Harnessing nonadiabatic excitations promoted by a quantum critical point: Quantum battery and spin squeezing

Abstract

Crossing a quantum critical point in finite time challenges the adiabatic condition due to the closing of the energy gap, which ultimately results in the formation of excitations. Such nonadiabatic excitations are typically deemed detrimental in many scenarios, and consequently several strategies have been put forward to circumvent their formation. Here, however, we show how these nonadiabatic excitations - originated from the failure to meet the adiabatic condition due to the presence of a quantum critical point - can be controlled and thus harnessed to perform certain tasks advantageously. We focus on closed cycles reaching the quantum critical point of fully connected models analyzing two examples. First, a quantum battery that is loaded by approaching a quantum critical point, whose stored and extractable work increases exponentially via repeating cycles. Second, a scheme for the fast preparation of spin squeezed states containing multipartite entanglement that offer a metrological advantage, analogous to a two-axis twisting scheme. The corresponding figure of merit in both examples crucially depends on the universal critical exponents and the scaling of the protocol in the vicinity of the transition. Our results highlight the rich interplay between quantum thermodynamics and metrology with critical nonequilibrium dynamics.