# Observation of a large, resonant, cross-Kerr nonlinearity in a free-space Rydberg medium

## TL;DR

This paper demonstrates a significant cross-Kerr nonlinearity in a free-space Rydberg medium, enabling effective cross-phase modulation for quantum information applications.

## Contribution

It reports the first experimental observation of a large, resonant cross-Kerr nonlinearity in a free-space Rydberg atom medium using electromagnetically induced transparency.

## Key findings

- Achieved a nonlinear phase shift of 8 mrad per nW of signal power.
- Measured a third-order nonlinear susceptibility of 10^{-8} m^2/V^2.
- Showcased potential for quantum non-demolition photon measurement.

## Abstract

We report the experimental observation of a cross-Kerr nonlinearity in a free-space medium based on resonantly-excited, interacting Rydberg atoms and electromagnetically induced transparency. The nonlinearity is used to implement cross-phase modulation between two optical pulses. The nonlinear phase written onto the probe pulse is measured to be as large as $8$ mrad per nW of signal power, corresponding to a $\chi^{(3)}$ of $10^{-8}\textrm{m}^2/\textrm{V}^2$. Potential applications range from optical quantum information processing to quantum non-demolition measurement of photon number.