Strong coupling QED of mesoscopic point contacts

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Quantum electrodynamics (QED), describing all the low energy physics around us, captures the interaction of matter (electrons) and the electromagnetic field (photons). In vacuum the coupling between electrons and photons is weak due to the smallness of the fine-structure constant, yet cavity Quantum electrodynamics (CQED), and in recent years also circuit quantum electrodynamics (cQED), have led to an unprecedented level of control over quantum states. While the bare electron-photon interaction remains weak, coherent coupling has been achieved by implementing high finesse cavities such that the ‘matter’ and the ‘field’ hybridize. Currently, two main challenges in atomic and condensed matter physics are addressed: Tailoring strong matter-light coupling ‘in the material’ on the one hand and exploring strongly correlated many-body systems on the other hand. Mesoscopic solid state circuits allow to attack simultaneously both strong light-matter interactions and many-body correlations due to their unique properties: Charge-charge correlations naturally exist and can be tuned, e.g. by tuning the transmission through coherent conductors, and light-charge interactions can, by circuit fabrication, reach effective fine structure constants on the order of 1. The main goal of this project is to investigate a novel domain of QED, namely out-of-equilibrium quantum circuits in presence of both strong electron-electron and electron-photon interactions. To attack this challenging goal, we will focus on circuits that combine many-body complexity with conceptual simplicity, namely, quantum point contacts (QPCs) and superconducting point contacts (SPCs). Both systems offer an exquisite experimental control over the electronic scattering at the single channel level which enables the tuning of the scattering amplitudes from the well-understood tunneling regime to the strongly correlated limit of a perfectly transmitting channel, thus QPCs implementing Fermi liquid correlations and SPC superconducting ones. We follow two directions: In one, the point contacts are coupled to optimized radiofrequency (RF) circuits enabling an efficient detection of photons emitted upon electron scattering; in two, high impedance RF circuits are employed to achieve strong QED coupling giving rise to multi-photon emission and thus strong back-action effects. Theoretical modelling and quantitative descriptions accompany are integral part of these realizations. This French-German project combines the complementary expertise of two experimental and two theory groups. It offers a unique research environment within this binational cooperation to attack a very timely and highly challenging research topic of relevance also beyond the mesoscopic solid state community.