A quick post on Quantum Levitation

Today in physics we were looking at magnetic fields, and our teacher decided he'd show us a video of quantum levitation, but wasn't really able to explain what was going on, so I thought I'd try and write a blog on it. If you've not come across it before, it's this: Quantum Levitation. Though I'd seen it before, it just clicked to me that it was something very similar to the Meissner Effect, a phenomena that I came across over the summer.

The Meissner effect occurs when you take a superconducting material cooled below its critical temperature and place it in a magnetic field. The magnetic field induces a current that flows through the material, which in turn creates a magnetic field. Much like more typical induction, the magnetic field created by the current acts in the opposite direction to the  external magnetic field, and cancels it out, thus stopping any magnetic fields from passing through the material: they instead pass round it. This can be seen in the diagram to the right, where a super conducting material has been surrounded by small bar magnets to illustrate the field lines of magnetic field. The strength of the current that flows around the superconducting material varies as the magnetic flux increases, meaning it's possible to measure tiny changes in magnetic fields. This property of super conducting materials has many applications, the most well-known of which is in superconducting quantum interference devices (or SQUID, for short), which are the main detectors in an MRI scanner.

So, how is this effect used in quantum levitation? Well, because of the Meissner effect, if you place a superconducting material above a magnetic plate, or on a track, as you saw in the video, it acts against the magnetic field with its own, and hovers. However, with typical examples of the Meissner effect, the superconductor doesn't stay locked as strongly as the one in the video, it wobbles unsteadily around a point. The "locking" is a consequence of both the thinness of the superconductor (which is usually made out of ruby or sapphire) and impurities in it: where there are inconsistencies in the structure, the magnetic field can pass through the superconductor, and hold it in place steadily. When the disc is placed in the field, the person putting there puts work in to move it into the magnetic field, but then as long as it stays on an equipotential (a line on which the strength of the field doesn't change), very little energy is required to get it to move around the track.

Sadly, practical uses of this idea are very hard to put into practice. The effect requires a track of magnets, and a superconducting material, which typically has to be cooled to very low temperature, meaning using this in something such as transport very expensive and impractical. However, just this year, it was reported in the scienfitic journal Advanced Materials that scientists have observed superconducting characteristics of graphite at room temperature, though this has yet to be completely confirmed, it's a step forward into seeing superconductors at more normal temperatures. If these obstacles can be overcome,  it's possible that quantum locking may one day be a practical solution to many problems. 

If you're interested in how this was made, a more in detail video by I think the guy in the first video here:
http://youtu.be/VyOtIsnG71U

The images were found on wikipedia's page on the Meissner effect