A quantum battery has been built within a quantum computer, a first step in determining whether such batteries could play a role in powering future quantum technologies.
Conventional batteries store energy because their components undergo electrochemical reactions, but quantum batteries rely on quantum bits, or qubits, which undergo changes in their quantum states. Some studies have shown that harnessing quantumness in this way can result in faster charging, but the practicality and usefulness of quantum batteries remain open questions.
“Many future quantum technologies will need their quantum versions of batteries,” says Dian Tan at Hefei National Laboratory in China. “While significant progress has been made in the development of quantum computation, communication and sensing, the energy-storage mechanisms for these quantum systems have not been fully explored.”
Tan and his colleagues built a battery using 12 qubits made from tiny superconducting circuits, each of which they could control with microwaves. Every qubit played the role of a battery cell and also interacted with its nearest neighbours.
The researchers could control those interactions, so they experimented with two different charging protocols. One mimicked how conventional, or classical, batteries get charged, so it didn’t use these quantum interactions, but the other protocol did. The team found that using quantum interactions between the qubits made the battery attain more power on average, more quickly.
“The quantum battery achieves maximum power that is up to twice as large as the classical charging power,” says team member Alan Santos at the Spanish National Research Council. It is important that this worked with each qubit interacting only with its nearest neighbour, he says, because that is standard for superconducting quantum computers, and engineering more of these advantageous interactions would be practically difficult.
James Quach at the Commonwealth Scientific and Industrial Research Organisation in Australia says that, until now, quantum battery charging experiments have used, for example, molecules instead of components of an existing quantum device. Quach and his colleagues have previously theorised that quantum computers powered by quantum batteries could be more efficient and easier to make larger, which would make them more powerful. “This was a theoretical idea that we proposed only recently, but the new work could really be used as the basis to power future quantum computers,” he says.
However, making precise comparisons between conventional and quantum batteries is difficult, says Dominik Šafránek at Charles University in the Czech Republic. In his view, there is currently no obvious way to translate the measured quantum battery advantages into unambiguously useful devices.
Kavan Modi at the Singapore University of Technology and Design says for qubits that interact only with their nearest neighbour, his team’s mathematical work has shown there can be only modest charging advantages, which could easily be cancelled out by other properties of real-life quantum computers, such as their noisiness or slow qubit control.
At the same time, quantum computers may end up being much more energetically costly than conventional computers, so studying how energy can be moved within them may become a necessity if we are to build very large quantum computers, says Modi.
Tan says he sees energy storage for quantum technologies, such as quantum computers, as the ideal use case for his team’s quantum battery. The researchers now want to combine their battery with a qubit-based quantum heat engine, which would produce energy that could then be stored in the battery, all within a quantum computer.
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