Friday, September 13, 2013

A study in Nature Communications last month reported the University of St Andrews near Edinburgh, Scotland was briefly home to the world’s fastest spinning manmade object. Physicists accelerated a microscopic sphere of atoms to 600 million revolutions per minute; it then, according to press coverage, disintegrated. Wikinews contacted the team to learn more.

The experiment was designed to explore the boundary between conventional physics, which applies to larger objects, and quantum physics, which applies only to extremely small objects. Subatomic particles obey a very different set of rules than the items we see every day, but the behaviour of particles at just above quantum levels remains enigmatic.

The team wanted to expand upon research using single atoms or molecules, instead constructing a four-micrometre thick sphere of calcium carbonate, in a crystaline form called vaterite, in a bid to examine systems containing over a million atoms. The ball was so small it could be manipulated using lasers; light beams exert a force called radiation pressure.

With the ball held within a vacuum by a laser trap, the scientists were able to apply a twisting force through the light’s polarisation (orientation) as it passed through the ball. The vacuum eliminated air resistance so that scientists could look for evidence of quantum friction, a proposed force that slows spinning particles without external assistance.

The spinning sphere turned into a miniature gyroscope, stabilising itself. The ball cooled as it span to ?233°C (?387°F, 40 Kelvin).

The research was carried out by Dr. Yoshihiki Arita, Dr. Michael Mazilu, and Professor Kishan Dholakia. Wikinews was able to ask Mazilu some questions about his research.

((Wikinews)) What first got you interested in researching quantum friction?

Michael Mazilu: The fundamental aspect that raised our interest is the mechanism that stops an object [rotating] infinitely fast in absence of friction. Quantum friction is one possible but debatable mechanism that will ultimately limit the rotation rate. One can also imagine other interesting mechanisms and we hope that future experiments will be able to conclusively distinguish between them.

((WN)) Press coverage has focused on the fact this is the fastest spinning manmade object ever created, but the aim of the experiment was to research quantum physics. How did you end up with this unusual record — was it by accident?

MM: From the beginning we wanted to go for a very fast rotating sphere to test the limits of transfer of angular momentum of light. The motivation was to explore if we can see [if] any anomaly arose as we rotated the particle faster and faster. The hope was to develop an experimental platform that would allow testing the boundary between classical and quantum physics. That this worked better than expected was a happy accident.

((WN)) How was the sphere manufactured, and how long did it take?

MM: The spheres are produced by mixing three chemical compounds together (CaCl2, MgSO4 and K2CO3) until the mixture becomes transparent. This happens in about 5 to 10 minutes and results in birefringent spherical vaterite crystals of 4.4 micrometer in diameter.

((WN)) How long did the sphere take to reach 600 million revs per minute and break up?

MM: The whole process takes about 10–20 minutes. It all depends on how fast we evacuate the vacuum chamber. If we do it too fast we risk [losing] the micro-gyroscope from the trap. With regard to the sphere breaking up: This is a working hypothesis that we are not able to prove yet. What we observe is that the signal corresponding to the rotating sphere disappears at 600 million RPM. We need further measures to verify if the sphere breaks up or if its motion is perturbed and it escapes in some slingshot or other motion.

((WN)) Could the high speeds attained be taken as evidence against quantum friction, as the sphere simply kept getting faster until it broke apart?

MM: This is a very interesting question. The particle keeps getting faster and faster until the signal disappears, however, just before this happens we observe that the slope of the acceleration changes. This could be seen as a signature of “quantum friction” but we need to look more closely. Alternatively, it might be a consequence of the sphere deforming at such high rotation rates.

((WN)) The experiment failed to conclusively prove quantum friction, but did it provide any evidence to support the theory?

MM: The main goal of the experiment was not to prove or disprove quantum friction but to develop a tool that might be useful to carry out these studies in the near future. Though the micro-gyroscope that we studied sounds like a simple system its behaviour and interaction with the laser beam is very complex. In order to use this experiment to prove or disprove quantum friction it is first necessary to completely understand and model its complex behaviour. We need therefore more extensive experimental studies and more precise simulations.

((WN)) How challenging is research of this sort? What kind of difficulties are encountered?

MM: One of the challenges in this experiment is that it brings together many different parts of physics such as vacuum science, optical micro-manipulation, thermodynamics and potentially quantum mechanics. The main difficulty experimentally and theoretically is to combine all these fields simultaneously and make them work together to create a “clean” system that can test ‘friction’ or other theories.

((WN)) Previous research on the boundary between conventional and quantum physics has used atoms and individual molecules. Why was a sphere in excess of a million atoms appropriate for this experiment? Would that not move further away, rather than closer to, the boundary between the two?

MM: Quantum physics should not just be the remit of the world of atoms or molecules but should apply at all scales in some way. One of the main drives in present quantum technology is to create what is called mesoscopic or macroscopic quantum states, that is quantum states that can be see in a microscope. It is in the hope to achieve this that we chose to work with the micrometer sized vaterite crystals. The other reason for the size of the sphere is that we experimentally found that smaller spheres are presently more difficult to levitate.

((WN)) How likely is this result to be an anomaly? Might a similar ball break up more quickly, or be unable to spin as fast?

MM: With respect to the sphere break-up, these are interesting questions. One can expect that, depending on the mechanical failure property of the sphere, it would breakup sooner or later. Optically, we can make the sphere rotate at any speeds smaller than the maximum speed. So it would be very interesting to fabricate a series of spheres that have same optical properties but different mechanical failure points.

((WN)) Where would you like to see the research go next? More spheres?

MM: Indeed, two or more spheres would bring an additional degree of freedom to the experiments that would allow the study of the rotation rate as a function of the distance between them. Some theoretical predictions suggest that quantum friction effects might be enhanced in this case.

((WN)) If confirmed, what applications might quantum friction have?

MM: It is relatively easy to dream up applications for an effect that has not been observed yet! In general, friction dissipates energy and is seen as a detrimental effect. However, there are applications that use friction in a useful way. Indeed, velocity dependent friction could also be used to slow down microscopic objects to the point where these objects would reach what is called the quantum ground state for their centre of mass. Creating these states on demand would bring quantum technology a step closer and might lead us to “couple” quantum mechanically [macroscopic] objects — a phenomenon more accurately termed entanglement.

((WN)) One follow-up question for publication: You said you found smaller spheres more difficult to levitate. Why is that?

MM: I have double checked the sphere size problem. While it might be more difficult to use smaller sphere in the experiment due to the trapping geometry, as it turns out this was a sphere synthesis problem. With our present method we were not able [to synthesise] smaller spheres.