Decoherence in a Pair of Long-Lived Cooper-Pair Boxes

TL;DR

This study investigates decoherence mechanisms in two superconducting Cooper-pair box qubits, revealing how coupling to microwave lines affects their relaxation times and identifying charge noise as a key decoherence source.

Contribution

The paper provides detailed measurements of relaxation and coherence times in two long-lived Cooper-pair box qubits, highlighting the inverse relationship between coupling strength and qubit lifetime, and suggests reducing coupling to enhance performance.

Findings

T1 times ranged from 30 to 200 microseconds in the first qubit.

Charge noise with a high-frequency cutoff of 0.2 MHz affects coherence.

Reducing coupling to the drive line could improve qubit lifetimes.

Abstract

We have investigated the decoherence of quantum states in two Al/AlOx/Al Cooper-pair boxes coupled to lumped element superconducting LC resonators. At 25 mK, the first qubit had an energy relaxation time T1 that varied from 30 us to 200 us between 4 and 8 GHz and displayed an inverse correlation between T1 and the coupling to the microwave drive line. The Ramsey fringe decay times T2* were in the 200-500 ns range while the spin echo envelope decay times Techo varied from 2.4-3.3 us, consistent with 1/f charge noise with a high frequency cutoff of 0.2 MHz. A second Cooper-pair box qubit with similar parameters showed T1=4-30 us between 4-7.3 GHz, and that the T1 and the coupling were again inversely correlated. Although the lifetime of the second device was shorter than that of the first device, the dependence on coupling in both devices suggests that further reduction in coupling should lead to improved qubit performance.