# Large collective Lamb shift of two distant superconducting artificial atoms

## TL;DR

This paper demonstrates a significant collective Lamb shift in two distant superconducting qubits, achieved by suppressing decay with a mirror, revealing strong virtual photon-mediated interactions.

## Contribution

The study reports the first observation of a large collective Lamb shift in distant superconducting qubits, surpassing linewidths and showing interaction without decay into the transmission line.

## Key findings

- Collective Lamb shift of 0.8% of qubit frequency observed
- Qubits interact via transmission line even without decay into it
- Decay suppression enhances visibility of the Lamb shift

## Abstract

Virtual photons can mediate interaction between atoms, resulting in an energy shift known as a collective Lamb shift. Observing the collective Lamb shift is challenging, since it can be obscured by radiative decay and direct atom-atom interactions. Here, we place two superconducting qubits in a transmission line terminated by a mirror, which suppresses decay. We measure a collective Lamb shift reaching 0.8% of the qubit transition frequency and exceeding the transition linewidth. We also show that the qubits can interact via the transmission line even if one of them does not decay into it.