Accurate characterization of nonlinear photonic integrated circuits is fundamentally limited by the inability of linear transmission measurements to distinguish input and output on/off-coupling efficiencies, which enter only through their product. We show that this degeneracy arises from an inherent input-output symmetry of linear optics and can be broken by exploiting the directional asymmetry of nonlinear processes. We introduce bidirectional nonlinear optical tomography (BNOT), which combines forward and backward pumping of complementary nonlinear interactions to uniquely infer individual on/off-coupling efficiencies from off-chip measurements. By jointly modeling the nonlinear response and detection statistics, BNOT enables unbiased reconstruction of on-chip nonlinear figures of merit with significantly reduced uncertainty, whereas conventional calibration yields systematically biased estimates. Monte Carlo simulations demonstrate reliable convergence of the inferred efficiencies across realistic noise and fluctuation regimes. Our results establish nonlinear directionality as a general symmetry-breaking resource for coupling-resolved characterization in integrated photonics, with immediate implications for scalable quantum light sources, frequency conversion, and precision optical metrology.

We present a variational quantum algorithm that solves the one-dimensional diffusion problem with a space-dependent diffusion constant $D(x)$. This problem is relevant for the exchange of hydroxide ion across a two-layer membrane in an alkaline electrolyzer, where the concentration of ${\text{OH}}^{-}$ ion determines the chemical stability for longer time periods. We use 16 to 64 grid points across the membrane, resulting from $n=4$ to 6 data qubits for the ideal statevector and shot-based quantum simulations implemented using Qiskit. For these qubit numbers, the depth of the parametric quantum circuit has been chosen to ensure sufficient expressibility. The state preparation requires particular attention since the diffusivity $D$ is piecewise constant in the different layers with discontinuities at the interface. Furthermore, we compare different classical optimization schemes with respect to their convergence in the variational quantum algorithms (VQA) method. We demonstrate the applicability of the quantum algorithm to a problem with nontrivial boundary conditions and jump conditions of the diffusion constant and outline possible extensions of the proof-of-concept application case of quantum computing. Our simulations show that pronounced hydroxide-ion concentration gradients, and thus chemical instabilities, can occur only when the ratio of diffusivity in both layers of the membrane exceeds approximately 50.

We present a gradient-based method to construct memory-efficient, high-fidelity, single-qubit gates for fluxonium qubits. These gates are constructed using a sequence of single-flux quantum (SFQ) pulses that are sent to the qubit through either capacitive or inductive coupling. The schedule of SFQ pulses is constructed with an on-ramp and an off-ramp applied prior to and after a pulse train, where the pulses are spaced at intervals equal to the qubit period. We reduce the optimization problem to the scheduling of a fixed number of SFQ pulses in the on-ramp and solve it by relaxing the discretization constraint of the SFQ clock as an intermediate step, allowing the use of the Broyden-Fletcher-Goldfarb-Shanno optimizer. Using this approach, gate fidelities of 99.99% can be achieved for inductive coupling and 99.9% for capacitive coupling, with leakage being the main source of coherent errors for both approaches.

Phononic nanodevices offer a promising route toward quantum technologies, as phonons combine strong confinement within matter with broad coupling capabilities to various quantum systems. In particular, the piezoelectric response of materials, such as lithium niobate, enables coupling between superconducting qubits and gigahertz-frequency phonons. However, bulk lithium niobate phononic devices typically rely on surface acoustic waves and are therefore inherently subject to leakage from the surface into the bulk substrate. Here, we explore the acoustic behavior of resonator cavities supporting GHz-frequency Lamb waves in a 200-nm-thick suspended lithium niobate layer. We characterize the acoustic response at both room and millikelvin temperatures. We find that our resonator cavities with strong confinement reach intrinsic quality factors of approximately 6000 at the single phonon level. We use the measured parameters of the resonators to model their coupling to a superconducting transmon qubit, allowing us to evaluate their potential as quantum acoustic devices.

Privacy is a fundamental requirement in distributed quantum sensor networks, where multiple clients estimate spatially distributed parameters using shared quantum resources while interacting with potentially untrusted servers. Despite its importance, existing privacy conditions rely on idealized quantum bounds and do not fully capture the operational constraints imposed by realistic measurements. Here, we introduce a universal operational privacy framework for distributed quantum sensing, formulated in terms of the experimentally accessible Fisher information matrix and applicable to arbitrary protocols characterized by singular information structures. The proposed condition provides a protocol-independent criterion, ensuring that no information about individual parameters is accessible to untrusted parties. We further experimentally demonstrate that a distributed quantum sensing protocol employing fewer photons than the number of estimated parameters simultaneously satisfies the universal privacy condition and achieves Heisenberg-limited precision. Our results establish universal operational constraints governing privacy in distributed quantum sensor networks and provide a foundation for practical, privacy-preserving quantum sensing beyond full-rank regimes.

Aluminum remains the central material for superconducting qubits, and considerable effort has been devoted to optimizing its deposition and patterning for quantum devices. However, postprocessing strategies focused on oxide removal of Nb- and Ta-based resonators using buffered oxide etch (BOE), which cannot be used for aluminum (Al). This challenge becomes particularly relevant for industry-scale fabrication with multichip bonding, where delays between sample preparation and cooldown require surface treatments that preserve low dielectric loss during extended exposure to ambient conditions. In this work, we investigate surface modification approaches for Al resonators subjected to a 24-h delay prior to cryogenic measurement. Passivation using self-limiting oxygen and fluorine chemistries was evaluated utilizing different plasma processes. Remote oxygen plasma treatment reduced dielectric losses, in contrast to direct oxygen plasma. A fluorine-based plasma process was developed that passivated the Al surface for subsequent BOE treatment. However, the fluorine content on the surface resulted in higher loss, identifying fluorine as an unsuitable passivation material for Al resonators. Above all, selective oxide removal using HF vapor and phosphoric acid yielded median dielectric losses as low as ${\tilde{\delta }}_{\mathrm{LP}}=5.7\times {10}^{-7}$ ($Q_{\mathrm{LP}}\approx 1.7\mathrm{M}$) with ${\tilde{\delta }}_{\mathrm{TLS}}=3.6\times {10}^{-7}$ ($Q_{\mathrm{TLS}}\approx 2.8\mathrm{M}$) in the single-photon regime. Selective oxide removal provides a promising pathway for robust Al-based qubit fabrication, as it preserves low dielectric losses for a 24-h delay before cooldown.

Reducing materials- and processing-induced decoherence is critical to the development of utility-scale quantum processors based on superconducting qubits. Here, we report on the impact of two fluorine-based wet etches, which we use to treat the silicon surface underneath the Josephson junctions (JJs) of fixed-frequency transmon qubits made with aluminum base metallization. Using several materials analysis techniques, we demonstrate that these surface treatments can remove germanium residue introduced by our JJ fabrication, with no other changes to the overall process flow. When combined with argon ion milling before JJ fabrication, these surface treatments result in significantly improved energy relaxation times for the highest-performing process, with median $T_1=334\mathrm{\mu }\mathrm{s}$, corresponding to quality factor $Q=6.6\times {10}^6$. This result suggests that the metal-substrate interface directly underneath the JJs was a major contributor to microwave loss in these transmon qubits prior to the integration of these surface treatments. Furthermore, this work illustrates how quantum device performance metrics, in conjunction with materials analysis targeted at identifying the impacts of specific processing steps, can be used to improve the performance of superconducting qubits.

We theoretically investigate a high-efficiency and broadband microwave storage and retrieval scheme employing a superconducting artificial chiral molecule embedded in a one-dimensional transmission line. By optimizing the parametric coupling, the chiral $\mathrm{\Lambda }$-type molecule enables near 100% storage efficiency and fidelity across a bandwidth of 100 MHz. Our results provide a feasible pathway toward implementing microwave quantum memories compatible with broadband quantum networks.

Measurement-induced quantum computation with continuous-variable cluster states utilizes teleportation to transmit and alter quantum states via measurement-and-feedforward control. One of the key challenges of this approach is the deterioration of quantum states caused by the noise added due to imperfect entanglement of the cluster. We analyze the propagation of a quantum non-Gaussian state with nonlinear squeezing through a small cluster state. We show that a nonlinear feedforward in the deterministic teleportation protocol reduces the added noise and improves the nonlinear squeezing transferred. In a probabilistic regime, the improvement can be manifested even with current experimental resources. Better processing of non-Gaussian states can bring us closer to the necessary interplay between cluster states and non-Gaussianity required by quantum computing.

Identity authentication between communicating parties is a fundamental prerequisite for secure communication. Classical authentication systems face serious threats from the advent of quantum computing. In contrast, quantum identity authentication (QIA) can provide security based on the fundamental principle of quantum mechanics and has consequently attracted significant research interest. However, currently most QIA schemes either belong to one-way identity authentication or suffer from information leakage during the identity authentication process. Here, we for the first time propose a bidirectional QIA scheme with zero-knowledge proof. This scheme will not disclose any information other than the authenticity of the statement itself, and it possesses the merits of simple structure and easy execution. Theoretical analysis has been conducted on the completeness, robustness, and zero-knowledge nature of the proposed scheme. Furthermore, corresponding experimental demonstrations have been carried out by utilizing entanglement sources. This work has effectively improved the practical security and practicability of QIA, paving the way for the construction of a large-scale quantum internet in the future.

Resonator-photon-induced dephasing presents a critical challenge for superconducting qubit coherence. This study examines the influence of the frequency and number of coherent photons on quantum coherence degradation. Our measurements show that the dephasing profile across photon frequencies closely follows the resonator’s spectral characteristics, establishing a direct link between dephasing dynamics and resonator-qubit interaction parameters. Of note, dynamical decoupling demonstrated robust performance in mitigating coherence decay under a wide range of photon conditions. This underscores its potential for addressing various forms of coherent photon noise, including ac Stark shifts and additional photon-mediated dephasing.

Practical quantum advantage hinges on executing deep quantum circuits within the coherence limits of noisy intermediate-scale quantum processors. The absence of native, high-fidelity multiqubit gates remains a major bottleneck, as their decomposition into single- and two-qubit gates leads to prohibitive depth and error overhead. Here, we propose a hardware-efficient protocol that directly implements a native controlled-controlled-Z gate in a tunable-coupler superconducting circuit. Our theoretical protocol activates a resonant three-qubit interaction via a time-frequency-correlated virtual process, explicitly relying on the dynamic resonant exchange within the $|101⟩\leftrightarrow |020⟩$ transition manifold. This approach is compatible with standard tunable-coupler architectures without requiring additional control resources. Through a systematic calibration workflow combining pulse shaping and active cancellation of residual phases, we demonstrate a gate fidelity exceeding 99% within 165 ns—significantly outperforming decomposed sequences. Comprehensive error budgeting confirms that the gate performance remains robust against realistic experimental imperfections. Furthermore, we show that this scheme can be naturally extended to a continuous $\mathrm{CCPhase}(\theta )$ gate set. This work provides a direct, high-fidelity route to three-qubit entanglement, offering promising prospects for efficient execution of quantum algorithms on near-term superconducting hardware.

High-quality quantum communications that enable important capabilities, such as distributed quantum computing and sensing, will require quantum repeaters for providing high-quality entanglement. To realize high-rate heralded entanglement for quantum repeaters, Chen et al. [Phys. Rev. Appl. 19, 054029 (2023)] proposed a scheme for heralded-multiplexed generation of quasideterministic entangled photon pairs, called zero-added-loss multiplexing (ZALM). Here, we propose a design of the ZALM source using time-bin entanglement and spectral shearing. Additionally, we provide an analysis of experimentally relevant spectral-shearing parameters to optimize the spectral multiplexing. Moreover, we experimentally verify the compatibility of time-bin pulses and spectral shearing, as supported by observation of no appreciable phase shift when the same shearing is applied to both time bins. These results expand the benefits of applying a ZALM source to time-bin entanglement use cases. Moreover, more fully demonstrating time-bin and spectral shearing compatibility clears a path toward a broader use of spectral shearing that provides a deterministic frequency shift of high utility.

Crosstalk is a key obstacle to scaling up quantum computers. It may arise from persistent qubit-qubit couplings or dynamically during gate operation, with the latter being particularly difficult to detect. Here, we introduce the perfect-entangler spectrum as a means to identify dynamic crosstalk leading to undesired entanglement. It leverages the geometric classification of two-qubit gates in terms of perfect entanglers. We exemplify the application of the spectroscopy for fixed-frequency transmons and parametrically driven gates. When scanning the frequency of a spectator qubit, peaks in the perfect-entangler spectrum signal dynamic crosstalk, and analysis of the peaks reveals the mechanisms causing the crosstalk. We discuss the experimental implementation of the crosstalk spectroscopy, which requires two two-qubit gate tomographies.

Recently proposed metastability-induced quantum batteries have shown particular promise for coherent microwave generation. However, achieving high-power coherent microwave generation in quantum batteries remains fundamentally challenging due to quantum correlations, aging, and self-discharging processes. For the cavity-quantum-electrodynamics-based quantum batteries, a further trade-off arises between strong spin-photon coupling for energy storage and sufficient output coupling for power delivery. To overcome these constraints, we introduce dissipation engineering as a dynamic control strategy that temporally separates energy storage and release. By suppressing emission during charging and rapidly enhancing the output coupling during discharging, we realize nanosecond microwave bursts with watt-level peak power. By optimizing three dissipation schemes, we improve work extraction efficiency of the quantum battery by over 2 orders of magnitude and achieve high-power compression factors outperforming the state-of-the-art techniques, establishing dissipation engineering as a pathway toward room-temperature, high-power coherent microwave sources.

We present a method to calculate the frequency components of a pump waveform driving a parametric oscillator, which realizes a desired frequency mixing or scattering between frequency modes. The method is validated by numerical analysis, and we study its sensitivity to added Gaussian noise. A series of experiments apply the method and demonstrate its ability to realize complex scattering processes involving many modes at microwave frequencies, including nonreciprocal mode circulation. We also present an approximate method to dynamically control mode scattering, capable of rapidly routing signals between modes in a prescribed manner. These methods are useful tools for encoding and manipulating continuous-variable quantum information with multimodal Gaussian states.

Electrons bound to the surface of liquid helium are an emerging quantum computing platform, offering the potential for highly mobile spin qubits that can be manipulated using complementary metal-oxide-semiconductor (CMOS)-fabricated devices. Here, as a step toward realizing this technology, we demonstrate selective two-dimensional shuttling of electrons across a helium film condensed on the surface of a CMOS control chip. The electrons are moved in packets containing, on average, several tens down to single electrons. We perform CCD-style electron shuttling in any of 128 transport microchannels, each of which links electron storage zones and sensing zones in the two-dimensional plane. This shuttling “unit cell” is repeated 32 times across the chip, allowing packet sensing to be performed in parallel. Despite the resulting gain in sensitivity, we find no evidence of electron loss after repeating shuttling sequences at least ${10}^9$ times. The device serves as a prototype quantum information processing platform that is readily scalable to control large monolithically integrated arrays of electron spins.

We propose a scalable dual-species fluxonium-transmon system that utilizes a central transmon to mediate high-fidelity gates and parity checks between two fluxonium qubits without the need for strong nonlocal interactions. This approach suppresses unwanted long-range interactions, which is critical for developing larger quantum processors. First, we analyze the performance of cross-resonance (CR) cnot gates between a fluxonium and a transmon. We show that even in the presence of a spectator qubit, these gates maintain high fidelity with coherent errors on the order of ${10}^{-5}$. We then demonstrate that these gates, when applied sequentially, enable high-fidelity parity checks and logical fluxonium-fluxonium cnot gates. In addition, the central transmon can facilitate the readout of the neighboring fluxoniums, consolidating multiple critical functions into a single ancilla. Our work establishes the viability of a dual-species architecture as a promising path toward fault-tolerant quantum computation.

The surface of three-dimensional topological insulators (TIs) constitutes an intriguing system for hosting Majorana zero modes (MZMs) when rendered superconducting via the proximity effect. To study the peculiar role of MZMs and the accompanied $4\pi$-periodic energy-phase relation in TI-based Josephson junctions (JJs), we constructed a superconducting flux qubit out of a JJ based on ${\text{Sn}}_{0.02}{\text{Bi}}_{1.08}{\text{Sb}}_{0.9}{\text{Te}}_2\mathrm{S}$ (Sn-doped BSTS). In the vicinity of the qubit’s sweet spot, we obtained a two-tone spectrum that differed from that of a conventional flux qubit. The analysis employing different models suggests the existence of 4π-periodic supercurrents in the junction. The results support the fact that superconducting flux qubits incorporating TI-based JJs may serve as a potential platform for the detection and further braiding of MZMs.