Decoherence times of universal two-qubit gates in the presence of broad-band noise

TL;DR

This paper analyzes how broadband noise affects the decoherence times of two-qubit gates in solid-state quantum devices, providing analytic expressions and optimal conditions for entanglement preservation.

Contribution

It introduces a multi-stage analytic approach to identify decoherence times for two-qubit gates under broadband noise, including 1/f spectra, and applies it to superconducting -SWAP gates.

Findings

Decoherence times depend on noise spectral properties and coupling symmetry.

Optimal operating conditions reduce sensitivity to noise.

Analytic formulas for entanglement decay in noisy environments.

Abstract

The controlled generation of entangled states of two quantum bits is a fundamental step toward the implementation of a quantum information processor. In nano-devices this operation is counteracted by the solid-state environment, characterized by a broadband and non-monotonic power spectrum, often 1/f at low frequencies. For single-qubit gates, incoherent processes due to fluctuations acting on different time scales result in peculiar short- and long-time behavior. Markovian noise gives rise to exponential decay with relaxation and decoherence times, T1 and T2, simply related to the symmetry of the qubit-environment coupling Hamiltonian. Noise with the 1/f power spectrum at low frequencies is instead responsible for defocusing processes and algebraic short-time behavior. In this paper, we identify the relevant decoherence times of an entangling operation due to the different decoherence channels originating from solid-state noise. Entanglement is quantified by concurrence, which we evaluate in an analytic form employing a multi-stage approach. The 'optimal' operating conditions of reduced sensitivity to noise sources are identified. We apply this analysis to a superconducting \sqrt{i-SWAP} gate for experimental noise spectra.