Coherent oscillations between classically separable quantum states of a superconducting loop

TL;DR

This paper reports the first observation of macroscopic quantum coherence oscillations in a superconducting loop with a large inductance, using innovative measurement techniques that could improve qubit coherence.

Contribution

It demonstrates reversible quantum tunneling between classically separable states in a superconducting circuit with unprecedented parameters and a novel dispersive readout scheme.

Findings

Observed MQC oscillations with sub-GHz frequency and high quality factor

Implemented a large inductance loop with a junction in the charging regime

Developed a dispersive readout method that reduces electromagnetic relaxation

Abstract

Ten years ago, coherent oscillations between two quantum states of a superconducting circuit differing by the presence or absence of a single Cooper pair on a metallic island were observed for the first time. This result immediately stimulated the development of several other types of superconducting quantum circuits behaving as artificial atoms, thus bridging mesoscopic and atomic physics. Interestingly, none of these circuits fully implements the now almost 30 year old proposal of A. J. Leggett to observe coherent oscillations between two states differing by the presence or absence of a single fluxon trapped in the superconducting loop interrupted by a Josephson tunnel junction. This phenomenon of reversible quantum tunneling between two classically separable states, known as Macroscopic Quantum Coherence (MQC), is regarded crucial for precision tests of whether macroscopic systems such as circuits fully obey quantum mechanics. In this article, we report the observation of such oscillations with sub-GHz frequency and quality factor larger than 500. We achieved this result with two innovations. First, our ring has an inductance four orders of magnitude larger than that considered by Leggett, combined with a junction in the charging regime, a parameter choice never addressed in previous experiments. Second, readout is performed with a novel dispersive scheme which eliminates the electromagnetic relaxation process induced by the measurement circuit (Purcell effect). Moreover, the reset of the system to its ground state is naturally built into this scheme, working even if the transition energy is smaller than that of temperature fluctuations. As we argue, the MQC transition could therefore be, contrary to expectations, the basis of a superconducting qubit of improved coherence and readout fidelity.