In active systems — processes driving chemical reactions, communication channels in biology, and systems driven by social dynamics — interactions are fundamentally nonreciprocal as the components modify and adjust their responses over time. Unlike these macroscopic systems, microscopic quantum systems rely solely on reciprocal interactions as they are typically modeled as isolated from drives and dissipation.

In our research, we explore how nonreciprocity arises in quantum systems interacting with and through their environment and how it alters the collective dynamics. We pioneered a novel experimental platform based on a tweezer array of silica nanoparticles, which interact through an optical binding force (also called light-induced dipole-dipole interaction). In our recent work, we observed how its nonlinear and nonreciprocal nature leads to collective non-Hermitian dynamics of two trapped nanoparticles, with limit cycle orbits arising in the position-velocity phase space of each particle (see associated figure). 

In the future, we aim to significantly increase the size of nanoparticle arrays and program graphs of nonreciprocal interactions. The scalable and tunable approach provides unique opportunities to study collective quantum optomechanical effects in the presence of nonreciprocal interactions.

This research is supported by the START Award of the Austrian Science Fund (FWF) (Grant number STA 175).

Light shining on polarizable objects, such as atoms, molecules, or dielectric nanoobjects, induces a dipole in them, and the objects coherently scatter some part of that light. If two objects are close, the scattered light can induce forces between them, the so-called optical binding or light-induced dipole-dipole forces. This force couples the objects' motion and is fundamentally nonreciprocal due to the non-conservative radiation pressure force, as demonstrated in our experiment for the first time.

In our future experiments, we aim to explore the quantum aspects of optical binding. We aspire to observe the effects of the quantum noise of light on collective particle motion and the limits of generating collective quantum states. In the long run, we plan to leverage optical interactions to realize entanglement between two or more nanoparticles.

This research is supported by the Austrian Science Fund (FWF) (Grant numbers: I 5111, STA 175 and PAT8785024) and the John Templeton Foundation.

Cavity optomechanics

Joint lab with Markus Aspelmeyer at the University of Vienna

When the interaction between an object — a two-level system or a solid mechanical oscillator — and an optical cavity mode becomes stronger than the dissipation and decoherence rates, approximations to the theory of light-matter interaction break down. Exciting phenomena, such as virtual photons or two-mode squeezed ground states of the strongly hybridized light and matter, can occur in this high-cooperativity, ultrastrong coupling regime in the experiment. Cavity optomechanics is an example of light-matter interaction, where the motion of a solid-state object influences the cavity mode, for example, by changing its resonance frequency.

In our work, we study the optomechanical effects arising from coupling the motion of an optically trapped silica nanoparticle to a high-finesse optical cavity. The particle behaves as an induced dipole that radiates light coherently with the drive. Optical cavities enhance the dipole radiation into the desired cavity mode, the so-called Purcell enhancement. The resulting optomechanical interaction, the coherent coupling, is inherently linear and yields strong coupling rates, resulting in interesting and unexplored consequences for the nanoparticle motion.

We strive to push this optomechanical interaction into the ultrastrong and deep-strong coupling regime by building optical cavities with ever-higher Purcell factors. We apply this interaction mechanism to create non-classical quantum states of motion of a single nanoparticle.

This research is supported by the Austrian Academy of Sciences (ÖAW) with the ESQ Discovery Grant "Ultrastrong Cavity Optomechanics" and by the funding of the Aspelmeyer group.