Light to Atom to Light Again

Reversible State Transfer between Light and a Single Trapped Atom

A.D. Boozer, A. Boca, R. Miller, T.E. Northup, and H.J. Kimble

PRL 98, 193601 (2007)

URL: http://link.aps.org/abstract/PRL/v98/e193601

This group from Cal Tech claims the first demonstration of the transfer of a coherent state between a photon and an atom, then from the atom back to a photon. This is a necessity for a quantum network.

Much work in quantum computing has gone into developing and manipulating qbits. It's one thing to have a single working qbit in isolation, or a pair, or 16 of them. But if you want to develop a quantum computer with a large number of qbits, or if you want to transfer the output state of your computation somewhere else, what do you do? The authors suggest that coherent light would be able to transfer superpositions of quantum states over optical fibers. Transmitting light over optical fibers is not so hard. The difficult step is turning an atomic state into a photon, transmitting the photon, then turning the photon into the same atomic state somewhere else.

In this letter, the Cal Tech group demonstrates "the reversible mapping of a coherent optical field to and from the hyperfine ground states of a single trapped cesium atom."

The prototype for their experiment is a 3-level atom. The atom has two ground states |a> and |b>, and an excited state |e>. The atom is in an optical cavity that couples |b> and |e>, and an external field couples |a> and |e>. If the external field is turned on slowly, the state |a,n> is transformed into |b,n+1> --- i.e., there is a transition between atomic states and a single photon is generated in the cavity. Slowly turning the field off reverses the transition.

If the cavity is empty, the process can be used to generate a single photon. The atom is prepared in state |a,0> and the field is slowly turned on. The resulting state is |b,1>. This is interesting, but transitions between single atomic states are not the building blocks of quantum computing. Entanglement and coherent superpositions of states are the tools of the trade. The useful thing about the process just described is that is works on a superposition of states:

(A |a> + B |b> ) |0> <---> |b> ( A |0> + B |1> )

This represents the transfer of a superposition of atomic states to a superposition of photon states, and all that was required is the turning off of a classical field.

The authors never mention the word "entangled" when discussing their experiments. I don't know what the exact definition of entangled states is, but I recall something about the impossibility of writing such a state as a product of states. If this is true, then the relation above does not describe the transfer of an entangled state from atom to photon. Still, it's a good first step!