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.

The helical edge of a quantum spin Hall insulator hosts a one-dimensional metallic state with spin-momentum locking, realizing a helical Luttinger liquid (HLL). While the gapless phase is well understood analytically through bosonization in its simplest form, more complex questions that demand numerical treatment, have remained largely inaccessible to address for a long time. The core obstacle is the fermion-doubling problem: any local and symmetry-preserving discretization of the Hamiltonian on a strictly one-dimensional lattice either introduces spurious low-energy modes or lifts the Dirac point, breaking the topological protection of the cone.
In this talk I describe how this obstruction can be circumvented using a tangent fermion discretization. This preserves time-reversal symmetry and spin-momentum locking at the lattice level, without invoking a two-dimensional bulk. I present the application of this method first to the gapless HLL, where numerical results are in quantitative agreement with bosonization predictions, establishing the framework as a reliable numerical tool. And then I turn to interaction-induced mass generation, including spontaneous time-reversal symmetry breaking in HLL and symmetric mass generation in 3-4-5-0 model, where full analytical treatment is out of reach.