Researchers at the National
Institute of Standards and Technology (NIST) and JILA, a joint institute
of NIST and the University of Colorado (CU) at Boulder, have made the first
tunable "noiseless" amplifier. By significantly reducing the uncertainty
in delicate measurements of microwave signals, the new amplifier could boost
the speed and precision of quantum computing and communications systems.
In the JILA/NIST “noiseless” amplifier, a long line of superconducting magnetic sensors (beginning on the right in this colorized micrograph) made of sandwiches of two layers of superconducting niobium with aluminum oxide in between, creates a 'metamaterial' that selectively amplifies microwaves based on their amplitude rather than frequency or phase. Credit: M. Castellanos-Beltran/JILA
Conventional amplifiers add unwanted “noise,” or random fluctuations,
when they measure and boost electromagnetic signals. Amplifiers that theoretically
add no noise have been demonstrated before, but the JILA/NIST technology, described
in an Oct. 5, 2008, advance online publication of Nature Physics,* offers better
performance and is the first to be tunable, operating between 4 and 8 gigahertz,
according to JILA group leader Konrad Lehnert. It is also the first amplifier
of any type ever to boost signals sufficiently to overcome noise generated by
the next amplifier in a series along a signal path, Lehnert says, a valuable
feature for building practical systems.
Noisy amplifiers force researchers to make repeated measurements of, for example,
the delicate quantum states of microwave fields—that is, the shape of
the waves as measured in amplitude (or power) and phase (or point in time when
each wave begins). The rules of quantum mechanics say that the noise in amplitude
and phase can’t both be zero, but the JILA/NIST amplifier exploits a loophole
stipulating that if you work with only one—phase—and ignore amplitude,
then the amplifier is theoretically capable of adding no noise. In reality,
the JILA/NIST amplifier adds about half the noise that would be expected from
measuring both amplitude and phase.
The JILA/NIST amplifier could enable faster, more precise measurements in certain
types of quantum computers—which, if they can be built, could solve some
problems considered intractable today—or quantum communications systems
providing “unbreakable” encryption. It also offers the related and
useful capability to “squeeze” microwave fields, trading reduced
noise in the signal phase for increased noise in the signal amplitude. By combining
two squeezed entities, scientists can “entangle” them, linking their
properties in predictable ways that are useful in quantum computing and communications.
Entanglement of microwave signals, as opposed to optical signals, offer some
practical advantages in computing and communication such as relatively simple
equipment requirements, Lehnert says.
The new amplifier is a 5-millimeter-long niobium cavity lined with 480 magnetic
sensors called SQUIDs (superconducting quantum interference devices). The line
of SQUIDs acts like a “metamaterial,” a structure not found in nature
that has strange effects on electromagnetic energy. Microwaves ricochet back
and forth inside the cavity like a skateboarder on a ramp. Scientists tune the
wave velocity by manipulating the magnetic fields in the SQUIDs and the intensity
of the microwaves. An injection of an intense pump tone at a particular frequency,
like a skateboarder jumping at particular times to boost speed and height on
a ramp, causes the microwave power to oscillate at twice the pump frequency.
Only the portion of the signal which is synchronous with the pump is amplified.
Funding for the research was provided by NIST, the National Science Foundation,
and a NIST-CU seed grant.
* M.A. Castellanos-Beltran, K.D. Irwin, G.C. Hilton, L.R. Vale and K.W. Lehnert.
Amplification and squeezing of quantum noise with a tunable Josephson metamaterial.
Nature Physics advance online publication.