Squeezed Light – the first real application starts now
Fig. 1: Roman Schnabel at the squeezed light experiment.
[This is an invited article based on recent works of the authors. -- 2Physics.com]
Authors: Roman Schnabel and Henning Vahlbruch
Affiliation: Institut für Gravitationsphysik der Leibniz Universität Hannover and Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut)
Not all light is the same. When looking at a light bulb, the light particles (photons) arrive at our eyes not in a well-organized stream, but in a chaotic fashion. Even in ultra-stable, single-color coherent laser beams, the photons are randomly distributed. This is demanded by the statistical nature of quantum physics. Similar to a rain shower, where many or just a few rain drops may hit the ground, sometimes a bunch of photons arrive, and sometimes just one. This fluctuation of the intensity, known as photon shot-noise, perturbs especially sensitive measurements.
However, quantum physics also allows for light with less (squeezed) photon noise. The very special property of such squeezed light is that the photons are not independent from each other but show quantum correlations. One way of interpreting squeezed light is the following. The occurrence of a photon, say on a photo-electric detector, is still unpredictable, because it obeys the statistical nature of quantum physics, but whenever a first photon has shown up, a second photon is guaranteed to follow a given time later. No wonder that the peculiar properties of squeezed light have been used to demonstrate quantum entanglement and teleportation .
A new world record in the strength of squeezing has been achieved this year by the team made up of Henning Vahlbruch, Roman Schnabel and co-workers at the Albert-Einstein-Institute (AEI) in Hanover/Germany. In their experiment an ultra-stable, infra-red laser beam was further refined by squeezing its photon shot-noise by a factor of ten (or 10 dB in the Decibel scale) . The first squeezed light sources were demonstrated in 1985. However, after 20 years of intensive research, doubts arose whether strong squeezing could ever be realized as required for imminent applications. The new experiment at the AEI showed that strong squeezing of light’s quantum noise is possible. If a 10 dB squeezed laser beam replaces a standard coherent laser beam in an optical measurement device, its noise variance would decrease by a factor of 10. Similarly, the new world-record technology can be used to improve entanglement and teleportation experiments.
At the heart of the AEI squeezing experiment was an artificially grown birefringent crystal, that was precisely cut and polished. Two laser beams were sent through the crystal. A green laser beam of wavelength 532 nm caused the electron clouds of the crystal's atoms to oscillate with the frequency of the green light. In this state, the crystal could redistribute photons from a second, infrared beam that had exactly twice the wavelength (1064 nm). When the infrared beam sent a large bunch of photons through the crystal, the crystal stored those photons, and only returned them to the infrared beam when the photon flux became less. In this way, a more regular photon distribution for the infrared laser beam was achieved, and the photon noise of the infra-red beam became squeezed.
Figure 2 shows a photograph of the original optical components of the squeezed light source: a magnesium oxide doped lithium niobate crystal pumped with green laser light. The crystal had a length of 6.5 mm. The additional mirror was used to form a resonator together with one dielectrically coated crystal surface. In the real experiment the crystal was inside a temperature controlled housing and not visible.
Figure 3 shows the result of the successful squeezing experiment. When the green pump light was switched on, the photon noise of the infrared laser dropped by 10 dB to the lower noise level (red). The experimental techniques used for this result are independent of the absolute laser power. In principle any laser beam of arbitrary power could be realized with the same quantum noise reduction of a factor of 10.
Squeezed light offers a lot of fantastic applications in quantum communication and optical quantum computation. However in gravitational wave detection, squeezed light will find its first real application. Gravitational wave detectors are sophisticated laser interferometers which use cutting-edge technology in order to observe tiny changes in space-time that originate from distant black hole binaries or neutron star mergers. The circulating laser powers in these interferometers reach up to several kilowatts in order to reach high measurement sensitivities. A further increase of laser power can in principle further improve the detectors. But there may be a better way.
Imagine that you had a fixed amount of money, say a billion Euros, to build the most sensitive gravitational wave detector. Squeezed light would certainly be one of your key technologies. In order to reduce thermal motions inside your detector, you would use cryogenic techniques to cool it to liquid Helium temperature (-269°C). At the same time you would want to use high laser powers to boost its sensitivity to gravitational waves. You would find that the high laser power inside the detector would heat it up, such that any operation at low temperatures would become technologically extremely challenging, if not impossible. The 10 dB squeezed light technology can solve this problem. It ensures the same high sensitivity to gravitational waves for only one tenth the laser power, and your ultra-sensitive gravitational wave detector could be cost-effectively realized.
All prototype experiments for the application of squeezed light in gravitational wave detectors have been rather successful , and the implementation in the gravitational wave detector GEO600  is currently in preparation. The routine use of squeezed light in GEO600 is envisaged for 2009.
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