Geometric Optimisation of Quantum Thermodynamic Processes

Abstract

Differential geometry offers a powerful framework for optimising and characterising finite-time thermodynamic processes, both classical and quantum. Here, we start by a pedagogical introduction to the notion of thermodynamic length. We review and connect different frameworks where it emerges in the quantum regime: adiabatically driven closed systems, time-dependent Lindblad master equations, and discrete processes. A geometric lower bound on entropy production in finite-time is then presented, which represents a quantum generalisation of the original classical bound. Following this, we review and develop some general principles for the optimisation of thermodynamic processes in the linear-response regime. These include constant speed of control variation according to the thermodynamic metric, absence of quantum coherence, and optimality of small cycles around the point of maximal ratio between heat capacity and relaxation time for Carnot engines.

Keywords: cooling; finite-time thermodynamics; heat engines; quantum thermodynamics; thermodynamic length.

Figure 1

We plot the upper bound of ${(\Delta S)}^2/\sigma$, given in (70), as a function of $g_x$ for different values of $g_y=\{0.5,1.5,2.4\}$. The point where $g_x=g_y\approx 2.4$ is the point where ${(\Delta S)}^2/\sigma$ is maximised (this can be easily checked numerically), which is also the point of maximum heat capacity C. The heat capacity and its maximum are also plotted in dashed lines. We take ${\tau }_{\mathrm{eq}}=1$.