# Quantum Bayesian Networks

## December 9, 2010

### Agile, Micron-Wide Spotlight

Filed under: Uncategorized — rrtucci @ 10:10 pm

Normally, I don’t say much about experimental stuff, because I’m not an experimental physicist, and I feel uncomfortable teaching outside my area of expertise, but this experiment is easy to explain and really cool. Check out:

“Independent individual addressing of multiple neutral atom qubits
with a micromirror-based beam steering system”
by C. Knoernschild, X. L. Zhang, L. Isenhower, A. T. Gill, F. P. Lu, M. Saffman, and J. Kim (ArXiv version available here)

Some scientists from Saffman’s group at the Univ. of Wisconsin (Madison, Wisc.) and from Duke Univ. (N.Carolina) have built a device that traps 5 NEUTRAL Rubidium-87 atoms and then shines a spotlight on one atom at a time. (so far, the NIST people have used ions)

The five Rb atoms are trapped in a straight line; adjacent ones are 8.7 micrometers apart. The diameter of the spotlight is about 5 micrometers. (bacteria are typically 1 micrometer in size, so this is a spotlight for bacteria) The spotlight can be moved from one atom to another in 10 to 20 microseconds.

The spotlight is so powerful 🙂 that it causes a Rb atom to transition between the $|0\rangle$ and $|1\rangle$ quantum states which define a qubit. In this case, these two states are defined as two hyperfine energy levels of the atom; more specifically, $|0\rangle := | F=1, m_F=0\rangle$ and $|1\rangle := |F=2, m_F=0 \rangle$. It takes about 0.1 microsecs for an atom to go from $|0\rangle$ to $|1\rangle$. This so called Rabi flopping can be induced in a single target atom, with minimal disturbance on the quantum state of adjacent atoms.

In the near future, these researchers plan to produce 2-qubit interactions and 2-dimensional arrays of qubits.

The last sentence of the paper is:

While we have focused on its use in neutral atom QIP, it is applicable to other qubit systems in a periodic lattice including atomic ions, diamond nitrogen-vacancy color centers, and quantum dots.

I find it awesome how good the experimentalists are getting at controlling single qubits. True, qubits floating in the middle of a vacuum cavity, held in place by ephemeral “tractor beams” of light, manipulated by a spotlight which is steered by mechanical means, is probably not the most sturdy and convenient way of implementing a scalable quantum computer. But we can certainly learn a lot of physics from such experiments, and this will illuminate the way towards more convenient devices.

Press release for paper here

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