# Observation and stabilization of photonic Fock states in a hot radio-frequency resonator

## TL;DR

This paper demonstrates the direct observation, control, and stabilization of photonic Fock states in a hot radio-frequency resonator using a superconducting qubit, advancing quantum thermodynamics and quantum interface technologies.

## Contribution

It introduces a method to observe and stabilize quantum states in a high-temperature RF resonator, extending circuit QED to new regimes.

## Key findings

- Successful stabilization of photonic Fock states.
- Real-time observation of re-thermalization dynamics.
- Quantum control of thermal photons at megahertz frequencies.

## Abstract

Detecting weak radio-frequency electromagnetic fields plays a crucial role in wide range of fields, from radio astronomy to nuclear magnetic resonance imaging. In quantum mechanics, the ultimate limit of a weak field is a single-photon. Detecting and manipulating single-photons at megahertz frequencies presents a challenge as, even at cryogenic temperatures, thermal fluctuations are significant. Here, we use a gigahertz superconducting qubit to directly observe the quantization of a megahertz radio-frequency electromagnetic field. Using the qubit, we achieve quantum control over thermal photons, cooling to the ground-state and stabilizing photonic Fock states. Releasing the resonator from our control, we directly observe its re-thermalization dynamics with the bath with nanosecond resolution. Extending circuit quantum electrodynamics to a new regime, we enable the exploration of thermodynamics at the quantum scale and allow interfacing quantum circuits with megahertz systems such as spin systems or macroscopic mechanical oscillators.