The Quantum Measurement Spintronic Engine: Using Entanglement to Harvest Vacuum Fluctuations

Several experimental reports have described electrical power output by electronic devices that channel spin-polarized currents across paramagnetic centers. Phononic radiation have been proposed as the source of the engine's energy, though other hypotheses, such as quantum vacuum fluctuations, should also be examined. This paper is the first of a series which will address these hypotheses. Herein, we investigate the more basic hypothesis that quantum vacuum fluctuations power a quantum engine that converts entanglement energy into useful electrical work. The system under review is composed of two atom-level quantum dots that are tunnel-coupled and exhibit a magnetic exchange interaction. This working substance is connected in series with two ferromagnetic electrodes. The engine cycle comprises two strokes. The thermalizing stroke puts the system into equilibrium with the electrode baths, leading to a release of electrical energy into the leads and to an increase in the system entropy due to entanglement. Then the measurement stroke breaks the entanglement between the two quantum dots, thereby reducing its entropy while energizing it on average. Using a perturbative master equation approach, we analytically demonstrate the efficiency of the engine, and we study the cycle numerically to gain insight into the relevant parameters to maximize power. Although the possibility of harvesting energy from the quantum vacuum fluctuations and the interactions with the baths is proven on paper and confirmed by numerical experiments, the efficiency remains low and is unstable. Our results indicate that quantum vacuum fluctuations alone are unlikely to be the energy source in the the quantum spintronic engine experiments that have been reported thus far.