Quantum state transmission and quantum information transmission (QIT) through fiber channels hold immense promise for advancing the scope of quantum information applications. However, besides unavoidable transmission loss, channel noise accelerates the decoherence of quantum states and severely limits the transmission distance in practical continuous-variable quantum state transmission. To address the issue of channel noise in metropolitan quantum fiber-optic links, we propose a scheme called dual-channel transferring after interference. This scheme exhibits immunity to additional channel noise while maintaining sufficient communication capacity. Based on this scheme, we experimentally achieve the transmission of a $-5.9$-dB squeezed state at 1.3 $\text{μ}\mathrm{m}$ through 3 km of optical fiber. We obtain a $-3.2$-dB output squeezed state and demonstrate that this decrease is solely attributable to channel loss, independent of channel noise. Incidentally, we achieve an improved measurement sensitivity of guided acoustic-wave Brillouin scattering noise in a fiber channel exploiting squeezed light. In addition, beyond its simplicity of installation and ease of operation, this approach also offers the highest transmission capacity compared with the existing QIT technologies. Our scheme is suitable for almost all modulation-free continuous-variable quantum information protocols, such as quantum secret sharing, quantum entanglement swapping, and some quantum key distribution schemes with entangled states, as well as participating in the construction of future quantum networks.

The accurate computation of properties of large molecular systems is classically infeasible and is one of the applications in which it is hoped that quantum computers will demonstrate an advantage over classical devices. However, due to the limitations of present-day quantum hardware, variational-hybrid algorithms introduced to tackle these problems struggle to meet the accuracy and precision requirements of chemical applications. Here, we apply the quantum computed moments (QCM) approach combined with a variety of noise-mitigation techniques to an eight qubit and spin-orbital representation of the water molecule (${\text{H}}_2\text{O}$). A noise-stable improvement on the variational result for a four-excitation trial state (circuit depth 25, 22 cz gates) was obtained, with the ground-state energy computed to be within $1.0\pm 0.8$ mHa of exact diagonalization in the 14 spin-orbital basis. Thus, the QCM approach, despite an increased number of measurements and noisy quantum hardware (cz error rates c.0.3% corresponding to expected error rates on the trial-state circuit of order 7%), is able to determine the ground-state energy of a nontrivial molecular system at the required accuracy (c.0.05%). To the best of our knowledge, these results are the largest calculations performed on a physical quantum computer to date to achieve millihartree agreement with exact diagonalization, and a promising indicator of how such hybrid approaches might scale to problems of interest in the low-error and fault-tolerant regimes as quantum computers develop.

We introduce a feasible scheme to produce high-rate long-distance entanglement that uses hybrid entanglement between continuous variables (CVs) and discrete variables (DVs). We show that hybrid entanglement can effectively remove the experimental limitations of existing CV and DV systems to produce long-range entanglement. We benchmark the resulting DV entangled states using an entanglement-based-quantum-key-distribution protocol. We show, using hybrid entangled states, that entanglement-based quantum key distribution is possible with standard telecommunication fibers for 300 km. The key idea is the use of the CV part, which can be adjusted to be robust regarding photon losses, for increasing the transmission distance, and the use of the DV part for achieving high secure key rates. Our results show that hybrid entangled states provide a clear advantage for practical generation of long-distance and high-rate entanglement that may lead to further applications in quantum information processing.

Silicon is a key semiconducting material for electrical devices and hybrid quantum systems where low temperatures and zero-spin isotopic purity can enhance quantum coherence. Electrical conductivity in $\mathrm{Si}$ is characterized by carrier freeze out at around 40 K allowing microwave transmission, which is a key component for addressing spins efficiently in silicon quantum technologies. In this work, we report an additional sharp transition of the electrical conductivity in a $\mathrm{Si}$-28 cylindrical cavity at around 1 K. This is observed by measuring microwave resonator whispering gallery mode frequencies and $Q$ factors with changing temperature and comparing these results with finite-element models. We attribute this change to a transition from a relaxation mechanism-dominated to a resonant phononless absorption-dominated hopping conduction regime. Characterizing this regime change represents a deeper understanding of a physical phenomenon in a material of high interest to the quantum technology and semiconductor device community and the impact of these results is discussed.

Predicting properties of large-scale quantum systems is crucial for the development of quantum science and technology. Shadow estimation is an efficient method for this task based on randomized measurements, where many-qubit random Clifford circuits are used for estimating global properties like quantum fidelity. Here we introduce the minimal Clifford measurement (MCM) to reduce the number of possible random circuits to the minimum, while keeping the effective postprocessing channel in shadow estimation. In particular, we show that MCM requires $2^n+1$ distinct Clifford circuits, and it can be realized by mutually unbiased bases, with $n$ as the total qubit number. By applying the $Z$-tableau formalism, this ensemble of circuits can be synthesized to the $-\mathrm{S}-\mathrm{CZ}-\mathrm{H}-$ structure, which can be decomposed to $2n-1$ fixed circuit modules, and the total circuit depth is at most $n+1$. Compared to the original Clifford measurements, our MCM reduces the circuit complexity and the compilation costs. In addition, we find the sampling advantage of MCM on estimating off-diagonal operators, and extend this observation to the biased-MCM scheme to enhance the sampling improvement further.

Research on optimal eavesdropping models under practical conditions will help to evaluate realistic risk when employing a quantum key distribution system for secure information transmission. Intuitively, fiber loss will lead to the optical energy leaking to the environment, rather than harvested by the eavesdropper, which also limits the eavesdropping ability while improving the quantum key distribution system performance in practical use. However, defining the optimal eavesdropping model in the presence of lossy fiber is difficult because the channel is beyond the control of legitimate partners and the leaked signal is undetectable. Here we investigate how the fiber loss influences the eavesdropping ability based on a teleportation-based collective attack model, which requires two distant stations and a shared entanglement source. We find that if the distributed entanglement is limited due to the practical loss, the optimal attack occurs when the two teleportation stations are merged to one and placed close to the transmitter site, which performs similar to the entangling-cloning attack but with a reduced wiretapping ratio. Assuming Eve uses the best available hollow-core fiber, the secret key rate in the practical environment can be 20–40% higher than that under ideal eavesdropping. While if the entanglement distillation technology is mature enough to provide high quality of distributed entanglement, the two teleportation stations should be distantly separated for better eavesdropping performance, where the eavesdropping can even approach the optimal collective attack. Under the current level of entanglement purification technology, the unavoidable fiber loss can still greatly limit the eavesdropping ability as well as enhance the secret key rate and transmission distance of the realistic system, which promotes the development of quantum key distribution systems in practical application scenarios.

We use a floating tunable coupler to mediate interactions between qubits on separate chips to build a modular architecture. We demonstrate three different designs of multichip tunable couplers using vacuum-gap capacitors or superconducting indium-bump bonds to connect the coupler to a microwave line on a common substrate and then connect to the qubit on the next chip. We show that the zero-coupling condition between qubits on separate chips can be achieved in each design and that the relaxation rates for the coupler and qubits are not noticeably affected by the extra circuit elements. Finally, we demonstrate two-qubit gate operations with fidelity at the same level as qubits with a tunable coupler on a single chip. Using one or more indium bonds does not degrade qubit coherence or impact the performance of two-qubit gates.

We present a unified approach for designing isolators based on temporally modulated coupled resonator networks. Our method leverages standard superconducting quantum interference device–based resonators as building blocks, arranged in series-coupled resonators to realize a wide range of on-chip nonreciprocal devices. The devices operate by translating microwave power into out-of-band intermodulation products in a direction-dependent way. Our theoretical studies demonstrate the effectiveness of the proposed approach, achieving isolators with near-zero insertion losses and isolation greater than 20 dB. To validate our findings, we implemented and measured a series-coupled three-resonator superconducting isolator using a single-layer superconducting process. At a base temperature of 20 mK, our device exhibited an insertion loss of 1.3 dB in the forward direction and isolation of up to 25 dB at the center frequency and greater than 15 dB across a bandwidth of 250 MHz in the reverse direction. Our approach promises to enable the design of a broad range of high-performance nonreciprocal devices for superconducting circuits.

Integrated Kerr quantum frequency combs (QFCs), with the potential to generate multiple and scalable quantum states, have emerged as a compact, stable, and fundamental resource for broadband entanglement. Here, we build a platform for designing bipartite entangled QFCs via an on-chip silicon nitride microring resonator. By establishing the system quantum dynamics of Kerr nonlinear microresonators, our platform can support up to 12 continuous-variable quantum modes, in the form of six simultaneously two-mode squeezed pairs under the influence of hysteresis. The entanglement degree of frequency-mode pairs is dependent on the resonator structure and ambient temperature. By adjusting the cavity temperature, we can optimize the performance of entanglement under certain injected pump power and pump detuning. Our comprehensive QFC designing process and entanglement distribution control may ameliorate the generation and optimization of entanglement.

Kernel function plays a crucial role in machine learning algorithms such as classifiers. In this paper, we aim to improve the classification performance and reduce the reading out burden of quantum classifiers. We devise a universally trainable quantum feature mapping layout to broaden the scope of feature states and avoid the inefficiently straight preparation of quantum superposition states. We also propose an improved quantum support vector machine that employs partially evenly weighted trial states. In addition, we analyze its error sources and superiority. As a promotion, we propose a quantum iterative multiclassifier framework for one-versus-one and one-versus-rest approaches. Finally, we conduct corresponding numerical demonstrations in the qiskit package. The simulation result of trainable quantum feature mapping shows considerable clustering performance, and the subsequent classification performance is superior to the existing quantum classifiers in terms of accuracy and distinguishability.

Efficient interconnection between distant semiconductor spin qubits with the help of photonic qubits offers exciting new prospects for future quantum communication applications. In this paper, we optimize the extraction efficiency of a novel interface between a singlet-triplet-spin-qubit and a photonic-qubit. The interface is based on a 220-nm-thick $\mathrm{Ga}\mathrm{As}/(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ heterostructure membrane and consists of a gate-defined double quantum dot (GDQD) supporting a singlet-triplet qubit, an optically active quantum dot (OAQD) consisting of a gate-defined exciton trap, a photonic crystal cavity providing in-plane optical confinement, efficient outcoupling to an ideal free-space Gaussian beam while accommodating the gate wiring of the GDQD and OAQD, and a bottom gold reflector to recycle photons and increase the optical extraction efficiency. All the essential components can be lithographically defined and deterministically fabricated on the $\mathrm{Ga}\mathrm{As}/(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ heterostructure membrane, which greatly increases the scalability of on-chip integration. According to our simulations, the interface provides an overall coupling efficiency of 28.7% into a free-space Gaussian beam, assuming a ${\mathrm{Si}\mathrm{O}}_2$ interlayer fills the space between the reflector and the membrane. The performance can be further increased by undercutting this ${\mathrm{Si}\mathrm{O}}_2$ interlayer below the photonic crystal. In this case, the overall efficiency is calculated to be 48.5%.

The number of superconducting qubits contained in a single quantum processor is increasing steadily. However, to realize a truly useful quantum computer, it is inevitable to increase the number of qubits much further by distributing quantum information among distant processors using flying qubits. A key element towards this goal is a deterministic quantum interaction between superconducting-atom and propagating microwave-photon qubits. Here, we confirm the bidirectional state transfer between them, which completes by simply reflecting the photon at the atom. The averaged fidelity of the photon-to-atom (atom-to-photon) state transfer reaches 0.826 (0.801), limited mainly by the lifetime of the atom qubit. The present technology would be useful for future distributed quantum computation with superconducting qubits.

Diamond tin-vacancy centers have emerged as a promising platform for quantum information science and technology. A key challenge for their use in more-complex quantum experiments and scalable applications is the ability to prepare the center in the desired charge state with the optical transition at a predefined frequency. Here we report on heralding such successful preparation using a combination of laser excitation, photon detection, and real-time logic. We first show that fluorescence photon counts collected during an optimized resonant probe pulse strongly correlate with the subsequent charge state and optical-transition frequency, enabling real-time heralding of the desired state through threshold photon counting. We then implement and apply this heralding technique to photoluminescence-excitation measurements, coherent optical driving, and an optical Ramsey experiment, finding strongly increased optical coherence with increasing threshold. Finally, we demonstrate that the prepared optical frequency follows the probe laser across the inhomogeneous linewidth, enabling tuning of the transition frequency over multiple homogeneous linewidths.

The dielectric loss of silicon nitride (${\mathrm{Si}}_3{\mathrm{N}}_4$) limits the performance of microwave-frequency devices that rely on this material for sensing, signal processing, and quantum communication. Using superconducting resonant circuits, we measure the cryogenic loss tangent of either as-deposited or high-temperature annealed stoichiometric ${\mathrm{Si}}_3{\mathrm{N}}_4$ as a function of drive strength and temperature. The internal loss behavior of the electrical resonators is largely consistent with the standard tunneling model of two-level systems (TLSs), including damping caused by resonant energy exchange with TLSs and by the relaxation of nonresonant TLSs. We further supplement the TLS model with a self-heating effect to explain an increase in the loss observed in as-deposited films at large drive powers. Critically, we demonstrate that annealing remedies this anomalous power-induced loss, reduces the relaxation-type damping by more than 2 orders of magnitude, and reduces the resonant-type damping by a factor of 3. Employing infrared absorption spectroscopy, we find that annealing reduces the concentration of hydrogen in ${\mathrm{Si}}_3{\mathrm{N}}_4$, suggesting that hydrogen impurities cause substantial dissipation.

Increasing the separation between semiconductor quantum dots offers scaling advantages by facilitating gate routing and the integration of sensors and charge reservoirs. Elongated quantum dots have been utilized for this purpose in GaAs heterostructures to extend the range of spin-spin interactions. Here, we study a MOS device where two quantum dot arrays are separated by an elongated quantum dot (340 nm long, 50 nm wide). We monitor charge transitions of the elongated quantum dot by measuring radiofrequency single-electron currents to a reservoir to which we connect a lumped-element resonator. We operate the dot as a single-electron box to achieve charge sensing of remote quantum dots in each array, separated by an edge-to-edge distance of 480 nm. Charge detection on both ends of the elongated dot at a coinciding setpoint demonstrates that the charge states are well distributed across its nominal length, supported by the simulated quantum mechanical electron density. Likewise, we show elongated-peripheral quantum dot tunnel couplings can exceed $20\mathrm{GHz}$, above the electron temperature, fulfilling the requirement for mediated exchange. Our results illustrate how single-electron boxes can be realized with versatile footprints that may enable compact quantum processor layouts, offering distributed charge sensing in addition to the possibility of mediated coupling.

Quantum state readout plays a pivotal role in quantum technologies, spanning applications in sensing, computation, and secure communication. In this work, we introduce an approach for efficiently reading populations of hybrid-spin states in the nitrogen-vacancy center of diamond using a single laser pulse, which utilizes the excited-state-level anticrossing mechanism at around 500 Gs. Reading spin-state populations through this approach achieves the same outcome as traditional quantum state diagonal tomography but significantly reduces the experimental time by an order of magnitude while maintaining fidelity. Moreover, this approach may be extended to encompass full-state tomography, thereby obviating the requirement for a sequence of spin manipulations and mitigating errors induced by decoherence throughout the procedure.

We present rapid and robust protocols for stimulated rapid adiabatic passage and quantum logic gates. Our gates are based on geometric phases acquired by instantaneous eigenstates of a slowly accelerating “inertial” Hamiltonian. To begin, we establish the criteria for inertial evolution and subsequently engineer pulse shapes that fulfill these conditions. These tailored pulses are then used to optimize geometric logic gates. We analyze a realization of our protocols with ${}^{87}\mathrm{Rb}$ atoms, resulting in gate fidelity that approaches the current state of the art, with marked improvements in robustness.