reported this week in the journal Physical Review Letters, MIT physicists have imposed a stringent set of traffic rules on atomic particles in a gas: Those spinning clockwise can move in only one direction, while those spinning counterclockwise can move only in the other direction.
Physical materials with this distinctive
property could be used in “spintronic” circuit devices that rely on spin
rather than electrical current for transferring information. The
correlation between spin and direction of motion is crucial to creating a
so-called topological superfluid, a key ingredient of some
The MIT team, led by Martin
Zwierlein, an associate professor of physics and a principal
investigator in the Research Laboratory of Electronics (RLE), produced
this spin-velocity correlation in an ultracold, dilute gas of atoms.
Just like electrons, the atoms in the gas are fermions, particles that
cannot share the same quantum state; as a consequence, each atom has to
have a different combination of spin and velocity.
process of sorting themselves into separate quantum states, the atoms
moving very fast to the left end up spinning one way, while those moving
very fast to the right end up spinning the other way. “What about the
atoms moving with a velocity in between these extremes?” Zwierlein asks.
“Quantum mechanics provides a surprising answer: They can
simultaneously spin both ways.”
Physical systems that correlate
spin and velocity could open the door to a novel approach to quantum
computers, largely hypothetical devices that would perform some types of
computations exponentially faster than conventional computers. They
derive this speed advantage by taking advantage of superposition, the
ability of tiny particles — such as the atoms spinning in both
directions at once — to inhabit more than one physical state at a time.
The chief obstacle in building quantum computers is that superposition is very difficult to maintain.
In theory, topological superfluids should give rise to particles called
Majorana fermions, which are much harder to knock out of superposition
than other particles.
In previous experiments, the RLE
researchers created a superfluid — a completely frictionless gas — of
lithium atoms. In their new experiment, the researchers used laser beams
to trap a cloud of lithium atoms about 50 micrometers in diameter. The
atoms were cooled to just a few billionths of a degree above absolute
zero. At such low temperatures, quantum mechanics describes the behavior
of the gas.
The researchers illuminated the gas with a pair of
laser beams, sorting the atoms into two lanes, each of which consists of
atoms with the same spin moving in the same direction. For the first
time in an atomic system, this correlation of atoms’ spins with their
velocities was directly measured.
“The combined system of
ultracold atoms and the light we shine on them forms a material with
unique properties,” says Lawrence Cheuk, lead author of the paper and a
graduate student in MIT’s physics department. “The gas acts as a quantum
diode, a device that regulates the flow of spin currents.”
Read more...in MIT News