Currently available superconducting quantum processors with interconnected transmon qubits are noisy and prone to various errors. The errors can be attributed to sources such as open quantum system effects and spurious interqubit couplings (crosstalk). Static $ZZ$ coupling between qubits in transmon architectures is always present and contributes to both coherent and incoherent crosstalk errors. Its suppression is therefore a key step toward enhancing the fidelity of quantum computation using transmons. Here, we propose the use of dynamical decoupling to suppress the crosstalk and we demonstrate the success of this scheme through experiments performed on several IBM quantum cloud processors. In particular, we demonstrate improvements in quantum memory as well as the performance of single-qubit and two-qubit gate operations. We perform open quantum system simulations of the multiqubit processors and find good agreement with the experimental results. We analyze the performance of the protocol based on a simple analytical model and elucidate the importance of the qubit drive frequency in interpreting the results. In particular, we demonstrate that the XY4 dynamical decoupling sequence loses its universality if the drive frequency is not much larger than the system-bath coupling strength. Our work demonstrates that dynamical decoupling is an effective, practical, and scalable way to suppress crosstalk and open-system effects, thus paving the way toward higher-fidelity logic gates in transmon-based quantum computers.

Quantum key distribution (QKD) is a crucial component for truly secure communication, which enables the leakage of information due to eavesdropper attacks to be analyzed. Although impressive progress was made in the field of long-distance implementations, user-oriented applications involving short-distance links have mostly remained overlooked. Recent technological advances in integrated photonics now enable developments towards QKD also for existing hand-held communication platforms. In this work we report on the design and evaluation of a hand-held free-space QKD system including a micro-optics-based sender unit. This system implements the BB84 protocol employing polarization-encoded faint laser pulses at a rate of $100\mathrm{MHz}$. Unidirectional beam tracking and live reference-frame alignment systems at the receiver side enable a stable operation over tens of seconds when aiming the portable transmitter to the receiver input by hand from a distance of about half a meter. The user-friendliness of our system was confirmed by successful key exchanges performed by different untrained users with an average link efficiency of about $20\mathrm{\%}$ relative to the case of the transmitter being stationarily mounted and aligned. In these tests we achieve an average quantum bit error ratio (QBER) of $2.4\mathrm{\%}$ and asymptotic secret key rates ranging from $4.0\mathrm{kbps}$ to $15.3\mathrm{kbps}$. Given its compactness, the versatile sender optics is also well suited for integration into other free-space communication systems enabling QKD over any distance.

Entangled microwave photons form a fundamental resource for quantum information processing and sensing with continuous variables. We use a low-loss Josephson metamaterial comprising superconducting, nonlinear, asymmetric inductive elements to generate frequency-entangled photons from vacuum fluctuations at a rate of 2 giga entangled bits per second spanning over the 4-GHz bandwidth. The device is operated as a traveling-wave parametric amplifier under Kerr-relieving biasing conditions. Furthermore, we demonstrate single-mode squeezing in such devices—$3.1\pm 0.7\mathrm{dB}$ below the zero-point level at half of modulation frequency.

Quantum networks facilitate the secure transmission of information between different users. Establishing communication links among multiple users in a scalable and efficient way is important for realizing a large-scale quantum network. Here we develop an energy-time entanglement-based dense wavelength division multiplexed network based on an integrated silicon nitride microring resonator, which offers a wide frequency span (covering at least the entire C-band) and narrow bandwidth modes ($\sim 650\mathrm{MHz}$). Six pairs of photons are selected to form a fully and simultaneously connected four-user quantum network. The observed quantum interference visibilities are well above the classical limits among all users. Each pair of users perform the BBM92 protocol for quantum key distribution. Our results pave the way for realizing large-scale quantum networks with integrated photonic architecture.

The interfacing of a superconducting microwave resonator with rare-earth-doped crystals presents a promising hybrid quantum system for applications including spin-assisted transducers and memories. The coupling strength between the spins of the rare-earth ions and the microwave photons is characterized by the cooperativity. Here, we report an ultrahigh cooperativity of $C\sim 650$ between rare-earth spins and a planar superconducting microwave resonator that features a highly uniform magnetic field for harnessing the strong anisotropic coupling strength of erbium-doped yttrium orthosilicate. This cooperativity rivals that from a bulk dielectric resonator and may pave a path for efficient coupling of spins with microwave photons on an integrated platform.

We propose a protocol for the deterministic generation of entanglement between two ensembles of nuclear spins surrounding two distant quantum dots. The protocol relies on the injection of electrons with definite polarization, their sequential interaction with the nuclear ensembles of each quantum dot for a short time, and the coherent transfer of each electron from one quantum dot to the other. Computing the exact dynamics for small systems, and using an effective master equation and approximate nonlinear equations of motion for larger systems, we are able to confirm that our protocol indeed produces entanglement for both homogeneous and inhomogeneous systems. Last, we analyze the feasibility of our protocol in several current experimental platforms.

The performance of a quantum processor depends on the characteristics of the device and the quality of the control pulses. Characterizing cloud-based quantum computers and calibrating the pulses that control them is necessary for high-fidelity operations. However, this time-intensive task eats into the availability of the device. Here, we show restless measurements with a dynamic repetition rate that speed-up calibration and characterization tasks. Randomized benchmarking is performed 5.3 times faster on the quantum device than when an active reset is used and without discarding any data. In addition, we calibrate a qubit with parameter scans and error-amplifying gate sequences and show speed-ups of up to a factor of 40 on the quantum device over active reset. Finally, we present a methodology to perform restless quantum process tomography that mitigates restless state preparation errors. These results reduce the footprint of characterization and calibration tasks. Quantum computers can thus either spend more time running applications or run calibrations more often to maintain gate fidelity.

In Si quantum dots, the valley degree of freedom, in particular the generally small valley splitting and the dot-dependent valley-orbit phase, adds complexities to the low-energy electron dynamics. In particular, tunnel coupling, a key knob in charge and spin qubit manipulations, is strongly dependent on the valley-orbit coupling in a Si double dot. Here we propose a four-level model that incorporates excited valley states to extract ground-state tunnel coupling information for a Si double quantum dot. This scheme is based on a charge-sensing measurement on a double dot in equilibrium, as proposed in the widely used protocol for a GaAs double dot [DiCarlo et al., Phys. Rev. Lett. 92, 226801 (2004)]. Our theory helps determine both intra- and intervalley tunnel coupling with high accuracy, and is robust against uncertainties in system parameters such as valley splittings in the individual quantum dots.

Quantum key distribution (QKD) provides a methodology for secure key distribution based on fundamental physical laws. Although significant experimental progress has been made, the previous QKD systems are mainly composed of discrete optical and electronic devices. Here, we present a prototype of a QKD transmitter by developing the technologies of integrated silicon photonics and electronics. The photonic chip integrates the essential encoding components for the decoy-state BB84 protocol using polarization encoding, while the electronic chips integrate the dedicated driving circuits. On the basis of these integrated chips, we build a desktop QKD system to verify their performance. Successful QKD experiments are conducted at a repetition frequency of 312.5 MHz using standard fiber and emulated channel loss and the intrinsic quantum bit error rate of the system is as low as 0.41%. The finite-key secret rate is 42.7 kbit/s for the 100-km fiber and 295.5 bit/s for the 40-dB emulated loss. Our results demonstrate the feasibility of QKD based on integrated optoelectronic devices and hopefully take a fundamental toward a system-in-a-package solution.

We study quantum secure direct communication using a general preshared quantum state and a generalization of dense coding. In this scenario, Alice is allowed to apply a unitary operation on the preshared state to encode her message, and the set of allowed unitaries forms a group. To decode the message, Bob is allowed to apply a measurement across his system and the system he receives. In the worst scenario, we guarantee that Eve obtains no information for the message even when Eve accesses the joint system between the system that she intercepts and her original system of the preshared state. For a practical application, we propose a concrete protocol and derive an upper bound of information leakage in the finite-length setting. We also discuss how to apply our scenario to the case with discrete Weyl-Heisenberg representation when the preshared state is unknown.

Properties of time-periodic Hamiltonians can be exploited to increase the dephasing time of qubits and to design protected one- and two-qubit gates. Recently, Huang et al. [Phys. Rev. Applied 15, 034065 (2021)] have shown that Floquet states offer a manifold of working points with dynamical protection larger than the few, usual, static sweet spots. Here, we show how Floquet theory, often used on systems with a single drive tone, can be used to describe approaches to robustly control Floquet qubits in the presence of multiple commensurate drive tones. Using this formulation, we introduce a longitudinal readout protocol to measure the Floquet qubit without the need of first adiabatically mapping the Floquet states back to the static qubit states, resulting in a significant speedup in the measurement time of the Floquet qubit. The analytical approach developed here can be applied to any Hamiltonian involving a small number of distinct drive tones, typical in the study of standard parametric gates for qubits outside of the rotating-wave approximation.

Quantum information theory sets the ultimate limits for any information-processing task. In rangefinding and LIDAR, the presence or absence of a target can be tested by detecting different states at the receiver. In this Paper, we use quantum hypothesis testing for an unknown coherent-state return signal in order to derive the limits of symmetric and asymmetric error probabilities of single-shot ranging experiments. We engineer a single measurement independent of the range, which in some cases saturates the quantum bound and for others is presumably the best measurement to approach it. This work bridges the gap between quantum information and quantum sensing and engineering and will contribute to devising better ranging sensors, as well as setting the path for finding practical limits for other quantum tasks.

Coupling a resonator to a superconducting qubit enables various operations on the qubit, including dispersive readout and unconditional reset. The speed of these operations is limited by the external decay rate of the resonator. However, increasing the decay rate also increases the rate of qubit decay via the resonator, limiting the qubit lifetime. Here, we demonstrate that the resonator-mediated qubit decay can be suppressed by utilizing the distributed-element, multimode nature of the resonator. We show that the suppression exceeds 2 orders of magnitude over a bandwidth of 600 MHz. We use this “intrinsic Purcell filter” to demonstrate a 40-ns readout with 99.1% fidelity and a 100-ns reset with residual excitation of less than 1.7%.

Degenerate parametric amplifiers (DPAs) exhibit the unique property of phase-sensitive gain and can be used to noiselessly amplify small signals or squeeze field fluctuations beneath the vacuum level. In the microwave domain, these amplifiers have been utilized to measure qubits in elementary quantum processors, search for dark matter, facilitate high-sensitivity spin resonance spectroscopy and have even been proposed as the building blocks for a measurement-based quantum computer. Until now, microwave DPAs have almost exclusively been made from nonlinear Josephson junctions, which exhibit high-order nonlinearities that limit their dynamic range and squeezing potential. In this work we investigate a microwave DPA that exploits a nonlinearity engineered from kinetic inductance. The device has a simple design and displays a dynamic range that is four orders of magnitude greater than state-of-the-art Josephson DPAs. We measure phase-sensitive gains up to $50\mathrm{dB}$ and determine that the device likely operates near the quantum noise limit. Additionally, we show that the higher-order nonlinearities that limit other microwave DPAs are almost nonexistent for this amplifier, which allows us to demonstrate its potential to perform squeezing by measuring the deamplification of coherent states by as much as 26 dB.

Coherent control of collective spin excitations (magnons) with acoustic phonons is a key technology for future hybrid spintronics devices. Surface acoustic waves (SAWs), owing to their high frequency, short wavelength, and low radiation loss, could provide an appropriate platform for integrating the devices on a chip. However, the tiny coupling efficiency in previous experiments using traveling SAWs limits the availability of this prospect. We report a planar cavity magnomechanical system, where standing acoustic waves (i.e., a SAW cavity) enhance the spatial and spectral power density to implement magnified magnon-phonon coupling. Excitation of spin-wave resonance involves significant acoustic power absorption approaching $90\mathrm{\%}$, whereas the collective spin motion exerts hybridization force on the cavity dynamics, leading to coupling cooperativity exceeding unity at room temperature in such a chip-scale device. The cavity magnomechanical system paves the way to the coherent acoustic control of magnons and the development of alternative spin-acoustic technologies for classical and quantum applications.

Benchmarking and characterizing quantum states and logic gates is essential in the development of devices for quantum computing. We introduce a Bayesian approach to self-consistent process tomography, called fast Bayesian tomography (FBT), and experimentally demonstrate its performance in characterizing a two-qubit gate set on a silicon-based spin qubit device. FBT is built on an adaptive self-consistent linearization that is robust to model approximation errors. Our method offers several advantages over other self-consistent tomographic methods. Most notably, FBT can leverage prior information from randomized benchmarking (or other characterization measurements), and can be performed in real time, providing continuously updated estimates of full process matrices while data are acquired.

Twin-field quantum key distribution (TFQKD) systems have shown great promise for implementing practical long-distance secure quantum communication due to its measurement-device-independent nature and its ability to offer fundamentally superior rate-loss scaling than point-to-point QKD systems. A surge of research and development effort in the last two years has produced many variants of protocols and experimental demonstrations. In terms of hardware topology, TFQKD systems interfering quantum signals from two remotely phase-locked laser sources are in essence giant Mach-Zehnder interferometers (MZIs) requiring active phase stabilization. Such configurations are inherently unsuitable for a TFQKD network, where more than one user pair share the common quantum measurement station, because it is practically extremely difficult, if not impossible, to stabilize MZIs of largely disparate path lengths, a situation that is inevitable in a multi-user-pair TFQKD network. On the other hand, Sagnac interferometer-based TFQKD systems exploiting the inherent phase stability of the Sagnac ring can implement asymmetric TFQKD, and are therefore eminently suitable for implementing a TFQKD network. In this work, we experimentally demonstrate a proof-of-principle multi-user-pair Sagnac TFQKD network where three user pairs sharing the same measurement station can perform pairwise TFQKD through time multiplexing, with channel losses up to $58.00$ dB, and channel loss asymmetry up to $15.00$ dB. In some cases, the secure key rates still beat the rate-loss bound for point-to-point repeaterless QKD systems, even in this network configuration. Our demonstration of this multi-user-pair TFQKD network is a step in advancing quantum-communication network technologies.

Measurements that occur within the internal layers of a quantum circuit—midcircuit measurements—are a useful quantum-computing primitive, most notably for quantum error correction. Midcircuit measurements have both classical and quantum outputs, so they can be subject to error modes that do not exist for measurements that terminate quantum circuits. Here we show how to characterize midcircuit measurements, modeled by quantum instruments, using a technique that we call quantum instrument linear gate set tomography (QILGST). We then apply this technique to characterize a dispersive measurement on a superconducting transmon qubit within a multiqubit system. By varying the delay time between the measurement pulse and subsequent gates, we explore the impact of residual cavity photon population on measurement error. QILGST can resolve different error modes and quantify the total error from a measurement; in our experiment, for delay times above $1000\mathrm{ns}$ we measure a total error rate (i.e., half diamond distance) of ${ϵ}_{\diamond }=8.1\pm 1.4\mathrm{\%}$, a readout fidelity of $97.0\pm 0.3\mathrm{\%}$, and output quantum-state fidelities of $96.7\pm 0.6\mathrm{\%}$ and $93.7\pm 0.7\mathrm{\%}$ when measuring $0$ and $1$, respectively.

Quantum nano-optics aims at transposing the concepts of quantum optics at the nanoscale. In this context, nitrogen-vacancy color centers in nanodiamonds are particularly interesting model sources as they emit single photons one by one with a broad spectral distribution between 600 and 800 nm. We deposit such quantum emitters at the extremities of silicon nanowires and analyze the effects of this interaction via spectrally resolved confocal microscopy. We demonstrate that the single-photon emission can be guided over several microns under conservation of their quantum statistics, while the silicon nanowires act as spectral filters for the quantum emission. Numerical simulations are in very good agreement with experiments, and indicate that the modal landscape of these nanowaveguides and the according coupling efficiencies with quantum emitters can be designed such that only light from emitters of specific orientations is efficiently guided through the nanowire, enabling a unique quantum state selectivity at a micrometer length scale. Our work opens the door to a genuine modal control of single-photon transfer in subwavelength waveguides, paving the way to future fundamental studies and toward integrated and multiwavelength photonics applications in quantum nano-optics.

Ferrite microwave circulators allow one to control the directional flow of microwave signals and noise, and thus play a crucial role in present-day superconducting quantum technology. They are typically viewed as a black box, with their internal structure neither specified nor used as a quantum resource. In this work, we show a low-loss waveguide circulator constructed with single-crystalline yttrium iron garnet in a three-dimensional cavity, and analyze it as a multimode hybrid quantum system with coupled photonic and magnonic excitations. We show the coherent coupling of its chiral internal modes with integrated superconducting niobium cavities, and how this enables tunable nonreciprocal interactions between the intracavity photons. We also probe experimentally the effective non-Hermitian dynamics of this system and its effective nonreciprocal eigenmodes. The device platform provides a test bed for implementing nonreciprocal interactions in open-system circuit QED.