A new and improved way to measure light has been unveiled by
physicists at NASA’s Jet Propulsion Laboratory and the California
Institute of Technology, both in Pasadena, Calif. 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, JPL researchers Drs.
Peter Day and Henry LeDuc, along with Dr. Jonas Zmuidzinas, a Caltech
physics professor, 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.
The researchers 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 said.
“In astrophysics, this will give you lots more information from every
photon you detect,” he explained. “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, which is
the relic of the intense light that filled the early universe. It’s
detectable today as an almost uniform glow of microwave radiation
coming from all directions.
Measurements of the radiation 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 radiation by structure in the early universe were detected by
the Cosmic Background Explorer satellite. In 2000, using new detectors
developed at JPL and Caltech, the Boomerang experiment, led by Caltech
physicist Dr. Andrew Lange, produced the first resolved images of
these patterns. Other experiments, most notably the Cosmic Background
Imager of Caltech astronomer Dr. Tony Readhead, and the Wilkinson
Microwave Anisotropy Probe, led by Dr. Charles L. Bennett of NASA’s
Goddard Space Flight Center, Greenbelt Md, have confirmed and extended
the 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 Dr. Alan Guth, a physicist at the Massachusetts
Institute of Technology, Cambridge, Mass., is a reality.
Ben Mazin and Anastasios Vayonakis, Caltech graduate students working
in Zmuidzinas’s lab, also contributed to the paper. The research was
funded by NASA’s Aerospace Technology Enterprise, the JPL Director’s
Research and Development Fund, and Caltech’s President’s Fund.
