Well, the short answer is 8mK (milli-Kelvin)! That is 8 thousands of a degree above the absolute zero, so just about as cold as it gets.
New kid on the blo(g&ck)
Here at bigQ, we are pretty excited to have now a BlueFors BF-LD400 dilution refrigerator installed in our newly refurbished optomechanics lab. In brief, optomechanics is the study of the interplay between vibrational degrees of freedom of micro-mechanical oscillators and radiation pressure of light. Reaching the point where this coupling can be studied and engineered at the level of single quanta of vibrations (phonons) and light (photons) one transits into the exciting regime of quantum optomechanics, where fundamental aspects of quantum mechanics can be tested. That’s where we want to be!
Why colder is better
Quantum systems are not known for their robustness and longevity, and micro-mechanical systems are no exception. For a mechanical system, the relevant figure of merit is the quality factor which gives the average number of coherent oscillations between each phonon exchange between the system and its surroundings. Fabricational creativity and equilibrism is currently driving the field forward with seven-league steps, and quality factors of 100,000,000 – previously just figments of theorists’ imagination – are being reported for microfabricated structures. However, a high quality factor doesn’t do it alone. It is as much the number of phonons present in the environment around the oscillator that determines its evolution and ability to preserve its state. And that number is directly related to the temperature of the environment. There is our motivation for cooling the whole experiment down to milli-Kelvin temperatures.
For the past six years, we have conducted optomechanical experiments on microtoroidal resonators at room temperature and made a number of successful demonstations of how the application of squeezed light (light for which the noise in one quadrature is reduced below the level of shot noise, while a corresponding increase in the conjugate quadrature noise ensures that Heisenberg’s uncertainty relation is observed at all times) can be used for quantum enhanced sensing and manipulation of micro-mechanical motion. But now we are stepping up the efforts and along with shifting from microtoroids to tethered membrane oscillators we are currently also establishing state-of-the-art facilities for quantum optomechanics at ultra high vacuum and ultra low temperatures. Our aim is to measure and control the motion of the trampoline-like oscillators at a level that allows us to steer the system into states showing distinctly quantum features.
Our BlueFors cryostat was installed during the first week of January 2018 and the first on-site tests showed that it is capable of cooling down to a base temperature below 8 milli-Kelvin. Later on, the cryostat was accompagnied by a new optical table that allows us to exploit fully the five free space viewports (yes, there is also vertical free space access though the bottom of the cryostat) to the mixing chamber plate.
Taking it to the test
Adding windows of course raises the base temperature because it allows thermal radiation to directly impact the mixing chamber plate temperature. Being researchers we are curious by nature and when it comes to tech-specs even more so. So, after getting to know our piece of equipment with a few cool-down and heat-up cycles we installed all the optical windows in the viewports to check how much the added heat load increases the base temperature. We almost didn’t dare to hope for it, but it still goes below 14mK with direct access to the cold. Evaluation result: PASSED!