Cavity mediated interactions between levitated particles

Optical trapping of nano- and microscale objects with lasers optical tweezer has been the focal point of the 2018 Nobel prize in Physics awarded to Arthur Ashkin. The technique is now used in diverse scientific fields, from trapping atoms in atomic physics to controlling bacteria or investigating DNA. More recently it has been employed in exciting research directions in the intermediary regime of nanoscale objects glass nanoparticles about 1000 times smaller than a grain of sand such as quantum optomechanics, stochastic thermodynamics, dark matter research and sensing. In quantum optomechanics optical resonators are used to enhance the interaction with the laser, which allows us to achieve a precise control of the motion of a trapped nanoparticle. For example, by stimulating photons to absorb the motional energy we can efficiently cool the particle motion to the lowest achievable temperature such that quantum effects for example, superposition can be observed with macroscopic objects at room temperature. This project aims to extend the optical control to a chain of identical particles in order to entangle their motions over a large distance, thus preparing a highly non-classical state of motion. In order to achieve this goal, we will implement known techniques from atomic physics in order to generate a trap array of optical tweezers. We will initially trap two particles and explore how their motion is coupled via photons, i.e. we will investigate optical binding between the particles. This force is yet to be fully explored for two dipoles, i.e. nanoparticles smaller than the laser wavelength, at nanoscale distances. Moreover, we plan to controllably charge nanoparticles in order to provide an additional direct coupling mechanism via Coulomb interaction, thus realizing arbitrary tuning of the system. As a final step in order to realize quantum states of motion we will combine the trap arrays with an optical cavity, which will allow us to cool and entangle two particles. This approach will have a large impact in fundamental research of quantum macroscopic physics. Furthermore, it is expected that our experiment provides a novel platform for other research directions, such as studies of optical binding, sensing of weak forces, (quantum) synchronization and quantum many-body physics.

Laser light can "trap" solid-state objects, such as glass particles the size of one-thousandth of a grain of sand, and isolate them from all other forces. When they are trapped in these so-called "optical tweezers," the laser can be used to manipulate the motion of a single particle down to the quantum level, where its motion freezes to the lowest possible temperature. This project aimed to trap two or more particles simultaneously, make them interact with each other, and create collective quantum states of motion. We built arrays of optical tweezers during the project, with each tweezer trapping exactly one spherical glass particle. The particles scatter the laser everywhere, and some scattered light hits other particles. Through this process, the particles can apply forces to each other. We demonstrated optical forces between two nanoparticles for the first time and have shown that they can also be nonreciprocal. We used the nonreciprocal forces to program novel non-Hermitian dynamics, where the particles started moving together in phase. Furthermore, we observed the electrostatic interaction between two charged glass particles, promising to entangle the motion of two particles in future works.