The Rydberg Quantum Nonlinear Optics project at Durham University led by Charles Adams has demonstrated a long-range interaction between individual optical photons that never touch and propagate in entirely different spatial “channels” comprised of separate optical media and spatial modes. The interaction occurs over distances of more than 10 µm, well above the diffraction limit for the optical resolution, as the photons are temporarily stored as strongly interacting collective Rydberg excitations in separate ultracold atom clouds.
Intrinsically, light does not interact with light: When two light beams cross, they normally pass straight through each other. However, some optical media show a nonlinear response to light and act differently depending on the incident intensity. If the nonlinearity affects the mediums optical susceptibility not only locally, but also in other regions of the medium, e.g. through diffusion processes, one speaks of nonlocal nonlinear optics (not to be confused with nonlocality in quantum mechanics). Unfortunately, nonlinear effects occur in most cases only at very high intensities making them unsuitable to mediate interactions between individual photons as they are e.g. required for optical quantum information processing.
Rydberg nonlinear optics, where the photons are mapped into highly excited Rydberg atoms that exhibit strong dipolar interactions by using quantum optical techniques such as electromagnetically-induced transparency and photon storage is an approach developed in recent years to overcome this problem. A single Rydberg atom - and thus also a single photon converted into a Rydberg excitation - can shift the energy of high lying Rydberg states of many other atoms in a region of more than ten µm and alter their interaction with subsequent photons thanks to the long interaction range.
In their recent experiment, the Durham team have directly demonstrated this long range character by storing photons in two side-by-side, optically trapped clouds of ultracold rubidium atoms with centres separated by more than 10 µm. The photons are propagating in independent, non-overlapping optical modes that are focussed to 1 µm into the storage media. This way an interaction has been realised that is not only nonlocal, but also “contactless” in the sense that neither the interacting photons nor the optical media never have to touch as the interaction is mediated thorugh free space via the exchange of virtual microwave photons.
While the photons are stored as a collective Rydberg excitation, the phase of the original photon is imprinted in the collective atomic excitation. Van-der-Waals interactions between the Rydberg excitations induce phase shifts between the individual atom pairs. Because the interaction strength and thus the phase shift acquired by each atom contributing to the collective state is strongly distance dependent, the phase relation of the collective states is not maintained and the direction in which the photons are re-emitted after Therefore, the effect of the interaction can be observed as an anti-correlated retrieval of photon pairs in their original optical modes.
The experiment has investigated the influence of the choice of Rydberg state, distance between the storage media, and interaction time on the retrieval and shown that these parameters allow to tune the interaction across channels independently from interactions between photons in the same channel. Due to the length scale of the interaction, it could be integrated into complex photonic circuits for quantum optical devices or be used as a platform for quantum simulation if scaled to larger arrays of channels.
H. Busche, P. Huillery, S. W. Ball, T. Ilieva, M. P. A. Jones, and C. S. Adams, “Contactless nonlinear optics mediated by long-range Rydberg interactions”, Nature Physics, advance online publication (2017).