Microchilled electrons.

Colder is better when an electronic circuit must operate accurately. Immersing such a device in liquid helium or using a gas compression and expansion cycle similar to the one in household refrigerators chills the electrons, narrowing their range of energies. The narrower the range, the more accurately a detector circuit registers a value.

Now, researchers have demonstrated a new way of refrigerating the electrons in tiny circuits, allowing them to reach lower temperatures than possible using conventional schemes. This technique is based on a superconducting circuit that siphons away "hot" electrons and replaces them with "cold" ones.

John M. Martinis of the National Institute of Standards and Technology in Boulder, Colo., and his coworkers describe the method in the Dec. 12, 1994 APPLIED PHYSICS LETTERS.

The microrefrigerator consists of a small copper strip between two superconducting contacts, one made of aluminum and the other of lead. Only electrons whose energy exceeds a certain value can pass through the copper-Aluminum junction. The overall effect is to lower the temperature of the electrons in the copper conductor.

In their initial demonstration, Martinis and his colleagues lowered the electron temperature from 100 to 85 millikelvins.

"One should not expect to see such refrigeration schemes appear soon in common laboratory equipment," Sean Washburn of the University of North Carolina at Chapel Hill comments in the Jan. 12 NATURE. "The superconducting electrode can remove only femtowatts of power, so it is not capable of cooling any but the tiniest of conductors, and then only slightly."

Nonetheless, such cooling may prove helpful for increasing the sensitivity of high-resolution particle and radiation detectors.