Quantum cooling below absolute zero

Abstract: Similar to other perovskites in its family, SrTiO$_3$ exhibits a significant softening of the ferroelectric mode with decreasing temperature, a behavior that typically heralds the onset of a ferroelectric transition. However, this material remains paraelectric down to 0K due to the Heisenberg quantum uncertainty principle that prevents atoms from localizing in the ferroelectric minima, a fundamental limit that appears impossible to overcome. This work shows that in the strong out-of-equilibrium regime induced by resonant mid-IR pulses, quantum fluctuations can be suppressed, effectively cooling the material below the 0K ground state and inducing a ferroelectric transition in SrTiO$_3$ that is otherwise impossible. The appearance of a metastable state, that is distinct from the conventional ground state, is the first demonstration of how it is possible to leverage and control quantum fluctuations with pulsed light to qualitatively alter the free energy landscape of a quantum system. We predict the conditions and system parameter under which induced non-equilibrium state can be long-lived and even permanent. In providing a quantitative description, based on first principles machine learned potential energy surface, we explain recent experimental observations of light-induced ferroelectric transition in this material.