Experiments with entangled photons and the introduction of pioneering quantum information science, which received the Nobel Prize in Physics this year, also brought a new theoretical concept to Indian scientists exploring the connections between the laws of thermodynamics and quantum information theory (QIT). This new concept could facilitate the use of quantum entanglement for futuristic energy storage technology. Scientists have theorized a concept called “ergotropy,” which represents the amount of work extractable from a system by keeping its entropy (a measure of the system’s randomness) constant.
This idea, if exploited, could open up ways to use quantum batteries in a way that is much more efficient than its classical counterpart. They proposed thermodynamic quantities that capture a signature in multipartite quantum systems called “true multipartite entanglement, where several particles behave as a single unit even when separated. According to thermodynamics, states of thermal equilibrium are completely passive because no work can be extracted from such a state, even if many copies are available. However, the situation becomes even more interesting when states are involved.
The local thermality or local passivity of such states does not always mean that the global state is thermal or passive, and thus a useful form of energy can be obtained in global operations. Ergotropic work can therefore be extracted from a composite quantum system in various ways. Individual parts can be probed locally to obtain useful energy that can be further stored in a battery for later use. Probing can also be performed on the entire composite system, resulting in more work being extracted. The difference between the extraction of work from the individual parts and the extraction of work from the composite system is called the ergotropic gap.
The ergotropic gap can be increased if the parts of the compound quantum system are prepared in an entangled state. This, in turn, provides an experimentally efficient method for detecting entanglement, which has created a useful resource for several protocols such as quantum teleportation, quantum super-dense coding, and secure quantum key distribution, the implications of which have profoundly influenced physics and computer science.
Dr. Manik Banik, scientist, S. N. Bose National Center for Basic Sciences, an autonomous research institute under the Ministry of Science and Technology along with his colleagues Dr. Mir Alimuddin (Çanakya Post-Doctoral Fellow) and Mr. Samgeeth Puliyil (BSMS project student from IISER TVM) turned their attention to real multipart entangled systems that have more drastic manifestations.
In their letter titled “Thermodynamic signatures of true multipart entanglement” published in Physical Review Letters, they pointed out that true entangled states, which are again of different types, can be detected using the ergotropic gap. In particular, they showed that appropriately defined ergotropic gap functions—minimum ergotropic gap, average ergotropic gap, ergotropic fill, and ergotropic volume—can serve as good measures of entanglement in multipartite systems. Importantly, their proposed quantifiers of entanglement are defined in terms of energy instead of entropy, which in turn allows these quantities to be measured in the laboratory.
