Electronics

With superconductors

“Superconducting electronics” is a very broad term, with very varied applications:

• Squid magnetometers. These devices enable very accurate measurements of the magnetic field.
• Current limiters. Superconducting circuits can be built so as to efficiently protect a plant (electrical plant…) from supply accidents (high voltage, short-circuit). These limiters react very quickly, guarantee an optimum protection of the plant and automatically reset when the accident is over.
• Electronic filters. A filter is an electronic circuit that selects some frequencies and blocks all the others. This kind of circuit is used in radio sets, for instance, to select the radio station we want to listen to; the radio set receives the signal of all the stations and the filter only allow the frequency of the selected station. Filters are very current components in electronic circuits. However, they are not flawless; they always tend not to block a small part of the non-selected signals, especially if the latter use a frequency that is not very different from the selected one. This is what causes the very annoying situation when a radio picks up two stations at the same time, because the filter has trouble differentiating the two signals.

With the development of telecommunications over the last couple of years, the most efficient filters are required. Yet, the electric resistance of the materials used to build these circuits is a limit for the quality of the filters: using superconductors is hence the solution to overcome this obstacle. However, the resistance of superconductors is not strictly equal to zero when we want to build a filter. Indeed, the electric resistance of a superconductor is only equal to zero when a constant current is used. For telecommunications applications, alternating currents are used, i.e. currents (or voltages) that oscillate very quickly (about 1 GHz for cell phones for instance, or one billion oscillations per second). When the current oscillates that quickly, the superconductor presents a very weak electric resistance; it is however very weak compared to the resistance of the metals that are usually used in regular electronic circuits.

Filters made of superconducting materials are hence more efficient than regular filters. They are used in relay antennas for cell phones and enable to pick up a signal at further distances from the antenna, hence increasing the range of the antenna. Several thousands relay antennas use superconducting filters in the United Stated, and a joint-venture was created with a Chinese firm to study the possible development of this technology in other countries. These filters could also be used in telecommunication satellites.

Picture representing two filters, one with 10 poles, the other with 14 poles, for the UMTS (central frequency = 1975 MHz, width = 10MHz). On the right, the answer of the 14-pole filter: the frequencies that were not selected are reduced by ten billions. The two main characteristics of this device are its weak insertion loss and the absence of regulator: the filter works at once!

Crédits : THALES & IRCOM - ANR "SUPRACOM"

RSFQ electronics (“Rapid Single Flux Quantum”) is currently under development. It is a binary sort of electronics that uses 0s and 1s (contrary to quantum electronics), but which does not use silicon transistors as a basic element. Instead of the transistor, RSFQ electronics uses Josephson junctions to manipulate magnetic flux quantum.

We use flux quantum to encode bits (0 or 1). The advantage of this technique is that Josephson junctions enable very fast speed use, up to 100 GHz, and that these circuits dissipate much less than average circuits. RSFQ electronics represents a potential huge gap for electronics but, besides low temperatures, it also requires a radical change of technology, which significantly delays any marketing possibility.