Physicists
at Harvard University have realized a new way to cool synthetic
materials by employing a quantum algorithm to remove excess energy. The
research, published this week in the journal Nature,
is the first application of such an “algorithmic cooling” technique to
ultra-cold atomic gases, opening new possibilities from materials
science to quantum computation.
“Ultracold
atoms are the coldest objects in the known universe,” explains senior
author Markus Greiner, associate professor of Physics at Harvard. “Their
temperature is only a billionth of a degree above absolute zero
temperature, but we will need to make them even colder if we are to
harness their unique properties to learn about quantum mechanics.”
Greiner
and his colleagues study quantum many-body physics, the exotic and
complex behaviors that result when simple quantum particles interact. It
is these behaviors which give rise to high-temperature
superconductivity and quantum magnetism, and that many physicists hope
to employ in quantum computers.
“We
simulate real-world materials by building synthetic counterparts
composed of ultra-cold atoms trapped in laser lattices,” says co-author
Waseem Bakr, a graduate student in physics at Harvard. “This approach
enables us to image and manipulate the individual particles in a way
that has not been possible in real materials.”
The catch is that observing the quantum mechanical effects that Greiner, Bakr and colleagues seek requires extreme temperatures.
“One
typically thinks of the quantum world as being small,” says Bakr, ” but
the truth is that many bizarre features of quantum mechanics, like
entanglement, are equally dependent upon extreme cold.”
The
hotter an object is, the more its constituent particles move around,
obscuring the quantum world much as a shaken camera blurs a photograph.
The
push to ever-lower temperatures is driven by techniques like “laser
cooling” and “evaporative cooling,” which are approaching their limits
at nanoKelvin temperatures. In a proof-of-principle experiment, the
Harvard team has demonstrated that they can actively remove the
fluctuations which constitute temperature, rather than merely waiting
for hot particles to leave as in evaporative cooling.
Akin
to preparing precisely one egg per dimple in a carton, this “orbital
excitation blockade” process removes excess atoms from a crystal until
there is precisely one atom per site.
“The
collective behaviors of atoms at these temperatures remain an important
open question, and the breathtaking control we now exert over
individual atoms will be a powerful tool for answering it,” said
Greiner. “We are glimpsing a mysterious and wonderful world that has
never been seen in this way before.”
Orbital excitation blockade and algorithmic cooling in quantum gases
