Protected cat states in a driven superfluid boson gas.*

We investigate the behavior of a one-dimensional Bose-Hubbard gas in both a ring and a hard-wall box, whose kinetic energy is made to oscillate with zero time average, which suppresses first-order particle hopping while allowing even higher-order processes [1]. At a critical value of the driving, the system passes from a Mott insulator to an exotic superfluid phase. The system in the ring has similarities to the Richardson pairing model which can be exploited to understand key features of the interacting boson problem [2]. The superfluid ground state is a macroscopic quantum superposition, or cat state, of two many-body states characterized by the preferential occupation of opposite momentum eigenstates. Interactions give rise to a reduction (or modified depletion) cloud that is common to both macroscopically distinct states [2]. Symmetry arguments permit a precise identification of the two orthonormal many-body branches forming the ground state. In the ring, the system is sensitive to variations of the effective flux but in such a way that the macroscopic superposition is preserved. We discuss other physical aspects that contribute to protect the catlike nature of the ground state. We study the robustness of the effect against variations in the signal shape and switching protocol [3].

[1] G. Pieplow, F. Sols, C. E. Creffield, New J. Phys. 20, 073045 (2018).<br>[2] G. Pieplow, C. E. Creffield, F. Sols, Phys. Rev. Research 1, 033013 (2019).<br>[3] J. Mateos, G. Pieplow, C. E. Creffield, F. Sols, arXiv:2005.05181.