Symmetry-protected topological (SPT) systems extend the Landau paradigm of quantum matter by admitting distinct phases that lack local order parameters, making them challenging to characterise using conventional experimental probes. While material platforms provide important evidence for SPT order through indicative signatures such as boundary states, experimental access is typically limited to such partial probes rather than fully diagnostic measurements of the underlying quantum state. Programmable quantum processors offer a powerful complementary approach, enabling direct access to non-local properties of many-body wavefunctions. However, preparing such states on digital hardware at scales sufficient to avoid finite-size effects is typically limited by the circuit depth required to capture non-trivial entanglement. Here, we use a tensor-network-based approximate quantum compiling (AQC) protocol to construct shallow quantum circuits (18-39 CNOT depth) that prepare 100-site ground states of the spin-1/2 bond-alternating Heisenberg chain across distinct SPT phases with 97.9-99.0% fidelity. Executing these circuits on IBM quantum hardware, we directly extract multiple non-local diagnostics of SPT order, including string order for lengths up to 20, persisting well beyond the decay of conventional two-point correlations, characteristic features of the entanglement spectrum, and symmetry-protected edge modes. The simultaneous observation of these independent diagnostics provides a scalable and programmable approach to preparing and characterising SPT phases on quantum processors. More broadly, this establishes digital quantum devices as flexible platforms for studying complex quantum matter and provides a practical foundation for exploring non-equilibrium dynamics in regimes that challenge classical computational methods.
Machine learning provides a powerful framework for predicting ground-state properties across families of quantum many-body problems, enabling amortized inference and reducing the cost of repeated simulations. Variational quantum algorithms (VQAs) are promising candidates for implementing such learnable solvers on quantum computers, yet rigorous guarantees for convergence and generalization remain scarce. Motivated by the fact that standard quantum phase estimation (QPE) provides provable performance when given a guiding state with non-trivial overlap with the ground state, we introduce a variational quantum algorithm with guiding states aiming towards predicting ground-state properties of quantum many-body systems. We then develop a proof technique—the linearization trick—that maps the training dynamics of the algorithm to those of a kernel model. This connection yields theoretical guarantees on both convergence and generalization for the VQA under the guiding state assumption. Our analysis shows that guiding states facilitate convergence, suppress finite-size error terms, and ensure stability across system dimensions. Finally, we validate our findings with numerical experiments on 2D random Heisenberg models.
Accurate state preparation and measurement (SPAM) is vital for performing high-fidelity quantum computation1–3. However, systems typically use individual readout hardware for each qubit, presenting a significant overhead to the scale-up of quantum computers2,4. In response, frequency-multiplexed simultaneous quantum non-demolition (QND) readout through a single channel has been investigated5–11. In this work, we investigate an alternative solution: time-multiplexed QND readout of multiple data qubits through a single readout channel. We present Bayesian estimation scheduling procedures, enabling initialisation and readout with fidelities comparable to separate readout channels. We show that up to four qubits can be measured and initialised using a single channel with fidelities >99.9% and >99.95% respectively, even when ancilla single-shot fidelities are ~95%. These results show that minimising data qubit flips enables high-fidelity SPAM for sequential QND readout of multiple qubits, even with poor single-shot readout, using only a single shared readout channel.
We explore the generation of nonclassical mechanical states by combining continuous measurement and feedback control. We find that feedback-induced spring softening can greatly enhance position squeezing, allowing squeezing despite measurement rates slower than the mechanical frequency. Conversely, even from a pure position measurement, we find that spring hardening can enable momentum squeezing, in a new regime we term the fast feedback regime. We interpret these effects as arising from correlations between the feedback drive and the measurement imprecision noise, which in turn effectively modify the measurement rates for position and momentum. Together, this significantly lowers the barrier to measurement-based preparation of nonclassical mechanical states at room temperature.
Abstract On-demand qubit-state initialization is a prerequisite for quantum computation. We demonstrate such a protocol in a device consisting of fixed-frequency transmon qubits pair-wise coupled via tunable couplers — an architecture that is also compatible with the surface code. We use tunable couplers to transfer any undesired qubit excitation to the readout resonator of the qubit, from which this excitation decays into the feedline. In total, the combination of multi-level qubit reset, leakage reduction, and coupler reset takes only 88 ns to complete. Our reset scheme is fast, unconditional, and achieves fidelities above 99%, thus enabling fixed-frequency qubit architectures as future implementations of fault-tolerant quantum computers.
We propose a scheme for deterministic single photon subtraction based on Single Photon Raman Interaction (SPRINT) with a three-level quantum emitter coupled to a chiral waveguide and present an analytical and numerical study of its feasibility. We show that single photon subtraction probability for pulsed input light in a coherent or a Fock state can approach unity in the ideal limit and discuss the potential of various reported quantum emitter-waveguide platforms as candidates for experimental realization of our scheme. The use of chiral slow-light photonic crystal waveguides coupled to Λ-type emitters can perform efficient photon subtraction. While integrating cold atoms to such systems remains technically challenging, solid-state emitters offer a potentially practical path to realize such photon subtraction.
Scalable quantum photonic technologies require the simultaneous production of highly indistinguishable multi-photon states with high probability. This need may be met, and the output probability significantly increased by multiplexed sources, which channel numerous low-probability heralded single-photon sources into a specified output mode using time or spatial multiplexing. However, conventional methods merely replicate these multiplexing units when scaled to multiple output modes, which is fundamentally inefficient because it discards many valid multi-photon events. To address this, we propose and experimentally demonstrate a novel multiplexing source architecture based on a multi-input/multi-output switching network, which can utilize combinatorial photon events. A super-exponential enhancement in multi-port single-photon creation is revealed by theoretical analysis, which significantly reduces the number of heralded single-photon sources needed. We demonstrate multiplexing with two output ports, supported by four integrated heralded single-photon sources. Our strategy has a generation probability of 1.60 times that of conventional methods and 2.58 times that of implementations without multiplexing. This multi-port multiplexed method opens a new avenue for establishing a solid foundation for advanced multi-photon quantum interference and large-scale quantum information processing.
A noteworthy discovery is that the minimal evolution time is smaller for parity-time ($${\mathcal{PT}}$$)-symmetric systems compared to Hermitian setups. Moreover, there is a significant acceleration of two-qubit quantum entanglement preparation near an exceptional point (EP), or spectral coalescence, within such a system. Nevertheless, an important problem often overlooked for quantum EP-based devices is their fidelity, greatly affected by the process of dissipation. We identify an inherent trade-off relation between the degree of entanglement and fidelity, and demonstrate that this limitation can be effectively overcome by harnessing an active $${\mathcal{PT}}$$-symmetric system, which possesses balanced gain and loss, enabling maximal entanglement with rapid speed, high fidelity, and greater resilience to non-resonant and coupling strength errors. Two- to six-partite entanglement is prepared as the concrete example to show (i) how to break this inherent fidelity-entanglement trade-off even with non-balanced gain and loss, and its resilience against non-resonant errors, in contrast to passive $${\mathcal{PT}}$$-symmetric dynamics; (ii) achieving speedups of many orders of magnitude, in contrast to conventional Hermitian dynamics. We use this approach to prepare 10-qubit entanglement to show the generality of this method. These findings highlight the potential of truly $${\mathcal{PT}}$$-symmetric devices as a versatile platform for generating and manipulating diverse quantum resources, with implications for advancing quantum information technologies.
Quantum key distribution (QKD) enables two remote parties to share encryption keys with information-theoretic security guaranteed by physical laws. Side-channel-secure QKD (SCS-QKD) has attracted considerable attention because it simultaneously removes source and detector side-channel vulnerabilities. Although a recent experiment demonstrated SCS-QKD over 50 km, practical implementation remains challenging due to imperfect vacuum preparation and finite-key constraints under coherent attacks. Here, following the theoretical framework of Jiang et al. [Phys. Rev. Res. 6, 013266 (2024)], we experimentally implement a practical SCS-QKD protocol using an imperfect whole-space source and rigorous finite-key analysis. Benefiting from a stable GHz-level system operating at 1.25 GHz, we extend the transmission distance to 200 km and achieve high secure key rates of 18.31 kbps, 2.55 kbps, and 196.03 bps at 100 km, 150 km, and 200 km, respectively. Our results establish a new distance record for SCS-QKD and demonstrate the feasibility of high-speed, long-distance, and practically secure quantum key distribution.
The causal structure of a set of events specifies which events influence which others. Determining this structure, known as causal inference, is a fundamental task in science. Reset-style intervention is commonly used to determine causal structure in classical scenarios. However, in the more fundamental quantum theory, there is evidence that projective measurements alone can suffice in some cases. We here extend those cases to two systems at two times, more general channels, and from fine-grained to less invasive coarse-grained projective measurements. The measurements are implemented by scattering circuits in a nuclear magnetic resonance platform. The measurement data are analysed via the pseudo-density matrix (PDM) formalism for measurements on systems at several times, in order to determine compatibility with given causal structures. We thereby successfully demonstrate causal inference from coarse-grained measurements alone, in a scenario with several qubits and two times.
Preparing long-range entangled states is critical for quantum information processing and many-body physics, but typically requires impractically deep circuits. Recent progress on adaptive circuits, which incorporate mid-circuit measurements and classical feedback, shows that adaptivity can greatly reduce depth and even prepare arbitrary states in constant depth. This advantage, however, comes with a trade-off between depth and ancillary qubits, motivating the study of adaptive state complexity: the minimal depth and number of ancillas needed to prepare a given state. In this work, we investigate adaptive state complexity for generic mixed states and general circuit architectures. Our approach analyzes the growth of correlation range in adaptive circuits and shows that insufficient depth or ancilla resources strictly limit the formation of long-range correlations. This yields rigorous bounds on approximate state and gate complexity given the correlation value. We demonstrate the generality and applicability of our approach with representative examples, including permutation-invariant states, Gibbs states of many-body Hamiltonians, and multi-qubit gates like the Toffoli gate.
Quantum networks enable secure communication, distributed computing, and precision sensing beyond classical capabilities. Recent advances have progressed from two-node links to multi-node architectures, enabling rigorous tests of multiparty quantum communication protocols. Among these, quantum secret sharing (QSS) leverages multipartite entanglement to distribute secrets such that only authorized parties can reconstruct them. Here, we present a three-node quantum communication testbed with a triangular topology, each edge implemented by a 1.3-meter-long transmission line, and use this testbed to demonstrate QSS. We demonstrate state transfer and entanglement generation between all node pairs, create genuine tripartite GHZ entanglement, and implement QSS of classical information. Our experiments show that the QSS protocol detects eavesdropping in situ, ensuring secure secret distribution on a superconducting platform.
We introduce a merging-based quantum repeater that departs from the conventional swapping paradigm by progressively growing multipartite entanglement. In contrast to swapping-based schemes, where a single failed operation often forces the entire protocol to restart, our approach reuses previously established entanglement through iterative gap-patching, thereby reducing waiting times, improving distribution rates, and introducing enhanced flexibility in the communication requests. We analyze this protocol in the context of probabilistic operations and a time-dependent dephasing noise model. We compare it with standard repeater protocols and demonstrate a clear advantage in secret-key rate across relevant operating regimes, underscoring its potential for practical quantum communication scenarios. These results establish merging-based repeaters as a promising alternative design principle for scalable and resource-efficient quantum-network architectures.
We introduce the random coupled-plaquette gauge model (RCPGM), which enables optimal accounting for Y-errors in decoding a surface code with noisy syndrome measurements. Using Parallel Tempering Monte Carlo simulations, we determine the code’s fundamental error thresholds. For phenomenological depolarizing data and bit-flip syndrome noise we determine a threshold of 6%, to be compared to the “uncoupled” random plaquette gauge model (RPGM) with a mere 4.3%. We then tackle the circuit-level noise scenario, where an approximate reduction technique allows us to exploit the RCPGM. Within the assumptions of the reduction technique, we find a threshold of 1.4%, to be compared to 0.7% when marginalizing Y-errors for the “uncoupled” RPGM. These results crucially enlarge the landscape of statistical mechanical mappings for quantum error correction. In particular they show further room for improvement of the surface code for fault-tolerant quantum computation and should be highly encouraging for practical decoder development.
Noise is usually treated as an obstacle to reliable quantum computation. Here, we show that noisy quantum data can instead enhance classical simulation by introducing the Noisy-device-enhanced Classical Simulation (NDE-CS) protocol. NDE-CS decomposes the expectation value of a target non-Clifford parameterized quantum circuit into a linear combination of expectation values of Clifford circuits. These Clifford circuits are sampled by replacing the rotation gates in the target circuit with Clifford analogs, while preserving the original gate layout and connectivity. The protocol learns highly efficient linear combinations from noisy expectation values on quantum hardware and uses them to reconstruct the noiseless target expectation value. Numerical simulations show that NDE-CS outperforms layer-wise stabilizer Monte Carlo baselines and, for two specific circuit families, requires substantially fewer resources than Sparse Pauli Dynamics (SPD), highlighting its complementary role to SPD. Lastly, we experimentally implement NDE-CS on a superconducting quantum processor, demonstrating that the protocol remains effective on real-world quantum hardware.
In the field of quantum communication, investigating the practical security of systems is conducive to their deployments in real-world scenarios. In this paper, we identify and experimentally demonstrate a side-channel vulnerability within continuous-variable quantum key distribution (CV-QKD) arising from the zero-order hold (ZOH) effect in digital-to-analog conversion. As digital-to-analog converters (DACs) are indispensable for modulation in CV-QKD transmitters, this leakage constitutes an intrinsic and widespread risk. We show that the ZOH-induced spectral side lobes allow an eavesdropper to extract secret information without disturbing the main signal band. Experimental validation on a CV-QKD platform reveals a leakage of 2.68 Mbit/s against a generated secret key rate of 4.73 Mbit/s. Crucially, we propose a defense strategy that completely eliminates this vulnerability, effectively restoring the system’s implementation security. By resolving this fundamental hardware limitation, our work bridges the gap between theoretical models and practical engineering, paving the way for robust, standardized quantum communication networks.
Here we review analytical methods for computing the entanglement distribution time in first-generation quantum networks. We describe and briefly discuss the main schemes for entanglement generation, distillation, and swapping, and their combination in repeater chains, providing formulae for mean values and probability functions under a unified framework. We also derive some new expressions that help draw a more comprehensive picture, and present some scheme comparisons for different operational scenarios.
We propose a framework for quantizing randomized quantum games in which entanglement is examined as a tunable fairness resource without relying on fine-tuned strategy parameters. As a case study of the Monty Hall problem, we show that initial entanglement between host and player registers neutralizes classical asymmetry at the distribution level, driving win-probability distributions under different strategic conditions to converge. Implemented on IBM’s Eagle processors via four-qubit encodings, our experiments span a continuum of entanglement strengths and employ extensive randomized trials over broad strategy ensembles. Although the mean no-switch probability remains fixed at its classical value, stronger entanglement progressively suppresses distributional signatures of asymmetric play, yielding increasingly uniform outcome statistics across adversarial conditions. Within the pure-state setting studied here, these findings provide evidence for an experimentally accessible route to quantify entanglement-enabled fairness in quantum games and suggest relevance to quantum information settings involving strategic decision-making under asymmetry.
Predicting the performance of a Quantum Neural Network (QNN) on a given dataset before its training can save critical resources. However, obtaining such a priori information is, in general, very challenging. The Quantum Neural Tangent Kernel (QNTK) has recently emerged as a tool to mathematically describe the behavior of QNNs. Here, we provide practical guidelines for using the QNTK to diagnose the performance of a given QNN model. In particular, we link QNTK spectral features to the training dynamics, and we show how a first-order kernel approximation can predict the expected inference performance. By analyzing the interplay between the number of model parameters and of training samples, we identify a mismatch at the interpolation threshold: while the QNTK exhibits a double descent in generalization error, we observe that QNNs do not show this behavior in practice. This suggests that QNTK diagnostics are mostly reliable for overparametrized models well above the interpolation regime. Extensive numerical simulations across different architectures and datasets validate our approach. Our results demonstrate that QNTK diagnostics yields useful information about QNN behavior for sufficiently deep circuits, and enable detecting and addressing potential shortcomings in model design.
Practical and robust control of the complex phases of qubit superposition states is necessary for high-fidelity quantum gates. Here we experimentally demonstrate such control in a trapped ultracold 40Ca+ ion, where the phase of qubit ground state is modified by optical driving to an excited state. Using a third spectator quantum level as reference, we experimentally measure the phases generated on the ground state amplitude by different driving schemes. While geometric phases exhibit intrinsic robustness to systematic control errors due to their dependence on global geometric properties (e.g., Berry curvature) rather than dynamical details, their resilience to decoherence remains a topic requiring critical clarification. In this work, we show that geometric phases are not in general immune to environment-induced decoherence, and their practical robustness, including to decoherence, depends on active error-suppression strategies. We implement different driving schemes and find that minimizing excited state decay is more decisive for their robustness on decoherence than whether the phase is of a geometric or dynamical character. This work established and demonstrated a new control strategy that mitigates decoherence effects in geometric/complex phase systems, compensating for their susceptibility to decoherence. We experimentally benchmark four phase-imprinting protocols under a tunable engineered dissipation channel and controlled systematic errors in a single trapped-ion qubit. Our central quantitative finding is that the sensitivity to decoherence is governed by the time-integrated excited-state population, rather than by whether the accumulated phase is labeled geometric or dynamical.
