Quantum phase transitions occur at temperatures close to absolute zero and are driven by quantum fluctuations. One paradigmatic example is the change from a superfluid to a Mott insulator of ultracold bosons. This transition typically is continuous, i.e., the system undergoes a smooth change.
Using a resonantly shaken lattice, we could however turn the Mott transition into a discontinuous or first-order transition . By modulating the position of the lattice, we hybridize the lowest two bands and drive a transition between a Mott insulator and a $\pi$-superfluid with a staggered order, where atoms dominantly occupy the band edge. Crucially, the transition can be discontinuous, as the non-staggered order in the Mott insulator is incompatible with the staggered order of the $\pi$-superfluid.
Furthermore, we observe metastability and hysteresis associated with the discontinuous transition, by monitoring how fast one phase changes into another. After crossing the discontinuous phase transition, the system remains in its initial, now metastable state, instead of following the evolution of the ground state. The eventual decay from the metastable state due to quantum fluctuations would be analogous to the false vacuum decay in the early universe.
In disordered or quasiperiodic media, the propagation of matter waves will be affected by (quasi)random interferences, forming a localized state known as the Anderson insulator. In the Bose-Hubbard model, the combination of interactions and disorder can give rise to the Bose glass phase, an insulating but compressible phase without long-range order that sits between the superfluid and the Mott insulator.
We have created a two-dimensional optical quasicrystal  and observed Anderson localization of non-interacting bosons above a critical lattice depth . The interplay of disorder and interactions significantly shifts the transition from the superfluid to the Bose glass. Our realization of this transition in the optical quasicrystal opens a new avenue to exploring intriguing transport phenomena and many-body localization in two dimensions.
 Bo Song et al. Realizing discontinuous quantum phase transitions in a strongly correlated driven optical lattice, Nature Physics 18, 259–264 (2022).
 Konrad Viebahn et al. Matter-wave diffraction from a quasicrystalline optical lattice, Phys. Rev. Lett. 122, 110404 (2019).
 Matteo Sbroscia et al. Observing localisation in a 2D quasicrystalline optical lattice, Phys. Rev. Lett. 125, 200604 (2020).
|Presenter name||Bo Song|
|How will you attend ICAP-27?||I am planning on in-person attendance|