Realizing discontinuous quantum phase transitions in a strongly correlated driven optical lattice

Discontinuous (first-order) quantum phase transitions and the associated metastability play central roles in diverse areas of physics, ranging from ferromagnetism to the false-vacuum decay in the early Universe1,2; yet, their dynamics are not well understood. Ultracold atoms provide an ideal platform for experimental simulations of quantum phase transitions3,4; so far, however, studies of first-order phase transitions have been limited to systems with weak interactions5,6,7,8, where quantum effects are exponentially suppressed. Here we realize a strongly correlated driven many-body system whose transition can be tuned from continuous to discontinuous. Resonant shaking of a one-dimensional optical lattice hybridizes the two lowest Bloch bands9,10, driving a novel transition from a Mott insulator to a superfluid with a staggered phase order. For weak shaking amplitudes, this transition is discontinuous and the system can remain frozen in a metastable state, whereas for strong shaking, it undergoes a continuous transition towards a superfluid. Our observations of this metastability and hysteresis agree with numerical simulations and pave the way for exploring the crucial role of quantum fluctuations in discontinuous transitions.