Nonlinear spectroscopy of photons bound to one atom

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.

Nonlinear Spectroscopy of Photons Bound to One Atom

In the presented paper, researchers demonstrate groundbreaking developments in optical nonlinearities at the quantum level, achieved through the confinement of a single atom within two highly reflective mirrors. This experimental setup provides an insightful exploration into the dynamics of atom-cavity interactions, expanding fundamental understanding in cavity quantum electrodynamics (QED).

Overview and Findings

Optical nonlinearities typically demand macroscopic media, posing challenges for quantum-level implementation. This study advances the field of quantum optics by demonstrating a nonlinear effect for a single atom encompassed by a cavity. At weak laser intensities, conventional vacuum-Rabi resonances are observed. However, at increased intensities, the cavity reveals additional resonances, identified as multi-photon transitions. This originates from the discrete nature of photon quantization within the cavity, dictating the characteristic frequencies of the atom-cavity system. The experiment underscores the unique properties of the atom-cavity composite, famously termed the "atom-cavity molecule," possessing an energy spectrum distinct from its components.

Detailed Phenomena and Methodology

The experimental setup involves single rubidium (${^{85}Rb}$) atoms localized inside the cavity, providing strong atom-cavity coupling with a coupling strength (g) reaching up to 11.2 MHz. The study meticulously maps out the energy-level spectrum, including pairs of dressed states, enabling spectroscopic probing of the state characterized by a two-photon resonance. The observation of transmission scaling with the square of laser intensity further cements the results' divergence from classical nonlinear saturation theories.

The authors exclude classical optical bistability theories, affirming the quantum nature of the nonlinearity by postulating that conventional saturation does not adequately account for the transmission patterns observed. Notably, the patterns cannot be reconciled through classical descriptions using single-photon transitions or Maxwell-Bloch equations, indicating a distinctive quantum effect. This experimental protocol demonstrates the elementary steps toward photon-photon interaction fostered by a single atom, paving the way for applications like two-photon gateways or single-photon transistors.

Implications and Future Research Directions

The implications of this study transcend mere academic interest; they offer vital contributions to the field of quantum information science. The demonstrated nonlinearity could fortify advancements in quantum logic devices, serving as an experimental basis for future investigations into quantum communication protocols and computational systems reliant on photon-based logic.

Future research might explore extending cooling techniques into three-dimensional traps for enhanced atom localization, intensifying strong coupling scenarios and enabling the production of complex multi-photon states. Additionally, the investigation of photon statistics and transmitted light spectra could reveal further insights, augmenting understanding of quantum state manipulations and interactions.

In conclusion, this study presents a significant leap toward harnessing quantum interactions at the atomic level, offering transformative potential for the development of quantum technologies in both theoretical exploration and practical applications.