quantum computing #2

Some of the other research Prof. Kimble's group is working on involves cavity QED. Though it may sound like a highly specialized field to some, it's basically the study of how light and matter interact in confined regions (as opposed to outer space). In the classical limit, it becomes Maxwell's equations for waveguides or resonant cavities (like a home microwave!). Technology for creating high quality factor (high-Q) resonant cavities plus improvements in magnetic trapping of atoms has created the possibility of seeing the interactions between photons and electronic levels on a quantum level.

An alternative approach is to use photonic crystals to create a high-Q cavity, and embed either one or a few atoms or quantum dots in the middle of the cavity. Photonic crystals have two attributes that are of great relevance: (1) they can effectively confine light to small regions with low losses and (2) they can be used to convey photons over long distances (one might even replace fiber optic cables connecting the internet between the US and Europe with photonic crystals).

The other piece of the puzzle is nonlinearity. Single photons interact very weakly with one another in vacuum (an order \alpha^2 process in quantum electrodynamics), but one can introduce matter that can mediate a strong interaction between the same two photons. Most of these nonlinearities come from electromagnetically induced transparency (EIT), a remarkable physical phenomena whereby two absorption processes in a 3-level atom "cancel" such that light at a probe frequency can pass through without absorption in the presence of strong nonlinearities (normally this would be impossible). That's where the name comes from. An additional wrinkle is that if you introduce a fourth atomic level, one can change where this cancellation takes place with a different frequency (the "gating" frequency). In our system, we envision sending in a single gating photon, which will then change the property of the atom(s) inside the resonant cavity such that a single probe photon will behave differently than it did before, e.g., it would be transmitted instead of reflected. As a result, one obtains an entangled state between two photons. This idea could be used as the basis for a quantum network. For more details, click here for our preprint.