# Research

We conduct quantum optics experiments using photons confined in optical dye-filled microcavities. Microcavities consist of two highly-reflecting mirrors, separated by a few µm (hence 'micro'-cavity), allowing for an efficient storage of light. Due to the light-matter interaction between the trapped photons and the dye molecules immersed between the cavity mirrors, quantum many-body states of light – such as Bose-Einstein condensates of photons – can form in the system. Thanks to the high level of control over the physical parameters in optical quantum gases (for example, we control the flow of light using lattice potentials, or tune thermalization, fluctuations, or dissipation), the experimental cavity platform presents a valuable toolbox and rich playground to explore collective phenomena emerging in complex systems by investigating systems of many coupled condensates.

### Read about our current research themes:

### Quantum gas in a box

Uniform quantum gases in a box present a current 'hot topic' in atomic molecular and optical (AMO) physics, enabling the systematic investigation of various quantum systems. This gives insight into the fundamental behaviour of quantum matter, ranging from weakly interacting Bose gases over superfluids to neutron stars.

Quantum gases provide a test bed for many-body physics under both in- and out-of-equilibrium settings. Experimental control over dimensionality, potential energy landscapes, or the coupling to reservoirs offers wide possibilities to explore different phases of matter.

In our lab, we study 2D quantum gases of photons in box potentials. In the finite-size homogeneous system, we observed BEC, as evidenced from the measured position and momentum distributions. By exerting a mechanical force onto the photon gas prepared in a regime around the phase transition and studying the density response, we measured both the bulk isothermal compressibility and the EOS of the optical quantum gas.

In the future, uniform optical gases may enable thermodynamic machines with light as a work medium. An additional perspective is the exploration of sound. The required dynamic manipulation of optical quantum gases can become feasible through, for example, electro-optic trap modulation or spatiotemporally resolved pumping of the dye reservoir. Beyond ideal gas theory, a nonvanishing healing length can be achieved by either adding Kerr media or exploiting the weakly dissipative nature of photon condensates.

Busley et al., *Science* **375**, 1403–1406 (2022)

### Topological physics

Topology is a powerful paradigm for the classification of phases of matter. Photon condensates enable a new approach to investigate the topology of *open systems*, which are coupled to the environment. With our ERC-funded project 'TopoGrand' we will explore links between photonics, condensed matter, and quantum computing.

Topological states are considered as a key resource in quantum technology. Microcavity photon condensates offer a conceptually new approach to engineer such topological states, which we will pursue in the TopoGrand project: Inducing topological phases and protected edge states solely by controlling reservoirs. This recipe will allow us to perform fundamental tests of the yet unexplored interplay between topology and reservoir-induced fluctuations. To date, understanding topology at finite temperature is one of the outstanding problems in physics.

Experimentally, we will develop a versatile room-temperature microcavity platform hosting one-dimensional arrays of photon condensates with tuneable tunnelling, coherence and single-site-resolved control of dissipation and gain. Non-Hermitian topological states will then be realised by controlling the pump laser masks. A technological appeal of this approach is the possibility to synthesise topological phases in a dynamical way. Reconfigurable topological phases, conveniently imprinted on topologically trivial photonic crystal structures inside dye-microcavities, can be envisaged for robust routing of light on photonic chips with on-demand updating.

See, e.g.: Z. Gong et al., *Phys. Rev. X* **8**, 031079 (2018)

### Quantum communication

In future quantum networks, coupled dye-filled optical microcavities producing entangled N-photon states can become a valuable resource for quantum communication between multiple users. Our project within the Cluster of Excellence "Matter and Light for Quantum Computing" (ML4Q) explores this new direction.

Even in classical communication, significant computational and power resources are devoted to establishing links between computing modules and to creating wide-area networks. The era of useful quantum computers is likely to start with relatively small processing units consisting of tens of logical qubits. Fully interconnecting these units to networks will enable novel applications in multi-user communications, known as multipartite networks.

Our ML4Q project “Multipartite quantum networks” aims at the experimental implementation of multipartite quantum networks using entangled N-photon sources based on rings of coupled photon condensates. With this system, we want to demonstrate protocols for conference quantum key distribution. Presently, experiments achieving multipartite photonic entanglement with high repetition rates are difficult and scarce. A milestone towards enabling multi-user quantum communication protocols requires quantum effects at the single-photon level. Implementing photon-photon interactions is a difficult task, but they could be utilised by second-order nonlinearities. Owing to the platforms’ modularity, the photonic system part can be directly interfaced with such nonlinear media.

See, e.g.: Epping et al., *New J. Phys.* **19**, 093012 (2017)