# Decoherence Dynamics of Complex Photon States in a Superconducting Circuit

## TL;DR

This paper reports the first detailed measurement of how complex photon states decohere in a superconducting circuit, revealing that decay is primarily driven by energy relaxation and dephasing, and matching theoretical predictions.

## Contribution

It introduces a method to prepare and measure specific quantum photon states in a superconducting resonator, enabling detailed analysis of decoherence dynamics.

## Key findings

- Decoherence dynamics are dominated by energy relaxation and dephasing.
- Quantum states decay in a manner consistent with theoretical models.
- The approach allows precise tracking of density matrix elements over time.

## Abstract

Quantum states inevitably decay with time into a probabilistic mixture of classical states, due to their interaction with the environment and measurement instrumentation. We present the first measurement of the decoherence dynamics of complex photon states in a condensed-matter system. By controllably preparing a number of distinct, quantum-superposed photon states in a superconducting microwave resonator, we show that the subsequent decay dynamics can be quantitatively described by taking into account only two distinct decay channels, energy relaxation and dephasing. Our ability to prepare specific initial quantum states allows us to measure the evolution of specific elements in the quantum density matrix, in a very detailed manner that can be compared with theory.