Decoherence of a Josephson qubit due to coupling to two level systems

TL;DR

This paper investigates how two level systems in the oxide barrier of Josephson junctions cause decoherence in qubits, affecting their coherence and Rabi oscillations, with implications for quantum computing stability.

Contribution

It introduces two specific decoherence mechanisms involving TLS interactions and fluctuations, providing a detailed theoretical analysis aligned with experimental observations.

Findings

Resonant coupling causes Rabi oscillation beating.

TLS decay leads to qubit decoherence via phonon emission.

Fluctuations induce 1/f noise, degrading qubit coherence.

Abstract

Noise and decoherence are major obstacles to the implementation of Josephson junction qubits in quantum computing. Recent experiments suggest that two level systems (TLS) in the oxide tunnel barrier are a source of decoherence. We explore two decoherence mechanisms in which these two level systems lead to the decay of Rabi oscillations that result when Josephson junction qubits are subjected to strong microwave driving. (A) We consider a Josephson qubit coupled resonantly to a two level system, i.e., the qubit and TLS have equal energy splittings. As a result of this resonant interaction, the occupation probability of the excited state of the qubit exhibits beating. Decoherence of the qubit results when the two level system decays from its excited state by emitting a phonon. (B) Fluctuations of the two level systems in the oxide barrier produce fluctuations and 1/f noise in the Josephson junction critical current I_o. This in turn leads to fluctuations in the qubit energy splitting that degrades the qubit coherence. We compare our results with experiments on Josephson junction phase qubits.