New quantum technologies: batteries with qubits and electromagnetic resonators

Abstract

In this thesis, an emerging quantum technology is studied as an open quantum system: the quantum battery (QB). This new technology has recently emerged as a promising tool for the thermodynamic control at the quantum scale [1 6]. A quantum battery is a quantum mechanical system that behaves as an efficient energy storage device. Its realization is motivated by the fact that genuine quantum effects such as entanglement or squeezing can typically boost the performances of classical protocols, e.g., by speeding up the underlying dynamics [7, 8]. These systems have been mostly studied neglecting the dissipation due to the interaction with the environment surrounding them. In this thesis, the main focus is to push forward the knowledge frontier in this regard, incorporating dissipation into the QB system. In order to do so, numerical simulations were performed to study if the collective effects that have been previously reported in QBs [9], still hold under dissipation. In particular, the system studied is made out of N non-mutually interacting two-level systems (qubits) charged via a single electromagnetic field mode in a resonator. This configuration is compared to N copies of a resonator with one qubit. The former is a collective QB while the latter is a parallel QB. The results show that the performance of parallel and collective QBs (for instance, the power) decreases under dissipation as expected. Nevertheless, the ratio between the power of the collective over the parallel QB increases with dissipation meaning that the deterioration in performance is smaller for the collective QB. More remarkably, it is found that the loss in performance due to dissipation can be reduced by scaling up the QB, which means equally increasing the injected energy and number of qubits. In many systems this is easier to do than decreasing dissipation. For example, nitrogen-vacancy centers in diamond (NV centers), which can be prepared to behave as spin qubits, may be in groups of hundreds in a sample of diamond [10]. This characteristic, together with its large values of decoherence time (time before losing the quantum phase) and longitudinal relaxation time (time before reaching thermal equilibrium) at room temperature [11], are the motivation to analyze, in this thesis, the feasibility of making QBs with NV centers. As a result of this analysis, it is concluded that for the type of QBs studied in this thesis, the technology is not yet good enough to realize a so called charger-based QB. Nonetheless, a general experimental restriction has been deduced, and the possibility of using NV centers for stable adiabatic QBs (not the focus of this thesis) has been identified as promising future work. The collective enhancements and performances were also studied in regard of charging energy, ergotropy (maximum amount of extractable energy with unitary operations), and transfer rate. For the first two, similar results as for the charging power are obtained. For the transfer rate, instead, it is found that its collective enhancement decreases and its performance increases as the dissipation rate increases. Last but not least, in the writing of this thesis, an effort to introduce new common nomenclature in the area of QBs has been done, as the literature is not completely consistent with the terms used up to now.