Cooling with Squeezed Light

SEM micrograph of a microtoroidal resonator similar to the one used for demonstration of quantum-enhanced feedback cooling. The silica torus forms a cavity for light which is modulated by the mechanical vibrations of the supporting disk. Light is coupled in and out of the system by bringing a tapered optical fiber in proximity of the torus. (Source: K. Rasmussen / DTU)

SEM micrograph of a microtoroidal resonator similar to the one used for demonstration of quantum-enhanced feedback cooling. The silica torus forms a cavity for light which is modulated by the mechanical vibrations of the supporting disk. Light is coupled in and out of the system by bringing a tapered optical fiber in proximity of the torus. (Source: K. Rasmussen / DTU)

How does the tightrope walker manage to maintain her balance and avoid that fatal drop from the sky? She carefully senses the motion of her body and vibrations of the rope and accor­dingly compen­sates any deviation from equi­librium by shifting her center of gravity. In a ther­mally excited system, the amplitude of the mechanical vibra­tions are directly linked to the system’s tempe­rature. Thus, by eliminating vibra­tions the system is cooled to a lower effec­tive tempe­rature.

In recent expe­riments at the Technical Univer­sity of Denmark DTU researchers have employed a quantum-enhanced feedback technique to dampen the motion of a micron-sized mechanical oscillator, thereby cooling its tempera­ture by more than 140 degrees below room tempe­rature. Most impor­tantly, this work demonstrates a novel appli­cation of squeezed light allowing an improved sensi­tivity to the mecha­nical motion and thereby a more efficient extrac­tion of infor­mation on how the damping feedback should be tailored.

In the expe­riment, the mechanical motion of a micro­toroidal resonator was conti­nuously sensed using laser light circu­lating inside the resonator. Using that infor­mation an electric feedback force that was always out of phase with the instan­taneous motion was tailored and applied. That is, when the motion was directed upwards the feedback force would counter­act this by pushing the toroid downwards and vice versa. Using classical laser light, this technique is ulti­mately limited by the intrinsic quantum noise of the probe laser, and that sets the classical limit for how efficient the feedback cooling can be.

As now demon­strated by DTU researchers, this limit can be surpassed by using quantum-engi­neered squeezed light. In the experiment, an improvement of more than twelve percent over the classical limiting tempe­rature was achieved. This improve­ment was limited by inef­ficiencies of the specific system resulting in a loss of information on the mechanical motion. The full potential of the demonstrated technique can be unfolded by appli­cation to state-of-the-art opto­mechanical systems, holding promises for reaching the motional quantum ground state of a mechanical oscil­lator in room tempe­rature experiments. Achieving this would pave the way for a plethora of new opto­mecha­nical investi­gations of fundamental quantum physics and constitute a crucial step towards development of new quantum techno­logies for sensing and infor­mation processing based on micro mecha­nical oscil­lators. (Source: DTU)

Reference: C. Schäfermeier et al.: Quantum enhanced feedback cooling of a mechanical oscillator using nonclassical light, Nat. Comms. 7, 13628 (2016); DOI: 10.1038/ncomms13628

Link: Dept. of Physics, Technical University of Denmark, Kgs Lyngby, Denmark

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