Quantum teamwork produces T-ray beam

By Mason Inman A long-sought device able to produce a beam of ‘T-rays’ that could revolutionise airport security and medical scans has been created by persuading normally independent quantum junctions to work together. The new gadget produces terahertz waves, or T-rays, which are sandwiched between infrared light and microwaves in the electromagnetic spectrum. Many researchers are trying to use them because, like microwaves, they can pass through many materials such as clothing, but provide much higher resolution images. But making terahertz waves is tricky. Lasers and microwave emitters can be pushed out of their usual ranges to emit them. But there remains a “terahertz gap” in the middle, between about 0.5 and 2 terahertz, which no device has been able to fill. Now an international team led by Ulrich Welp at Argonne National Laboratory in Illinois, US, have started to close the gap. To make a powerful beam they coordinated teams of quantum devices that had previously been uncooperative. Josephson junctions are made from a sandwich of superconducting material with an insulating filling. They can produce terahertz waves when voltage applied to the superconductors makes a “current tunnel” through the insulating layer. Single junctions produce feeble amounts of radiation, though. Previous devices could only muster around a millionth of a millionth of a watt (a picowatt), and to make matters worse, researchers have struggled to work the junctions in sync. Now Welp and colleagues made hundreds of junctions work together, creating a beam of laser-like terahertz light with 10,000 times more power (about half a microwatt). The team used a high-temperature semiconductor called BSCCO, which naturally contains stacks of Josephson junctions in its structure. It comprises of superconducting sheets, a couple of atoms thick, separated by 1.5 nanometer insulating gaps. “We were able to pack in a huge number of Josephson junctions” in each crystal, Welp says. In a strip of the material about one micron tall, 100 microns wide, and 300 microns long, they fitted in more than 600 junctions. The usually unruly junctions were tamed with a carefully chosen voltage applied across the superconductor. That created a stationary electromagnetic wave that coordinated the junctions’ actions. “That was the trick,” Welp says. “People were never able to synchronize all these junctions before.” “It’s analogous to a laser,” he adds, which also use reflecting cavities to provide feedback that makes molecules, such as those of noble gases, emit synchronised light waves. By using different size crystals, they were able to fire T-ray beams of 0.36 to 0.85 terahertz, covering about a third of the terahertz gap. They aim to decrease the gap further by making their crystals narrower, Welp says, and also plan to increase the power output. The new study is a significant step forward, says August Yurgens of Chalmers University of Technology in Gothenburg, Sweden. “Attempts to synchronize many Josephson junctions for producing radiation have so far not been very successful.” “If the power output were boosted up to 1 to 10 milliwatts, it would be a very promising niche device”, complementing other devices that create terahertz radiation at other frequencies, Yergens says. The frequencies covered by the new device are some of the more useful for imaging. “You have to be slightly below one terahertz to take full advantage of such radiation,” he adds. Journal reference: Science (vol 318,
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