Electric charges can exist in isolation, and positive protons and negative electrons are capable of independent action. Magnets, on the other hand, always seem to come in pairs of north and south poles. However, several theories center on individual magnetic charges—known as monopoles—and use them to help explain why electric charges come in distinct quantities. Nobody has yet seen a magnetic monopole in particle form, though the search continues.

Rather than searching for an instance of a monopole, others are trying to model them using quantum simulation: the universal character of quantum physics can allow one physical system to behave exactly like another. Thus, in lieu of hunting for particles that are monopolar, M. W. Ray, E. Ruokokoski, S. Kandel, M. Möttönen, and D. S. Hall emulated the behavior of a north magnetic charge using ultracold atoms. The result was behavior described as a Dirac magnetic monopole, something never before seen. This experiment relied on the quantum character of monopoles and might provide hope that isolated magnetic charges could exist in nature.

Quantum simulations work like simulations run on an analog computer: researchers construct electric circuits that obey the same basic mathematical equations as a more complicated physical phenomenon, which allows them to emulate the complicated system without trying to solve the (possibly unsolvable) equations that describe it. A quantum simulation lets physicists substitute a controllable physical system for one that might be too challenging to ever construct in the lab.

This process works because all quantum systems are governed by the same basic rules. Any two physical systems—no matter how different in composition—can be used as proxies for each other, so long as their basic quantum states behave the same way. Thus, an atom with two internal energy states can behave like a magnet; a collection of atoms in a trap can collectively act like a complex material; and vortices in a Bose-Einstein condensate can emulate a magnetic monopole.

A pole, apart

If you ionize an atom, you get a free negative charge (electron) and positive charge (the atom with more protons than electrons). But if you break a bar magnet in half, you obtain two smaller bar magnets: you can't isolate the north and south poles. Although magnetic fields arise from phenomena associated with charges, there's a curious asymmetry between electricity and magnetism. While particles having a single type of charge are ubiquitous, magnetic monopoles are either rare or nonexistent. Yet magnetic fields are generated by phenomena associated with charges: the spin of electrons, the flow of electric charge through wires, or similar phenomena.

Quarks, protons, electrons, and the like all bear electric charges that are specific multiples of the fundamental charge. The very strange physicist Paul Dirac realized that if even a single magnetic monopole particle exists somewhere in the cosmos, it would explain why electric charges only come in certain quantities. For that reason, some particle theories predict that monopoles were produced in large numbers early in the history of the cosmos, but the rapid expansion spread them out until they became fairly rare today.

According to theory, monopoles have strange magnetic properties, including an odd quantum effect known as a Dirac string. (This should not be confused with either a superstring or a cosmic string.) The string itself isn't a physical object, instead, it affects any quantum particles passing by, causing a distinctive interference pattern. The combination of Dirac strings and the magnetic field from a monopole would make ordinary particles like electrons act as sensitive probes of monopole behavior—should we ever spot one in the wild.

Emulating monopoles

Since we can't seem to find one, though, some researchers decided to emulate monopole behavior using an analogous quantum system. They used a Bose-Einstein condensate: a collection of very cold atoms that behaves like a single quantum system. In this case, they used rubidium atoms that act like a permanent magnet (a ferromagnet for the physics fans in the crowd), with specific spin properties.

Within the Bose-Einstein condensate, the equations governing the fluid's velocity and vorticity—rotational rate and variation—correspond exactly to those of the magnetic potential (akin to voltage) and magnetic field. By tuning these properties, the researchers created something that behaved exactly like a magnetic Dirac monopole, including the fact that it exhibited the predicted Dirac string.

This approach provided an improvement over earlier monopole work, which used more exotic systems (yes, a cloud of super-cold rubidium atoms isn't all that exotic). The specifically quantum character of the Bose-Einstein condensate meant the researchers were able to emulate the full predicted behavior of monopoles for the first time. This particular experiment also used very sensitive instruments and tightly controlled experimental conditions, which allowed the physicists to see behavior that would be otherwise hidden by other effects.

What does this research say about the existence of monopole particles in the Universe? Nothing directly, but because quantum systems are proxies for each other, creating monopole-like behavior in a Bose-Einstein condensate is a strong hint that such particles could still be out there for us to find. Future experiments should be able to probe the magnetic behavior more deeply, creating a full emulation of processes that could have birthed elementary particles in the early cosmos.

Nature, 2013. DOI: 10.1038/nature12954 (About DOIs).