Focus: Filtering Atoms by Their Spin
Researchers have demonstrated a laser-based spin filter for cold atoms. The experiment allows atoms in one spin state to move through a narrow channel, while it blocks atoms in a different spin state. The scheme lets the experimenters control the strength of the interaction between the two spin types and dictate whether the force is attractive or repulsive. The setup could be used to study spin-filtering effects relevant to the development of future spintronic devices, which will use currents of electrons with a preferential spin alignment and are expected to be superior to conventional electronics in some ways. The experiment might also help researchers search for Majorana fermions—elusive particles that act as their own antiparticles.
For decades, researchers have used magnetic fields and lasers to control the interactions between cold atoms trapped in a cloud. The atoms can stand in for electrons in a solid, so these systems have served as simulators of condensed-matter phenomena like superconductivity and supersolidity. Researchers would also like to simulate effects in which the movement of particles depends on their spin. These effects are central to the behavior of topological insulators—materials that conduct electricity only on their surfaces—and they are also important for devices that produce spin-filtered currents for spintronics research. To simulate spin-dependent phenomena, researchers have added another control knob for cold atoms by manipulating their atomic spins with lasers .
For these simulations, they would like to control both interactions and spins simultaneously, but such experiments have proven challenging. “Every time you manipulate cold atoms with lasers you induce heating,” says Laura Corman of the Swiss Federal Institute of Technology (ETH) in Zurich. The heating can kick atoms out of a trap or hide phenomena that require ultralow temperatures. To mitigate heating problems, Corman’s team looked for a way to create a spin-dependent effect using a laser that hits only a small region within a cold-atom cloud.
The team prepared about 200,000 lithium-6 atoms in two hyperfine states—the “spin-up” and “spin-down” states. Using a combination of lasers and magnetic fields, they confined the atoms to a region containing two lobes joined by a small constriction called a quantum point contact (QPC). They then illuminated the QPC with a laser beam that acted like a magnetic field, shifting the energies of quantum states inside the QPC differently for atoms with different spins. As a consequence, the probability for atoms to cross the QPC depended on their spin.
To show that this arrangement behaved as a spin filter, the team tracked the number of atoms of each spin species on both sides of the QPC over time. They started with 50% more atoms on one side, which generated a sort of “pressure” pushing the atoms toward the less-populated side. While the number of spin-up atoms remained constant on both sides, spin-down atoms moved from one side to the other at a rate of about 800 atoms per second.
The team also demonstrated that they could vary the interactions between atoms of different spin states and still observe the spin filter effect. By changing the magnetic field, they could make opposite spins attract or repel each other and could tune the interaction strength over a broad range. The changes in interaction resulted in subtle yet measurable changes in the transport of atoms with different spins through the filter.
Quantum optics theorist Andrew Daley of the University of Strathclyde in the UK calls the result “a milestone for our ability to model spin-dependent transport phenomena with cold atoms.” Theoretical physicist Rembert Duine of Utrecht University in the Netherlands says that the scheme could help researchers vet new ideas for spintronics, as they consider, for instance, materials with various degrees of electron-electron interactions.
Corman and her colleagues, however, have a longer-term goal in mind. “It’s a far-fetched idea, but what really excites us is the possibility of using this platform to search for Majorana fermions,” says Corman. Majoranas—elementary particles that are their own antiparticles—have never been seen, but researchers believe that analogs of these unusual particles could be found in materials in the form of collective “quasiparticle” states of electrons. There are experimental and theoretical hints that Majoranas could be hosted by a nanowire in contact with a superconductor. “With the addition of some ingredients that have been demonstrated independently, our setup could mimic such a nanowire configuration, while offering the possibility to fine tune the microscopic parameters and the geometry of the system,” Corman says.
Matteo Rini is the Deputy Editor of Physics.
- In 2012, two studies (see 27 August 2012 Viewpoint) showed that lasers can introduce a connection between the atoms’ spins and their motion, known as spin-orbit coupling.