Published online 14 November 2007 | Nature | doi:10.1038/news.2007.242

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The most accurate measurement ever made

Physicists get down to the theoretical limit of precision.

Measured up: precision is tricky at the quantum scale.Getty

A measurement has been made on a quantum system at the greatest precision theoretically possible, according to a paper published today in Nature1.

The team, led by Geoff Pryde of Griffith University in Brisbane, Australia, has managed to measure the interference between two light waves as they beat slightly out of step, with a precision that is limited only by Heisenberg’s uncertainty principle, the most fundamental and unavoidable source of ‘fuzziness’ in the quantum world.

The Heisenberg uncertainty principle comes out of the mathematics describing the quantum world, in which particles exist in a 'superposition' of several states simultaneously, and have both a particle and a wave nature. It quantifies the fact that it is impossible to measure some pairs of properties of a quantum particle simultaneously as precisely as we like — for example, position and momentum.

Eliminating imprecision

In the real world, there are usually additional sources of imprecision in making measurements, and physicists would like to eliminate these as far as possible.

One case where that is important is in devices called interferometers, which measure changes of synchronization between two light beams. This allows ultra-accurate distance measurements, because small changes in the distances travelled by two light beams can make their wavy oscillations become out of step. Interferometry is being used, for example, to look for the tiny effects of putative gravitational waves on light from distant stars. Usually, a quantum effect called ‘shot noise’ adds extra imprecision to these measurements, above that due to the Heisenberg uncertainty limit.

Schemes for eliminating this imprecision have tended to depend on using quantum states that are hard to create. In an interferometer where the two beams being compared are split and sent along separate channels before being recombined, one such approach involves using ‘entangled’ photons and ensuring that a bunch of many photons goes down just one arm or the other, with the other arm being empty.

A large group of photons moving together like this effectively behaves like a single photon with a shorter wavelength, which helps to reduce shot noise.

Single photons

But Pryde and his coworkers in Australia have demonstrated a way of reaching the Heisenberg limit of measurement precision without needing these elusive states: by looking at photons traversing an interferometer's arms one at a time. The key is to avoid making measurements that determine which arm the photon is in, until the beams are recombined at the end. This allows the shot noise to be more or less smoothed away.

Their system is much easier to arrange than making a bunch of many entangled photons. It produces a much weaker light signal that would be hard to measure — but the Australian team boosts it by looping the photons through the device many times while preserving their quantum state.

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"It is really surprising that using only single photons, they can achieve the Heisenberg limit," says Časlav Brukner, a quantum optics expert at the University of Vienna.

Jonathan Dowling,a physicist at Louisiana State University in Baton Rouge confesses that he was very skeptical that it could be made to work when the team announced their plans at a conference at the start of the year. "I am now forced to eat my hat," Dowling says.

"This work could lead to significant improvements in high precision interferometry," says Brukner. It cannot yet handle the kind of light-beam intensities needed in gravity-wave detectors, but might find applications soon in high-sensitivity optical sensors and imaging technologies. 

  • References

    1. Higgins, B. L., Berry, D. W., Bartlett, S. D., Wiseman, H. M. & Pryde, G. J. Nature 450, 393–396 (2007).&nbs
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