Quantum mechanics

Bose–Einstein condensate

Spin (aerodynamics)

Engineering

Metallurgy

Materials science

Ferromagnetism

Embedded system

Coupling (piping)

Atomic physics

Computer science

Energetic neutral atom

Ultracold atom

Mechanical engineering

Thermodynamics

Physics

Fermion

Topological insulator

Atom (system on chip)

Condensed matter physics

Quantum

State of matter

Electron

Spin–orbit interaction

Spintronics

Boson

Nature

Spin-orbit coupling describes the interaction between a quantum particle's spin and its momentum, and is important for many areas of physics from spintronics to the quantum spin Hall effect and topological insulators. However, in systems of ultracold neutral atoms, there is no coupling between the spin and the centre-of-mass motion of the atom. Lin et al. use lasers to engineer such spin-orbit coupling in a neutral atomic Bose–Einstein condensate, the first time this has been achieved for any bosonic system. This should lead to the realization of topological insulators in fermionic neutral atom systems. Spin–orbit coupling describes the interaction between a quantum particle's spin and its momentum, and is important for many areas of physics such as spintronics and topological insulators. However, in systems of ultracold neutral atoms, there is no coupling between the spin and the centre of mass motion of the atom. This study uses lasers to engineer such spin–orbit coupling in a neutral atomic Bose–Einstein condensate, the first time this has been achieved for any bosonic system. This should lead to the realization of topological insulators in fermionic neutral atom systems. Spin–orbit (SO) coupling—the interaction between a quantum particle’s spin and its momentum—is ubiquitous in physical systems. In condensed matter systems, SO coupling is crucial for the spin-Hall effect1,2 and topological insulators3,4,5; it contributes to the electronic properties of materials such as GaAs, and is important for spintronic devices6. Quantum many-body systems of ultracold atoms can be precisely controlled experimentally, and would therefore seem to provide an ideal platform on which to study SO coupling. Although an atom’s intrinsic SO coupling affects its electronic structure, it does not lead to coupling between the spin and the centre-of-mass motion of the atom. Here, we engineer SO coupling (with equal Rashba7 and Dresselhaus8 strengths) in a neutral atomic Bose–Einstein condensate by dressing two atomic spin states with a pair of lasers9. Such coupling has not been realized previously for ultracold atomic gases, or indeed any bosonic system. Furthermore, in the presence of the laser coupling, the interactions between the two dressed atomic spin states are modified, driving a quantum phase transition from a spatially spin-mixed state (lasers off) to a phase-separated state (above a critical laser intensity). We develop a many-body theory that provides quantitative agreement with the observed location of the transition. The engineered SO coupling—equally applicable for bosons and fermions—sets the stage for the realization of topological insulators in fermionic neutral atom systems.

Spin–orbit-coupled Bose–Einstein condensates