Ultra-weak acoustic interactions of temporal cavity solitons

Solitons are ubiquitous nonlinear waves that manifest themselves across a plethora of physical systems. They are known to interact in a variety of ways, elastically or inelastically. Despite the vast number of systems supporting solitons, the past two decades have shown optics to constitute the platform of choice for the systematic study of their interactions [1]. Soliton interactions can be either short-range or long-range, the latter being mediated by the coupling of light with a non-local material response. The weakest interactions reported are long-range, but observations are typically limited by the distance or duration across which the solitons can be maintained. Here we report what we believe is by far the weakest interaction between solitons ever observed. To circumvent the limitation imposed by a host material with a finite size, our experiment utilizes temporal cavity solitons (CSs) recirculating in a coherently-driven optical fiber loop [2]. We observe two such CSs, separated by up to 8000 times their width, changing their temporal separation by a few attoseconds per round-trip of the 100 m-long resonator. The acoustically-mediated [3] interactions are so weak that they require an effective propagation distance of the order of an astronomical unit, i.e., tens of millions of kilometers, to truly manifest themselves.