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Squeezed light mutes quantum noise

Silicon zip reduces energy fluctuations in laser beams to improve sensitivity of optical motion sensors.

Oskar Painter spends his time carving silicon blocks into shapes that interact with light in strange ways. The latest of these — what Painter calls “Legos for adults” — squeezes the light from laser beams to push the limits of what they can measure.

By eliminating some of the noise caused by quantum effects, researchers can use squeezed light to illuminate movements too small to see with normal light. Painter’s silicon sculpture, built on a microchip, could boost the sensitivity of sensors that use lasers to monitor motion, such as the gyroscopes that keep track of an aircraft’s orientation.

“We have an opportunity to push the performance of these sensors by orders of magnitude,” says Painter, an applied physicist at the California Institute of Technology in Pasadena, whose team reports how it squeezes light on page 185 (ref. 1).

All light is plagued by quantum noise, especially at the low powers typically required by sensors. These energy fluctuations blur the defined peaks of classical light waves, fundamentally limiting the precision of measurements.

Squeezing the light can suppress some noise, but Heisenberg’s uncertainty principle demands a trade-off. A squeeze that reduces noise in one dimension — the height of a light wave’s peaks, for instance — must be balanced by a stretch that adds noise in another, such as the distance between the peaks. Researchers therefore have to match the direction of the squeezing to the direction of the measurement.

Efforts to put light-squeezing to use have so far focused on gravitational-wave detectors, which search for faint ripples in space-time by timing laser beams as they bounce between mirrors 4 kilometres apart. Passing ripples should stretch or compress the laser beams ever so slightly. But measurements with normal laser light are limited by quantum noise, and have so far failed to detect any disturbances attributable to gravitational waves.

Hoping to improve the next generation of measurements, researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Hanford, Washington, added a dose of squeezed light by passing laser light through a crystal. In July, they reported that they had achieved a sensitivity better than the standard limit imposed by quantum noise2. This represents a step towards the ultimate goal of doubling LIGO’s sensitivity, says team member Nergis Mavalvala, a physicist at the Massachusetts Institute of Technology in Cambridge. “We have to work hard to strip the noise out of the light,” she says.

Painter’s silicon device potentially offers a simpler way to squeeze light, although only at frequencies too high to be useful for gravitational-wave detectors. The device looks like a zip; photons bouncing around between its two arms push them apart with a force dictated by the amount of noise in the light. As the size of the gap changes, the zip tunes the frequency of the light — just as a finger sliding along a guitar string changes the pitch of the sound produced — and squeezes out some of the fluctuations.

The prototype tends to leak light, so it can suppress only about 5% of the noise. “The absolute level of squeezing is relatively low,” says Warwick Bowen, a physicist at the University of Queensland in Brisbane, Australia. Painter says he will next be working with higher-quality zips, which could cut out as much as 90% of the noise.

But he will have some competition. A team at JILA in Boulder, Colorado, a joint institute of the University of Colorado and the US National Institute of Standards and Technology, has already created a vibrating silicon nitride membrane that boasts a 32% reduction in noise. JILA physicist Cindy Regal and her colleagues will report their work in a paper under review at Physical Review X (ref. 3). “It has been technically challenging to get to this regime,” says Regal.

References

  1. 1

    Safavi-Naeini, A. H. et al. Nature 500, 185–189 (2013).

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  2. 2

    The LIGO Scientific Collaboration Nature Photon. 7, 613–619 (2013).

  3. 3

    Purdy, T. P., Yu, P.-L., Peterson, R. W., Kampel, N. S. & Regal, C. A. Preprint at http://arxiv.org/abs/1306.1268 (2013).

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Powell, D. Squeezed light mutes quantum noise. Nature 500, 131 (2013). https://doi.org/10.1038/500131a

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