Integrating quantum key distribution (QKD) on the existing optical fiber network through dense wavelength-division multiplexing (DWDM) can greatly reduce the costs of quantum channels and improve the scalability and flexibility of the quantum communication network. In the scenarios of an access network or local area network with a large number of users, the connected optical fibers are relatively short and the quantum signal can be adjacent to the classical channels. In this case, the four-wave-mixing noise generated by the classical lights will have a severe impact on the performance of the QKD. We establish a theoretical model to characterize the excess noise in continuous-variable QKD induced by the four-wave mixing, and experimentally demonstrate the validity of the theoretical predictions. In addition, the influence of cross-phase-modulation noise between the carrier of the quantum signal and classical data channels on the QKD is investigated. To suppress the effect of the four-wave-mixing noise, an unequal channel spacing technique is employed. We realize coexistence of entangled state-based continuous-variable QKD and five adjacent DWDM classical channels with 10 dBm launch power and nonreturn-to-zero on-off-keying modulation at 2.5 Gb/s (10 Gb/s).

Quantum cryptographic conferencing (QCC) holds promise for distributing information-theoretic secure keys among multiple users over a long distance. Limited by the fragility of Greenberger-Horne-Zeilinger (GHZ) states, QCC networks based on directly distributing GHZ states over a long distance still face a big challenge. Another two potential approaches are measurement device-independent QCC and conference-key agreement with single-photon interference, which were proposed on the basis of the postselection of GHZ states and the postselection of the W state, respectively. However, implementations of the former protocol are still heavily constrained by the transmission rate $\eta$ of optical channels and the complexity of the setups for postselecting GHZ states. Meanwhile, the latter protocol cannot be cast as a measurement device-independent prepare-and-measure scheme. Combining the idea of postselecting GHZ states and recently proposed twin-field quantum-key-distribution protocols, we report a QCC protocol based on weak coherent-state interferences named “phase-matching quantum cryptographic conferencing,” which is immune to all detector side-channel attacks. The proposed protocol can improve the key-generation rate from $O({\eta }^N)$ to $O({\eta }^{N-1})$ compared with the measurement device-independent QCC protocols. Meanwhile, it can be easily scaled up to multiple parties due to its simple setup.

Gradual loss of the entanglement of a twin beam containing around 25 photon pairs with the increasing external noise is experimentally investigated. The entanglement is quantified by the nonclassicality depths and the nonclassicality counting parameters related to several nonclassicality criteria. The reduction of intensity moments of the analyzed multimode twin beams to single-mode ones allows determination of the negativity as another quantifier of the entanglement. Both the raw photocount histograms and the reconstructed photon-number distributions are analyzed in parallel.

Recent empirical results using quantum-annealing hardware have shown that midanneal pausing has a surprisingly beneficial impact on the probability of finding the ground state for a variety of problems. A theoretical explanation of this phenomenon has thus far been lacking. Here we provide an analysis of pausing using a master-equation framework, and derive conditions for the strategy to result in a success probability enhancement. The conditions, which we identify through numerical simulations and then prove to be sufficient, require that relative to the pause duration the relaxation rate is large and decreasing right after crossing the minimum gap, small and decreasing at the end of the anneal, and is also cumulatively small over this interval, in the sense that the system does not thermally equilibrate. This establishes that the observed success probability enhancement can be attributed to incomplete quantum relaxation, i.e., is a form of beneficial nonequilibrium coupling to the environment.

Measurement-based feedback control is central in quantum computing and precise quantum control. Here we realize a fast and flexible field-programmable-gate-array-based feedback control in an Xmon- superconducting-qubit system. The latency of room-temperature electronics is custom-optimized to be as short as 140 ns. Projective measurement of a signal qubit produces a feedback tag to implement a conditional pulse gate on the qubit. In a feedforward process, the measurement-based feedback tag is brought to a different target qubit for conditional control. In a two-qubit experiment, the feedback and feedforward controls are simultaneously implemented in consecutive steps. A quantum number is then generated by the signal qubit, and a random walk of the target qubit is correspondingly triggered and realized on the Bloch sphere. Our experiment provides a conceptually simple and intuitive benchmark for feedback control in a multiqubit system. The feedback system can be further scaled up for more-complex feedback-control experiments.

Current density distributions in active integrated circuits result in patterns of magnetic fields that contain structural and functional information about the integrated circuit. Magnetic fields pass through standard materials used by the semiconductor industry and provide a powerful means to fingerprint integrated-circuit activity for security and failure analysis applications. Here, we demonstrate high spatial resolution, wide field-of-view, vector magnetic field imaging of static magnetic field emanations from an integrated circuit in different active states using a quantum diamond microscope (QDM). The QDM employs a dense layer of fluorescent nitrogen-vacancy (N-$V$) quantum defects near the surface of a transparent diamond substrate placed on the integrated circuit to image magnetic fields. We show that QDM imaging achieves a resolution of approximately $10\mu \mathrm{m}$ simultaneously for all three vector magnetic field components over the $3.7\times 3.7{\mathrm{mm}}^2$ field of view of the diamond. We study activity arising from spatially dependent current flow in both intact and decapsulated field-programmable gate arrays, and find that QDM images can determine preprogrammed integrated-circuit active states with high fidelity using machine learning classification methods.

We study electric dipole spin resonance caused by subterahertz radiation in a multilevel finite-size quantum dot formed in a nanowire, focusing on the range of driving electric field amplitudes where a strong interplay between the Rabi spin oscillations and tunneling from the dot to continuum states can occur. A strong effect of the tunneling on the spin evolution in this regime occurs due to the formation of mixed spin states. As a result, the tunneling strongly limits possible spin manipulation times. We demonstrate a backaction of the spin dynamics on the tunneling and position of the electron. The analysis of the efficiency of the spin manipulation in terms of the system energy shows that tunneling decreases this efficiency. Fourier spectra of the time-dependent expectation value of the electron position show a strong effect of the spin-orbit coupling on their low-frequency components. This result can be applied to operational properties of spin-based nanodevices and can extend the range of possible spin resonance frequencies to the subterahertz domain.

There are diverse interdisciplinary applications for nanoscale-resolution electrometry of elementary charges under ambient conditions. These include characterization of two-dimensional electronics, charge transfer in biological systems, and measurement of fundamental physical phenomena. The nitrogen-vacancy center in diamond is uniquely capable of such measurements, however electrometry thus far has been limited to charges within the same diamond lattice. It has been hypothesized that the failure to detect charges external to diamond is due to quenching and surface screening, but no proof, model, or design to overcome this has yet been proposed. In this work we affirm this hypothesis through a comprehensive theoretical model of screening and quenching within a diamond electrometer and propose a solution using controlled nitrogen doping and a fluorine-terminated surface. We conclude that successful implementation requires further work to engineer diamond surfaces with lower surface-defect concentrations.

Quantum metrology aims to enhance the precision of various measurement tasks by taking advantages of quantum properties. In many scenarios, precision is not the sole target; the acquired information must be protected once it is generated in the sensing process. Considering a remote sensing scenario where a local site performs cooperative sensing with a remote site to collect sensitive information at the remote site, the loss of sensing data inevitably causes sensitive information to be revealed. Quantum key distribution is known to be a reliable solution for secure data transmission; however, it fails if an eavesdropper accesses the sensing data generated at a remote site. In this study we demonstrate that, by sharing entanglement between local and remote sites, secure quantum remote sensing can be realized, and the secure level is characterized by asymmetric Fisher information gain. Concretely, only the local site can acquire the estimated parameter accurately with Fisher information approaching 1. In contrast, the accessible Fisher information for an eavesdropper is nearly zero even if he or she obtains the raw sensing data at the remote site. This achievement is primarily due to the nonlocal calibration and control of the probe state at the remote site. Our results explore one significant advantage of “quantumness” and extend the notion of quantum metrology to the security realm.

The topologically protected one-way edge modes for light show a way to manipulate the interaction between a single photon and a quantum emitter. We systematically study single-photon quantum optics in a topological waveguide supporting the frequency-dependent one-way edge mode at the boundary of a dynamically modulated photonic system, and compare the results of the single-photon interacting dynamics with the quantum emitter in a unidirectional waveguide. As a major difference, a photon in a topological waveguide propagates around the resonant site coupled to the quantum emitter due to the topological protection. Our results show that, despite different subtleties, photon-emitter interactions for one photon at the edge mode and for one photon propagating in a unidirectional waveguide hold similar features. In addition, we consider the interaction between the photon and a $\mathrm{\Lambda }$-type three-level quantum emitter in the topological waveguide, where we show the nonlocal correlated photon-state generation as well as the single-photon reflection with the frequency conversion. Our work provides insight for understanding the interaction between the topologically protected unidirectional-propagating photon and the quantum emitter, which holds promise for potential applications in quantum-information processing.

As the advances in quantum hardware bring us into the noisy intermediate-scale quantum (NISQ) era, one possible task we can perform without quantum error correction using NISQ machines is the variational quantum eigensolver (VQE) due to its shallow depth. A specific problem that we can tackle is the strongly interacting Fermi-Hubbard model, which is classically intractable and has practical implications in areas like superconductivity. In this paper, we outline the details about the gate sequence, the measurement scheme, and the relevant error-mitigation techniques for the implementation of the Hubbard VQE on a NISQ platform. We perform resource estimation for both silicon spin qubits and superconducting qubits for a 50-qubit simulation, which cannot be solved exactly via classical means, and find similar results. The number of two-qubit gates required is on the order of 20 000. Hence, to suppress the mean circuit-error count to a level such that we can obtain meaningful results with the aid of error mitigation, we need to achieve a two-qubit gate error rate of approximately ${10}^{-4}$. When searching for the ground state, we need a few days for one gradient-descent iteration, which is impractical. This can be reduced to around 10 min if we distribute our task among hundreds of quantum-processing units. Hence, implementing a 50-qubit Hubbard model VQE on a NISQ machine can be on the brink of being feasible in near term, but further optimization of our simulation scheme, improvements in the gate fidelity, improvements in the optimization scheme and advances in the error-mitigation techniques are needed to overcome the remaining obstacles. The scalability of the hardware platform is also essential to overcome the runtime issue via parallelization, which can be done on one single silicon multicore processor or across multiple superconducting processors.

Nitrogen-vacancy ($\mathrm{N}$-$V$) centers in diamond are among the best-studied quantum-based sensors for a wide range of applications. Here, we demonstrate an optical polarization-based method for full vector magnetometry using ensembles of $\mathrm{N}$-$V$ centers. The anisotropy of the electric dipole moments of the $\mathrm{N}$-$V$ centers enables the assignment of the different magnetic field split transitions in the optically detected magnetic resonance spectra by appropriately setting the optical polarization in the excitation and/or detection of the $\mathrm{N}$-$V$ photoluminescence signal. By further employing the optical-selection rules for the driving microwave field, we show how the full vector of the investigated magnetic field can be restored without the need to apply external bias fields. We discuss different $\mathrm{N}$-$V$-center orientation implementations and limitations toward common approximations used in the $\mathrm{N}$-$V$ magnetometry literature for our approach.

High-dimensional quantum key distribution (QKD) allows achievement of information-theoretic secure communications, providing high key-generation rates, which cannot, in principle, be obtained by QKD protocols with binary encoding. Nonetheless, the amount of experimental resources needed increases as the quantum states to be detected belong to a larger Hilbert space, thus raising the costs of practical high-dimensional systems. Here, we present an alternative scheme for fiber-based four-dimensional QKD, with time and phase encoding and one-decoy state technique. Quantum state transmission is tested over different channel lengths up to 145 km of standard single-mode fiber, evaluating the enhancement of the secret key rate in comparison to the three-state two-dimensional BB84 protocol, which is tested with the same experimental setup. Our scheme allows measurement of the four-dimensional states with a simplified and compact receiver, where only two single-photon detectors are necessary, thus making it a cost-effective solution for practical and fiber-based QKD.

We report high-resolution photocurrent (PC) spectroscopy of a single self-assembled $\mathrm{InAs}$/$\mathrm{GaAs}$ quantum dot embedded in an $n$-$i$ Schottky device in an applied vector magnetic field. The PC spectra of positively charged excitons ($X^{+}$) and neutral excitons ($X^0$) are obtained by two-color resonant excitation. With an applied magnetic field in the Voigt geometry, the double-$\mathrm{\Lambda }$ energy-level structure of $X^{+}$ and the dark states of $X^0$ are observed clearly in the PC spectra. In the Faraday geometry, the PC amplitude of $X^{+}$ decreases and is then quenched with increasing magnetic field, which provides a new way to determine the relative sign of the electron and hole $g$-factors. With an applied vector magnetic field, the electron and hole $g$-factor tensors of $X^{+}$ and $X^0$ are obtained. The anisotropy of the hole $g$-factors of both $X^{+}$ and $X^0$ is larger than that of the electron $g$-factors.

In condensed matter systems, coherent backscattering and quantum interference in the presence of time-reversal symmetry lead to well-known phenomena, such as weak localization (WL) and universal conductance fluctuations (UCFs). Here we use multipass Landau-Zener transitions at the avoided crossing of a highly coherent superconducting qubit to emulate these phenomena. The average and standard deviations of the qubit transition rate exhibit a dip and peak when the driving waveform is time-reversal symmetric, analogous to WL and UCFs, respectively. The higher coherence of this qubit enabled the realization of both effects, in contrast to the earlier work by Gustavsson et al. [Phys. Rev. Lett. 110, 016603 (2013)], who successfully emulated UCFs, but did not observe WL. This demonstration illustrates the use of nonadiabatic control to implement quantum emulation with superconducting qubits.

To build a global quantum-communication network, low-transmission, fiber-based communication channels can be supplemented by using a free-space channel between a satellite and a ground station on Earth. We construct a system that generates hyperentangled photonic “ququarts” and measures them to execute multiple quantum-communication protocols of interest. We successfully execute and characterize superdense teleportation, a modified remote-state preparation protocol that transfers more quantum information than standard teleportation, for the same classical information cost, and moreover, is in principle deterministic. Our measurements show an average fidelity of $0.94\pm 0.02$, with a phase resolution of approximately $7^{\circ }$, allowing reliable transmission of $>{10}^5$ distinguishable quantum states. Additionally, we demonstrate the ability to compensate for the Doppler shift, which would otherwise prevent sending time-bin encoded states from a rapidly moving satellite, thus allowing the low-error execution of phase-sensitive protocols during an orbital pass. Finally, we show that the estimated number of received coincidence counts in a realistic implementation is sufficient to enable faithful reconstruction of the received state in a single pass.

Among some of the current uses of dc superconducting quantum interference devices (SQUIDs) are qubit readouts and sensors for probing the properties of quantum materials. We present a gradiometric niobium SQUID design with state-of-the-art sensitivity in the femtotesla range, which can be tuned to specific readout requirements. The sensor is a next-generation fractional SQUID with a tightly optimized input coil and a combination of various measures known for restraining parasitic resonances and other detrimental effects. Our design combines the practical usefulness of well-defined pickup loops for well-defined imaging kernel and tunable probing applications with a fractionalization approach to reduce undesired inductances. In addition, our modeling predicts small dimensions for these planar sensors. These features make them of relevance for material studies and for the detection of magnetic fields in small volumes, e.g., as part of a cryogenic scanning quantum imaging apparatus for efficient diagnostics and quantum device readouts. This manuscript will benefit scientists and engineers working on quantum-computing technologies by clarifying potential general misconceptions about dc SQUID optimization alongside the introduction of the flexible compact dc SQUID design.

Quantum sensing uses quantum systems as sensors to capture weak signals and is providing opportunities in science and technology. The biggest challenge in quantum sensing is decoherence due to the coupling between the sensor and the environment. The dissipation will destroy the quantum coherence and reduce the performance of the sensor. Here we show that quantum sensing can be realized under dissipation by engineering the steady state of the sensor. We demonstrate this protocol with a magnetometer based on ensemble nitrogen-vacancy centers in diamond, while neither high-quality initialization or readout of the sensor nor sophisticated dynamical decoupling sequences are required. Thus our method provides a concise and decoherence-resistant means of quantum sensing. The frequency resolution and precision of our magnetometer are far beyond the coherence time of the sensor. Furthermore, we show that the dissipation can be engineered to improve the performance of our sensor. By increasing the laser pumping, magnetic signals in a broad audio-frequency band from dc up to 140 kHz can be tackled by our method. Besides the potential applications in magnetic sensing and imaging on the microscopic scale, our results may provide opportunities for improvement of a variety of high-precision spectroscopies based on other quantum sensors.