PASADENA, Calif.—A new and improved way to measure light has been unveiled by physicists at the California Institute of Technology and the Jet Propulsion Laboratory. The technology exploits the strange but predictable characteristics of superconductivity, and has a number of properties that should lead to uses in a variety of fields, from medicine to astrophysics.
Reporting in the October 23 issue of Nature, Caltech physicist Jonas Zmuidzinas and his JPL colleagues outline the specifications of their superconducting detector. The device is cleverly designed to sidestep certain limitations imposed by nature to allow for very subtle and precise measurements of electromagnetic radiation, which includes visible light, radio signals, X-rays, and gamma rays, as well as infrared and ultraviolet frequencies.
At the heart of the detector is a strip of material that is cooled to such a low temperature that electrical current flows unimpeded—in other words, a superconductor. Scientists have known for some time that superconductors function as they do because of electrons in the material being linked together as "Cooper pairs" with a binding energy just right to allow current to flow with no resistance. If the material is heated above a certain temperature, the Cooper pairs are torn apart by thermal fluctuations, and the result is electrical resistance.
Zmuidzinas and his colleagues have designed their device to register the slight changes that occur when an incoming photon—the basic unit of electromagnetic radiation—interacts with the material and affects the Cooper pairs. The device can be made sensitive enough to detect individual photons, as well as their wavelengths (or color).
However, a steady current run through the superconducting material is not useful for measuring light, so the researchers have also figured out a way to measure the slight changes in the superconductor's properties caused by the breaking of Cooper pairs. By applying a high-frequency microwave field of about 10 gigahertz, a slight lag in the response due to the Cooper pairs can be measured. In fact, the individual frequencies of the photons can be measured very accurately with this method, which should provide a significant benefit to astrophysicists, as well as researchers in a number of other fields, Zmuidzinas says.
"In astrophysics, this will give you lots more information from every photon you detect," he explains. "There are single-pixel detectors in existence that have similar sensitivity, but our new detector allows for much bigger arrays, potentially with thousands of pixels."
Such detectors could provide a very accurate means of measuring the fine details of the cosmic microwave background radiation (CMB). The CMB is the relic of the intense light that filled the early universe, detectable today as an almost uniform glow of microwave radiation coming from all directions.
Measurements of the CMB are of tremendous interest in cosmology today because of extremely faint variations in the intensity of the radiation that form an intricate pattern over the entire sky. These patterns provide a unique image of the universe as it existed just 300 thousand years after the Big Bang, long before the first galaxies or stars formed. The intensity variations are so faint, however, that it has required decades of effort to develop detectors capable of mapping them.
It was not until 1992 that the first hints of the patterns imprinted in the CMB by structure in the early universe were detected by the COBE satellite. In 2000, using new detectors developed at Caltech and JPL, the BOOMERANG experiment led by Caltech physicist Andrew Lange produced the first resolved images of the these patterns. Other experiments, most notably the Cosmic Background Imager of Caltech astronomer Tony Readhead, have confirmed and extended these results to even higher resolution. The images obtained by these experiments have largely convinced the cosmology research community that the universe is geometrically flat and that the theory of rapid inflation proposed by MIT physicist Alan Guth is a reality.
Further progress will help provide even more detailed images of the CMB—ideally, so detailed that individual fluctuations could be matched to primordial galaxies—as well as other information, including empirical evidence to determine whether the CMB is polarized. The new detector invented by Zmuidzinas and Henry G. LeDuc, a co-author of the paper, could be the breakthrough needed for the new generation of technology to study the CMB.
In addition, the new superconducting detector could be used to scan the universe for dark matter, and in X-ray astronomy for better analysis of black holes and other highly energetic phenomena, in medical scanning, in environmental science, and even in archaeology.
Other Caltech faculty are beginning to investigate these additional applications for the new detector. Assistant professor of physics Sunil Golwala is targeting dark-matter detection, while associate professor of physics and astronomy Fiona Harrison is pursuing X-ray astronomy applications.
The lead author of the paper is Peter Day, who earned his doctorate at Caltech under the direction of condensed-matter physicist David Goodstein and is now a researcher at JPL. In addition to LeDuc, also a researcher at JPL and leader of the JPL superconducting device group, the other authors are Ben Mazin and Anastasios Vayonakis, both Caltech graduate students working in Zmuidzinas's lab.
The work has been supported in part by NASA's Aerospace Technology Enterprise, the JPL Director's Research and Development Fund, the Caltech President's Fund, and Caltech trustee Alex Lidow.
