Cavity Optomechanics (Hedwig Eerkens, Frank Buters, Gesa Welker, Kier Heeck, Sven de Man, Dirk Bouwmeester)

Schematic of trampoline resonator (left) and actual resonator inside a vibration isolation stage (right). Sample fabricated by Matthew Weaver.

 

Schrödinger's mirror

Where is the boundary between classical physics and quantum mechanics? How big can you make an object and still observe quantum mechancial behaviour? Optomechanics is one of the research fields where these questions are investigated.

 

When light hits a mechanical object (for example the end mirror of an optical cavity), it excerts a radiation pressure force on it. On the other hand, a motion of this mechanical object will modulate the phase of the light in the cavity. Via this optomechanical interplay we can use light to control the mechanical motion of a so-called trampoline resonator, a 80 μm Bragg mirror hanging from SiN arms. One of our longterm goals is demonstrating that this macroscopic mirror behaves like a quantum mechanical object. One possible experiment would be to put it into a Schrödinger cat state, a macroscopic superposition of moving and not moving at the same time! The whole project is done in close collaboration with UC Santa Barbara, where our samples are produced in state of the art cleanroom facilities.

Cooling towards the quantum mechanical groundstate

Another demonstration of its non-classical behaviour would be to bring the mirror into its quantum mechanical ground state. As our trampoline resonator has its ground state at a temperature of about 15 μK, this can only be achieved by a combination of optical sideband cooling and cryogenic cooling.

 

We have already achieved optical cooling factors of 10.000, which means we can use our laser to cool the thermal motion perpendicular to the mirrors surface down to 30mK. Currently we are working on combining this with the cryogenic cooling. Feel free to drop by if you want to learn more about our cooling techniques!

 

 

 

The animated figure shows the effect of optical sideband cooling: Measuring the power spectral density allows us to calculate the effective temperature of our mirror from the area below the power spectral density curve. The cooling depends on the detuning of the laser frequency with respect to the cavity resonance frequency.

 

Technical challenges

Our experiments are partially done at cryogenic temperatures and they are very vibration sensitive. Even femtometer movements can be picked up by our setup. We are working in a dilution refrigerator with a pulse tube cryostat, which has a lot of inherent vibrations. Developing better vibration isolation systems therefore remains a constant challenge. We do so in close collaboration with the fine mechanical department.

 

 

Springs for vibration reduction in our dilution refrigerator (left) and fiber interferometer to measure vibrations (right).