Chris Amey’s senior comps talk, “Superconductors, SQUIDs, and Applications” was not for the Biology department. Nor was it an English department talk about 20,000 Leagues Under the Sea. It wasn’t even a Religion comps.
In this case SQUID refers to Superconducting QUantum Interference Device and Chris is a physics major.
“Looking at the audience, I think most of you have taken quantum,” Chris said, just before launching into an explanation of electron tunneling. He was probably right: most of the people in the basement of Olin that day were either professors or senior year physics majors. Yet he made the subject of his comps accessible to reporters who have never taken a physics class at Carleton, such as myself.
He started off with the idea of a superconducting metal. The colder a metal gets, the less resistance it has to the flow of electricity. Then, at a particular temperature (usually a few degrees above absolute zero), the resistance drops to zero. Very cold metals become substances with no electrical resistance at all.
Metals with zero electrical resistance can start doing some pretty weird things. For one thing, superconductors are extremely diamagnetic – they repel magnetic fields. Chris did a demonstration of this effect with a certain alloy that’s designed to become a superconductor at a temperature that can be reached in a lab, namely, by pouring liquid nitrogen on it. He’d tell the audience the alloy’s name, but “it’s a really, really long word.” He poured liquid nitrogen out of a special thermos onto the alloy, then placed a strong magnet on top of it. The magnet floated. When he gave it a push with a pair of tongs, it spun in midair.
(I would like to see anybody in the Economics department make stuff float.)
Another bizarre property of superconductors is that they can sustain an electrical current without a voltage. Chris says the best way to explain this phenomenon is that nothing’s pushing these electrons, but there’s no resistance to stop them, so they just keep on going.
Voltless electrical current makes Josephson Junctions possible (though for half the talk I thought they were Joseph’s Injunctions). Take these flowing, voltless electrons and put a very thin barrier in their way. Because electrons are subatomic particles, can “tunnel.” Sometimes, if there is an energy barrier in an electron’s way, it will spontaneously wind up on the other side, without ever having gone over the barrier. There is a small probability that this will ever happen, but the probability is not zero, and it can be measured. Put two Joseph’s Injunctions – sorry, Josephson Junctions – together in an electrical circuit, and you get a device that can detect very, very small magnetic fields. That is a SQUID.
SQUIDs are living proof that physics has many practical applications. For one thing, these devices can be used in magnetoencephalography (MEG), a technology that can image brain activity much more accurately than electrodes on the scalp can by detecting the tiny magnetic fields that the brain produces. Magnetotellurics SQUIDs can measure magnetic fields from the magnetosphere and how they interact with the Earth to figure out the density of things under the Earth’s crust. This technology is useful for finding oil. SQUIDs can also be used for basic physics research and may someday hold the key to quantum computing. So next time you’re about to eat calamari at a restaurant, remember to respect the SQUID.