A Closer Look at Quantum Levitation

A post by Tianyou Xu

A few weeks ago at the ASTC (Association of Science-Technology Centers) Annual Conference held in Baltimore, a recording of a demo "quantum levitation" was made and uploaded onto Youtube. Since then, it has become a viral sensation, having drawn in over 5.5 million views in less than 10 days! If you haven't already seen it, you can watch it here.

The clip was a demonstration of the impossible; it was shown that an object could be both levitated and locked in 3-dimensional space with any preferred orientation. Of course, that object was a superconducting slate of sapphire and the space in which it froze was above some sort of a prepared magnetic plate - i.e. things one wouldn't commonly find - but that fact shouldn't detract from it any wonder.

How does it work?

Let's start with the setup. The magnets placed along the tracks are permanent rare-earth magnets. Despite the name, these magnets are essentially the same as those you would find on your refrigerator, except that they are much stronger. These magnets produce a magnetic field that - if you can imagine - runs perpendicular to the plane of the magnet and diverges with (a) distance along that axis and (b) distance from that axis. Since these magnets are rather flat and the disk is placed close to the surface, let us assume that the magnetic field runs uniformly up and down. The disk used in the experiment was composed of a single crystal sapphire wafer, coated with a very thin layer (~ 1um) of yttrium barium copper oxide, YBCO¹.

At room temperature, nothing spectacular occurs. The magnetic field lines that run normal to the plane penetrates right through the disk. In other words, if you were to drop the disk from some distance above the magnet, it would simply crash down - no levitation.

However, things become more interesting once the disk is supercooled. Supercooling is the process through which the temperature of a material is lowered below a critical point such that it becomes a superconductor. For the yttrium barium copper oxide coated sapphire, this temperature is approximately -185 C¹. An interesting note here is that once a material becomes a superconductor, it gains the capacity to conduct electricity without any resistance; hence yields no energy dissipation. (For example, resistance of copper is why our computers gets so warm!)

Another phenomenon exhibited by superconductors is that they expel nearly all magnetic flux, meaning that the field lines that once ran through the object can no longer do so. Indeed, these field lines must now bend around the object. This phenomenon is known as the Meissner effect. See figure below. By repelling the magnetic field the object is, by translation, repelling the magnet. Therefore, when this force of repulsion surpasses that of gravity, the object is said to be levitating.

But, what allows it to be locked in space? By itself, the force of repulsion due to the exclusion of magnetic field lines should only levitate the object, but not introduce any stability to it - think about it! Therefore, fixing an object in space with a magnetic field must involve another element...

Here we introduce the concept of flux pinning. Flux pinning describes the exceptional case when magnetic field lines do penetrate the superconductor. This phenomenon often occurs in circumstances where the superconductor is extremely thin or when the material carries some structural defects. In these cases, field lines are able converge into the region of defect, passes through the material, and diverges out the other side^1,2. See picture below.

And this is the source of ‘quantum locking'. The pinning of flux tubes through the superconductor destroys local superconductivity present in the region. As a result, the rest of the superconductor becomes resistant to changes in spatial orientation so as to preserve the flux pinning to these weak localities. By doing so, the superconductor locks itself in space².

A note: As aspiring physicists - or simply children in awe in this case - my classmates and I tracked down and demanded an explanation from our professor the following day. Although we were humbly informed that in order to really understand the concepts of the phenomenon one required experience in quantum field theory, we were nonetheless satisfied with the explanation provided (i.e. what's above). However, if you are interested in learning more about the properties of superconductors and flux pinning, here is an excellent source to check out: http ://www .josephshoer .com /academic /files /PhD _Shoer .pdf