With the rapid advancement of quantum computing, hybrid quantum-classical machine learning has shown numerous potential applications at the current stage, with expectations of being achievable in the noisy intermediate-scale quantum (NISQ) era. Quantum reinforcement learning, as an indispensable study, has recently demonstrated its ability to solve standard benchmark environments with formally provable theoretical advantages over classical counterparts. However, despite the progress of quantum processors and the emergence of quantum computing clouds, implementing quantum reinforcement learning algorithms utilizing parameterized quantum circuits (PQCs) on NISQ devices remains infrequent. In this work, we take the first step towards executing benchmark quantum reinforcement problems on real devices equipped with at most 136 qubits on the BAQIS Quafu quantum computing cloud. The experimental results demonstrate that the policy agents can successfully accomplish objectives under modified conditions in both the training and inference phases. Moreover, we design hardware-efficient PQC architectures in the quantum model using a multi-objective evolutionary algorithm and develop a learning algorithm that is adaptable to quantum devices. We hope that the Quafu-RL can be a guiding example to show how to realize machine learning tasks by taking advantage of quantum computers on the quantum cloud platform.

We develop universal quantum computing models that form a family of quantum von Neumann architectures, with modular units of memory, control, CPU, and internet, besides input and output. This family contains three generations characterized by dynamical quantum resource theory, and it also circumvents no-go theorems on quantum programming and control. Besides universality, such a family satisfies other desirable engineering requirements on system and algorithm design, such as modularity and programmability, hence serves as a unique approach to building universal quantum computers.

Atomic nonlinear interferometry has wide applications in quantum metrology and quantum information science. Here we propose a nonlinear time-reversal interferometry scheme with high robustness and metrological gain based on the spin squeezing generated by arbitrary quadratic collective-spin interaction, which could be described by the Lipkin-Meshkov-Glick (LMG) model. We optimize the squeezing process, encoding process, and anti-squeezing process, finding that the two particular cases of the LMG model, one-axis twisting and two-axis twisting outperform in robustness and precision, respectively. Moreover, we propose a Floquet driving method to realize equivalent time reverse in the atomic system, which leads to high performance in precision, robustness, and operability. Our study sets a benchmark for achieving high precision and high robustness in atomic nonlinear interferometry.

Quantum enhanced metrology has the potential to go beyond the standard quantum limit and eventually to the ultimate Heisenberg bound. In particular, quantum probes prepared in nonclassical coherent states have recently been recognized as a useful resource for metrology. Hence, there has been considerable interest in constructing magnetic quantum sensors that combine high resolution and high sensitivity. Here, we explore a nanoscale magnetometer with quantum-enhanced sensitivity, based on $^{123}$Sb ($I=7/2$) nuclear spin doped in silicon, that takes advantage of techniques of spin-squeezing and coherent control. With the optimal squeezed initial state, the magnetic field sensitivity may be expected to approach 6 aT$\cdot $Hz$^{-1/2}\cdot$cm$^{-3/2}$ and 603 nT$\cdot $Hz$^{-1/2}$ at the single-spin level. This magnetic sensor may provide a novel sensitive and high-resolution route to microscopic mapping of magnetic fields as well as other applications.

Scaling up spin qubits in silicon-based quantum dots is one of the pivotal challenges in achieving large-scale semiconductor quantum computation. To satisfy the connectivity requirements and reduce the lithographic complexity, utilizing the qubit array structure and the circuit quantum electrodynamics (cQED) architecture together is expected to be a feasible scaling scheme. A triple-quantum dot (TQD) coupled with a superconducting resonator is regarded as a basic cell to demonstrate this extension scheme. In this article, we investigate a system consisting of a silicon TQD and a high-impedance TiN coplanar waveguide (CPW) resonator. The TQD can couple to the resonator via the right double-quantum dot (RDQD), which reaches the strong coupling regime with a charge-photon coupling strength of ${g_0}/({2\pi})=175$ ${\rm MHz}$. Moreover, we illustrate the high tunability of the TQD through the characterization of stability diagrams, quadruple points (QPs), and the quantum cellular automata (QCA) process. Our results contribute to fostering the exploration of silicon-based qubit integration.

One-way quantum computation focuses on initially generating an entangled cluster state followed by a sequence of measurements with classical communication of their individual outcomes. Recently, a delayed-measurement approach has been applied to replace classical communication of individual measurement outcomes. In this work, by considering the delayed-measurement approach, we demonstrate a modified one-way CNOT gate using the on-cloud superconducting quantum computing platform: Quafu. The modified protocol for one-way quantum computing requires only three qubits rather than the four used in the standard protocol. Since this modified cluster state decreases the number of physical qubits required to implement one-way computation, both the scalability and complexity of the computing process are improved. Compared to previous work, this modified one-way CNOT gate is superior to the standard one in both fidelity and resource requirements. We have also numerically compared the behavior of standard and modified methods in large-scale one-way quantum computing. Our results suggest that in a noisy intermediate-scale quantum (NISQ) era, the modified method shows a significant advantage for one-way quantum computation.

The performance of Nb superconducting quantum devices is predominantly limited by dielectric loss at the metal-air interface, where Nb$_2$O$_5$ is considered the main loss source. Here, we suppress the formation of native oxides by in-situ deposition of a TiN capping layer on the Nb film. With TiN capping layers, no Nb$_2$O$_5$ forms on the surface of the Nb film. The quality factor $Q_{\rm i}$ of the Nb resonator increases from $5.6\times10^{5}$ to $7.9\times10^{5}$ at low input power and from $6.8\times10^{6}$ to $1.1\times10^{7}$ at high input power. Furthermore, the TiN capping layer also shows good aging resistance in Nb resonator devices, with no significant performance fluctuations after one month of aging. These findings highlight the effectiveness of TiN capping layers in enhancing the performance and longevity of Nb superconducting quantum devices.

Counterdiabatic driving (CD) offers a fast and robust route to manipulate quantum systems, which has widespread applications in quantum technologies. However, for higher-dimensional complex systems, the exact CD term involving the spectral properties of the system is difficult to calculate and generally takes a complicated form, impeding its experimental realization. Recently, many approximate methods have been proposed for designing CD passages in many-body systems. In this topical review, we focus on the CD formalism and briefly introduce several experimental constructions and applications of approximate CD driving in spin-chain models with nuclear magnetic resonance (NMR) systems.

Reducing the control error is vital for high-fidelity digital and analog quantum operations. In superconducting circuits, one disagreeable error arises from the reflection of microwave signals due to impedance mismatch in the control chain. Here, we demonstrate a reflection cancelation method when considering that there are two reflection nodes on the control line. We propose to generate the pre-distortion pulse by passing the envelopes of the microwave signal through digital filters, which enables real-time reflection correction when integrated into the field-programmable gate array (FPGA). We achieve a reduction of single-qubit gate infidelity from 0.67% to 0.11% after eliminating microwave reflection. Real-time correction of microwave reflection paves the way for precise control and manipulation of the qubit state and would ultimately enhance the performance of algorithms and simulations executed on quantum processors.

Magneto-optical traps (MOTs) composed of magnetic fields and light fields have been widely utilized to cool and confine microscopic particles. Practical technology applications require miniaturized MOTs. The advancement of planar optics has promoted the development of compact MOTs. In this article, we review the development of compact MOTs based on planar optics. First, we introduce the standard MOTs. We then introduce the grating MOTs with micron structures, which have been used to build cold atomic clocks, cold atomic interferometers, and ultra-cold sources. Further, we introduce the integrated MOTs based on nano-scale metasurfaces. These new compact MOTs greatly reduce volume and power consumption, and provide new opportunities for fundamental research and practical applications.

The application of the vector magnetometry based on nitrogen-vacancy (NV) ensembles has been widely investigated in multiple areas. It has the superiority of high sensitivity and high stability in ambient conditions with microscale spatial resolution. However, a bias magnetic field is necessary to fully separate the resonance lines of optically detected magnetic resonance (ODMR) spectrum of NV ensembles. This brings disturbances in samples being detected and limits the range of application. Here, we demonstrate a method of vector magnetometry in zero bias magnetic field using NV ensembles. By utilizing the anisotropy property of fluorescence excited from NV centers, we analyzed the ODMR spectrum of NV ensembles under various polarized angles of excitation laser in zero bias magnetic field with a quantitative numerical model and reconstructed the magnetic field vector. The minimum magnetic field modulus that can be resolved accurately is down to $\sim 0.64 $ G theoretically depending on the ODMR spectral line width (1.8 MHz), and $\sim 2 $ G experimentally due to noises in fluorescence signals and errors in calibration. By using $^{13}$C purified and low nitrogen concentration diamond combined with improving calibration of unknown parameters, the ODMR spectral line width can be further decreased below 0.5 MHz, corresponding to $\sim 0.18 $ G minimum resolvable magnetic field modulus.

A maximal photon number entangled state, namely NOON state, can be adopted for sensing with a quantum enhanced precision. In this work, we designed silicon quantum photonic chips containing two types of Mach-Zehnder interferometers wherein the two-photon NOON state, sensing element for temperature or humidity, is generated. Compared with classical light or single photon case, two-photon NOON state sensing shows a solid enhancement in the sensing resolution and precision. As the first demonstration of on-chip quantum photonic sensing, it reveals the advantages of photonic chips for high integration density, small-size, stability for multiple-parameter sensing serviceability. A higher sensing precision is expected to beat the standard quantum limit with a higher photon number NOON state.

It is well known that the time-dependent Schrrödinger equation can only be solved exactly in very rare cases, even for two-level quantum systems. Thus, finding the exact quantum dynamics under a time-dependent Hamiltonian is not only fundamentally important in quantum physics but also facilitates active quantum manipulations for quantum information processing. In this work, we present a method for generating nearly infinite analytically assisted solutions to the Schrödinger equation for a qubit under time-dependent driving. These analytically assisted solutions feature free parameters with only boundary restrictions, making them applicable in a variety of precise quantum manipulations. Due to the general form of the time-dependent Hamiltonian in our approach, it can be readily implemented in various experimental setups involving qubits. Consequently, our scheme offers new solutions to the Schrödinger equation, providing an alternative analytical framework for precise control over qubits.

We study the enhanced sensing of weak anharmonicities in a gain-based cavity-magnon-waveguide coupled system. By dissipatively coupling the two subsystems through a mediating waveguide, the Hamiltonian of the system is tailored to be anti-parity-time symmetric. Unique to the gain condition, the eigenvalues exhibit two singularities with linewidth suppression, distinguishing them from those of gain-free systems. Under the gain condition, a counter-intuitive bistable signature emerges even at low drive powers. As the effective gain approaches a certain value, this bistability yields a significantly enhanced spin-current response of the magnon mode. Consequently, the sensitivity, quantified by an enhancement factor, is enhanced remarkably compared to the linewidth suppression scenario. Moreover, the high enhancement factor can be sustained over a broad gain-bandwidth and also stays large even when the coherent coupling becomes considerably strong. Based on the integrated cavity-magnon-waveguide systems, this scheme can be used for sensing different physical quantities related to the Kerr-type nonlinearity and has potential applications in high-precision measuring microwave-signal nonlinearities.

As superconducting quantum computing continues to advance at an unprecedented pace, there is a compelling demand for the innovation of specialized electronic instruments that act as crucial conduits between quantum processors and host computers. Here, we introduce a microwave measurement and control system (M$^{2}$CS) dedicated to large-scale superconducting quantum processors. M$^{2}$CS features a compact modular design that balances overall performance, scalability and flexibility. Electronic tests of M$^{2}$CS show key metrics comparable to commercial instruments. Benchmark tests on transmon superconducting qubits further show qubit coherence and gate fidelities comparable to state-of-the-art results, confirming M$^{2}$CS's capability to meet the stringent requirements of quantum experiments running on intermediate-scale quantum processors. The compact and scalable nature of our design holds the potential to support over 1000 qubits after upgrade in stability and integration. The M$^{2}$CS architecture may also be adopted to a wider range of scenarios, including other quantum computing platforms such as trapped ions and silicon quantum dots, as well as more traditional applications like microwave kinetic inductance detectors and phased array radar systems.

This work demonstrates a micron-sized nanosecond current pulse probe using a quantum diamond magnetometer. A micron-sized diamond crystal affixed to a fiber tip is integrated on the end of a conical waveguide. We demonstrate realtime visualization of a single 100 nanosecond pulse and discrimination of two pulse trains of different frequencies with a coplanar waveguide and a home-made PCB circuit. This technique finds promising applications in the display of electronic stream and can be used as a pulse discriminator to simultaneously receive and demodulate multiple pulse frequencies. This method of detecting pulse current is expected to provide further detailed analysis of the internal working state of the chip.

In distributed quantum computing (DQC), quantum hardware design mainly focuses on providing as many as possible high-quality inter-chip connections. Meanwhile, quantum software tries its best to reduce the required number of remote quantum gates between chips. However, this “hardware first, software follows” methodology may not fully exploit the potential of DQC. Inspired by classical software-hardware co-design, this paper explores the design space of application-specific DQC architectures. More specifically, we propose AutoArch, an automated quantum chip network (QCN) structure design tool. With qubits grouping followed by a customized QCN design, AutoArch can generate a near-optimal DQC architecture suitable for target quantum algorithms. Experimental results show that the DQC architecture generated by AutoArch can outperform other general QCN architectures when executing target quantum algorithms.

A high-precision and tunable mass detection scheme based on a double-oscillator optomechanical system is proposed. By designating one of the oscillators as the detection port, tiny mass signals can be probed through the frequency shift of the output spectrum, utilizing the system's optomechanically induced transparency (OMIT) effect. By solving the output of the optical mode, we demonstrate that the system exhibits two OMIT windows due to the double-oscillator coupling, with one window being strongly dependent on the mass to be detected. Characterizing the spectrum around this window enables high magnification and precise detection of the input signal under nonlinear parameter conditions. Additionally, our scheme shows resilience to environmental temperature variations and drive strength perturbations.