# Theoretical Physics

**14 May 2014**

**Time:**15:00 to 16:00

**Location:**EC Stoner SR 7.83

Andreas Kurcz (Madrid, Spain)

Unconventional lattice models with ultrastrong circuit-QED

The ability to single out microwave photons using superconducting strip-line resonators is a magnificent tool in order to probe artificial polariton physics. In this context, I will demonstrate that circuit-QED indeed offers an ideal playground in order to simulate the existing many-body machinery from interacting bosons to spin-photon waves and many body-spectroscopy. In particular, I would like to introduce a family of interesting lattice models that is based on the idea of tunable coupling engineering [1] for ultrastrong coupled microwave resonators. The states of interest combine local nonlinearities in each of the cavities with a counter-rotating term that induces two-mode squeezing among cavities. I will discuss the competition between squeezing, nonlinearity and external driving, demonstrating that the physics of this model cannot be accounted with a simple mean-field theory and it is not at all like the conventional Bose-Hubbard model. I furthermore introduce a new form of quantum hybrid, where quantum magnetism arises non-perturbatively. The setup exhibits a class of spin-boson lattice models [2, 3] that reveal a phase transition of the Ising type [4], where both qubits and cavities spontaneously polarise and where the cavity-qubit detuning acts as the equivalent of a magnetic field. I will demonstrate a many-body correlated mean field theory that accurately predicts the nature of the ground state and that can reproduce the low-energy excitations of the model. This ansatz captures this phenomenon all the way, from a perturbative dispersive regime where photons can be traced out, to the non-perturbative ultrastrong coupling regime where photons must be treated on the same footing as qubits.

[1] B. Peropadre. et. al., Phys. Rev. B 87, 134504 (2013).

[2] D. Porras et. al., Phys. Rev. Lett. 108, 235701 (2012).

[3] M. Schiro et. al., Phys. Rev. Lett. 109, 053601 (2012).

[4] P. Pfeuty, Ann. Phys. 57, 79 (1970).

© Copyright Leeds 2011