Nonlinear spectroscopy of photons bound to one atom

TL;DR

This paper demonstrates a quantum nonlinearity using a single atom in a cavity, showing photon-photon interactions and providing evidence of the quantum nature of the atom-cavity system, with potential applications in quantum information processing.

Contribution

It introduces a novel quantum nonlinear spectroscopy technique for a single atom in a cavity, revealing photon-number-dependent resonances and photon-photon interactions.

Findings

Observation of vacuum-Rabi resonances at low intensity

Detection of an additional resonance at higher intensities

Transmission increases quadratically with laser intensity

Abstract

Optical nonlinearities typically require macroscopic media, thereby making their implementation at the quantum level an outstanding challenge. Here we demonstrate a nonlinearity for one atom enclosed by two highly reflecting mirrors. We send laser light through the input mirror and record the light from the output mirror of the cavity. For weak laser intensity, we find the vacuum-Rabi resonances. But for higher intensities, we find an additional resonance. It originates from the fact that the cavity can accommodate only an integer number of photons and that this photon number determines the characteristic frequencies of the coupled atom-cavity system. We selectively excite such a frequency by depositing at once two photons into the system and find a transmission which increases with the laser intensity squared. The nonlinearity differs from classical saturation nonlinearities and is direct spectroscopic proof of the quantum nature of the atom-cavity system. It provides a photon-photon interaction by means of one atom, and constitutes a step towards a two-photon gateway or a single-photon transistor.