Cooling Light...

Schematic of the Michelson–Sagnac interferometer used in the cooling experiment. Laser light enters from the left and is split into two beams that make their way round a triangular path and create an optical cavity. The mirror to be cooled is the square object that breaks the beams in the middle of the cavity. The output port is at the bottom of diagram where the light encounters a signal-recycling mirror before travelling to a detector (not shown). (Courtesy: Andreas Sawadsky and Roman Schnabel/Leibniz University of Hannover)

Topics: Laser, Modern Physics, Optics, Quantum Computer, Quantum Mechanics

A new technique for cooling a macroscopic object with laser light has been demonstrated by a team of physicists in Germany and Russia. Making clever use of the noise in an optical cavity, which normally heats an object up, the technique could lead to the development of "stable optical springs" that would boost the sensitivity of gravitational-wave detectors. It could also be used to create large quantum-mechanical oscillators for studying the quantum properties of macroscopic objects or to create components for quantum computers.

Physicists already have ways of cooling tiny mirrors by placing them in an optical cavity containing laser light. When the mirror is warm, it vibrates – creating a series of "sidebands" that resonate with light at certain frequencies. The first lower sideband has a frequency equal to the difference between the resonant frequency of the cavity and the vibrational frequency of the mirror. So when a photon at that frequency enters the cavity, it can be absorbed and re-emitted with an extra quantum of vibrational energy. As a result of this "dispersive coupling" process, the mirror cools because energy from it is removed.

Dispersive coupling works best when the bandwidth of the cavity is much smaller than the vibrational frequency of the mirror. This is possible for relatively small mirrors with vibrational frequencies in the hundreds of megahertz. However, for more massive mirrors with vibrational frequencies in the hundreds of kilohertz, optical cavities with sufficiently narrow bandwidths are simply not available.

Physics World:
Physicists reveal new way of cooling large objects with light
Hamish Johnston, editor of physicsworld.com