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Atomic physics

Neutral atoms put in charge

Nature volume 462, pages 584585 (03 December 2009) | Download Citation

An elegant experiment shows that atoms subjected to a pair of laser beams can behave like electrons in a magnetic field, as demonstrated by the appearance of quantized vortices in a neutral superfluid.

Ultracold gases of atoms — a million times thinner than air and a million times colder than interstellar space — allow the observation and control of many-body quantum phenomena at macroscopic scales. They can thus serve as model materials1 for condensed-matter systems in which such phenomena arise. From superfluids (fluids that flow without friction) to insulators, and from weakly to strongly interacting systems, a great variety of fundamental states of matter can be realized, observed in real time and probed with the precision of atomic physics. But there seems to be one obvious limitation: atoms are neutral, a fact that in principle precludes the observation of a wealth of phenomena tied to charged particles, for example their behaviour in a magnetic field. On page 628 of this issue, Lin et al.2 get round this problem and in striking fashion demonstrate their ability to create synthetic magnetic fields for a neutral ultracold atomic system. In their study, a collective state of matter termed a Bose–Einstein condensate develops quantized vortices of magnetic flux similar to mini-tornadoes.

Magnetic fields are directly tied to the rotational motion of charged particles: an electron placed in a uniform magnetic field rotates about the field axis. One way to mimic the effect of a magnetic field on a cloud of neutral atoms is thus to make the cloud rotate. But this approach comes with limitations. Achieving the equivalent of a large magnetic field requires fast rotation, which causes atoms to fly apart. Also, the method requires rotationally symmetrical confinement of the gas and cannot be applied to atoms trapped in a static optical lattice — an artificial 'crystal' of light used to model solid-state systems1. A more direct way of mimicking effective magnetic fields should thus not be restricted to certain system geometries and configurations.

To get a picture of what is needed, consider what happens to a paddle wheel placed in a river (Fig. 1). If the river flows with uniform velocity across the paddle wheel, it will not rotate. But if the water on one side of the paddle wheel flows faster than that on the other side or in a different direction, or if there is a swirl (vortex) in the flow, the paddle wheel will rotate. In this analogy, the axis and speed of rotation represent the direction and strength of the magnetic field, and the flow pattern is called a vector potential. Just as the paddle wheel rotates in the presence of vortices in the flow, so a cloud of atoms will experience a magnetic field if a vector potential that carries vorticity is imprinted on the system.

Figure 1: Paddle-wheel analogy for magnetic fields.
Figure 1

a, A paddle wheel placed in water flowing with uniform velocity (blue arrows) does not rotate. b, If the water flows non-uniformly and in a different direction on one side of the paddle wheel from that on the other, the wheel rotates (green arrow). The flow pattern is analogous to the 'vector potential' created in Lin and colleagues' experiment2 to generate a synthetic magnetic field for an ultracold cloud of atoms known as a Bose–Einstein condensate. The rotation axis (purple arrow) and the speed with which the paddle wheel rotates correspond respectively to the direction and strength of the magnetic field. c, d, Images of the Bose–Einstein condensate before (c) and after (d) application of the synthetic magnetic field. The appearance of quantized vortices (d) is a direct demonstration of a synthetic magnetic field.

This is precisely what Lin and colleagues2 achieved in their experimental set-up, which is an inspired realization of earlier proposals3,4,5,6. Using a pair of laser beams, they first imprinted the equivalent of a river's uniform flow — a uniform vector potential — on their ultracold cloud of atoms7,8. Next, by judiciously tuning the lasers in a spatially dependent way, they endowed the vector potential with a swirl: atoms on different sides of the cloud experienced a different vector potential. This non-uniform flow created a synthetic magnetic field.

Quantum mechanics predicts a wide range of peculiar effects for systems subjected to magnetic fields. One dramatic example, the Meissner effect, is found in superconductors, in which currents of electron pairs flow without resistance. In the Meissner effect, a superconductor placed in a magnetic field expels the field from its interior. But beyond a certain critical field strength, many superconductors, termed type II superconductors, allow magnetic flux to pierce their interior in the form of tiny quantized vortices or flux tubes. This is a direct consequence of the fact that superconductors can be described by a single collective macroscopic wavefunction that is shared among all of their constituent electron pairs. Such 'flux quantization' is the principle behind the working of SQUIDs (superconducting quantum interference devices), the extremely accurate magnetometers that are used to measure small magnetic fields.

Superconductors are nothing other than charged superfluids. The behaviour of superconductors in a magnetic field is thus directly analogous to that of neutral superfluids under rotation. Observations of a rotating lattice of vortices in neutral superfluids such as Bose–Einstein condensates9 and gases of fermionic atoms10 (particles with half-integer spin, such as electrons) have in fact served as direct proof of these systems' superfluidity. Lin et al.2 applied their novel technique of creating synthetic magnetic fields, which do not require the system to rotate, to a Bose–Einstein condensate, and found that the system develops a striking array of vortices — the smoking gun for magnetic-flux quantization. This is the first time that a stable, long-lived, non-rotating array of vortices has been formed in a neutral superfluid.

In charged systems, many intriguing magnetic phenomena arise at large magnetic fields. One example is given by fractional quantum Hall states — collective states of matter in which electrons behave as if their elementary charge is only a fraction of their actual charge. Reaching the required field strengths in neutral systems is, however, probably beyond the reach of experiments that rely on rotation to mimic the effect of such fields. Lin and colleagues' experiment2 offers a new approach to generate synthetic fields and might point the way towards observing fractional quantum Hall and other exotic states of matter in neutral systems. Of course, the task of extending the authors' technique to strong magnetic fields won't be easy. Their technique also comes with shortcomings: for example, the finite lifetime of the atomic cloud in the presence of the laser beams; a limit on the highest possible vector potential; and the difficulty of extending the approach to fermionic systems. That said, the challenges that accompany the authors' technique might be easier to tackle than those that plague the rotational approach.

Lin and colleagues' spectacular demonstration2 of a synthetic magnetic field for neutral atoms signals the advent of synthetic electrodynamics in the field of ultracold atomic gases. Future applications of their method might include a measurement of the superfluid fraction in ultracold atomic gases11 and the creation of unusual quantum states in two-dimensional optical lattices at high effective fields5. The demonstration of quantum Hall physics in such lattices6 using the authors' approach might also be within reach. Their work opens up exciting avenues for producing novel many-body quantum systems.

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  1. Martin Zwierlein is at the MIT-Harvard Center for Ultracold Atoms, Research Laboratory of Electronics, and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.  zwierlein@mit.edu

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