Why in the News?

A new study in Physical Review Letters finds that in quantum clocks the main cost of timekeeping comes from measurement rather than the clockwork itself, reshaping ideas in quantum metrology.

What are Quantum Clocks?

• Concept: Quantum clocks are timekeeping devices based on microscopic quantum systems whose transitions – atomic jumps, tunnelling events, or energy-level shifts – act as clock ticks.
• Quantum Nature: Unlike classical clocks, their evolution is probabilistic, allowing temporary backward ticks due to quantum fluctuations while still needing a mechanism to mark irreversible flow of time.
• Irreversibility Requirement: A functional clock must create a permanent record distinguishing past from future, despite underlying reversible quantum dynamics.
• Role of Measurement: Their precision depends on both internal quantum transitions and the classical measurement system used to read them, since measurement converts quantum events into usable time signals.
• Double Quantum Dot Model: In setups using double quantum dots (DQDs), a single electron tunnels between two nanoscale sites; each tunnelling event forms a discrete tick.
• Quantum Dot Basis: Quantum dots – recognised by the 2023 Nobel Prize in Chemistry – can confine single electrons precisely, enabling well-resolved quantum transitions.
• Entropy and Precision: The clock’s internal entropy rises with precision; at equilibrium (equal forward and backward ticks), entropy is zero and the system loses its ability to mark time.

Recent Findings and Implications:

• New Demonstration (2025): A Physical Review Letters study built a working quantum clock using a double quantum dot and separately measured entropy from the clockwork and from the measurement process.
• Key Result: The entropy generated by measurement (via DC sensing and RF reflectometry) was nine orders of magnitude higher than the entropy needed for the electron-tunnelling clock itself.
• Zero-Entropy Clockwork Still Works: Even when the quantum system produced no entropy, continuous measurement still created an irreversible classical record, allowing timekeeping.
• Core Insight: The arrow of time in quantum clocks arises mainly from the classical measurement interface, not from the quantum dynamics.
• 2023 Theoretical Link: Supports earlier findings that quantum measurement is inherently invasive and energy-costly, and that increasing measurement frequency does not always improve accuracy.

Implications:

• Thermodynamic Cost: Extracting information from any quantum system has an energy and entropy cost, affecting quantum sensing, quantum metrology, and clock design.
• Application Outlook: Ultra-precise atomic clocks may be improved by lower-entropy measurement systems, leading to more efficient next-generation timekeeping.
• Quantum Technologies: Insights are crucial for scalable quantum computers, where reading qubits must be precise yet thermodynamically minimal.
• Conceptual Implication: Suggests that the microscopic arrow of time emerges from creating readable, irreversible records, rather than solely from quantum evolution itself.

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