Scientists at the University of Oxford have discovered that reading a tiny quantum clock requires up to a billion times more energy than operating the clock itself. This finding, published on November 14 in Physical Review Letters, highlights the significant thermodynamic cost of measurement in quantum timekeeping. The research challenges assumptions about energy efficiency in quantum devices and links observation to time's irreversibility.
A team led by the University of Oxford built a miniature quantum clock using single electrons hopping between two nanoscale regions, known as a double quantum dot. Each electron hop acts as a clock tick, mimicking traditional timekeeping at the quantum scale. To monitor these ticks, the researchers employed two methods: one detecting tiny electric currents and the other using radio waves to observe subtle system changes. These techniques convert quantum events into classical, recordable information.
Calculations revealed a stark imbalance: the entropy generated by the measurement devices—amplifying and reading the clock's signals—can be up to a billion times greater than that produced by the clock's own operation. This measurement process introduces irreversibility, which the researchers identify as the key factor giving time its forward direction. Previously, such measurement costs were considered negligible in quantum physics.
Lead author Professor Natalia Ares from the University of Oxford's Department of Engineering Science stated: "Quantum clocks running at the smallest scales were expected to lower the energy cost of timekeeping, but our new experiment reveals a surprising twist. Instead, in quantum clocks the quantum ticks far exceed that of the clockwork itself."
Co-author Vivek Wadhia, a PhD student in the same department, added: "Our results suggest that the entropy produced by the amplification and measurement of a clock's ticks, which has often been ignored in the literature, is the most important and fundamental thermodynamic cost of timekeeping at the quantum scale. 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 far more efficiently, as nature does."
Co-author Florian Meier, a PhD student at Technische Universität Wien, noted: "Beyond quantum clocks, the research touches on deep questions in physics, including why time flows in one direction. 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."
The study, involving collaborators from TU Wien and Trinity College Dublin, suggests that efficient measurement designs could enhance quantum sensors, navigation systems, and other time-dependent technologies. It shifts focus from improving clock components to optimizing information extraction.