The nitrogen-vacancy ($\mathrm{N}$-$V$) defect in diamond is a versatile quantum sensor, being able to measure physical quantities such as magnetic field, electric field, temperature, and pressure. In the present work, we demonstrate multiplexed sensing of magnetic field and temperature using a N-V ensemble in diamond. The dual-frequency-driving technique we employ is based on frequency-division multiplexing, which enables the sensing of both measurables in real time. The pair of $\mathrm{N}$-$V$ resonance frequencies for dual-frequency driving must be selected to avoid coherent population trapping of $\mathrm{N}$-$V$ spin states. With enhanced optical collection efficiency higher than 50% and a type 1b diamond crystal with a natural abundance of ${}^{13}\mathrm{C}$ spins, we achieve sensitivities of about $70\mathrm{pT}/\sqrt{\mathrm{Hz}}$ and $25\mu \mathrm{K}/\sqrt{\mathrm{Hz}}$ simultaneously. We demonstrate a high isolation factor of 34 dB in the $\mathrm{N}$-$V$ thermometry signal against the magnetic field; and we provide a theoretical description for the isolation factor. This work paves the way for extending the application of $\mathrm{N}$-$V$ diamond sensors into more demanding conditions.

Ising machines (IMs) are physical systems designed to find solutions to combinatorial optimization (CO) problems mapped onto the IM via the coupling strengths between its binary spins. Using its intrinsic dynamics and different annealing schemes, the IM relaxes over time to its lowest-energy state, which is the solution to the CO problem. IMs have been implemented on different platforms, and interacting nonlinear oscillators are particularly promising candidates. Here we demonstrate a pathway towards an oscillator-based IM using arrays of nanoconstriction spin Hall nano-oscillators (SHNOs). We show how SHNOs can be readily phase binarized and how their resulting microwave power corresponds to well-defined global phase states. To distinguish between degenerate states, we use phase-resolved Brillouin-light-scattering microscopy and directly observe the individual phase of each nanoconstriction. Micromagnetic simulations corroborate our experiments and confirm that our proposed IM platform can solve CO problems, showcased by how the phase states of a $2\times 2$ SHNO array are solutions to a modified max-cut problem. Compared with the commercially available D-Wave ${\mathrm{Advantage}}^{\mathrm{TM}}$, our architecture holds significant promise for faster sampling, substantially reduced power consumption, and a dramatically smaller footprint.

Superconducting nanowire single-photon detectors (SNSPDs) offer high-quantum-efficiency and low-dark-count-rate single-photon detection. In a growing number of cases, large magnetic fields are being incorporated into quantum microscopes, nanophotonic devices, and sensors for nuclear and high-energy physics that rely on SNSPDs, but superconducting devices generally perform poorly in large magnetic fields. Here, we demonstrate robust performance of amorphous SNSPDs in magnetic fields of up to $\pm 6$ T with a negligible dark-count rate and unchanged quantum efficiency at typical bias currents. Critically, we also show that the SNSPD can be used as a magnetometer with a sensitivity of better than $100\mu \mathrm{T}/\sqrt{\mathrm{Hz}}$ and as a thermometer with a sensitivity of $20\mu \mathrm{K}/\sqrt{\mathrm{Hz}}$ at 1 K. Thus, a single-photon detector integrated into a quantum device can be used as a multifunctional quantum sensor capable of describing the temperature and magnetic field on chip simply by varying the bias current to change the operating modality from single-photon detection to thermometry or magnetometry.

Currently, thermally excited magnons are being intensively investigated, owing to their potential in computing devices and thermoelectric conversion technologies. We report the detection of a thermal magnon current propagating in a magnetic insulator yttrium iron garnet under a temperature gradient using a quantum sensor: electron spins associated with nitrogen-vacancy (N-V) centers in diamond. A thermal magnon current is observed as modified Rabi-oscillation frequencies of N-V spins hosted in a beam-shaped bulk diamond that is resonantly coupled with coherent magnon propagating over a long distance. Additionally, using a nanodiamond, alteration in N-V spin-relaxation rates, depending on the applied temperature gradient, are observed under nonresonant N-V excitation conditions. The demonstration of probing a thermal magnon current mediated by coherent magnons via N-V spin states serves as a basis for creating a device platform that hybridizes spin caloritronics and spin qubits.

Quantum key distribution (QKD), which enables the secure distribution of symmetric keys between two legitimate parties, is highly useful in future network security. An access network that connects multiple end users with one network backbone can be combined with QKD to build security for end users in a scalable and cost-effective way. In this paper, we show that a downstream-access network can be implemented using continuous-variable (CV) QKD. The security analysis is conducted independently with other network nodes, which greatly relax the trustfulness requirement on the network nodes. The security is then obtained against other network parties even when the quantum signals are broadcast throughout the network. The passive beam splitter is sufficient to deterministically distribute the quantum signals, and standard CV QKD systems can be directly fitted in the downstream-access network, which makes it more applicable for practical implementations. Numerous simulation results are provided to demonstrate the performance of the CV QKD downstream-access network, where up to 64 end users are shown to be feasible to access the network. We provide the security-analysis framework for realizing CV QKD in the downstream-access network, which will boost the diversity for constructing practical CV QKD networks.

The nitrogen-vacancy ($\mathrm{N}$-$V$) center in diamond has a wide range of potential applications in quantum metrology, communications, and computation. The key to its use lies in how large the optical spin contrast is and the associated fidelity of the spin-state readout. In this paper, we propose a mechanism for improving contrast with a spin-to-charge protocol that relies on the use of an external electrode and cryogenic temperatures to discretize the diamond conduction band for spin-selective resonant photoionization. We use effective-mass theory to calculate the discrete eigenenergies in this system and use them to formulate a spin-to-charge protocol that involves resonant photoionization out of the $\mathrm{N}$-$V$ ground state into the diamond conduction band. The major sources of broadening, which guide the design of the experiment, are also addressed. With this mechanism, we calculate an optical spin contrast that and an associated spin-readout fidelity of 85%. This significant improvement can be applied to a number of cryogenic quantum technologies.

Single-photon detectors operating on satellites for use in a quantum communications network can incur large dark-count-rate increases from the natural radiation environment of space. Displacement damage to the material lattice of a detector from the ionizing radiation can result in a permanent dark-count increase in the detector. In this work, we analyze the radiation-induced dark-count rate of a silicon single-photon avalanche diode onboard a satellite at different orbiting altitudes, as well as, the additional radiation from a nuclear-disturbed environment caused by a high-altitude nuclear explosion. For detectors on low Earth orbit satellites, protons are the biggest source of radiation damage and are best mitigated by choosing an orbit that minimizes exposure when passing through the South Atlantic Anomaly and Polar Cusps. Detectors on medium Earth orbit and geostationary orbit satellites, if shielded by more than 10 mm of aluminum, provide the best platform in terms of the least amount of radiation damage to the detectors. In the event of a high-altitude nuclear explosion, the artificial radiation belts produced by the explosion will cause too much damage to silicon single-photon detectors on low Earth orbit satellites and render them unfit for quantum communications in less than a day. Higher orbit satellites will only suffer minor dark-count-rate increases from the artificial radiation belts.

We present a simple approach to produce multiple correlated photon pairs through spontaneous parametric down-conversion. We develop Gaussian masks to subdivide the pump beam before passing it through a nonlinear medium. In this way, we are able to observe simultaneous separate down-converted emission cones with spatial overlap. The technique we employ could be used to greatly increase the dimensionality of entangled photonic systems generated from spontaneous parametric down-conversion, affording greater scalability to optical quantum computing than previously explored.

The coherent transduction between microwave and optical frequencies is critical to interconnect superconducting quantum processors over long distances. However, it is challenging to establish such a quantum interface with high efficiency and small added noise based on the standard direct conversion scheme. Here, we propose an electro-optic transduction system based on continuous-variable quantum teleportation. Reliable quantum-information transmission can be realized with an arbitrarily small cooperativity, in contrast to the direct conversion scheme, which requires a large minimum cooperativity. We show that the teleportation-based scheme maintains a significant rate advantage robustly for practical thermal noise and all values of cooperativity. We further investigate the performance in the transduction of complex quantum states such as cat states and Gottesman-Kitaev-Preskill (GKP) states and show that a higher fidelity can be achieved with the teleportation-based scheme. Our scheme significantly reduces the device requirement, and makes quantum transduction between microwave and optical frequencies feasible in the near future.

We investigate the ultrastrong tunable coupler for coupling of superconducting resonators. The obtained coupling constant exceeds 1 GHz, and the wide-range tunability is achieved for both antiferromagnetics and ferromagnetics from $-1086$ to 604 MHz. The ultrastrong coupler is composed of a rf superconducting quantum interference device (SQUID) and dc SQUID as tunable junctions, which connect to resonators via shared aluminum thin-film meander lines enabling such a huge coupling constant. The spectrum of the coupler obviously shows the breaking of the rotating-wave approximation, and our circuit model treating the Josephson junction as a tunable inductance reproduces the experimental results well. The ultrastrong coupler is expected to be utilized in quantum annealing circuits and/or noisy intermediate-scale quantum devices with dense connections between qubits.

We propose a one-step scheme to implement the time-optimal nonadiabatic holonomic three-qubit controlled gates in Rydberg atoms by modulating Rabi frequencies of driving fields on the target atom. We perform the time-optimal operations following the brachistochrone curve, after which the scheme manifests completely different characteristics in contrast to the conventional nonadiabatic holonomic quantum computation (NHQC) that all operations require exactly the same evolution time, even for small angular rotations. Since the present three-qubit controlled-gate scheme significantly reduces the evolution time, the influence of decoherence can be greatly reduced. On the other hand, the robustness on the systematic error can also be guaranteed by the feature of geometric phase. The simulation through master equation verifies the high fidelity and robustness of our scheme. Although we use the Rydberg-atom system to verify the feasibility of our scheme, any quantum platforms capable of achieving the same form of effective Hamiltonian is feasible, which means the proposed three-qubit time-optimal-control nonadiabatic holonomic gates is independent of the particular physical platform and may have the potential for wider application.

We study the impact of experimental imperfections in integrated optical circuits. We discuss the emergence of a moderate biased error in path encoding, and investigate its correlation with properties of the optical paths. Our analysis connects and deepens previous studies in this direction, revealing potential issues in high-precision tests and optical implementations of machine learning.

Quantum holonomic gates hold built-in resilience to local noises and provide a promising approach for implementing fault-tolerant quantum computation. We propose to realize high-fidelity holonomic $(N+1)$-qubit controlled gates using Rydberg atoms confined in optical arrays or superconducting circuits. We identify the scheme, deduce the effective multibody Hamiltonian, and determine the working condition of the multiqubit gate. Uniquely, the multiqubit gate is immune to systematic errors, i.e., laser parameter fluctuations and motional dephasing, as the $N$ control atoms largely remain in the very stable qubit space during the operation. We show that $C_N$-not gates can reach the same level of fidelity at a given gate time for $N\le 5$ under a suitable choice of parameters, and the gate tolerance against errors in systematic parameters can be further enhanced through optimal pulse engineering. In the case of Rydberg atoms, the proposed protocol is intrinsically different from typical schemes based on Rydberg blockade or antiblockade. Our study paves an alternative way to build robust multiqubit gates with Rydberg atoms trapped in optical arrays or with superconducting circuits. It contributes to current efforts to develop scalable quantum computation with trapped atoms and fabricable superconducting devices.

The goal of quantum metrology is the precise estimation of parameters using quantum properties such as entanglement. This estimation usually consists of three steps: state preparation, time evolution during which information of the parameters is encoded in the state, and readout of the state. Decoherence during the time evolution typically degrades the performance of quantum metrology and is considered to be one of the major obstacles to realizing entanglement-enhanced sensing. We show, however, that under suitable conditions, this decoherence can be exploited to improve the sensitivity. Assume that we have two axes, and our aim is to estimate the relative angle between them. Our results reveal that the use of Markovian collective dephasing to estimate the relative angle between the two directions affords Heisenberg-limited sensitivity. Moreover, our protocol based on Markovian collective dephasing is robust against environmental noise: it is possible to achieve the Heisenberg limit by applying the collective dephasing even under the effect of independent dephasing. Our counterintuitive proposal with the decoherence leads to alternative applications in quantum metrology.

Measurement-based quantum computing (MBQC) promises an alternative approach to quantum computation that has natural compatibility with error-correction codes at the cost of a polynomial increase in physical qubits. MBQC implementations have previously focused on photonic systems where two-qubit gates are difficult. On the other hand, semiconductor spin qubit systems offer fast two-qubit gates via the exchange interaction. To explore the benefits of MBQC on spin systems, two hybrid measurement-exchange schemes for full qubit control with semiconductor double-quantum-dot spin qubits are considered. Protocol 1 fully realizes the cluster-state approach to MBQC but requires singlet-triplet qubits in a magnetic field gradient. Protocol 2 implements a direct measurement-based replacement for more traditional gate-based encoded operations, without need for magnetic field gradients. Both protocols demonstrate full single- and two-qubit control through a combination of interqubit and intraqubit exchange and measurement onto the singlet-triplet basis. We show that both schemes suppress individual qubit spin-state leakage errors and offer fast gate times, up to known phase and Pauli corrections.

The ongoing rapid progress towards quantum technologies relies on new hybrid platforms optimized for specific quantum computation and communication tasks, and researchers are striving to achieve such platforms. We study theoretically a spin qubit exchange-coupled to an anisotropic ferromagnet that hosts magnons with a controllable degree of intrinsic squeezing. We find this system to physically realize the quantum Rabi model from the isotropic to the Jaynes-Cummings limit with coupling strengths that can reach the deep-strong regime. We demonstrate that the composite nature of the squeezed magnon enables concurrent excitation of three spin qubits coupled to the same magnet. Thus, three-qubit Greenberger-Horne-Zeilinger and related states needed for implementing Shor’s quantum error-correction code can be robustly generated. Our analysis highlights some unique advantages offered by this hybrid platform, and we hope that it will motivate corresponding experimental efforts.

The conversion and interaction between quantum signals at the single-photon level are essential for scalable quantum photonic information technology. Using a fully optimized periodically poled lithium niobate microring, we demonstrate ultraefficient sum-frequency generation on a chip. The external quantum efficiency reaches $(65\pm 3)\mathrm{\%}$ with only $(104\pm 4)$-$\mu \mathrm{W}$ pump power. At peak conversion, $3\times {10}^{-5}$-noise photon is created during the cavity lifetime, which meets the requirement of quantum applications using single-photon pulses. Using a pump and signal in single-photon coherent states, we directly measure the conversion probability produced by a single pump photon to be ${10}^{-5}$, which is a significant improvement from the state of the art, and the photon-photon coupling strength to be 9.1 MHz. Our results mark steady progress toward quantum nonlinear optics at the ultimate single-photon limit, with potential applications in highly integrated photonics and quantum optical computing.