On the path toward large-scale quantum information processing, the ability to implement diverse types of two-qubit gates is preferable for device structure designs and quantum circuit compilations. Here, taking advantage of the inherent Heisenberg interaction between spin qubits, we propose and verify a fast composite two-qubit gate scheme for spin qubits based on semiconductor quantum dots. Theoretical results indicate that the gate scheme facilitates the simultaneous implementation of various types of two-qubit gates, including five types of essential two-qubit gates, iswap-family gates, and fermionic simulation gates. Furthermore, we observe an excellent match between the measured and simulated results, representing the preliminary experimental demonstration of the composite gate scheme. We believe that, with this versatile composite two-qubit gate scheme, the available broad-spectrum two-qubit operations will allow us to efficiently utilize the hardware resources and accelerate the advancement of noisy intermediate-scale quantum computing.

We present the design and experimental characterization of a kinetic inductance traveling-wave parametric amplifier (KI-TWPA) for subgigahertz frequencies. KI-TWPAs amplify signals through mixing processes supported by the nonlinear kinetic inductance of a superconducting transmission line. The device described here uses a compactly meandered $\mathrm{Ti}\mathrm{N}$ microstrip transmission line to achieve the length needed to amplify subgigahertz signals. It is operated in a frequency-translating mode where the amplified signal tone is terminated at the output of the amplifier, and the idler tone at approximately 2.5 GHz is brought out of the cryostat. By varying the pump frequency, a gain of up to 22 dB was achieved in a tunable range from about 450 to 850 MHz. Using $\mathrm{Ti}\mathrm{N}$ as the nonlinear element allows for a reduction of the required pump power by roughly an order of magnitude relative to $\mathrm{Nb}\mathrm{Ti}\mathrm{N}$, which has been used for previous KI-TWPA implementations. This amplifier has the potential to enable high-sensitivity and high-speed measurements in a wide range of applications, such as quantum computing, astrophysics, and dark matter detection.

For superconducting quantum processors, stable high-fidelity two-qubit operations depend on precise flux control of the tunable coupler. However, the pulse distortion poses a significant challenge to the control precision. Current calibration methods, which often rely on microwave crosstalk or additional readout resonators for coupler excitation and readout, tend to be cumbersome and inefficient, especially when couplers only have flux control. Here, we introduce and experimentally validate a pulse calibration scheme that exploits the strong coupling between qubits and couplers, eliminating the need for extra coupler readout and excitation. Our method directly measures the short-time and long-time step responses of the coupler flux pulse transient, enabling us to apply predistortion to subsequent signals using fast Fourier transformation and deconvolution. This approach not only simplifies the calibration process, but also significantly improves the precision and stability of the flux control. We demonstrate the efficacy of our method through the implementation of diabatic controlled-$Z$ (cz) and $i$swap gates with fidelities of $99.61\mathrm{\%}\pm 0.04\mathrm{\%}$ and $99.82\mathrm{\%}\pm 0.02\mathrm{\%}$, respectively, as well as a series of diabatic controlled-phase gates with high fidelities characterized by cross-entropy benchmarking. The consistency and robustness of our technique are further validated by the reduction in pulse distortion and phase error observed across multilayer cz gates. These results underscore the potential of our calibration and predistortion method to enhance the performance of two-qubit gates in superconducting quantum processors.

The heavy holes in $\mathrm{Ge}$/$\mathrm{Ge}\text{−}\mathrm{Si}$ heterostructures show highly anisotropic gyromagnetic response with in-plane $g$ factors $g_{x,y}^{\ast }\lesssim 0.3$ and out-of-plane $g$ factor $g_z^{\ast }\gtrsim 10$. As a consequence, Rabi hot spots and dephasing sweet lines are extremely sharp and call for a careful alignment of the magnetic field in $\mathrm{Ge}$ spin-qubit devices. We investigate how the $g$ factors can be engineered by strains. We show that uniaxial strains can raise in-plane $g$ factors above unity while leaving $g_z^{\ast }$ essentially constant. We discuss how the etching of an elongated mesa in a strained buffer can actually induce uniaxial (but inhomogeneous) strains in the heterostructure. This broadens the operational magnetic field range and enables spin manipulation by shuttling holes between neighboring dots with different $g$ factors.

Designing quantum systems with the measurement speed and accuracy needed for quantum error correction using superconducting qubits requires iterative design and testing informed by accurate models and characterization tools. We introduce a single protocol, with few prerequisite calibrations, to measure the dispersive shift, resonator linewidth, and drive power used in the dispersive readout of superconducting qubits. We find that the resonator linewidth is poorly controlled, with a factor of 2 variation between the maximum and minimum measured values, and this variation is likely to require focused attention in future quantum error correction experiments. We also introduce a protocol for measuring the efficiency of the readout system using the same power levels as are used in typical qubit readout, and without the need to measure the qubit dephasing. We routinely run these protocols on chips with tens of qubits to a hundred qubits, driven by automation software with little human interaction. Using the extracted system parameters, we find that a model based on these parameters predicts the readout signal-to-noise ratio to within 10% over a device with 54 qubits.

Quantum frequency conversion is a widely used technique to interface atomic systems with the telecom band in order to facilitate propagation over longer distances in fiber. Here, we demonstrate the difference-frequency conversion from 606 nm to 1552 nm of microsecond-long weak coherent pulses at the single-photon level compatible with $\mathrm{Pr}$${}^{3+}$:$\mathrm{Y}$${}_2$$\mathrm{Si}\mathrm{O}$${}_5$ quantum memories, with a high signal-to-noise ratio (SNR). We use a single-step difference-frequency-generation process with a continuous-wave pump at 994 nm in a periodically poled lithium niobate ($\mathrm{Mg}\mathrm{O}$:ppLN) waveguide and ultranarrow spectral filtering down to a bandwidth of 12.5 MHz. With this setup, we achieve the conversion of weak coherent pulses of duration up to 13.6 µs with a device efficiency of about 25 % and an SNR >460 for 10-$\text{μ}\mathrm{s}$-long pulses containing one photon on average. This SNR is large enough to enable a high-fidelity conversion of qubits emitted from an emissive quantum memory based on $\mathrm{Pr}$${}^{3+}$:$\mathrm{Y}$${}_2$$\mathrm{Si}\mathrm{O}$${}_5$ and to realize an interface with quantum processing nodes based on narrow-linewidth cavity-enhanced trapped ions.

Synthetic spin-orbit coupling (SOC) has achieved significant advancements in silicon-based electron spin qubits, positioning it as a promising platform for quantum computation. However, SOC also introduces challenges in coherence and quantum information exchange. To address these challenges and achieve practical scalability, on-demand switchable SOC is crucial. Yet, it remains underexplored due to the nonvolatility of micromagnets. Spin valves, which are known for their ultrafast magnetization switching and have been primarily used in magnetic storage, offer a potential solution. In this paper, we propose an alternative approach by utilizing the stray field of spin valves to enable switchable SOC in silicon. This switchable architecture provides a larger coherent region for spin arrays. It also makes long-sought coherent information exchange via swap operations and resonant quantum state transfer (QST) easily surpasses the fault-tolerance threshold when considering realistic experimental parameters. By leveraging these advantages, we have developed a modular quantum chip that successfully integrates the operation, coherence, and scalability of spin qubits. Combined with recent experimental advancements in spin qubits and spin valves, our proposal demonstrates high feasibility for large-scale silicon quantum computation.

We present the implementation and performance analysis of a portable rack-mounted stand-alone warm-vapor quantum memory system that also includes the laser package, control electronics, and data-processing hardware. The optical memory is based on long-lived hyperfine ground states of cesium, which are connected to an excited state via the $D_1$ line at 895 nm in a $\mathrm{\Lambda }$-configuration. The memory is operated with weak coherent pulses containing on average $<1$ photons per pulse. The long-term stability of the memory efficiency and storage fidelity is demonstrated at the single-photon level together with operation in a nonlaboratory environment.

In conventional variational quantum eigensolvers (VQEs), trial states are prepared by applying series of parametrized gates to a reference state, with the gate parameters being varied to minimize the energy of the target system. Recognizing that the gates are intermediates, which are ultimately compiled into a set of control pulses to be applied to each qubit in the lab, the recently proposed ctrl-VQE algorithm takes the amplitudes, frequencies, and phases of the pulse as the variational parameters used to minimize the molecular energy. In this work, we explore how all 3 degrees of freedom interrelate with one another. To this end, we consider several distinct strategies to parametrize the control pulses, assessing each one through numerical simulations of a transmonlike device. For each parametrization, we contrast the pulse duration required to prepare a good ansatz, and the difficulty to optimize that ansatz from a well-defined initial state. We deduce several guiding heuristics to implement practical ctrl-VQE in hardware, which we anticipate will generalize for generic device architectures.

Certain quantum sensing protocols rely on qubits that are initialized, coherently driven in the presence of a stimulus to be measured, then read out. The most widely employed driving sequences act locally in the frequency domain. We introduce and experimentally demonstrate a generalized set of sequences that effect a measurement in any fractional Fourier domain, i.e., along a linear trajectory of arbitrary angle through the time-frequency plane. Combined with linear frequency modulation, this scheme could help separate signal from background in nanoscale spectroscopy and other applications.

Quantum information is very fragile to environmentally and operationally induced imperfections. Therefore, the construction of practical quantum computers requires quantum error-correction techniques to protect quantum information. In particular, encoding a logical qubit into the large Hilbert space of an oscillator is a hardware-efficient way of correcting quantum errors. In this strategy, selective number-dependent arbitrary phase (SNAP) gates are vital for universal quantum control. However, the quality of SNAP gates is considerably limited by the small coupling-induced nonlinearity of the oscillator. Here, to resolve this limitation, we propose a robust scheme based on quantum optimal control via functional theory, by designing an appropriate trajectory for a target operation. Besides, we combine the geometric phase approach with our trajectory design scheme to minimize the decoherence effect, by shortening the gate time. Numerical simulation shows that both errors can be significantly mitigated and that the robustness of the geometric gate against both $X$ and $Z$ errors can be maintained. Therefore, our scheme provides a promising alternative for fault-tolerant quantum computation.

Platforms based on transmission lines are nowadays employed for the simulation of standard phenomena in quantum electrodynamics and quantum field theory. In this work, we propose a hybrid platform, in which a right-handed transmission line is connected to a left-handed transmission line by means of a superconducting quantum interference device. We examine the interaction between the two transmission lines, as well as the excitation flow along the composite platform. We show that, by activating specific resonance conditions, this platform can be used as a quantum simulator of different phenomena in quantum optics, multimode quantum systems, and quantum thermodynamics.

Superconducting qubits are a key contender for quantum computing elements, but they often face challenges like noise and decoherence from two-level systems (TLSs). Tantalum ($\mathrm{Ta}$) qubits are notable for their long $T_1$ coherence times, nearing milliseconds, presumably due to fewer TLSs, though the cause of this is unclear. We explore this by analyzing the vacuum-oxide interface using density functional theory, particularly comparing $\mathrm{Nb}$ oxide (${\mathrm{Nb}}_2{\mathrm{O}}_5$) and $\mathrm{Ta}$ oxide (${\mathrm{Ta}}_2{\mathrm{O}}_5$). We discover that ${\mathrm{Ta}}_2{\mathrm{O}}_5$ forms a smoother surface with fewer dangling $\mathrm{O}$ atoms and structural TLSs than ${\mathrm{Nb}}_2{\mathrm{O}}_5$. The greater atomic mass of $\mathrm{Ta}$ also lowers the tunnel splittings of these structural TLSs below the qubit’s operating frequency. Furthermore, using external electric fields or ${\mathrm{SO}}_2$ passivation can significantly reduce defect-related TLSs on $\mathrm{Nb}$ surfaces, potentially improving their coherence times.

Shortcuts to adiabaticity (STA) enables fast and robust coherent control of quantum system, which has been well placed in quantum technologies. In particular, inverse-engineering STA provides much more freedom for the optimization of shortcut, which alleviates the complexity for experimental realization. Here, we implement a STA technique, known as invariant-based inverse engineering, to speed up the adiabatic control of the electron triplet ground state of a single nitrogen-vacancy (N-$V$) center. The microwave pulses to drive inversely engineered STA are obtained with space-curve quantum control, where the evolution of spin transition is mapped to a three-dimensional closed space curve and the design of shortcut is obtained by optimization over the space curve. We demonstrate the fast and high-fidelity drive of dipole-forbidden transition between two spin sublevels of the ground state. Moreover, we demonstrate the robustness of the spin transition by introducing the detuning of driving microwave field. The acceleration and robustness is further confirmed by the comparison with two traditional Raman control schemes. Our results suggest invariant-based inverse engineering is powerful for fast and robust manipulation of N-$V$ system, and thus benefits quantum sensing and quantum computation based on the N-$V$ platform.

Integrated photonics plays a pivotal role in quantum communication devices, offering exceptional robustness, scalability, and performance. The growing maturity of silicon photonics technology offers significant potential for miniaturization and cost-effective implementation of quantum key distribution (QKD). While the practical security of lithium niobate modulators in QKD systems has been extensively examined, the adoption of a new modulation mechanism by silicon photonic modulators necessitates a thorough investigation into their impact on the security of QKD systems. For the first time, we reveal that the two-photon absorption effect in silicon photonic intensity modulators will introduce vulnerabilities, enabling an eavesdropper (Eve) to execute malicious attacks on QKD systems. Through experimental demonstration, we show that by externally injecting femtosecond pulses, Eve can manipulate the intensities of decoy states sent by legitimate users. Based on the experimental results, we find that in the presence of the attack, legitimate users consistently overestimate the secret key rate predicted by the security proof. Our work represents an initial exploration into a new security aspect for the design and standardization of integrated QKD systems.

The rotating-wave approximation (RWA) is ubiquitous in the analysis of driven and coupled resonators. However, the limitations of the RWA seem to be poorly understood and in some cases the RWA disposes of essential physics. We investigate the RWA in the context of electrical circuits. Using a classical Hamiltonian approach, we find that by balancing electrical and magnetic components of the resonator drive or resonator-resonator coupling, the RWA can be made exact. This type of balance, in which the RWA is exact, has applications in superconducting qubits, where it suppresses nutation normally associated with strong Rabi driving. In the context of dispersive readout, balancing the qubit-resonator coupling changes the qubit leakage induced by the resonator drive but does not remove it in the case of the transmon qubit.

Reference-frame-independent measurement-device-independent quantum key distribution (RFI MDI QKD) can resist side-channel attacks against detectors and slow variations of reference frames, thus presenting an advantageous approach for long-distance secure quantum communication. Nevertheless, if an eavesdropper implements a Trojan-horse attack (THA) by injecting bright light into sources, the back-reflected light may carry encoding information into the channel, thereby causing serious information leakage. In this paper, we theoretically evaluate the finite-key security of decoy-state RFI MDI QKD against a THA by employing the reference-state technique and a particular concentration inequality. Additionally, dual-parameter optimization and multibasis-estimation strategies are introduced to reduce statistical fluctuations effectively. Our work demonstrates that RFI MDI QKD can maintain a better performance even in the presence of large reference-frame drifts and a THA, representing a key advancement in strengthening the security of quantum communication in real-world implementations.