The earliest systems had been intended to allow precise realization of a single dc voltage. Of course, there was still much more to do. "As a result," Benz says, "agreement of dc voltage measurements made in different labs improved by four orders of magnitude." Those measurements now differ by no more than a few parts in 10 10. In 1984, the first 1 V standard was produced, and by the late 1980s array technology was sufficiently advanced that 10 V superconducting integrated circuits had been successfully demonstrated, and JVS systems were at work in major national metrology institutes worldwide. Not surprisingly, the evolution of the modern Josephson standard has moved through several stages as microfabrication techniques have improved, and scientists have explored different materials and configurations. Each of those junctions has to be virtually identical and receive nearly the same microwave power and they all must be addressed uniformly for the device to work. Each individual junction, whose barrier thickness is on the order of 40 nanometers, produces no more than a few tens of microvolts so arrays of hundreds of thousands of junctions are necessary to generate the voltages relevant to industry and electronics research. Traditionally, however, the devices have been extraordinarily challenging to make and use. The chip contains about 300,000 superconducting Josephson junctions located along coplanar waveguides which run horizontally. The integrated circuit for NIST's programmable AC/DC 10-volt standard. "Every system based on these effects produces exactly the same voltage as every other system," Benz says. So the Josephson voltage standards (JVS) are truly intrinsic standards. The voltage "quantization" effect is independent of the environment or material composition of the microfabricated junctions. Because microwaves can be controlled to exquisite precision, so can the resulting voltage. When microwaves are additionally applied to the junction, the junction oscillations lock to that frequency to produce voltage steps with magnitudes determined only by the microwave frequency. When the junction is biased with a current to generate a voltage across the junction, it will produce an alternating current at a frequency precisely determined only by the voltage. The extraordinary precision of the standard relies on a quantum-mechanical phenomenon characteristic of Josephson junctions*, which consist of two superconductors separated by a thin barrier through which pairs of electrons can tunnel. We can make primary standards, programmable from 0 to 10 volts, which are simple and cheap enough that every lab can have one. If dissemination of the new, automated system proceeds as envisioned, then within our lifetimes there will no longer be a need for voltage transfer standards that have to be shipped off periodically for re-calibration. "We wanted a standard that was considerably easier to use. "The only problem with those is that they require an expert PhD physicist to operate," Rudman says. These new systems will empower a very wide range of researchers and businesses around the globe."Īlready, a very similar – if less user-friendly – version of NIST's programmable Josephson voltage standards (PJVS) have been installed in India, Brazil, and Taiwan and China is interested in multiple units, says Dave Rudman, leader of PML's Quantum Devices Group. "And now, finally, we have also been able to incorporate a very high degree of automation as well. "Getting to this point has required three decades of extensive research and development in a host of fields, including materials science, microwave engineering, superconducting technology, and electronics and system integration," says Sam Benz, leader of the Quantum Voltage System Development and Dissemination project in the Quantum Electronics and Photonics Division. PML researchers are on the verge of reaching a long-sought major goal: Providing the world with a programmable quantum voltage standard that has an uncertainty of less than 1 part per billion, never needs calibration, and is sufficiently automated that it can be used in developing countries by non-experts. The chip containing Josephson junctions is at the lower end of the rod. Sam Benz demonstrates the relatively small amount of equipment required for the newly automated voltage standard.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |