# Two-level system as a quantum sensor of absolute power

## TL;DR

This paper introduces a method to use a two-level quantum system as a non-intrusive, tunable sensor for absolute photon power calibration across various frequencies, enhancing quantum technology applications.

## Contribution

It proposes four novel methods for photon flux sensing using a two-level system, enabling precise power calibration without disturbing propagating waves.

## Key findings

- Demonstrated calibration of transmission line attenuation in dilution refrigerators.
- Analyzed four sensing methods for photon flux measurement.
- Showed the sensor's operation over a wide frequency range.

## Abstract

A two-level quantum system can absorb or emit not more than one photon at a time. Using this fundamental property, we demonstrate how a superconducting quantum system strongly coupled to a transmission line can be used as a sensor of the photon flux. We propose four methods of sensing the photon flux and analyse them for the absolute calibration of power by measuring spectra of scattered radiation from the two-level system. This type of sensor can be tuned to operate in a wide frequency range, and does not disturb the propagating waves when not in use. Using a two-level system as a power sensor enables a range of applications in quantum technologies, here in particular applied to calibrate the attenuation of transmission lines inside dilution refrigerators.