Coherent frequency conversion in a superconducting artificial atom with two internal degrees of freedom

TL;DR

This paper demonstrates a superconducting artificial atom with two internal modes that can coherently convert frequencies through strong nonlinear coupling, enabling advanced quantum control in superconducting circuits.

Contribution

It introduces a superconducting artificial atom with two internal degrees of freedom and shows coherent frequency conversion via mode coupling.

Findings

Large energy splitting between coupled states

Observation of coherent frequency conversion

Decoupling of junction dynamics in a dc-SQUID

Abstract

By adding a large inductance in a dc-SQUID phase qubit loop, one decouples the junctions' dynamics and creates a superconducting artificial atom with two internal degrees of freedom. In addition to the usual symmetric plasma mode ({\it s}-mode) which gives rise to the phase qubit, an anti-symmetric mode ({\it a}-mode) appears. These two modes can be described by two anharmonic oscillators with eigenstates $\ket{n_{s}}$ and $\ket{n_{a}}$ for the {\it s} and {\it a}-mode, respectively. We show that a strong nonlinear coupling between the modes leads to a large energy splitting between states $\ket{0_{s},1_{a}}$ and $\ket{2_{s},0_{a}}$. Finally, coherent frequency conversion is observed via free oscillations between the states $\ket{0_{s},1_{a}}$ and $\ket{2_{s},0_{a}}$.