High-Coherence Kerr-cat qubit in 2D architecture

TL;DR

This paper demonstrates a high-coherence Kerr-cat qubit integrated into a 2D superconducting circuit, achieving high fidelity readout and control, with potential for scalable fault-tolerant quantum computing.

Contribution

The authors develop a scalable 2D architecture with an on-chip band-block filter, enabling high-coherence Kerr-cat qubits without strong drives, and demonstrate high-fidelity readout and control.

Findings

Quantum non-demolition readout fidelity of 99.6%

Bit-flip time exceeds 1 ms for up to 10 photons

Linear decrease in phase-flip time with cat size

Abstract

The Kerr-cat qubit is a bosonic qubit in which multi-photon Schrodinger cat states are stabilized by applying a two-photon drive to an oscillator with a Kerr nonlinearity. The suppressed bit-flip rate with increasing cat size makes this qubit a promising candidate to implement quantum error correction codes tailored for noise-biased qubits. However, achieving strong light-matter interactions necessary for stabilizing and controlling this qubit has traditionally required strong microwave drives that heat the qubit and degrade its performance. In contrast, increasing the coupling to the drive port removes the need for strong drives at the expense of large Purcell decay. By integrating an effective band-block filter on-chip, we overcome this trade-off and realize a Kerr-cat qubit in a scalable 2D superconducting circuit with high coherence. This filter provides 30 dB of isolation at the qubit frequency with negligible attenuation at the frequencies required for stabilization and readout. We experimentally demonstrate quantum non-demolition readout fidelity of 99.6% for a cat with 8 photons. Also, to have high-fidelity universal control over this qubit, we combine fast Rabi oscillations with a new demonstration of the X(90) gate through phase modulation of the stabilization drive. Finally, the lifetime in this architecture is examined as a function of the cat size of up to 10 photons in the oscillator achieving a bit-flip time higher than 1 ms and only a linear decrease in the phase-flip time, in good agreement with the theoretical analysis of the circuit. Our qubit shows promise as a building block for fault-tolerant quantum processors with a small footprint.