The new amplifier consists of a superconducting material (niobium titanium nitride) coiled into a double spiral 16 millimeters in diameter. Image: Peter Day/Caltech |
Researchers at the California Institute of
Technology (Caltech) and NASA’s Jet Propulsion Laboratory (JPL) have developed
a new type of amplifier for boosting electrical signals. The device can be used
for everything from studying stars, galaxies, and black holes to exploring the
quantum world and developing quantum computers.
“This amplifier will redefine what it
is possible to measure,” says Jonas Zmuidzinas, Caltech’s Merle Kingsley
Professor of Physics, the chief technologist at JPL, and a member of the
research team.
An amplifier is a device that increases the
strength of a weak signal. “Amplifiers play a basic role in a wide range
of scientific measurements and in electronics in general,” says Peter Day,
a visiting associate in physics at Caltech and a principal scientist at JPL.
“For many tasks, current amplifiers are good enough. But for the most
demanding applications, the shortcomings of the available technologies limit
us.”
Conventional transistor amplifiers—like the
ones that power your car speakers—work for a large span of frequencies. They
can also boost signals ranging from the faint to the strong, and this so-called
dynamic range enables your speakers to play both the quiet and loud parts of a
song. But when an extremely sensitive amplifier is needed—for example, to boost
the faint, high-frequency radio waves from distant galaxies—transistor
amplifiers tend to introduce too much noise, resulting in a signal that is more
powerful but less clear.
One type of highly sensitive amplifier is a
parametric amplifier, which boosts a weak input signal by using a strong signal
called the pump signal. As both signals travel through the instrument, the pump
signal injects energy into the weak signal, therefore amplifying it.
About 50 years ago, Amnon Yariv, Caltech’s
Martin and Eileen Summerfield Professor of Applied Physics and Electrical
Engineering, showed that this type of amplifier produces as little noise as
possible: The only noise it must produce is the unavoidable noise caused by the
jiggling of atoms and waves according to the laws of quantum mechanics. The
problem with many parametric amplifiers and sensitive devices like it, however,
is that they can only amplify a narrow frequency range and often have a poor
dynamic range.
But the Caltech and JPL researchers say
their new amplifier, which is a type of parametric amplifier, combines only the
best features of other amplifiers. It operates over a frequency range more than
ten times wider than other comparably sensitive amplifiers, can amplify strong
signals without distortion, and introduces nearly the lowest amount of
unavoidable noise. In principle, the researchers say, design improvements
should be able to reduce that noise to the absolute minimum. Versions of the
amplifier can be designed to work at frequencies ranging from a few gigahertz
to a terahertz (1,000 GHz). For comparison, a gigahertz is about 10 times
greater than commercial FM radio signals in the U.S., which range from about 88
to 108 MHz (1 GHz is 1,000 MHz).
“Our new amplifier has it all,”
Zmuidzinas says. “You get to have your cake and eat it too.”
The team described the new instrument in Nature Physics.
One of the key features of the new
parametric amplifier is that it incorporates superconductors—materials that allow
an electric current to flow with zero resistance when lowered to certain
temperatures. For their amplifier, the researchers are using titanium nitride
(TiN) and niobium titanium nitride (NbTiN), which have just the right
properties to allow the pump signal to amplify the weak signal.
Although the amplifier has a host of
potential applications, the reason the researchers built the device was to help
them study the universe. The team built the instrument to boost microwave
signals, but the new design can be used to build amplifiers that help
astronomers observe in a wide range of wavelengths, from radio waves to X rays.
For instance, the team says, the instrument
can directly amplify radio signals from faint sources like distant galaxies,
black holes, or other exotic cosmic objects. Boosting signals in millimeter to
submillimeter wavelengths (between radio and infrared) will allow astronomers
to study the cosmic microwave background—the afterglow of the big bang—and to
peer behind the dusty clouds of galaxies to study the births of stars, or probe
primeval galaxies. The team has already begun working to produce such devices
for Caltech’s Owens Valley Radio Observatory (OVRO) near Bishop, California,
about 250 miles north of Los Angeles.
These amplifiers, Zmuidzinas says, could be
incorporated into telescope arrays like the Combined Array for Research in
Millimeter-wave Astronomy at OVRO, of which Caltech is a consortium member, and
the Atacama Large Millimeter/submillimeter Array in Chile.
Instead of directly amplifying an
astronomical signal, the instrument can be used to boost the electronic signal
from a light detector in an optical, ultraviolet, or even X-ray telescope,
making it easier for astronomers to tease out faint objects.
Because the instrument is so sensitive and
introduces minimal noise, it can also be used to explore the quantum world. For
example, Keith Schwab, a professor of applied physics at Caltech, is planning
to use the amplifier to measure the behavior of tiny mechanical devices that operate
at the boundary between classical physics and the strange world of quantum
mechanics. The amplifier could also be used in the development quantum
computers—which are still beyond our technological reach but should be able to
solve some of science’s hardest problems much more quickly than any regular
computer.
“It’s hard to predict what all of the
applications are going to end up being, but a nearly perfect amplifier is a
pretty handy thing to have in your bag of tricks,” Zmuidzinas says. And by
creating their new device, the researchers have shown that it is indeed
possible to build an essentially perfect amplifier. “Our instrument still
has a few rough edges that need polishing before we would call it perfect, but
we think our results so far show that we can get there.”
