Saturday, December 10, 2005

Progress With Photons, and Jitter and Skew

[This posting serves as both the fourth installment in our series on scalability, and as the papers-of-the-week entry.]

Nature this week has three intriguing papers on progress in quantum information processing with photons. I'm at home, without access to our institutional Nature subscription, so all I can read is the first paragraph. Grumble.

Jeff Kimble's group at Caltech reports on Measurement-induced entanglement for excitation stored in remote atomic ensembles. Use 10^5 atoms at each of two sites, and quantum interference when one of them emits a photon creates a single entangled state. Hmm. "One joint excitation" is the phrase they use, but I'm a little fuzzy on why they're not creating a Bell state. I'm looking forward to reading the full paper.

The other two papers, from Kuzmich's group at Georgia Tech and Lukin's at Harvard, are on the use of atomic ensembles to store qubits that can be inserted and retrieved as single photons. These have the possibility to serve as memories, or at least as latches, for photons, providing an important tool for addressing problems I think are under-appreciated: jitter and skew. Without the ability to regenerate the timing of signals propagating through a large circuit, you can't claim to have scalability.

Clock handling is one of the most complex problems in classical chip design. Signal propagation across a chip requires significant amounts of time. It's subject to two significant error processes: jitter, in which the timing of a single signal varies from moment to moment as a result of noise, voltage fluctuations, etc., and skew, where different members of a group of signals arrive at different times because their path lengths vary. (Both of these problems are substantially worse in e.g., your SCSI cables.) We can also talk about "clock skew" as being a problem between regions of a chip.

In a quantum computer, as in a classical one, there are going to be times when we want things to be in sync. We may need pair of photons to arrive at the same place at the same time, for example. Electromagnetically induced transparency (EIT), sometimes called "stopped light", is an excellent candidate for helping here. A strong control beam (our latch clock) is focused on an atomic ensemble. Whether the beam is on or off can determine whether a separate photon (the data signal) is allowed to pass through the material, or is held in place. Thus, it can be used to align multiple photons, releasing them all to move out of the ensemble(s) at the same time. Up until now, EIT experiments have been done on classical waves; these are, as I understand it, the first reports of doing similar things for a single photon.

Prof. Harris at Stanford is one of the leading lights (sorry) in EIT. The Lukin group is another, and Prof. Kozuma of Tokyo Institute of Technology has some promising-looking work.

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