# Detecting virtual phothons in ultrastrongly coupled superconducting quantum circuits

**Authors:** L. Giannelli, E. Paladino, M. Grajcar, G. S. Paraoanu, G. Falci  
**arXiv:** [2302.10973](https://arxiv.org/abs/2302.10973)  
**Date:** 2023-02-21  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2302.10973v1/

## Key points

- Study of light-matter interaction is crucial for quantum technologies
- Solid-state devices can explore "ultrastrong" coupling regimes
- Exotic phenomena occur due to entangled vacuum containing virtual photons
- Ground-state virtual photon detection has been a challenge for over a decade
- L. Giannelli et al. designed an unconventional superconducting quantum circuit and protocol for detecting virtual photons
- Highly efficient, faithful, and selective conversion of virtual photons into real ones was achieved
- Breakthrough in detecting virtual photons opens up new possibilities for exploring ultrastrongly coupled systems and understanding fundamental physics behind emerging quantum technologies
- Authors hope their work will inspire further research and lead to practical applications in future quantum technologies

## Summary

The study of light-matter interaction is crucial for the development of quantum technologies. Solid-state devices can explore new regimes where coupling strengths are "ultrastrong," meaning they are comparable to the energies of the subsystems. In these ultrastrongly coupled systems, exotic phenomena occur due to the fact that the entangled vacuum contains virtual photons. These virtual photons lack conservation of the number of excitations, which is a witness to ultrastrong coupling breaking the U(1) symmetry. Despite over a decade of research, detecting ground-state virtual photons has remained a challenge. However, in this work by L. Giannelli et al., an unconventional "light fluxonium"-like superconducting quantum circuit implemented by superinductors and a protocol of coherent amplification were designed to yield a highly efficient, faithful, and selective conversion of virtual photons into real ones. The design allows for highly efficient conversion while maintaining selectivity and faithfulness in detecting virtual photons. This breakthrough in detecting virtual photons opens up new possibilities for exploring ultrastrongly coupled systems and understanding their fundamental physics behind emerging quantum technologies. The authors hope that their work will inspire further research in this field and lead to practical applications in future quantum technologies. This could enable researchers to gain insights into exotic phenomena occurring in ultrastrongly coupled systems as well as provide potential applications for emerging quantum technologies.