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Devices, engines and circuits: quantum engineering with cold atoms

Periodic Reporting for period 4 - DECCA (Devices, engines and circuits: quantum engineering with cold atoms)

Reporting period: 2021-08-01 to 2022-07-31

Predicting and understanding the behavior of quantum systems comprising a large number of constituents strong interacting with each other is one of the hardest question in physics. In the last decades, quantum technologies have been developed to address this question using ‘programmable’ quantum systems, that allows to readout the properties rather than calculating them by classical means. DECCA represents a new platform for such quantum simulation, combining for the first time two of the most important pillars of this technology: strongly correlated Fermi gases on the one hand, and cavity quantum electrodynamics on the other hand. Both aspects are well understood separately, but their combination allows first to use the best of both for applications, but also leads to new questions and opportunities.

The development of quantum technologies is one of the most active frontier of technologies worldwide, and the EU is a leader in this area with the Quantum flagship in particular. DECCA fits in this framework, by offering a new type of technology for quantum simulation. It not only pushes the technology frontier, but also offers new opportunities for quantum technologies: the original approach that we developed makes a new type of problems from material science accessible to quantum simulations. Last, the expertise gathered in the project has been widely disseminated through teaching and large audience publications, allowing potentially the community to build upon it for future generations of scientific or industrial technologies.
Over the course of the project, we have designed, assembled and operated the world first and only experimental setup combining strongly correlated Fermi gases with cavity quantum electrodynamics.

We have systematically explored the different regime of operation of this new type of quantum system: first, in the regime where the cavity photons are interacting resonantly with the atoms of the Fermi gas, we have observed the expected normal mode structure, and its scaling with the experimental parameters.

Throughout this exploration we encountered several unexpected resonances, which we traced back to the coupling of Fermion pairs with photons of the cavity. This new type of light-matter interaction, never observed before, led to the discovery of pair-polaritons, a new type of coherent quasiparticle. In turn, these new excitations could be leveraged to retrieve information about quantum correlations in a weakly destructive way. This finding had a high impact on the community, and exemplifies the opportunities open by the combination of two types of existing technologies (Fermi gases and cavity quantum electrodynamics).

We then turned to the exploration of the dispersive regime, were the atoms interact with cavity photons through their index of refraction. We first demonstrated the use of this regime for repeated, weakly destructive measurement, an enabling technology for the investigation of quantum dynamics, which originally motivated the design of the setup. There, we could also observe the hallmark of this type of coupling, namely strong optomechanical non-linearities. Again, the unique features of the combined Fermi-gas cavity system allowed for a new mapping between the correlations of the quantum gas and the cavity photons, which we could use to measure response functions in regimes not accessible by any other mean.

The last months of the project revealed yet another opportunity, namely the use of cavity photons to induce new phases of matter in the gas. We think that this promises transformative changes in the quantum simulation of strongly correlated matter by enabling the exploration of fundamental mechanisms encountered in the most complex examples of quantum materials.

The results have been widely disseminated in the academic scientific literature and at international conferences.
The experiment remains at the moment the only one worldwide with the capability to combine cavity quantum electrodynamics with strongly correlated Fermions. This brings DECCA one step ahead of the state of the art in quantum simulation.

The project is now completed, and the results have been summarized in the previous section. Building upon this setup, we expect future work to explore further the physics of strongly correlated material. Indeed, the cavity can be used to induce a new type of order, ‘charge-density waves’, into the system, in a fully controlled way. Therefore, our system is ideally positioned to study the interplay of this charge density wave order with order types of phases occurring in quantum gases, in particular superfluidity. This problem is standing out in many-body physics for decades, and occurs generically in many strongly correlated materials, such as high temperature superconductors or bilayer graphene. We believe the the DECCA project has the potential to contribute to solving this issue, thanks to the unprecedented level of control available in our machine.
Artist view of the experimental system developped in the project
Artist view of atom pairs interacting with light in a cavity (credits: Ella Maru studio)