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Quantum magnetism in an ion trap

Camera images of a ytterbium ion crystal in different magnetic states (bright orange="up"; dark="down"). Courtesy of C. Monroe's quantum simulation lab.

Physicists can engineer a quantum magnet using lasers and ion qubits.  Ions are charged particles that interact strongly via the Coulomb force, which is an attraction/repulsion that decreases as particles separate. When a handful of positively charged ions are thrown together, they repel each other, and, for an oblong ion trap, form a linear crystal. (Images of ion traps can be found in our newly launched media galleries.) Each ion has two internal energy states that make up a qubit. Laser beams can manipulate the Coulomb force to create tunable, long range magnetic-like interactions, where each ion qubit represents a tiny magnet.

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Imagine that invisible springs connect the ions together. Vibrations occurring on one side of the crystal affect the entire crystal. This is called collective motion and is harnessed to generate a force that depends on how a magnet is oriented (which state the qubit is in). Chris Monroe's group at JQI can program this state-dependent force by simultaneously applying multiple laser beams, whose colors (frequencies) are specially chosen with respect to the internal vibrations of the ion crystal. The amount of influence each magnet has on the rest of the chain primarily depends on the choice of laser frequencies. The crystal geometry has little to do with the interactions. In fact, for some laser configurations the ions that are farthest apart in space interact most strongly. The team can ‘at will’ modify how the different collective modes contribute to magnetic order by merely changing the laser colors and/or the ion separation. 

Phenomena due to this type of magnet-magnet interaction alone can be explained without quantum physics. An additional uniform magnetic field, (here created with yet another laser beam), is necessary for introducing quantum phase transitions and entanglement. This added magnetic field induces quantum fluctuations that can drive the system into different energy levels.

The crystal can easily form various antiferromagnetic combinations, instead of the simple nearest neighbor antiferromagnet (up-down-up-down).  In fact, with a few technical upgrades, researchers can potentially engineer situations where the magnets can reside in an exponentially large number of antiferromagnetic states, generating massive quantum entanglement that accompanies this frustration.