Highly reflecting mirrors are an essential component of the most precise optical interferometers, which are used, for example, for the detection of gravitational waves or in optical atomic clocks (to reduce the linewidth of lasers). These applications are inherently limited by Brownian noise, where the thermal motion of the particles in the mirror leads to fluctuations of the reflected light wave. It is expected that novel GaAs/AlGaAs Bragg reflectors will substantially reduce this noise contribution. Due to newly discovered noise processes, however, the improvement at low temperatures is only small compared to conventional mirrors.
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