Quantum Clock Measurement Costs Requires Huge Energy
Quantum Clock Measurement Costs Rise: Reading a Nanoscale Timer Takes a Billion Times More Energy Than Running It
Quantum Clock Measurement Costs
A groundbreaking Oxford University study revealed a surprising insight regarding quantum timekeeping thermodynamics: Measurement uses more energy than operation for quantum clocks. Reading the clock and converting its feeble signals into data can require up to a billion times more energy than the clock mechanism.
This discovery will impact quantum technology, notably sensors and navigation systems that use accurate and energy-efficient internal clocks.
Energy Mystery at Quantum Scale
Timepieces like atomic oscillators and pendulums use irreversible physical processes. At the quantum level, irreversible processes are weaker or nearly nonexistent, making exact timekeeping harder. The thermodynamics of quantum clocks is mostly unknown.
Any clock that keeps time generates entropy and heat, per the second law of thermodynamics. These are usually caused by irreversible processes like springs winding down or gravity pulling sand grains. However, co-author Florian Meier suggested a nanoscale clock with reversible “ticking” motions that cause no entropy. With each quantum dot hop, an electron may record a tick. This seemed to contradict clocks' need to generate entropy and expend energy.
Building the Microclock
To solve this dilemma and calculate the thermodynamic cost of sustaining time at the quantum scale, researchers directly studied the cost of measurement.
The team, which included Technische Universität Wien and Trinity College Dublin, developed a tiny clock using two nanoscale regions dubbed double quantum dots. This device employed single electrons to jump between zones, so each electron leap was a clock "tick". The electrons on the left and right dots (states L and R) and three unoccupied states (state 0) comprised the clock cycle.
Radio waves and small electric currents were used to track these quantum ticks. The sensors were crucial in both situations, translating faint quantum signals (electron leaps) into classical data. This is called the “quantum-to-classical transition”.
The Entropic Cost of Watching
The researchers meticulously calculated the measurement device and quantum clockwork (double quantum dot) entropy, or energy dissipated.
A shocking difference was found. The clock mechanism used much less energy than reading the clock and recording its tiny signals. It was found that the quantum clock's cycle produced a billion times less entropy than monitoring and recording the clock signal. Co-author Vivek Wadhia says the entropy created by amplifying and monitoring the clock's ticks is a simple thermodynamic cost that has often been overlooked.
This massive entropy creation resolved the contradiction. Even if the quantum ticking motion produces zero net entropy, the sensor dot creates entropy during measurement, hence the second rule of thermodynamics is not broken.
Future Quantum Design Focus Shift
This discovery disproves the idea that quantum mechanics' measurement cost can be trivialised. Quantum clocks were intended to minimise timekeeping energy costs, however senior scientist Professor Natalia Ares found that measurement energy exceeds clockwork energy.
The paper suggests shifting focus to better quantum clocks. Instead of finding better quantum systems, researchers should develop smarter, energy-efficient tick measurement methods. Ares expects the experiment to reveal ways to improve quantum technologies. Quantum electrical systems expert Edward Laird acknowledged that the mechanism between a clock's ticking and its classical ticks must be included in any correct energy cost analysis.
Measurement shapes time
Beyond technological implications, the work presents major physics theoretical questions, including time directionality.
The findings link information science and energy physics by showing that measuring beyond the clock drives time forward and makes it irreversible. Time is directed by observation.
The researchers propose that this energy imbalance may be beneficial over time. Measurements with greater energy could provide a better understanding of the clock's operation by logging every minor change instead of just a tick count.
Wadhia concluded that nanoscale device efficiency principles must be understood to construct autonomous devices that can compute and keep time as well as natural processes.
