### Conveners

#### Excited-state quantum phase transitions

- Pavel Cejnar
- Pavel Stransky

#### Excited-state quantum phase transitions

- Pavel Cejnar
- Pavel Stransky

The talk will overview some recent contributions to the ESQPT field. The main attention will be focused on the recent generalization of the ESQPT concept to systems with continuous spectra, particularly to the one-dimensional tunneling problem [1,2].

References:

[1] P. Stránský, M. Šindelka, M. Kloc, P. Cejnar, Phys. Rev. Lett. 125, 020401 (2020).

[2] P. Stránský, M. Šindelka, P....

We will present a model of a quantum collective spin weakly coupled to a spin-polarized Markovian environment which displays extremely exotic spectral properties [1]. As a function of the environment magnetization the spectrum is divided in to two regions by a divergence in the density of Liouvillian eigenvalues. This Liouvillian spectral phase transition is the analog of the ESQPTs in closed...

ESQPTs have an important drawback compared with quantum and thermal phase transitions: it is not easy to define two different phases, with different equilibrium properties, separated by the corresponding critical energy. In this talk, I will review our work [1], where we proposed a way to address this issue. We find an operator which is a constant of motion only in one of these two phases....

During recent years there has been a growing interest in the different kinds of phase transition that many-body quantum systems may exhibit as well as in the thermodynamic properties associated to the resulting quantum phases. Besides the well-known quantum phase transition, occurring in the ground-state of a physical system as a certain control parameter is varied, and its generalization to...

It is well known that ESQPTs can either enhance or suppress survival probability of the initial state after a quantum quench. These effects, theoretically demonstrated particularly in the Lipkin model, the Tavis-Cummings model and the Rabi model, see e.g. Refs. [1,2], represent a possible experimental test of criticality in the excited domain. We will show the common origin of both these...

Recent advances in the control of superconducting circuits opened new possibilities for the investigation of fundamental quantum phenomena. In this work, we demonstrate the transition into the ``Schrödinger-cat regime'' of a Kerr-oscillator as it is progressively dressed by squeezed microwave light.

We discover that the lifetime of the degenerate ground states in our experiment increases in...

Kerr nonlinear resonators have been used to study dynamical bifurcation, to squeeze quantum fluctuations, and to prepare Schroedinger cat states. They can be realized by coupling a superconducting two-level system to a microwave cavity. This is also the setup for the transmon qubit adopted by companies such as IBM, Google, and Rigetti. We show that the experimentally detected spectrum of a...

We will discuss the metamorphosis of the spectrum and of the eigenstates of the Bose-Hubbard Hamiltonian at the transition from integrability to (quantum) chaos,

addressing universal as well as distinctive properties in the chaotic domain.

We will further contrast the spectral information with dynamical features directly accessible in cold atom experiments, with special attention to...