LABORATOIRE DE PHYSIQUE ET MODELISATION DES MILIEUX CONDENSES

The superradiant phase transition (SRPT) in the Dicke model, originally predicted in 1973 and studied for more than 50 years, leads to a striking equilibrium phenomenon: strong quantum squeezing at the critical point [1]. Unlike nonequilibrium squeezing, thermal-equilibrium quantum squeezing is robust against loss and decoherence, as it emerges in the most stable state of the system. While the original photonic SRPT remains unobserved, its magnonic realization has been confirmed in ErFeO3 [2].
In this talk, we present recent theoretical results on the observation of thermal-equilibrium quantum squeezing. We also propose a scheme to generate non-Gaussian states (particularly, Schrödinger-cat states) from the thermal-equilibrium quantum squeezed states. Both results are formulated within the Dicke model and discussed with a particular focus on ErFeO3.

References
[1] K. Hayashida, et al., Sci. Rep. 13, 2526 (2023).
[2] D. Kim, et al., Sci. Adv. 11, eadt1691 (2025); M. Bamba, et al., Commun. Phys. 5, 3 (2022); X. Li, et al., Science 361, 794 (2018).

Superconducting circuits (SCs) are quantum devices that mimic the behavior of atomic systems even though they are made up of macroscopic microwave circuit elements. Their tunability, high coherence, and strong coupling has led to their rapid development as a leading implementation of quantum hardware. Traditional SCs are made using known superconductors such as aluminum or niobium, but the integration of novel superconductors as part of the circuit can lead to new scientific insights and new capabilities. Such hybrid circuits are ideal sensors, capable of measuring the superconducting gap structures of novel materials using micron-sized samples, which is especially useful for interface superconductors or van-der-Waals flakes which cannot be probed with bulk techniques. Furthermore, the unique quantum properties of unconventional superconductors can be utilized to make a new class of quantum devices. This talk will present recent results where we explore van-der-Waals superconductors in cuprate and kagome systems with hybrid circuits, and a path towards utilizing them in new hybrid devices for quantum technology.