Coherence-limited digital control of a superconducting qubit using a Josephson pulse generator at 3 K

TL;DR

This paper demonstrates that a Josephson pulse generator at 3 K can effectively control superconducting qubits with high fidelity, offering a scalable alternative to traditional room-temperature electronics by reducing decoherence and improving gate performance.

Contribution

The study introduces a 3 K Josephson pulse generator for qubit control, achieving high-fidelity gates and scalable design, advancing cryogenic quantum computing hardware.

Findings

Qubit coherence times are maintained with JPG control.

Gate error per operation is approximately 0.46%.

JPG control shows an order of magnitude improvement over previous methods.

Abstract

Compared to traditional semiconductor control electronics (TSCE) located at room temperature, cryogenic single flux quantum (SFQ) electronics can provide qubit measurement and control alternatives that address critical issues related to scalability of cryogenic quantum processors. Single-qubit control and readout have been demonstrated recently using SFQ circuits coupled to superconducting qubits. Experiments where the SFQ electronics are co-located with the qubit have suffered from excess decoherence and loss due to quasiparticle poisoning of the qubit. A previous experiment by our group showed that moving the control electronics to the 3 K stage of the dilution refrigerator avoided this source of decoherence in a high-coherence 3D transmon geometry. In this paper, we also generate the pulses at the 3 K stage but have optimized the qubit design and control lines for scalable 2D transmon devices. We directly compare the qubit lifetime $T_1$, coherence time $T_2^*$ and gate fidelity when the qubit is controlled by the Josephson pulse generator (JPG) circuit versus the TSCE setup. We find agreement to within the daily fluctuations for $T_1$ and $T_2^*$, and agreement to within 10% for randomized benchmarking. We also performed interleaved randomized benchmarking on individual JPG gates demonstrating an average error per gate of $0.46$% showing good agreement with what is expected based on the qubit coherence and higher-state leakage. These results are an order of magnitude improvement in gate fidelity over our previous work and demonstrate that a Josephson microwave source operated at 3 K is a promising component for scalable qubit control.