Researchers believe that micromasers can be an excellent model for future quantum batteries. Quantum technologies, or technological devices created by building and manipulating quantum mechanical systems, have recently become a reality. The most prominent example is undoubtedly quantum computers, in which the unit of information, the bit, is replaced by its quantum mechanical counterpart, informally known as the qubit.
In contrast to classical computers, quantum computers promise to use the full quantum mechanical features of qubits to address and solve computational problems that classical computers would be unable to address. For example, the Canadian company Xanadu recently claimed that its quantum computer was able to solve a computational task that would have taken 9000 years using state-of-the-art supercomputers in just 36 microseconds.
Quantum technologies require energy to function. In the last ten years, researchers have developed the concept of quantum batteries, which are quantum mechanical systems used as energy storage devices.
In the very recent past, researchers at the Center for Theoretical Physics of Complex Systems (PCS) within the Institute for Basic Science (IBS), South Korea have been able to put tight constraints on the possible charging performance of a quantum battery.
The IBS PCS researchers and their collaborator demonstrated that micromasers have properties that make them ideal models of quantum batteries. One of the major concerns when attempting to use an electromagnetic field to store energy is that the electromagnetic field could absorb an enormous amount of energy, potentially much more than is required.
Specifically, they showed that a collection of quantum batteries can lead to an enormous improvement in charging speed compared to a classical charging protocol. This is thanks to quantum effects, which allow the cells in quantum batteries to be charged simultaneously.
Despite these theoretical achievements, the experimental realizations of quantum batteries are still scarce. The only recent notable counter-example used a collection of two-level systems (very similar to the qubits just introduced) for energy storage purposes, with the energy being provided by an electromagnetic field (a laser).
Given the current situation, it is clearly critical to develop new and more accessible quantum platforms that can be used as quantum batteries. With this motivation in mind, researchers from the same IBS PCS team decided to revisit a previously studied quantum mechanical system: the micromaser, in collaboration with Giuliano Benenti (University of Insubria, Italy).
A micromaser is a device that uses an atom beam to pump photons into a cavity. In simple terms, a micromaser is a configuration specific to the experimental model of quantum battery mentioned above: energy is stored in the electromagnetic field, which is charged by a stream of qubits sequentially interacting with it.
The IBS PCS researchers and their collaborator demonstrated that micromasers have properties that make them ideal models of quantum batteries. One of the major concerns when attempting to use an electromagnetic field to store energy is that the electromagnetic field could absorb an enormous amount of energy, potentially much more than is required. Using a simple analogy, this would be analogous to a phone battery that, when plugged in, continues to charge indefinitely. In this case, forgetting that the phone is plugged in could be extremely dangerous because there is no mechanism to stop the charging.
Luckily, the team’s numerical results show that this cannot happen in micromasers. The electromagnetic field reaches quickly a final configuration (technically called a steady state), whose energy can be determined and decided a priori when building the micromaser. This property ensures protection from the risks of overcharging.
In addition, the researchers showed that the final configuration of the electromagnetic field is in a pure state, which means that it brings no memory of the qubits that have been used during the charging. This latter property is particularly crucial when dealing with a quantum battery. It ensures that all the energy stored in the battery can be extracted and used whenever necessary, without the need of keeping track of the qubits used during the charging process.
Finally, it was demonstrated that these appealing features are robust and are not destroyed when the specific parameters defined in this study are changed. This property is clearly important when attempting to build an actual quantum battery because flaws in the construction process are simply unavoidable.
Interestingly, in a parallel series of papers, Stefan Nimmrichter and his colleagues demonstrated that quantum effects can make the micromaser charging process faster than classical charging. In other words, they demonstrated the presence of the previously mentioned quantum advantage while charging a micromaser battery.
All of these findings point to the micromaser as a promising new platform for the development of quantum batteries. The fact that these systems have already been implemented in experimental realizations for many years may provide a significant boost in the development of new accessible prototypes of quantum batteries.
To that end, the IBS PCS researchers and Giuliano Benenti are currently launching a collaborative effort with Stefan Nimmrichter and his colleagues to further investigate these promising models. This new research collaboration hopes to be able to finally benchmark and experimentally test the performance of micromaser-based quantum battery devices.
