Complete optomechanical system has reduced the quantum noise at room temperature

Physicists have made a device which is able at room temperature to suppress to 15 percent of the quantum noise in the light beam. The material is the main element of the installation — mirrors in the complete optomechanical cavity allows to practically eliminate the effects of thermal fluctuations and measurements without additional cooling system. In the future this result will ease the work of high — precision instruments, in particular, gravitational detectors. A study published in the journal Nature Physics, Preprint available at arXiv.org.

The nature of quantum phenomena is that some pairs of physical quantities cannot simultaneously measure with arbitrary accuracy — the product of the standard deviations (uncertainty in value) of these parameters has a lower limit, which is set by the uncertainty relation. To suppress the natural quantum noise and increase the accuracy of measurements, physics of the lead system in a special compressed state — that is, reduce the scatter of one magnitude at a cost of increased scatter another (more on this can be found in the material “quantum Sharpener pencil”).

In practice, in a compressed condition often produce electromagnetic radiation — in this case a pair of values are the intensity of the wave, which is related to the number of photons and the phase of oscillations, which depends on time. To improve the accuracy of measurements scientists use the complete optomechanical resonators are special optical cavity with moving boundary (for example, from fixed and moving mirrors). A problem in such installations is thermal noise — the natural chaotic movement of charge carriers, which also interfere with measurements. Before it becomes possible to suppress the quantum noise, it is necessary to create conditions for its observation — that is, to make a minor thermal fluctuations. With this purpose, the installation, as a rule, is cooled to cryogenic temperatures (typically tens of degrees Kelvin), but for long-term and continuous experiments (as, for example, the search of gravitational waves) it is highly desirable to perform measurements at room temperature.

Scientists from Austria and the USA under the leadership of Nancy Agarwal (Nancy Aggarwal) from the Massachusetts Institute of technology have developed and conducted an experiment on the creation and observation of a squeezed state of light. The main element of the installation was complete optomechanical resonator with a size of about a centimeter, which consisted of two mirrors: a fixed diameter also of about an inch, and rolling, with a diameter of about 70 micrometers is comparable to the thickness of a human hair. A movable mirror, the authors have produced 46 of alternating layers of gallium arsenide (GaAs) and gallium arsenide aluminum-gallium (Al0.92Ga0.08As) and attached to the GaAs crystal length of 55 µm, which served as a spring with a high q (about 16 thousand). The materials of the movable mirror and its mounting characterized by a high regularity of the atomic structure — due to this, the influence of thermal noise for them slightly, and elements of these crystals can be used to install even at room temperature.

In the resonator, the researchers sent the laser beam, the natural fluctuations in the number of photons in it led to the fact that at different time points on the movable mirror from the radiation acted different power (caused by the momentum transfer of photons). Fluctuations of this force, in turn, forced the mounting of the mirror a little to compress and decompress — as a result of changing the distance between the mirrors, and therefore the time the light spent to overcome it, and time-dependent phase of the light wave. In other words, in the resonator was any correlation between the phase fluctuations and number of photons — the light moved into the compressed condition.

Released from the resonator beam physics mixed with a reference beam (which has received by using beam splitters from the same laser) and recorded on the photodetector. The phase difference between the beams, which in the absence of any fluctuations would be determined only by the difference of distances traversed by them and would remain constant in real experience to have experienced small fluctuations — that waves of different superimposed on top of each other and the detector has a noise. The authors checked it and compared with the natural level of quantum noise (for standard condition of light), which are separately measured on the same setup, excluding from the scheme of the experience of the compression process of the world.

According to the results of measurements, scientists have determined that the installation allows at room temperature to reduce the quantum noise in the range of signal frequencies from 30 to 70 kHz with a maximum attenuation of about 15% near 45 kHz. According to the authors, the achievement of such results in the future will greatly facilitate the conduct of experiments that explore the squeezed state of light or use it for other measurements (such experiments is the work of gravitational detectors). Materials with an ordered structure will allow scientists not to worry about the cooling of the devices, and the compact size of the complete optomechanical system to easily include it in various settings.

Earlier the same team of authors was able at room temperature to reproduce the quantum noise experiment largely repeated the present, however, was not aimed at the suppression of noise. At the end of last year it became known about how squeezed light improved the properties of gravitational antennas, and most recently about how quantum fluctuations change the position coronarographies mirrors in the LIGO interferometer.

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