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 A scanning electron microscope image of an aluminum and silicon nitride resonator coupled to a superconducting single electron transistor (SSET). Researchers watched the resonator move through a phenomenon known as quantum back-action. Pic: news.cornell.edu
The problem comes in finding the dividing line between the two
worlds -- or even in establishing that such a line exists. To that end,
Keith Schwab, associate professor of physics who moved to Cornell this
year from the National Security Agency, and colleagues have created a
device that approaches this quantum mechanical limit at the largest
length-scale to date.
And surprisingly, the research also has shown how researchers can lower the temperature of an object -- just by watching it.
The results, which could have applications in quantum computing,
cooling engineering and more, appear in the Sept. 14 issue of the
journal Nature.
The device is actually a tiny (8.7 microns, or millionths of a
meter, long; 200 nanometers, or billionths of a meter, wide) sliver of
aluminum on silicon nitride, pinned down at both ends and allowed to
vibrate in the middle. Nearby, Schwab positioned a superconducting
single electron transistor (SSET) to detect minuscule changes in the
sliver's position.
According to the Heisenberg uncertainty principle, the precision of
simultaneous measurements of position and velocity of a particle is
limited by a quantifiable amount. Schwab and his colleagues were able
to get closer than ever to that theoretical limit with their
measurements, demonstrating as well a phenomenon called back-action, by
which the act of observing something actually gives it a nudge of
momentum.
"We made measurements of position that are so intense -- so strongly
coupled -- that by looking at it we can make it move," said Schwab.
"Quantum mechanics requires that you cannot make a measurement of
something and not perturb it. We're doing measurements that are very
close to the uncertainty principle; and we can couple so strongly that
by measuring the position we can see the thing move."
The device, while undeniably small, is -- at about ten thousand
billion atoms -- vastly bigger than the typical quantum world of
elementary particles.
Still, while that result was unprecedented, it had been predicted by
theory. But the second observation was a surprise: By applying certain
voltages to the transistor, the researchers saw the system's
temperature decrease.
"By looking at it you cannot only make it move; you can pull energy
out of it," said Schwab. "And the numbers suggest, if we were to keep
going on with this work, we would be able to cool this thing very cold.
Much colder than we could if we just had this big refrigerator."
The mechanism behind the cooling is analogous to a process called
optical or Doppler cooling, which allows atomic physicists to cool
atomic vapor with a red laser. This is the first time the phenomenon
has been observed in a condensed matter context.
Schwab hasn't decided if he'll pursue the cooling project. More
interesting, he says, is the task of figuring out the bigger problem of
quantum mechanics: whether it holds true in the macroscopic world; and
if not, where the system breaks down.
For that he's focusing on another principle of quantum mechanics --
the superposition principle -- which holds that a particle can
simultaneously be in two places.
"We're trying to make a mechanical device be in two places at one
time. What's really neat is it looks like we should be able to do it,"
he said. "The hope, the dream, the fantasy is that we get that
superposition and start making bigger devices and find the breakdown."
Source: Cornell University
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