Reading a Quantum Clock Costs More Energy Than Actually Running One

Quantum technologies—devices that operate according to quantum mechanical principles—promise to bring users some groundbreaking innovations in whichever context they appear. Ironically, the same principles often create complications preventing these supposedly amazing devices from really taking off.

A new study, published on November 14 in Physical Review Letters, further cements this problem by demonstrating another, somewhat unexpected obstacle—the act of measurement itself. For the experiment, physicists constructed a microscopic quantum clock and found that the energy required to read quantum clocks can rise up to a billion times greater than what’s needed to run the clock.

The findings highlight something “often ignored in the literature,†or the cost of observation in quantum mechanics, according to the study. At the same time, the extra energy could present an opportunity to create more informative, ultra-precise clocks—if physicists can find such a way, that is.

“Quantum clocks running at the smallest scales were expected to lower the energy cost of timekeeping, but our new experiment reveals a surprising twist,†said Natalia Ares, study senior author and a physicist at Oxford University in the UK, in a release. “Instead, in quantum clocks the quantum ticks far exceed that of the clockwork itself.â€

Some (extremely condensed) background

Time is an extremely difficult concept in quantum mechanics; its influence is weak or almost irrelevant in the quantum realm. Still, real-life devices are subjected to real-life phenomena that change according to time. For researchers, that means future quantum devices—such as sensors or navigation systems—must contain ultra-precise internal clocks to minimize issues.

And then there’s the measurement problem, with the famous Schrödinger’s cat thought experiment best exemplifying this phenomenon. Quantum systems can exist in a superposition of various states, but when an observer tries to measure that system, there is only one answer. So the cat could be dead or alive, but we won’t know until we open that box.

A normal clock automatically generates heat—and therefore entropy, or a measure of order—as it ticks and records the passage of time. The effect of the heat is usually so tiny that it doesn’t matter in most cases, leading most quantum researchers to ignore the effects of a clock’s ticks for quantum devices, according to the researchers.

Measuring the quantum ticks

For their experiment, the team created a quantum clock running on two electrons hopping between two different regions. Each jump was equivalent to a “tick†of a regular clock. They tracked the changes in tiny electric currents and radio waves—two different quantum signals—and translated these changes into classical data for timekeeping. Then, the researchers compared the energy costs of the entropy created by the bouncing electron “ticks†and the energy required to measure these ticks.

Surprisingly, they discovered that the latter “not only dwarfs the former but also unlocks greatly increased precision,†according to the paper. That is, setting efficiency aside, the extra measurement energy actually allowed the team to more precisely control the clock.

Looking ahead, understanding such dynamics could be useful for synchronizing time-related operations inside advanced computers, Edward Laird, a physicist at the University of Lancaster in the UK uninvolved in the new work, told Physics Magazine. The findings raise more fundamental questions about whether the very act of observation is what gives time direction, added the researchers.

“By showing that it is the act of measuring—not just the ticking itself—that gives time its forward direction, these new findings draw a powerful connection between the physics of energy and the science of information,†explained Florian Meier, study co-lead author and a postdoctoral student at the Technische Universität Wien in Austria, in the statement.

As the researchers note in the paper, energy efficiency has been a consistent issue in the design of quantum technologies. So it’s intriguing that, as it stands, the paper could be taken as an invitation to look away from the hardware and revisit some inherent paradoxes in theoretical quantum mechanics.