Condensed Matter

I will talk about the 1D quantum breakdown model in boson and spin systems, which has an exponential U(1) symmetry with charge decaying exponentially in space. We show that the ground state of the model exhibits a phase transition from a symmetric paramagnetic phase to a quantum breakdown condensate phase that spontaneously breaks the exponential U(1) symmetry. Unconventionally, the condensate phase does not have gapless Goldstone modes, making the spontaneous symmetry breaking stable at zero temperature in 1D. The condensate phase also has an exponentially large ground state degeneracy, which resembles a quantum glass but requires no disorder. This challenges the existing classification schemes of quantum phases of matter.

Preparation of thermal and ground states of many-body systems is a central challenge for quantum processors, needed e.g. as the starting point for many quantum physics experiments or for quantum chemistry applications. In this talk, I will discuss recent work on efficient state preparation using engineered system-bath physics. First, I will overview results from the Google experiment [Science 383 6689 2024], where a version of the dissipative algorithm was used to prepare low-energy states of quantum magnetic systems. I will then explain how to modify the algorithm to accurately prepare thermal and ground states [arXiv:2506.21318 & PRX Quantum 6, 010361 2025]: the resulting algorithm is suitable for near-term quantum devices, and exhibits partial robustness to noise owing to the dissipative nature. We demonstrate its efficiency numerically for ground states of 1d chain and ladder systems, and thermal states of the 2D quantum Ising model, providing perturbative arguments for its more general validity.

Although many-body localization (MBL) is most widely studied for systems with random potentials, various interesting phenomena occur naturally in quasiperiodic potentials. In particular, there is the lack of low-disorder rare regions which may destabilize MBL, the existence of a regime which is non-ergodic but with extended states, and a regime where all single-particle orbitals are critical. In this talk, I will give a theoretical picture of these phenomena (and some generalizations) based on the structure of the quasiperiodic potentials. First, I will show that regularly spaced deep wells in the potential create a separation of particle and information spreading timescales, leading to the non-ergodic extended behavior. Secondly, I will demonstrate that local mirror symmetries in the potential give rise to a hierarchy of resonances in the presence of interaction, leading to many-body critical states. Finally, I will discuss how such resonances may lead to rare-region effects in quasiperiodic systems, even though there is no low-disorder region in such systems.