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A quantum stopwatch that measures time by avoiding all except one final measurement would be the most accurate allowed by nature, suggests a new study.

To measure the time taken by a series of events, an everyday classical stopwatch measures the elapsed time for each event and adds them up. Each individual measurement does not disturb a classical clock.

That鈥檚 not the case with a quantum clock. If you were to stop and restart a quantum stopwatch, effectively introducing measurements, you鈥檇 disturb the fragile quantum state and lose precision.

鈥淓very measurement of time is subject to a fundamental uncertainty,鈥� says at the University of Oxford. 鈥淭he quantum stopwatch performs a single measurement, so it is affected only once by the uncertainty of time measurements. For a sequence of 100 events, the quantum stopwatch is 100 times more precise than a classical stopwatch.鈥�

He and his colleagues have shown how to build a quantum stopwatch that can avoid the accumulation of errors caused by individual measurements.

Atoms as clock hands

A quantum clock involves preparing, say, an atom, such that it oscillates between two different states, where each state is a different quantum superposition of two energy levels of the atom. One oscillation can be thought of as a single rotation of the hand of a clock. A measurement of time involves determining the quantum state of the atom as it oscillates, and thus the angle the clock hand would make.

But quantum mechanics decrees that each measurement gives a random kick to the clock hand. 鈥淚f we have a series of measurements, each time we are kicking the system and amplifying the fluctuations,鈥� says Chiribella.

For each event, instead of measuring the angle of the clock hand (and hence the time taken by the event), the quantum state of the clock is transferred to quantum memory. The information is stored as qubits, where a qubit can itself be in a superposition of 0 and 1. The transfer to memory is not a measurement, so it does not cause a random fluctuation in the clock. To restart the stopwatch, the information is moved back from memory, restoring the atom to its exact prior quantum state 鈥� analogous to setting the clock hand at its previous position.

Quantum memory

The idea is simple, except for one big problem. 鈥淨uantum memory is a very expensive resource,鈥� says Chiribella. 鈥淗ow much memory do you need for a given precision?鈥�

The researchers showed that the memory needed to preserve the clock鈥檚 quantum state is related to the precision you want to achieve: the more accurate you want the clock to be, the more memory you need, not unlike in a classical computer, where the greater the precision of, say, pi, the more memory you need to store its decimal digits.

Given this result, the team developed a scheme to build such a quantum stopwatch that uses the minimum amount of memory that is allowed by the laws of quantum mechanics.

鈥淸They] derive the ultimate quantum limit on the amount of memory needed to record time with a prescribed accuracy,鈥� says 聽at the University of Pavia in Italy. 鈥淭his is an important result.鈥�

Chiribella thinks that such a system could one day be used for, say, measuring the length of some intermittent biological activity, such as the firing of a neuron, with great precision.

Read more: Why now doesn鈥檛 exist, and other strange facts about time