Time-Reversal Symmetry and Universal Conductance Fluctuations in a Driven Two-Level System

TL;DR

This study demonstrates how a driven superconducting qubit exhibits universal conductance fluctuation-like behavior, with transition rate fluctuations strongly dependent on the symmetry of the driving waveform, revealing the role of time-reversal symmetry in quantum interference.

Contribution

It introduces a controllable artificial two-level system to study the effects of time-reversal symmetry breaking on quantum interference phenomena.

Findings

Transition rate fluctuations peak when the drive waveform is symmetric.

Interference patterns in the qubit transition rate resemble universal conductance fluctuations.

Control over time-reversal symmetry via drive waveform symmetry.

Abstract

In the presence of time-reversal symmetry, quantum interference gives strong corrections to the electric conductivity of disordered systems. The self-interference of an electron wavefunction traveling time-reversed paths leads to effects such as weak localization and universal conductance fluctuations. Here, we investigate the effects of broken time-reversal symmetry in a driven artificial two-level system. Using a superconducting flux qubit, we implement scattering events as multiple Landau-Zener transitions by driving the qubit periodically back and forth through an avoided crossing. Interference between different qubit trajectories give rise to a speckle pattern in the qubit transition rate, similar to the interference patterns created when coherent light is scattered off a disordered potential. Since the scattering events are imposed by the driving protocol, we can control the time-reversal symmetry of the system by making the drive waveform symmetric or asymmetric in time. We find that the fluctuations of the transition rate exhibit a sharp peak when the drive is time-symmetric, similar to universal conductance fluctuations in electronic transport through mesoscopic systems.