Thread by @UroshDelic

Our paper in which we demonstrate the vast tunability of optical interactions between levitated glass nanoparticles is published in Science! science.org/doi/10.1126/sc… #ScienceResearch 🧵1/18

Researchers in levitated optomechanics aim to exploit some of the unique features of #opticallevitation for novel quantum control techniques in order to create macroscopic quantum states of the particle motion. 2/18

One such feature is that particles smaller than the laser wavelength behave as “dipoles”: they polarize in phase with the laser. The particle radiates some of this light – like an antenna – in phase with the trapping laser as well. This is what we call “coherent scattering”. 3/18

Cooling of motion by coherent scattering into an optical resonator has been proposed and demonstrated for atoms 20 years ago and has recently been applied to levitated nanoparticles to cool them into the quantum ground state (science.org/doi/10.1126/sc…). 4/18

Trap two particles in the same laser beam and they will scatter light between each other until they self-arrange in a “particle crystal”. This so-called “optical binding” of particles has been an active field of research for the previous 30 years. 5/18

We change the geometry and trap glass particles in parallel, coherent optical tweezers, where we can independently tune optical powers, frequencies, phases, as well as the interparticle distance. This provides us with a lot of possibilities to engineer particle interactions! 6/18

#Experimentaldetails: We create the laser beams with a spatial light modulator and trap two particles far away from each other. Then we move them close to each other and the interaction increases. 7/18

We measure the interaction rate by doing a sweep of the particle oscillation frequencies: we decrease the frequency of one particle and increase the other. In absence of any interactions, the frequencies just cross each other like this: 8/18

When one turns on the interaction the two frequencies don’t cross but avoid each other; this is called the “mode splitting”. The size of the gap is related to the interaction rate “g”, and the two particles move together in a combination of breathing and swaying motions. 9/18

The system looks quite symmetric: particles are identical and the optical powers the same. However, we noticed that the particles interact differently if we break the system’s symmetry by setting the laser phases to different values. 10/18

For example, the left particle can push the right one but the right one doesn’t push the left (non-reciprocal interaction). Systems like that have been pursued in optomechanics for building optical circulators without magnetic fields. 11/18

In this case, we don’t observe the mode splitting but “mode attraction”: the two particles start to move with same mechanical frequencies! This might have interesting applications in force sensing. 12/18

We also show that we can turn off the optical interactions and observe the electrostatic (Coulomb) interaction. In this case, particles are charged with around 100 charges each in order to observe a strong interaction. 13/18

Coulomb interaction is interesting to use to entangle two nanoparticles; check our recent preprint where our collaborators from Uni Duisburg-Essen (@stickler_ben) develop the theory and sketch out the experimental feasibility: arxiv.org/abs/2204.13684 14/18

Although we focus on 1D motion in our work, each nanoparticle in fact has three translational degrees of freedom and they all couple to each other; one could easily extend to a synthetic network in 3D with a 1D chain of particles. And there's also rotations! 15/18

The benefits of the rich interaction toolbox, easy scalability to a large number of particles and operation in the quantum regime allows us to explore complex dynamics and quantum collective effects in a ensemble of particles. 16/18

This work has been the result of an amazing collaboration between the experiment and theory with @stickler_ben, funded by @FWF_at, @dfg_public and @ERC_Research. 17/18

We recommend reading the fantastic Science perspective by Julen Simon Pedernales (science.org/doi/10.1126/sc…) and the amazing news feature by Davide Castelvecchi (@dcastelvecchi) for Nature (nature.com/articles/d4158…). 18/18