Unexpectedly allowed transition in two inductively coupled transmons

TL;DR

This paper reports the experimental observation of an unexpected zero-two transition in a circuit of two inductively coupled transmons, enabled by strong mode coupling and wave function superposition, challenging typical selection rules.

Contribution

It demonstrates a novel transition in coupled transmons caused by Josephson-induced mode coupling, revealing new quantum behavior.

Findings

Observed zero-two transition with magnetic flux dependence

Transition disappears at zero magnetic flux

Strong mode coupling enables superposition of states

Abstract

We present experimental results in which the unexpected zero-two transition of a circuit composed of two inductively coupled transmons is observed. This transition shows an unusual magnetic flux dependence with a clear disappearance at zero magnetic flux. In a transmon qubit the symmetry of the wave functions prevents this transition to occur due to selection rule. In our circuit the Josephson effect introduces strong couplings between the two normal modes of the artificial atom. This leads to a coherent superposition of states from the two modes enabling such transitions to occur.