A framework for realizing a microscopic, highly precise and energy-efficient quantum clock

Over the past decades, physicists have been trying to develop increasingly sophisticated and precise clocks to reliably measure the duration of physical processes that unfold over very short periods of time, helping to validate various theoretical predictions. These include so-called quantum clocks, timekeeping systems that leverage the principles of quantum mechanics to measure time with extremely high precision.

After meeting at the Quantum Thermodynamics Conference (QTD) 2023 in Vienna, a team of researchers at Technische Universität Wien (TU Wien), the Austrian Academy of Sciences, Chalmers University of Technology and the University of Malta set out to realize a new microscopic quantum clock that could keep time precisely and while dissipating significantly less energy. Their framework for developing this clock was presented in a paper published in Nature Physics.

“The main principle is the possibility to trade off a clock’s precision with its resolution (in a new and genuinely quantum manner),” Marcus Huber, senior author of the paper, told Tech Xplore. “Like in an hourglass, we can, instead of using individual sand grains as ticks, wait until a sufficient quantity has fallen through. The resulting time units will be resolved more precisely, at the expense of having to wait longer.

“The same could, for instance, also be said for mechanical clocks with a ratchet moving the second hand and the second hand moving the minute hand. But every one of these fundamental events (be it sandgrain, second-hand or probing the electromagnetic field of a laser stabilized on atomic transitions) comes at a thermodynamic cost.”

Every time an irreversible event happens, energy is dissipated from time-keeping technologies. Therefore, to increase the precision of clocks, one also needs to increase entropy (i.e., their disorder or randomness).

The relationship between the precision of classical clocks—as well as that of most simple quantum clocks—and their thermodynamics is linear. This means that to double their precision, one also needs to double entropy dissipation (i.e., the irreversible loss of their usable energy).

“Our quantum many body clock’s main feature is that it allows these underlying events to be transported coherently, without ever producing an irreversible event in the process,” said Huber. “Like as if the second hand of a clock would be moving unobserved and undisturbed, it still increases the minute hand’s precision by concluding a pass. In that way, coherent, quantum many-body transport enables almost entropy-free increase in precision (at least only logarithmically with the number of events that would classically just be observed).”

A characterizing feature of the quantum clock design introduced by Huber and his colleagues is that it relies on dissipation-free coherent quantum transport, a wave-like transfer of quantum states. This could allow it to dissipate significantly less entropy than previously introduced quantum clocks, thus boosting its energy-efficiency.

“As a result, a single revolution of the quantum clock’s hand involves significantly less dissipation than that of a classical clock, whose hand continuously produces entropy during its motion,” explained Florian Meier, first author of the paper.

The quantum clock design proposed by this team of researchers challenges the long-held assumption that increasing the precision of time-keeping devices comes at a cost, namely an increase in energy dissipation. In the future, it could thus open new exciting possibilities for the realization of ultra-precise and yet energy-efficient quantum time-keeping systems.

“At present, dissipation is not the main limitation in the performance of state-of-the-art clocks,” said Meier. “However, as clock technology continues to advance, we are approaching a point where dissipation could become a significant factor affecting precision. A useful analogy comes from classical computing: for many years, heat dissipation was considered negligible, but in today’s data centers that process vast amounts of information, it has become a major practical concern. In a similar way, we anticipate that for certain applications of high-precision clocks, dissipation will eventually impose limits.”

While the researchers predict that dissipation will unavoidably limit the extent to which the precision of quantum clocks can be improved, their paper outlines fundamental physics principles that could help to minimize dissipation in the future. As part of their next studies, Huber, Meier and their colleagues would like to start testing prototypes of their quantum clock in experimental settings, to further validate its potential.

“We at Chalmers are currently building one such prototype using superconducting circuits based on Josephson junctions,” said Simone Gasparinetti, senior author of the team who is currently working on experimentally realizing the quantum clock. “These circuits are similar to those used to build a quantum computer by companies such as IBM or Google. Realizing a quantum clock based on these circuits requires a different approach, but we are confident that a proof-of-concept demonstration of the exponential advantage found in the paper is within reach of our technology.”

By linking time and thermodynamics to many-body effects (e.g., the coherent transport of excitations), these researchers’ works opens a wide range of possibilities for the use of quantum effects in the field of nanoscale thermodynamics. As part of their future studies, they hope to further demonstrate the advantages of their quantum clock design in experimental settings.

“We want to explore experimental implementations as well as using such high-precision autonomous processes for direct temporal control of quantum devices, avoiding dissipation,” added Huber.”

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