The set of quantum logic gates that can be easily implemented is fundamental to the performance of quantum computers, as it governs the accuracy of basic quantum operations and dictates the complexity of implementing quantum algorithms. Traditional approaches to extending gate sets often require operating devices outside the ideal parameter regimes used to realize qubits, leading to increased control complexity while offering only a limited set of gates. Here we experimentally demonstrate a unified and versatile gate scheme capable of generating arbitrary two-qubit gates using only an exchange interaction and qubit driving on a superconducting quantum processor. We achieve high fidelities averaging 99.38% across a wide range of commonly used two-qubit unitaries, enabling precise multipartite entangled state preparation. Furthermore, we successfully produce a B gate, which efficiently synthesizes the entire family of two-qubit gates. Our results establish that fully exploiting the capabilities of the exchange interaction can yield a comprehensive and highly accurate gate set. With maximum expressivity, optimal gate time, demonstrated high fidelity and easy adaption to other quantum platforms, our unified control scheme offers the prospect of improved performance in quantum hardware and algorithm development.

Magneto-optical double-resonance magnetometers are attractive candidates for sensitive measurements of magnetic fields but suffer from a fundamental problem known as the “dead zone”: certain magnetic field directions where the magnetometer loses its sensitivity. We present a dead-zone-free double-resonance magnetometer with a single linearly polarized laser, which keeps uniform signal amplitude and isotropic sensitivity among varying magnetic field directions. The isotropic magnetometer can be switched between two modes, i.e., $M_z$ mode insensitive to phase shifts and $M_x$ mode with a higher upper bound of bandwidth. The dead-zone-free magnetometer exhibits a magnetic field noise floor of $0.9\mathrm{pT}/\sqrt{\mathrm{Hz}}$ and $1.2\mathrm{pT}/\sqrt{\mathrm{Hz}}$ in $M_z$ and $M_x$ modes, respectively. The proposed double-resonance magnetometer, benefiting from its compact single-beam structure, isotropic sensitivity, and capability of dual modes, would be useful for high-sensitivity magnetic field measurements in mobile applications.

Scalar atomic magnetometer (SAM) with high sensitivity is designed to measure the total magnetic field and has been widely applied in aeromagnetic surveys. However, the measured total magnetic field of SAM is influenced by the magnetic disturbances due to the permanent, induced and eddy-current fields of the aircraft and heading errors of SAM. To reduce the magnetic disturbances caused by the aircraft maneuvers, a model was proposed by Tolles and Lawson and has been used in aeromagnetic compensation. However, the heading error of SAM deviates the compensation coefficients estimated by the Tolles-Lawson model. In this letter, we propose a linear model of heading error for multi-cell SAMs and analyze the new aeromagnetic compensation model in which the geomagnetic gradient and heading error are both taken into account. With using a ⁴He magnetometer consisting of three orthogonal cells, the experimental results show that our proposed model has better performances than the previous model in terms of the figure of merit (FOM) and standard deviation (STD).

The ability to fast reset a qubit state is crucial for quantum information processing. However, to actively reset a qubit requires engineering a pathway to interact with a dissipative bath, which often comes at the cost of reduced qubit protection from the environment. Here, we present a multipurpose architecture that enables fast reset and protection of superconducting qubits during control and readout. In our design, two on-chip diplexers are connected by two transmission lines. The high-pass branch provides a flat passband for convenient allocation of readout resonators above the qubit frequencies, which is preferred for reducing measurement-induced state transitions. In the low-pass branch, we leverage a standing-wave mode below the maximum qubit frequency for a rapid reset. The qubits are located in the common stopband to inhibit dissipation during coherent operations. We demonstrate resetting a transmon qubit from its first excited state to the ground state in 100 ns, achieving a residual population of 2.7%. The reset time may be further shortened to 27 ns by exploiting the coherent population inversion effect. We further extend the technique to resetting the qubit from its second excited state. Our approach promises scalable implementation of fast reset and qubit protection during control and readout, adding to the toolbox of dissipation engineering.

We present both a theoretical and experimental study of the magneto-optical spectra induced with intensity-modulated resonant laser light that synchronously pumps and detects the metastable ${}^4\mathrm{He}$ atoms in the presence of a quasistatic magnetic field. We extend previous work and derive complete analytical expressions for the resonance signals by taking into account arbitrary magnetic-field directions and arbitrary light polarizations. The analytical results are derived by solving the Liouville equation using the irreducible tensor formalism on the assumption of low light power. We discuss the potential application of the derived analytical results in constructing and optimizing a dead-zone-free all-optical atomic magnetometer.

We propose a compact ⁴He magnetometer realizing magnetic field measurement and laser-frequency stabilization simultaneously in a single ⁴He atomic cell. The frequency stabilization scheme is based on the asymmetric line shape of magnetic resonance which is induced by spatially inhomogeneous light shifts. We investigate the asymmetric line shape of the magnetic resonance signal theoretically and experimentally in laser pumped ⁴He magnetometer with the magneto-optical double-resonance configuration. Notice that, due to the asymmetric line shape, the in-phase component of the magnetic resonance signal is shown to have a linear dependence with respect to the laser frequency detuning and is used to actively lock the laser frequency to the resonant point. The method reduces the complexity of the system and improves the stability of the magnetometer, making the laser-pumped ⁴He magnetometer more compact and portable.