University of Rochester

Engineers Observe "Heartbeat" of Superconductors

October 18, 1994

Engineers have observed for the first time the movement of the smallest packet of magnetic charge possible in a superconductor, a find that makes it possible to measure the "heartbeat" of a new type of ultrafast superconducting electronics.

University of Rochester engineers will report their observation of a single flux quantum (SFQ) pulse at the Applied Superconductivity Conference in Boston October 18.

"A single flux quantum pulse is something we've all known is there, but no one until now has observed it directly because it's so fast and the signal is so faint," says Thomas Hsiang, professor of electrical engineering. "It's very exciting." His team was able to pluck out single SFQ pulses from the 50 billion that flow through a circuit during the blink of an eye.

SFQ pulses are at the heart of a new type of ultrafast electronics that engineers around the country are creating. The signals move so fast, in fact, that engineers have a difficult time finding ways to test the new circuitry. But detecting and tracing these bundles of magnetism is key for engineers trying to build complex superconducting circuits.

"How can you test a circuit if it's faster than any equipment you can buy?" asks Professor Marc Feldman. "All you know is that either it works, or it doesn't. Now we have a non- invasive way to check for SFQ signals."

Feldman is head of a Rochester team that is using SFQs as the "information bit" in a new kind of logic circuit where engineers shuttle about tiny bundles of magnetism to perform computations. Even though the circuitry is based on single pulses of magnetism, up to now no one has been able to observe them directly.

The work is part of a five-year Rochester effort to build a large digital filter using superconducting circuitry. Engineers hope to build thousands of logic elements and put them together to show that this new kind of logic can work up to 100 times faster than today's fastest semiconductor circuits. This project and a similar effort underway at the State University of New York at Stony Brook are funded by the Department of Defense through the University Research Initiative (URI) program. Researchers at several companies and national laboratories are building SFQ circuits as well.

"You can think of a single flux quantum pulse as a fundamental particle that cannot be broken down into anything smaller," says Feldman. "It's like the atom: We've all known that atoms were there, but it's only been recently that we've been able to observe them directly, with a scanning tunneling microscope." While inert single flux quanta have been observed before, none has ever been seen "on the fly," says Feldman.

To observe the SFQ pulses, scientists had to develop a system to detect very faint signals very quickly. Hsiang and his graduate students used an ultrafast laser to trigger an electrical pulse that was converted by superconducting circuitry into an SFQ pulse. Then they used an optoelectronic crystal as a sort of "listening device" to detect slight changes as the pulse traveled down a micro transmission line, and they used another laser beam to detect changes in the crystal. All this was done inside a cryogenic system at a temperature of just 1.8 degrees above absolute 0.

"It's like shaking a rope at one end and then trying to detect the pulse at the other," says Feldman. "But SFQ pulses are much more rapid and much weaker."

When a flux quantum moves, it causes a very tiny change in voltage. It is this tiny voltage pulse -- one millivolt lasting just two picoseconds (one picosecond is one-millionth of one- millionth of one second) -- that engineers saw. Hsiang says that while he and others have detected faster electrical signals, no one has ever been able to observe such a weak short-lived signal.

"An added significance is that this could be the beginning of a new concept of optoelectronics, where we can combine both optics and superconducting electronics into a single ultrafast system," says Hsiang.

Working on the SFQ pulse detection project, besides Feldman and Hsiang, are faculty member Roman Sobolewski and graduate students Marc Currie, Chia-Chi Wang, and Douglas Jacobs-Perkins, all in the Department of Electrical Engineering. Much of the work was done in the University's new Center for Optoelectronics and Imaging and the Laboratory for Laser Energetics. tr