An extremely high-quality green laser beam has allowed a record number of photons to be placed in a specific order for use in gravitational wave detectors. Ordering the photons reduced fluctuations in the intensity of the light by 90 percent. This so-called "squeezed light" has applications in secure message encryption, optical data transmission and astronomy.

The laser beam was produced by researchers at the Max Planck Institute for Gravitational Physics (also known as the Albert Einstein Institute) in Hanover and Leibniz University of Hanover, who used double-refraction crystals, an infrared laser beam and green laser light.

"The green laser prepares the crystal so that the infrared laser light can be squeezed," said Roman Schnable, a junior professor at both the university and the institute. The green light polarizes the crystal, causing the electron cloud of the crystal's atoms to oscillate with the frequency of the green light. The crystal can then store photons from the infrared beam. The stored photons are replaced into the infrared laser beam when the photon flux becomes less, achieving a more regular photon distribution, he said.

Schnable said he and his colleagues set a new world record by reducing the photon noise (squeezing) by 90 percent. They had previously set a record when they moved individual photons by up to half a second in a laser beam to achieve a more regular photon distribution.

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Squeezed light source: A crystal that is illuminated with green light places photons of an infrared laser beam (not visible) in a specific order, thereby reducing the photon noise in that infrared laser. (Image courtesy Roman Schnabel/MPI for Gravitational Physics)

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They have used their squeezed light to improve the sensitivity of the GEO600 gravitational wave detector, a telescope built by Germany and Britain, said Schnabel.  Other observatories with interferometers that can detect these waves include LIGO (Laser Interferometer Gravitational-Wave Observatory) in the US and the Virgo detector at the European Gravitational Observatory in Cascina, Italy.

Gravitational waves are created when massive objects are accelerated, such as when black holes coalesce or when neutron stars vibrate. Until now, there has been no direct detection of gravitational waves due to their weakness. It is hoped that squeezed light can improve the wave detectors' sensitivity greatly and ultimately help solve some mysteries of the universe.

A gravitational wave detector is composed of two perpendicular tunnels through which a laser beam is reflected back and forth. The beams are rejoined, resulting in a so-called interference pattern. If a gravitational wave impinges on the detector, one of the interferometer arms will be elongated while the other will be shortened. This changes the path length of the laser beams, and also changes the interference pattern. Sensitive measuring devices detect such small changes in the interference pattern and allow the researchers to identify a passing gravitational wave, at least in theory.

Light produced by an ordinary lightbulb or laser is not the same as that of squeezed laser light. The latter is particularly valuable to scientists since the intensity of the light -- the number of photons -- is essentially held constant over a certain period of time. In the everyday light of a lightbulb or even standard laser beams, these photons are randomly distributed, with some photons arriving at the destination later than others. This fluctuation of the intensity -- photon noise -- disturbs especially sensitive measurements.

"The statistical nature of the quantum physics is not violated. The appearance of photons remains probabilistic, however, we can connect the photons pair-wise so that they arrive within regular intervals (an effect is known as entanglement)," Schnabel said.

Normally the intensity fluctuations in a laser beam are not apparent because each photon carries such a tiny amount of energy. Physicists at the Albert Einstein Institute decided to look more closely at the beam to notice the fluctuations. They injected a laser beam into gravitational wave detector GEO600 in order to measure extremely small changes in the distance between two mirrors, with the goal of directly observing gravitational waves. The measurements were so sensitive that the photon noise of the laser beam was clearly visible.

All current gravitational wave detectors use infrared laser light. But the detectors were greatly improved by using the extremely uniform laser beams that he and his colleagues can produce, Schnable said.

"Using the squeezed light that we can generate, we can extend the reach of gravitational wave detectors by a factor of three," he said. The uniform intensity of squeezed light also means the interference pattern remains extremely regular. As a result, the gravitational wave detector becomes more sensitive to weak waves and can probe deeper into space. This would enable the observation of black-hole collisions at the edge of the universe, something not possible with current detectors.

Squeezed light can also be applied to secure optical data transmissions, Schnabel said. The encrypted transmissions would be secure because any outside interference would degrade the highly ordered sequence of photons, degrading the information contained in the transmission.

"We are only at the beginning of investigating the applications to quantum cryptography," he said.

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