Superconductors can convert a dc voltage into an oscillating current. This strange quantum mechanical effect occurs in a Josephson junction, a device that can also emit electromagnetic radiation. Now researchers have unearthed some unexpected behavior of this radiation. The March 1PRL demonstrates that an array of Josephson junctions can act like a laser. Like atoms in a laser, the Josephson junctions emit coherent light only after a threshold number of junctions is activated. Although groups of junctions in the past have been shown to work together as classical units, this experiment is the first hint that a collection of Josephson junctions can synchronize its emitted radiation in a quantum mechanical way. The arrays might also prove to be efficient sources of microwave radiation for use in research and industry.
A Josephson junction consists of two layers of a superconductor separated with a thin insulating film. Applying a dc voltage between the superconductors generates a current that oscillates at a frequency proportional to the voltage. In the 1970s researchers pointed out that Josephson junctions behave like radiating atoms. An atom spits out a photon when an atomic electron drops from a higher energy level to a lower one, and the frequency of the photon depends on the difference in energy levels. In the same way, a Josephson junction emits photons when electrons tunnel from the high-voltage end of the junction to the low-voltage end. Experiments had also found that several Josephson junctions linked together could emit coherent radiation, but the effect was explained by treating the connections between the junctions like a classical circuit. The radiation was synchronized, those experiments concluded, because of a physical bond between the junctions, just as two pendulums that are connected with a spring will eventually swing in time.
Chris Lobb and Paola Barbara of the University of Maryland and their colleagues wanted to study radiation from Josephson junction arrays that were not classically linked. They fabricated a rectangular array of 36 × 3 junctions from layers of niobium, aluminum, and aluminum oxide, about in size. Close to the array, they placed a detector that could measure the radiation emitted.
The researchers found that when fewer than 14 rows of junctions were switched on, no radiation was measured. With more than 14 rows switched on, however, the array produced radiation of a few millimeters in wavelength, and the radiated power depended on the square of the number of radiating junctions. “When the results of the experiment first came out, we didn’t know how to explain them,” says Barbara. “We had not thought about a connection with a laser.” But laser-type behavior of the array, Barbara says, seems to best explain their results because–just as in a real laser–coherent emission is possible only if the number of radiating units is large enough that losses can be overcome.
Some physicists, however, think that a more detailed classical description of the array might also be able to explain the experiment. “This is intriguing data,” says Sam Benz, of the National Institute of Standards and Technology in Boulder, Colorado. “The paper will generate a lot of discussion on the quantum versus classical nature of the array.”