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'Squeezed Light' Record Set

Photonics.com
Jan 2008
HANOVER, Germany, Jan. 29, 2008 -- 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.
SqueezedLight.jpg
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)
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.

For more information, visit: www.aei.mpg.de/english/contemporaryIssues/home/index.html


GLOSSARY
beam
1. A bundle of light rays that may be parallel, converging or diverging. 2. A concentrated, unidirectional stream of particles. 3. A concentrated, unidirectional flow of electromagnetic waves.
crystal
A solid with a structure that exhibits a basically symmetrical and geometrical arrangement. A crystal may already possess this structure, or it may acquire it through mechanical means. More than 50 chemical substances are important to the optical industry in crystal form. Large single crystals often are used because of their transparency in different spectral regions. However, as some single crystals are very brittle and liable to split under strain, attempts have been made to grind them very...
detector
1. A device designed to convert the energy of incident radiation into another form for the determination of the presence of the radiation. The device may function by electrical, photographic or visual means. 2. A device that provides an electric output that is a useful measure of the radiation that is incident on the device.
interference
1. The additive process whereby the amplitudes of two or more overlapping waves are systematically attenuated and reinforced. 2. The process whereby a given wave is split into two or more waves by, for example, reflection and refraction of beamsplitters, and then possibly brought back together to form a single wave.
interferometer
An instrument that employs the interference of lightwaves to measure the accuracy of optical surfaces; it can measure a length in terms of the length of a wave of light by using interference phenomena based on the wave characteristics of light. Interferometers are used extensively for testing optical elements during manufacture. Typical designs include the Michelson, Twyman-Green and Fizeau interferometers. The basic interferometer components are a light source, a beamsplitter, a reference...
light
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
optical
Pertaining to optics and the phenomena of light.
photon
A quantum of electromagnetic energy of a single mode; i.e., a single wavelength, direction and polarization. As a unit of energy, each photon equals hn, h being Planck's constant and n, the frequency of the propagating electromagnetic wave. The momentum of the photon in the direction of propagation is hn/c, c being the speed of light.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
quantum
Smallest amount into which the energy of a wave can be divided. The quantum is proportional to the frequency of the wave. See photon.
wave
1. An undulation or vibration; a form of movement by which all radiant energy of the electromagnetic spectrum is estimated to travel. 2. A type of surface defect, usually due to improper polishing.
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