Exploiting environmental resonances to enhance qubit quality factors

TL;DR

This paper investigates how environmental resonances affect qubit dephasing times and quality factors, demonstrating that tuning the resonance frequency can significantly enhance qubit coherence in solid-state systems.

Contribution

It introduces a method to analyze qubit-environment interactions using flow equations and shows how tuning environmental resonances improves qubit quality factors.

Findings

Quality factors depend strongly on environmental resonance frequency.

Tuning the resonance frequency below the qubit frequency enhances coherence.

Flow equations effectively diagonalize the spin-boson model with Lorentzian spectral functions.

Abstract

We discuss dephasing times for a two-level system (including bias) coupled to a damped harmonic oscillator. This system is realized in measurements on solid-state Josephson qubits. It can be mapped to a spin-boson model with a spectral function with an approximately Lorentzian resonance. We diagonalize the model by means of infinitesimal unitary transformations (flow equations), and calculate correlation functions, dephasing rates, and qubit quality factors. We find that these depend strongly on the environmental resonance frequency $\Omega$; in particular, quality factors can be enhanced significantly by tuning $\Omega$ to lie below the qubit frequency $\Delta$.