Building large-scale quantum computers from smaller modules offers a solution to many formidable scientific and engineering challenges. Nevertheless, engineering high-fidelity interconnectsbetween modules remains challenging. In recent years, quantum state transfer (QST) has provided a way to establish entanglement between two separately packaged quantum devices. However, QST is not a unitary gate, thus cannot be directly inserted into a quantum circuit, which is widely used in recent quantum computation studies. Here we report a demonstration of a direct CNOT gate realized by the cross resonance (CR) effect between two remotely packaged quantum devices connected by a microwave cable. We achieve a CNOT gate with fidelity as high as 99.15±0.02%. The quality of the CNOT gate is verified by cross-entropy benchmarking (XEB) and further confirmed by demonstrating Bell-inequality violation. This work provides a new method to realize remote two-qubit gates. Our method can be used not only to achieve distributed quantum computing but also to enrich the topology of superconducting quantum chips with jumper lines connecting distant qubits. This advancement gives superconducting qubits broader application prospects in the fields of quantum computing and quantum simulation.
Correlated errors can significantly impact the quantum error correction, which challenges the assumption that errors occur in different qubits independently in both space and time.Superconducting qubits have been found to suffer correlated errors across multiple qubits, which could be attributable to ionizing radiations and cosmic rays. Nevertheless, the direct evidence and a quantitative understanding of this relationship are currently lacking. In this work, we propose to continuously monitor multi-qubit simultaneous charge-parity jumps to detect correlated errors and find that occur more frequently than multi-qubit simultaneous bit flips. Then, we propose to position two cosmic-ray muon detectors directly beneath the sample box in a dilution refrigerator and successfully observe the correlated errors in a superconducting qubit array triggered by muons. By introducing a lead shielding layer on the refrigerator, we also reveal that the majority of other correlated errors are primarily induced by gamma rays. Furthermore, we find the superconducting film with a higher recombination rate of quasiparticles used in the qubits is helpful in reducing the duration of correlated errors. Our results provide experimental evidence of the impact of gamma rays and muons on superconducting quantum computation and offer practical insights into mitigation strategies for quantum error correction. In addition, we observe the average occurrence rate of muon-induced correlated errors in our processor is approximately 0.40 min−1cm−2, which is comparable to the muon event rate detected by the muon detector with 0.506 min−1cm−2. This demonstrates the potential applications of superconducting qubit arrays as low-energy threshold sensors in the field of high-energy physics.
Berry curvature is an imaginary component of the quantum geometric tensor (QGT) and is well studied in many branches of modern physics; however, the quantum metric as a real componentof the QGT is less explored. Here, by using tunable superconducting circuits, we experimentally demonstrate two methods to directly measure the quantum metric tensor for characterizing the geometry and topology of underlying quantum states in parameter space. The first method is to probe the transition probability after a sudden quench, and the second one is to detect the excitation rate under weak periodic driving. Furthermore, based on quantum-metric and Berry-curvature measurements, we explore a topological phase transition in a simulated time-reversal-symmetric system, which is characterized by the Euler characteristic number instead of the Chern number. The work opens up a unique approach to explore the topology of quantum states with the QGT.
We experimentally explore the topological Maxwell metal bands by mapping the momentum space of condensed-matter models to the tunable parameter space of superconducting quantum circuits.An exotic band structure that is effectively described by the spin-1 Maxwell equations is imaged. Three-fold degenerate points dubbed Maxwell points are observed in the Maxwell metal bands. Moreover, we engineer and observe the topological phase transition from the topological Maxwell metal to a trivial insulator, and report the first experiment to measure the Chern numbers that are higher than one.