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.
Quantum computation architecture based on d-level systems, or qudits, has attracted considerable attention recently due to their enlarged Hilbert space. Extensive theoretical and experimentalstudies have addressed aspects of algorithms and benchmarking techniques for qudit-based quantum computation and quantum information processing. Here, we report a physical realization of qudit with upto 4 embedded levels in a superconducting transmon, demonstrating high-fidelity initialization, manipulation, and simultaneous multi-level readout. In addition to constructing SU(d) operations and benchmarking protocols for quantum state tomography, quantum process tomography, and randomized benchmarking etc, we experimentally carry out these operations for d=3 and d=4. Moreover, we perform prototypical quantum algorithms and observe outcomes consistent with expectations. Our work will hopefully stimulate further research interest in developing manipulation protocols and efficient applications for quantum processors with qudits.
Implementing arbitrary single-qubit gates with near perfect fidelity is among the most fundamental requirements in gate-based quantum information processing. In this work, we fabrica transmon qubit with long coherence times and demonstrate single-qubit gates with the average gate error below 10−4, i.e. (7.42±0.04)×10−5 by randomized benchmarking (RB). To understand the error sources, we experimentally obtain an error budget, consisting of the decoherence errors lower bounded by (4.62±0.04)×10−5 and the leakage rate per gate of (1.16±0.04)×10−5. Moreover, we reconstruct the process matrices for the single-qubit gates by the gate set tomography (GST), with which we simulate RB sequences and obtain single-qubit fedlities consistent with experimental results. We also observe non-Markovian behavior in the experiment of long-sequence GST, which may provide guidance for further calibration. The demonstration extends the upper limit that the average fidelity of single-qubit gates can reach in a transmon-qubit system, and thus can be an essential step towards practical and reliable quantum computation in the near future.
Significant progress has been made in building large-scale superconducting quantum processors based on flip-chip technology. In this work, we use the flip-chip technology to realizea modified transmon qubit, donated as the „flipmon“, whose large shunt capacitor is replaced by a vacuum-gap parallel plate capacitor. To further reduce the qubit footprint, we place one of the qubit pads and a single Josephson junction on the bottom chip and the other pad on the top chip which is galvanically connected with the single Josephson junction through an indium bump. The electric field participation ratio can arrive at nearly 53% in air when the vacuum-gap is about 5 microns, and thus potentially leading to a lower dielectric loss. The coherence times of the flipmons are measured in the range of 30-60 microseconds, which are comparable with that of traditional transmons with similar fabrication processes. The electric field simulation indicates that the metal-air interface’s participation ratio increases significantly and may dominate the qubit’s decoherence. This suggests that more careful surface treatment needs to be considered. No evidence shows that the indium bumps inside the flipmons cause significant decoherence. With well-designed geometry and good surface treatment, the coherence of the flipmons can be further improved.