Observation of three-photon bound states in a quantum nonlinear medium (1709.01478v2)

Abstract: Bound states of massive particles, such as nuclei, atoms or molecules, constitute the bulk of the visible world around us. In contrast, photons typically only interact weakly. We report the observation of traveling three-photon bound states in a quantum nonlinear medium where the interactions between photons are mediated by atomic Rydberg states. Photon correlation and conditional phase measurements reveal the distinct bunching and phase features associated with three-photon and two-photon bound states. Such photonic trimers and dimers possess shape-preserving wavefunctions that depend on the constituent photon number. The observed bunching and strongly nonlinear optical phase are quantitatively described by an effective field theory (EFT) of Rydberg-induced photon-photon interactions, consistent with the presence of a substantial effective three-body force between the photons. These observations demonstrate the ability to realize and control strongly interacting quantum many-body states of light.

Bound States of Photons in a Quantum Nonlinear Medium

The paper "Observation of three-photon bound states in a quantum nonlinear medium" presents a significant advancement in the paper of photonic interactions in quantum systems. The researchers achieved the first experimental observation of three-photon bound states, expanding on previous work that documented two-photon bound states. This paper is grounded in the domain of quantum nonlinear optics and is propelled by the pursuit of developing quantum matter from light, with potential applications spanning quantum communication and metrology.

The experiment utilized an ultracold atomic gas to act as a quantum nonlinear medium. This medium leverages Rydberg-blockaded electromagnetically induced transparency (EIT) to mediate interactions between photons through atomic Rydberg states. In this configuration, the interactions are not only enhanced but also tunable, which is crucial for observing bound states—quasi-particles that propagate as solitary units despite the inherent dispersive nature of the medium.

Experimental Design and Results

The researchers employed a quantum probe field coupled to a Rydberg state of rubidium atoms, located in an optical dipole trap, to generate conditions conducive to photon interactions. The system is engineered to maintain effective one-dimensional behavior for photons, where the photon-photon interaction strength is modulated by the Rydberg blockade mechanism.

Key observations include distinct photon bunching and nonlinear phase shifts corresponding to three-photon and two-photon bound states. Notably, the third-order photon correlation function $g^{(3)}$ revealed a marked bunching effect. The likelihood of detecting three photons in close temporal proximity was significantly higher compared to a scenario involving uncorrelated photons. This increased probability—a factor of six—is indicative of a strong binding force among the three photons, effectively forming a trimer state over the traditionally understood dimer formation.

The nontrivial interactions manifest as significant conditional phase changes, where the phase shift observed with three coincident photons exceeded that of any two-photon combinations. A comprehensive analysis utilizing effective field theory (EFT) and a slow-light Hamiltonian was employed to model these interactions. The EFT accurately described the system by incorporating a contact interaction paradigm, capturing the essence of the effective three-body forces that arise in such multicomponent photonic systems.

Implications and Future Directions

The successful demonstration of three-photon bound states opens intriguing avenues for future research. Extending the medium length and optimizing atomic densities could lead to an exacerbation of the solitonic bound-state components, potentially isolating them from scattering contributions. Experimentally realizing larger photonic clusters, possibly even higher-order photon bound states, remains a tantalizing prospect. Additionally, expanding the system into higher dimensions could pave the way for observing complex phenomena like photonic Efimov states.

The paper also proposes leveraging these controlled interactions to explore phases of matter that are not easily achievable with traditional materials—specifically, exploring self-organized phases in open quantum systems. These advances underscore the growing capability of quantum photonics to serve as a versatile platform for both fundamental inquiries and technological applications in the quantum domain.