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06 Feb 2008
Reducing the quantum noise in a laser beam by a factor of ten could increase the sensitivity of gravitational wave detectors and be used in quantum key cryptography.
A German team has produced a beam of "squeezed" 1064 nm light with a record low level of noise. "We achieved a quantum noise reduction level of 90%," Roman Schnabel of the Max Planck Institute for Gravitational Physics and Leibniz University in Hanover told optics.org. "Until now a good result was a reduction by a factor of four, down to 25%. Our reduction by a factor of 10 is a world record." (Phys Rev Lett 100 033602.)
The electric field of a light beam always carries some inherent quantum noise, fluctuations in amplitude and phase caused by the intrinsic quantum nature of the photons. But Schnabel's team has found a way to squeeze out this noise by using a double-refraction crystal, an infrared laser beam and green laser light.
"The green laser prepares the crystal by causing the electron cloud of the crystal's atoms to oscillate with the frequency of the green light," explained Schnabel. "In this state, the crystal can then store photons sent in by the infrared beam." When the photon flux is then reduced these stored photons are replaced back into the beam, which in turn achieves a more regular photon distribution.
As a result, phase fluctuations are almost eliminated from the beam. At a pump power of 650 mW the noise in the infrared laser beam was reduced by more than 10 dB below the vacuum noise level. The researchers believe that the squeezing strength in their set-up is limited by optical losses rather than phase fluctuations, and so could be improved further.
Quantum noise can perturb sensitive measurements, such as those made by the laser interfermometer gravitational wave observatory (LIGO). All current LIGO detectors use infrared lasers to search for the tiny interference patterns that are predicted to appear when gravitational waves interact with the beams, but the extreme weakness of the waves makes them almost impossible to detect.
According to Schnabel, the uniform intensity of squeezed light could make these detectors more sensitive and allow it to probe deeper into space where the elusive waves are created. "Using squeezed light we can extend the reach of gravitational wave detectors by a factor of three."
Another application could be quantum information and cryptography. "Squeezed states have been used to demonstrate several quantum information protocols, construct entangled states of light and demonstrate quantum teleportation," said Schnabel. "They are a possible resource for secure quantum key distribution and for generating cluster states for quantum computing."
"The uses of squeezed light become more momentous with stronger squeezing," he commented. "We are only at the beginning of investigating these applications."
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