We present a scalable scheme to achieve photon distillation, i.e., the preparation of a single photon with reduced indistinguishability error out of multiple imperfect photons, based on multiphoton interference in Fourier matrices. Our scheme achieves arbitrary error reduction in a single step, removing the need to concatenate multiple rounds of the protocol. Our scheme has resource costs linear in the error reduction, and a constant probability of success for arbitrary error reductions. We find that the scheme both raises the threshold of error correcting codes in photonic quantum computers, as well as reduces the resources required to construct a photonic quantum computer, making it highly promising for implementation in such devices.

We introduce state-of-the-art protocols to distill indistinguishable photons, reducing distinguishability error rates by a factor of $n$, while using a modest amount of resources scaling only linearly in $n$. Our resource requirements are significantly lower and the protocols have fewer hardware requirements than in previous works, making large-scale distillation experimentally feasible for the first time. This efficient reduction of distinguishability error rates has direct applications to fault-tolerant linear optical quantum computation, potentially leading to improved thresholds for photon loss errors and allowing smaller code distances, thus reducing overall resource costs. Our protocols are based on Fourier transforms on finite Abelian groups, special cases of which include the discrete Fourier transform and Hadamard matrices. This general perspective allows us to unify previous results on distillation protocols and introduce a large family of efficient schemes. We utilize the rich mathematical structure of Fourier transforms, including symmetries and related suppression laws, to quantify the performance of these distillation protocols both analytically and numerically. Finally, our work resolves an open question concerning suppression laws for the $n$-photon discrete Fourier transform: the suppression laws are exactly characterized by the well-known zero transmission law if and only if $n$ is a prime power.

Spins associated to solid-state color centers are a promising platform for investigating quantum computation and quantum networks. Recent experiments have demonstrated multiqubit quantum processors, optical interconnects, and basic quantum error-correction protocols. One of the key open challenges towards larger-scale systems is to realize high-fidelity universal quantum gates. In this work, we design and demonstrate a complete high-fidelity gate set for the two-qubit system formed by the electron and nuclear spin of a nitrogen-vacancy center in diamond. We use gate set tomography (GST) to systematically optimize the gates and demonstrate single-qubit gate fidelities of up to $99.999(1)\mathrm{\%}$ and a two-qubit gate fidelity of $99.93(5)\mathrm{\%}$. Our gates are designed to decouple unwanted interactions and can be extended to other electron-nuclear spin systems. The high fidelities demonstrated provide opportunities towards larger-scale quantum processing with color-center qubits.

Quantum algorithms implemented on near-term devices require qubit mapping due to noise and limited qubit connectivity. In this paper we propose a strategy called algorithm-oriented qubit mapping (AOQMAP) that aims to bridge the gap between exact and scalable mapping methods by utilizing the inherent structure of algorithms. While exact methods provide optimal solutions, they become intractable for large circuits. Scalable methods, like swap networks, offer fast solutions but lack optimality. AOQMAP bridges this gap by leveraging algorithmic features and their association with specific device substructures to achieve depth-optimal and scalable solutions. The proposed strategy follows a two-stage approach. First, it maps circuits to subtopologies to meet connectivity constraints. Second, it identifies the optimal qubits for execution using a cost function and performs postselection among execution results across subtopologies. Notably, AOQMAP provides both scalable and optimal solutions for variational quantum algorithms with fully connected two-qubit interactions on common subtopologies including linear, T-, and H-shaped, minimizing circuit depth. Benchmarking experiments conducted on IBM quantum devices demonstrate significant reductions in gate count and circuit depth compared to Qiskit, Tket, and swap network. Specifically, AOQMAP achieves up to an 82% reduction in circuit depth and an average 138% increase in success probability. This scalable and algorithm-specific approach holds the potential to optimize a wider range of quantum algorithms.

Dissipative stabilization of cat qubits autonomously suppresses for bit-flip errors by ensuring that reservoir-engineered two-photon losses dominate over other mechanisms, inducing phase-flip errors. To describe the latter, we derive an effective master equation for an asymmetrically threaded superconducting quantum interference device–based superconducting circuit used to stabilize a dissipative cat qubit. We analyze the dressing of relaxation processes under drives in time-dependent Schrieffer-Wolff perturbation theory for weakly anharmonic bosonic degrees of freedom and in numerically exact Floquet theory. We find that spurious single-photon decay rates can increase under the action of the parametric pump that generates the required interactions for cat-qubit stabilization. Our analysis feeds into mitigation strategies that can inform current experiments, and the methods presented here can be extended to other circuit implementations.

Cavity magnonics is a promising field focusing on the interaction between spin waves (magnons) and other types of signal. In cavity magnonics, isolation of magnons from the cavity to allow signal storage and processing fully in the magnonic domain is highly desired, but its realization is often hindered by the lack of necessary tunability of the interaction. This work shows that by using the collective mode of two yttrium iron garnet spheres and applying Floquet engineering, magnonic signals can be switched on demand to a magnon dark mode that is protected from the environment, enabling a variety of manipulation over the magnon dynamics. Our demonstration can be scaled up to systems with an array of magnonic resonators, paving the way for large-scale programmable hybrid magnonic circuits.

One method for scaling up quantum systems is to adopt a modular approach. In the ion-trap architecture, an efficient photonic interface between independent linear ion traps would allow for such an expansion. To this end, an optical cavity with a small mode volume can be used to enhance the photon-emission probability from the ion. Miniature fiber-based Fabry-Perot cavities have been integrated into three-dimensional Paul traps that hold a single ion, but an efficient interface between an optical cavity and a linear trap that can keep multiple ions has remained elusive. This presents a barrier for combining the benefits of the motional coupling in a chain of ions with an optical interface between ion traps. In this paper, we show that simple electrically conductive shielding of the fibers could provide a substantial advantage in mitigating the adverse effects of stray charges and motional heating by dielectrics. We also reveal that while conductive shields are not compatible with the conventional radio frequency (rf) drives in ion traps, using two rf signals with opposite phases can solve this issue. Furthermore, the role played by the symmetry of the electrodes when incorporating an element that disrupts the translational symmetry of a linear trap is elucidated analytically. As a result, it is realized that a two-dimensional implementation of a linear ion trap such as a surface trap is inherently unsuitable for integrating a shielded miniature optical cavity due to the lack of geometrical symmetry. Based on the insights obtained through this analysis, we identify essential components and propose a design strategy for successful integration of a miniature optical cavity into a linear ion trap.

In the circuit quantum electrodynamics architectures, to realize a long-range quantum network mediated by a flying photon, it is necessary to shape the temporal profile of emitted photons to achieve a high transfer efficiency between two quantum nodes. In this work, we demonstrate an alternative single-rail and dual-rail time-bin-shaped photon generator without additional flux-tunable elements, which can act as a quantum interface of a point-to-point quantum network. In our approach, we adopt a qubit-resonator-transmission-line configuration, and the effective coupling strength between the qubit and the resonator can be varied by parametrically modulating the qubit frequency. In this way, the coupling is directly proportional to the parametric modulation amplitude and covers a broad tunable range beyond 20 MHz for the sample we used. Additionally, when emitting shaped photons, we find that the spurious frequency shift (−0.4 MHz) due to parametric modulation is small and can be readily calibrated through chirping. We develop an efficient photon field measurement setup based on the data-stream processing of GPU. Utilizing this system, we perform photon temporal profile measurement, quantum state tomography of photon field and quantum process tomography of single-rail quantum state transfer based on a heterodyne-measurement scheme. The single-rail encoding state-transfer fidelity of shaped photon emission is $90.32\mathrm{\%}$, and that for unshaped photons is $97.20\mathrm{\%}$, respectively. We believe that the fidelity of shaped photon emission is mainly limited by the qubit coherence time. The results demonstrate that our method is hardware efficient, simple to implement, and scalable. It could become a viable tool in a high-quality quantum network utilizing both single-rail and dual-rail time-bin encoding.

Entanglement generation between remote qubit systems is the central tasks for quantum communication. Future quantum networks will have to be compatible with low-loss telecom bands and operate with large separation between qubit nodes. Single-click heralding schemes can be used to increase entanglement rates at the cost of needing an optically phase-synchronized architecture. In this paper we present such a phase synchronization scheme for a metropolitan quantum network, operating in the low-loss telecom L band. To overcome various challenges such as communication delays and optical power limitations, the scheme consists of multiple tasks that are individually stabilized. We characterize each task, identify the main noise sources, motivate the design choices, and describe the synchronization schemes. The performance of each of the tasks is quantified by a transfer-function measurement that investigates the frequency response and feedback bandwidth. Finally we investigate the resulting optical phase stability of the fully deployed system over a continuous period of 10 h, reporting a short-term stability standard deviation of $\sigma \approx {30}^{\circ }$ and a long-term stability of the average optical phase to within a few degrees. The scheme presented served as a key enabling technology for a nitrogen-vacancy-center-based metropolitan quantum link. This scheme is of interest for other quantum network platforms that benefit from an extendable and telecom-compatible phase-synchronization solution.

This paper develops a protocol to synchronize clocks on board a network of satellites equipped with quantum resources. We show that, in such a constellation, satellites reinforce each other’s sync capabilities, forming a common clock that is more stable and precise than its constituents. We envision the resulting network as a master clock able to distribute time across the globe, providing the basis for a future quantum global navigation satellite system or a space-based quantum network. As an example of its capabilities, we show that a constellation of 50 satellites equipped with modest quantum resources, and distributed amongst five orbits at an altitude of 500 km, allows the synchronization of clocks spread across the globe at subnanosecond precision.

Extracting information from quantum systems is crucial in quantum physics and information processing. Methods based on randomized measurements, such as shadow estimation, show advantages in effectively achieving such tasks. However, randomized measurements require the application of random unitary evolution, which unavoidably necessitates frequent adjustments of the experimental setup or circuit parameters, posing challenges for practical implementations. To address these limitations, positive operator–valued measurements (POVMs) have been integrated to realize real-time single-setting shadow estimation. In this work, we advance the POVM-based shadow estimation by reducing the controlled-not (cnot)-gate count for the implementation circuits of informationally complete POVMs (IC POVMs), in particular, symmetric IC POVMs (SIC POVMs), through the dimension-dilation framework. We show that any single-qubit minimal IC POVM can be implemented with the use of at most two cnot gates, while a SIC POVM can be implemented with only one cnot gate. In particular, we provide a concise form of the compilation circuit of any SIC POVM along with an efficient algorithm for the determination of gate parameters. Moreover, we apply the optimized compilation circuit to shadow estimation, showcasing its noise-resilient performance and highlighting the flexibility in compiling various SIC POVMs. Our work paves the way for the practical applications of qubit IC POVMs on quantum platforms.

On the quest to understand and reduce environmental noise in $\mathrm{Si}$ spin qubits, hyperfine interactions between electron and nuclear spins impose a major challenge. Silicon is a promising host material because one can increase the spin coherence time by removing spinful ${}^{29}\mathrm{Si}$ isotopes. As more experiments rely on isotopic purification of $\mathrm{Si}$, the role of other spinful atoms in the device should be clarified. This is not a straightforward task, as the hyperfine interactions with atoms in the barrier layers are poorly understood. We use density-functional theory to determine the hyperfine tensors of both $\mathrm{Si}$ and $\mathrm{Ge}$ in a crystalline epitaxial $\mathrm{Si}$/$\mathrm{Si}$$\mathrm{Ge}$ quantum well as well as $\mathrm{Si}$ and $\mathrm{O}$ atoms in an amorphous $\mathrm{Si}$/${\mathrm{Si}\mathrm{O}}_2$ (MOS) interface structure. On the basis of the results, we estimate the dephasing time $T_2^{\ast }$ due to magnetic noise from the spin bath and show that the coherence is limited by interactions with non-$\mathrm{Si}$ barrier atoms to a few microseconds in $\mathrm{Si}$/$\mathrm{Si}$$\mathrm{Ge}$ (for nonpurified $\mathrm{Ge}$) and about 100 $\text{μ}\mathrm{s}$ in $\mathrm{Si}$-MOS. Expressing these numbers alternatively, we find in $\mathrm{Si}$/$\mathrm{Si}$$\mathrm{Ge}$ the interactions with $\mathrm{Ge}$ dominate below a ${}^{29}\mathrm{Si}$ content of 1000 ppm, and, due to low natural concentration of the spinful oxygen isotopes, the interactions with oxygen in $\mathrm{Si}$-MOS become significant only below a ${}^{29}\mathrm{Si}$ content of 1 ppm.

Owing to its repeaterlike rate-loss scaling, twin-field quantum key distribution (TF-QKD) has repeatedly exhibited in the laboratory its superiority for secure communication over record fiber lengths. Field trials pose a new set of challenges, however, which must be addressed before the technology’s roll-out into the real world. Here, we verify in the field the viability of using independent optical frequency combs, installed at sites separated by a straight-line distance of 300 km, to achieve a versatile TF-QKD setup that has no need for optical frequency dissemination and thus enables an open and network-friendly fiber configuration. Over 546- and 603-km symmetric links, we record a finite-size secure key rate (SKR) of 0.53 bit/s and an asymptotic SKR of 0.12 bit/s, respectively. Of practical importance, the setup is demonstrated to support 44-km fiber asymmetry in the 452-km link. Our work represents an important step toward incorporation of long-haul fiber links into large quantum networks.

Developing Hamiltonian models for quantum processors with many qubits on the same chip is crucial for advancing quantum computing technologies. Stray couplings between qubits lead to errors in gate operations. This study underscores the importance of incorporating lattice Hamiltonians into quantum circuit design. By comparing many-body effects with two-body stray couplings, we show how adjusting circuit parameters can increase two-qubit-gate fidelity. We find that loosely decoupled qubits result in weaker stray interactions and higher gate fidelity, challenging conventional assumptions. We investigate the scenario where three-body $ZZZ$ interaction surpasses two-body $ZZ$ interactions, highlighting the transformative potential of lattice Hamiltonians for novel multiqubit gates. Moreover, we investigate the cross-resonance gate within the lattice-Hamiltonian framework and examine the impact of microwave pulses on stray coupling. This emphasizes the necessity of developing a comprehensive theoretical framework that includes lattice interactions, which are now critical given the sophistication of contemporary quantum hardware. These insights are vital for developing fault-tolerant quantum computing and next-generation quantum processors.

Fault-tolerant quantum computation requires low physical-qubit gate errors. Many approaches exist to reduce gate errors, including both hardware- and control-optimization strategies. Dynamically corrected gates are designed to cancel specific errors and offer the potential for high-fidelity gates, but they have yet to be implemented in singlet-triplet spin qubits in semiconductor quantum dots, due in part to the stringent control constraints in these systems. In this work, we experimentally implement dynamically corrected gates designed to mitigate hyperfine noise in a singlet-triplet qubit realized in a $\mathrm{Si}$/$\mathrm{Si}\mathrm{Ge}$ double quantum dot. The corrected gates reduce infidelities by about a factor of 3, resulting in gate fidelities above $0.99$ for both identity and Hadamard gates. The gate performances depend sensitively on pulse distortions, and their specific performance reveals an unexpected distortion in our experimental setup.

Photonic addressing of superconducting circuits has been proposed to overcome wiring complexity and heat-load challenges. However, such superconducting-photonic links suffer from an efficiency-noise trade-off that limits scalability. This trade-off arises because increasing power conversion efficiency entails reducing optical power, which makes the converted signal susceptible to shot noise. We analyze this trade-off and find that the laser-driven qubit gate infidelity scales inversely with the number of photons used. While methods such as nonlinear detection or squeezed light could mitigate this effect, we consider generating higher-frequency electrical signals, such as millimeter waves (100 GHz), using laser light. At these higher frequencies, circuits have higher operating temperatures and cooling power budgets and alleviate constraints posed by the trade-off. Therefore, we demonstrate an optically driven cryogenic millimeter-wave source with a maximum power efficiency of $8\times {10}^{-5}$ that can generate a maximum of $0.7\text{μ}\mathrm{W}$ of 80 GHz power, with a $1200$-thermal-photon equivalent of added noise at $4\mathrm{K}$. Using this source, we perform frequency-domain spectroscopy of superconducting $\mathrm{Nb}$$\mathrm{Ti}$$\mathrm{N}$ resonators at 80–90 GHz. Our results show a promising approach to lessen the efficiency-noise constraints on superconducting-photonic links, while leveraging the benefits of photonic signal delivery. Further optimization of power efficiency and noise at high frequencies could make scalable photonic control of superconducting qubits viable at temperatures exceeding $1\mathrm{K}$.

Large-scale programmable trapped ion hardware, featuring high gate fidelity and long coherence times, is promising for realizing a practical fault-tolerant quantum computer. However, individual addressing (IA) methods, which are useful for implementing programmable gates in near-term quantum devices, can lead to undesired errors between the target ions and neighboring ions. In this work, we present a low-crosstalk optical addressing system based on multiple optical objectives and acousto-optic deflectors (AODs) with a symmetrical configuration. Two counterpropagating Raman operation beams are both tightly focused, generating an overlapping spot with a waist radius of approximately $1\text{μ}\mathrm{m}$, to address the target ion. As a result, IA crosstalk, characterized by Rabi-rate crosstalk on the spectator ion, is measured to be $1.19(5)\times {10}^{-3}$, with the two ions separated by approximately 5.5 $\text{μ}\mathrm{m}$. This low-crosstalk optical addressing system holds promise for high-fidelity entangling operations, and the symmetrically configured AODs in our method can be readily extended to two dimensions to address a two-dimensional ion crystal.

Optically interfaced solid-state defects are promising candidates for quantum communication technologies. The ideal defect system would feature bright telecom emission, long-lived spin states, and a scalable material platform, simultaneously. Here we use one such system, vanadium (${\mathrm{V}}^{4+}$) in silicon carbide, to establish a potential telecom spin-photon interface within a mature semiconductor host. This demonstration of efficient optical spin polarization and readout facilitates all-optical measurements of temperature-dependent spin relaxation times ($T_1$). By using this technique, and lowering the temperature from approximately 2 K to approximately 100 mK, we observe a remarkable 4-orders-of-magnitude increase in spin $T_1$ across all measured sites, with site-specific values ranging from 57.1 ms to 27.9 s. Furthermore, we identify the underlying relaxation mechanisms, which involve a two-phonon Orbach process, indicating the opportunity for strain tuning to enable qubit operation at higher temperatures. These results position ${\mathrm{V}}^{4+}$ in $\mathrm{Si}\mathrm{C}$ as a prime candidate for scalable quantum nodes in future quantum networks.

We demonstrate coherent control of a three-electron exchange-only spin qubit with the quantum dots arranged in a close-packed triangular geometry. The device is tuned to confine one electron in each quantum dot, as evidenced by pairwise charge stability diagrams. Time-domain control of the exchange coupling is demonstrated and qubit performance is characterized using blind randomized benchmarking, with an average single-qubit gate fidelity $F=99.84\mathrm{\%}$. The compact triangular device geometry can be readily scaled to larger two-dimensional quantum dot arrays with high connectivity.

On-demand storage and retrieval of quantum information in coherent light-matter interfaces is a key requirement for future quantum networking and quantum communication applications. Alkali vapor memories offer scalable and robust high-bandwidth storage at high repetition rates, which makes them a natural fit to interface with solid-state single-photon sources. Here, we experimentally realize a room-temperature ladder-type atomic vapor memory that operates on the $\mathrm{Cs}$ D1 line at 895 nm. We provide a detailed experimental characterization and demonstration of on-demand storage and retrieval of weak coherent laser pulses with an average of 0.06(2) photons per pulse at a high signal-to-noise ratio of 830(80). The memory achieves a maximum internal efficiency of ${\eta }_{\text{int}}=15(1)\mathrm{\%}$ and an estimated $1/e$ storage time of ${\tau }_s\approx 32$ ns. Benchmark properties for the storage of single photons from inhomogeneously broadened state-of-the-art solid-state emitters are estimated from the performance of the memory. Together with the immediate availability of high-quality $\mathrm{In}\mathrm{Ga}\mathrm{As}$ quantum dots emitting at 895 nm, these results provide clear prospects for the development of a heterogeneous on-demand quantum light interface.