A one-way street for spinning atoms

Elementary particles have a fundamental property called 'spin' that determines how they align in a magnetic field. MIT researchers have created a new physical system in which atoms with clockwise spin move in only one direction, while atoms with counterclockwise spin move in the opposite direction.

Elementary particles have a property called “spin” that can be thought of as rotation around their axes. In work 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 quantum-computing proposals.

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.

In the 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
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