Quantum optical binding of levitated nanoparticles

Laser light can manipulate and trap objects smaller than the laser wavelength. These optical tweezers are regularly used to study Brownian dynamics of colloids in gases and liquids, control samples in biology, and to trap and control single atoms and atom clouds. Since 2010, optical trapping has been used to trap single dielectric nanoobjects in an ultrahigh vacuum and realize quantum control of their motion, e.g., quantum ground-state cooling or squeezing. Optical tweezers can easily be assembled into arrays of arbitrary geometries, extending optical trapping to multiple sites occupied by atoms or nanoobjects that can mutually interact. Such a system can be used to realize entanglement, study topological physics, or improve the sensitivity to external forces. Tunable interactions are critical to all these applications. In arrays of optically trapped silica nanoparticles, scattered laser light can couple the nanoparticle motion through optical binding forces. The resulting interaction can be nonreciprocal, such that one particle drives the second one, but there is no force in the opposite direction. Nonreciprocal interactions have been proposed for nonreciprocal refrigeration, enhanced sensing, and studying non-Hermitian physics. In this system, they fundamentally arise from the light bath common to both particles, i.e., the scattered light, which might limit their application in the quantum regime. While many optomechanical experiments utilize nonreciprocal interactions, all have been operated in the classical regime, where the effects stemming from the quantum nature of light are neglected. The project QBind will explore the quantum limits of the recently demonstrated nonreciprocal optical binding forces between two optically trapped silica nanospheres. Furthermore, it will probe how quantum-limited interactions limit the performance of nonreciprocal refrigeration.