-atom structures.

Broadband and low-loss superconducting switches can facilitate large-scale quantum information processors and cryogenic detectors by dynamically reconfiguring the connectivity of their circuits. The time-dependent connectivity is enabled by the nonlinearity of lossless Josephson-junctions, which are often wired into superconducting loops to be controlled by magnetic flux. However, this approach needs a power-consuming constant flux bias and dynamic flux actuation, both of which are hard to isolate from other switches or flux-sensitive elements, limiting their integration density. Here, we design and characterize a microwave switch that implements a persistent current bias and direct-current actuation to reduce static power consumption, actuation energy, and potential crosstalk to other devices. We show that persistent current associated with tens of flux quanta is stable and long-lived, reducing the need for on-the-fly tuning. We further demonstrate that our switch has desirable performance for superconducting-circuit-based quantum information processing, including an off mode with more than 20 dB isolation comparable to commercial ferrite isolators, power handling larger than 100 pW sufficient for resonator readout tones and amplifier pumps, and modulation bandwidth broader than 600 MHz useful for multiplexing schemes.

Optimal measurements for quantum multiparameter estimation are complicated by the uncertainty principle. Generally, there is a trade-off between the precision with which different parameters can be simultaneously estimated. The task of determining the minimum achievable estimation error is a central task of multiparameter quantum metrology. For estimating parameters encoded in pure quantum states, the ultimate limit is known but is given by the solution of a nontrivial minimization problem. We present an alternative expression for the achievable bound for two-parameter estimation with pure states that is considerably simpler. We also determine the optimal measurements, completing the problem of two-parameter estimation with pure-state probes. To demonstrate the utility of our result, we determine the precision limit for estimating displacements using grid states.

Photonic quantum gates are fundamental building blocks for a wide range of optical quantum information processing tasks. We propose an efficient linear-optical scheme for implementing a postselected three-qubit fan-out gate (also known as a controlled-not-not gate) using only two ancillary single photons and linear optical elements. The scheme can be generalized to an $n$-qubit fan-out gate, requiring $(n-1)$ ancillary single photons and $(3n-3)$ polarizing beam splitters (PBSs), with a success probability of ${(\frac{1}{4})}^{n-1}$. Compared with the standard gate decomposition approach, which requires $(2n-2)$ ancillary single photons, $(5n-5)$ PBSs, and yields a success probability of ${(\frac{1}{8})}^{n-1}$. Our scheme significantly reduces the resource overhead and improves the success probability. We further evaluate the gate performance and demonstrate improved robustness compared with gate decomposition-based methods.

In trapped-ion chains, multipartite Greenberger-Horne-Zeilinger (GHZ) states can be prepared natively with the help of a single bichromatic laser pulse. However, higher-order terms in the expansion in the Lamb-Dicke parameter $\eta$ limit the GHZ state preparation infidelity for rectangular and bell-like pulses to the order of ${\eta }^4$. For tens of ions, the infidelity caused by out-of-Lamb-Dicke effects can reach several percents. We propose an amplitude and phase-modulated pulse shape, an “echoed lemniscate pulse,” which cancels this contribution into error in the leading order. For the proposed pulse, the infidelity scales as ${\eta }^6$. The improved scaling is achieved because of a special phase trajectory of a collective motional mode following the figure-eight curve (lemniscate). We demonstrate that the lemniscate pulse allows achieving lower infidelity than bell-like pulses, which can be as low as ${10}^{-4}$ for 20-ion chains.

We introduce a simulation-free approach to fidelity estimation in chaotic circuits implemented on universal quantum processors, based on the order statistics of measured output probabilities. The approach requires only the most frequently observed measurement outcomes and builds on analytical results for the order statistics of Haar-random quantum states derived here. Modeling noise via an effective depolarizing channel, we construct maximum-likelihood estimators that infer circuit fidelity from a small number of top-ranked outcomes, without requiring classical amplitude calculations. The method is benchmarked on publicly available data from Google’s 12-qubit Sycamore experiment and supported by numerical simulations, providing a framework for fidelity estimation in intermediate-scale quantum devices.

In donor qubit arrays, each electron is not confined to a specific donor: long-range Coulomb interactions enable changes in gate biases near one donor to modify the charge configurations of others. This nonlocal crosstalk arises not only from electrostatic effects but also from interactions that are strong across the donor array. Here we investigate the strong interaction-induced charge redistribution as a function of gate voltages in $2\times 2$ phosphorus donor arrays in silicon. We construct a multiorbital Hubbard model of the donor array, parametrized by density functional theory, atomic tight-binding, and effective-mass theory, and we solve this Hubbard model with the density matrix renormalization group. We find that an effective two-orbital subspace provides an accurate description at experimentally relevant donor separations, enabling tractable many-body calculations. An interaction-induced charge redistribution is discovered that is not detectable in total-charge stability diagrams and cannot be captured by noninteracting models. We show that these features are protected by same-symmetry correlation energies. The findings have deep implications for operating donor-array-based quantum technology hardware.

Mapping quantum approximate optimization algorithm (QAOA) circuits with nontrivial connectivity in fixed-layout quantum platforms, such as superconducting quantum processing units (QPUs), requires a transpilation process to match the circuit to the hardware layout. This step is critical for reducing error rates on noisy QPUs. Two approaches that improve the resources required for such transpilation are the SWAP network and parity twine chains (PTC), which reduce the two-qubit gate count and circuit depth needed to represent fully connected circuits. In this work, we introduce a simulated annealing-based method that further reduces the encoding overhead of PTC and SWAP networks for QAOA circuits with nonfully connected two-qubit interactions. The method is benchmarked against various transpilers, including the Qiskit SAT mapper, demonstrating that beyond specific connectivity thresholds it achieves significant reductions in both two-qubit gate count and circuit depth. For example, for a 120-qubit QAOA instance with 25% connectivity, our method achieves an 87% reduction in depth and a 29% reduction in two-qubit gates compared with the Qiskit transpiler. Finally, the practical impact of PTC encoding is validated by benchmarking QAOA on the ibm_fez and ibm_kingston devices, showing improved performance for systems of up to 20 qubits.

Radio-frequency (rf) reflectometry provides a useful method for sensitive and fast detection of dynamic processes in quantum systems. We use this method to detect excitation of the quantized (Rydberg) states of the vertical motion in a system of surface-bound electrons on liquid helium. The Rydberg transition is detected as a change in the rf reflection from a resonant circuit coupled to microwave-excited electrons. It is found that the observed reflection response drastically differs from an expected state-dependent dispersive shift of the resonant frequency. To elucidate the origin of the observed response, the result is compared with an independent reflection measurement on the same electron system modulated by an electrostatic potential and with a numerical simulation using the Green’s function method. The findings suggest that the observed response to the Rydberg resonance must be attributed to the in-plane motion of the photoexcited many-electron system rather than to the Rydberg transitions of independent electrons. Our work demonstrates that rf reflectometry could be a useful tool to study dynamic processes, such as quantum ac transport and collective excitations, in this system.

High-fidelity two-qubit gates are essential for scalable quantum computing. We present a scheme based on superconducting transmon qubits and a control pulse delivery protocol that enables arbitrary controlled-phase gates modulated solely by an independent arbitrary waveform generator pulse. We combined a tunable coupler design with bipolar flux-pulsing to demonstrate a controlled-phase gate, achieving a peak fidelity of 99.5%. Our gates inherit the advantages of both approaches: minimal residual $ZZ$ coupling, built-in echo-like low-frequency noise protection, and time-scale control pulse reproducibility, while remaining easy to calibrate. We optimize the system’s energy levels to mitigate leakage to the coupler and suppress residual interactions. Numerical simulations of the scheme as three qutrits indicate that an error below $1\times {10}^{-3}$ is achievable. We confirm the scalability potential of the proposed scheme on high-fidelity four-qubit and eight-qubit quantum processors.

Heralded multiphoton entanglement generation is a central bottleneck for photonic quantum computing, where resource costs typically skyrocket with target size. We explore efficient methods for generating photon states with tunable entanglement, providing a flexible tool for quantum state engineering. We introduce a theoretical framework that has been numerically validated, demonstrating the capacity to generate GHZ-like states incrementally from nonlogical intermediate states. We demonstrate that in certain scenarios—such as reducing the resource cost for building large maximally entangled GHZ states—these variable-entanglement states can outperform their fixed-entanglement counterparts. By adjusting intermediate states and optimizing interferometer schemes, we improve photon number cost efficiency of GHZ-like states generation. Our findings indicate that while not a universal solution, nonmaximally entangled states offer practical advantages for specific photonic quantum information tasks.

Fault-tolerant modular quantum computing requires stabilizer measurements across the modules in a quantum network. For this, entangled states of high quality and rate must be distributed. Currently, two main types of entanglement distribution protocols exist, namely emission-based and scattering-based, each with its own advantages and drawbacks. On the one hand, scattering-based protocols with cavities or waveguides are fast but demand stringent hardware such as high-efficiency integrated circulators or strong waveguide coupling. On the other hand, emission-based platforms are experimentally feasible but have so far relied on Bell-pair fusion with extensive use of slow two-qubit memory gates, limiting thresholds to approximately 0.16%. Here, we consider a fully distributed surface code using emission-based entanglement schemes that generate GHZ states in a single shot, i.e., without the need for Bell-pair fusions. We show that our optical setup produces Bell pairs, $W$ states, and GHZ states, enabling both memory-based and optical protocols for distilling high-fidelity GHZ states with significantly improved success rates. Furthermore, we introduce protocols that completely eliminate the need for memory-based two-qubit gates, achieving thresholds of approximately 0.19% with modest hardware enhancements, increasing to above approximately 0.24% with photon-number-resolving detectors. These results show the feasibility of emission-based architectures for scalable fault-tolerant operation.

Quantum metrology promises measurement precision beyond the classical limit by using suitably tailored quantum states and detection strategies. However, scaling up this advantage is experimentally challenging, due to the difficulty of generating high-quality large-scale probes. Here, we build a photonic setup that achieves enhanced precision scaling by manipulating the probe’s dynamics through operations performed in a coherently controlled order. Our setup applies an unknown rotation and a known orbital angular momentum increase in a coherently controlled order, in a way that reproduces a hybrid quantum switch involving gates generated by both discrete and continuous variables. The unknown rotation angle $\theta$ is measured with precision scaling as $1/4ml$ when a photon undergoes a rotation of $2m\theta$ and an angular momentum shift of $2l\hslash$. With a practical enhancement factor as high as 2317, we achieve a normalized precision of approximately equal to ${10}^{-4}$ rad per photon. The precision enhancement consumes only a linearly increasing number of applications of the gates while achieving a nonlinear scaling of the precision. We further indicate that this nonlinear enhancement roots in an in-depth exploration of the Heisenberg uncertainty principle (HUP). The very quantum noncommutativity that imposes fundamental limitations on measurement precision under the HUP is transformed into a valuable quantum resource that enables the observed nonlinear enhancement.

In autonomous quantum error correction, the lifetime of a logical bosonic qubit can be extended beyond its physical constituents without feedback measurements. Leveraging autonomous error correction, we propose a gate-based microwave quantum repeater (GBMQR) with encoded bosonic grid states. Each repeater station comprises a transmon and two bosonic resonators: one resonator serves as a stationary quantum memory utilizing autonomous error correction and the other serves as an information bus for entanglement generation. Entanglement is generated sequentially through the successful absorption of a microwave photon wave packet. This method enables deterministic entanglement generation, in contrast to a probabilistic mixing of two heralding signals on a balanced beam splitter. Furthermore, our GBMQR employs an all-bosonic entanglement swapping Bell-state measurement. This is implemented via a bosonic controlled-$\mathrm{Z}$ gate and two separate $X$-basis projective homodyne measurements on the stationary stored code words. Our approach circumvents mode-mismatch losses associated with routing and interfering heralding modes on a beam splitter and confines losses to those arising from stationary storage. We evaluate the performance of the proposed quantum repeater by calculating its secret key rate under realistic laboratory environments. Moreover, we explicitly demonstrate that at the stationary damping rate of ${\kappa }_{\text{damp}}^{-1}=40\mathrm{ms}$, GBMQR can achieve entanglement generation and swapping success probabilities of approximately 0.75 and 0.58, respectively, surpassing the hallmark success probability of $1/2$ set by ideal linear beam-splitter-based Bell-state measurements. The proposed device can be implemented using currently available superconducting microwave technology and is suited for secure chip-to-chip communication and distributed quantum computing.