A new study from the University of Oxford reveals a counterintuitive reality in quantum timekeeping: the energy required to measure a quantum clock far surpasses the energy needed to operate it. Published in Physical Review Letters, this finding challenges long-held assumptions about efficiency in quantum systems and has significant implications for the design of future quantum technologies.
The Paradox of Quantum Clocks
Traditional clocks, whether pendulum-based or atomic, rely on irreversible processes to mark time. However, at the quantum scale, these processes become weak or nearly absent, making precise timekeeping a challenge. Researchers have long sought ways to build more efficient quantum clocks, assuming that improvements in the underlying quantum systems would yield the greatest gains. This new research demonstrates that the true bottleneck lies not in the clock itself, but in the act of observation.
How Measurement Drives Energy Dissipation
The Oxford team constructed a microscopic clock using single electrons hopping between two nanoscale regions (a double quantum dot). Each jump represents a “tick.” To detect these ticks, they employed two methods: measuring tiny electric currents and using radio waves to sense changes in the system. Both methods convert quantum signals into classical data—a quantum-to-classical transition.
Their calculations revealed that the energy required to read a quantum clock is up to a billion times greater than the energy consumed by the clock itself. This overturns the assumption that measurement costs in quantum physics can be ignored. The very act of observation, it turns out, is what gives time its direction by making it irreversible.
Implications for Future Quantum Devices
This discovery doesn’t mean more efficient clocks require better quantum systems. Instead, research should prioritize smarter, more energy-efficient methods of measurement. As lead author Professor Natalia Ares (University of Oxford) explains, “Quantum clocks at the smallest scales were expected to lower the energy cost of timekeeping, but our new experiment reveals a surprising twist. In quantum clocks, the quantum ticks far exceed that of the clockwork itself.”
The imbalance, however, could be a feature, not a flaw. The excess energy from measurement can provide more detailed information about the clock’s behavior, potentially enabling highly precise timekeeping. Co-author Vivek Wadhia (University of Oxford) emphasizes that “the entropy produced by the amplification and measurement of a clock’s ticks…is the most important and fundamental thermodynamic cost of timekeeping at the quantum scale.”
Beyond Timekeeping: A Fundamental Insight
This research touches on deeper questions in physics, including the arrow of time. By demonstrating that measurement, not just ticking, drives time’s forward direction, the findings connect the physics of energy to the science of information. Co-author Florian Meier (TU Wien) suggests that the next step is to understand the principles governing efficiency in nanoscale devices so that we can design autonomous devices that compute and keep time more efficiently, as nature does.
The study underscores a critical realization: in the quantum realm, the act of knowing fundamentally alters the system being observed. This insight has far-reaching implications, extending beyond timekeeping to any quantum technology reliant on precise measurement
