September 25, 2009

Quantum Computers Powered By Photon Machine Guns

New Scientist reports of a new solution to increasing the numbers of entangled qubits for quantum computers. It will be a revolutionary advance for photonic quantum computing.

Raising the number of qubits has proven tricky because of the difficulty of reliably producing entangled particles. Now a team has designed a system that should fire out barrages of entangled photons with machine-gun regularity

Rudolph and Netanel Lindner at the Technion-Israel Institute of Technology in Haifa have designed the blueprint for a system that fires out large numbers of entangled photons on demand. They call it a "photonic machine gun"

Rudolph and Lindner initially estimated that their device would be able to fire out 12 qubits on demand. "Talking to various experimentalists I think we were a bit conservative," says Rudolph. "The current collection efficiencies might make detection of 20 to 30 entangled photons feasible, which would take us beyond what we can fit into the memory of a classical computer."

They say that a practical version could be built within a few years. "It's only within the last year or so that the [nanofabrication] technology has made this feasible," Rudolph says.

Abstract: Proposal for Pulsed On-Demand Sources of Photonic Cluster State Strings from Phys. Rev. Lett. 103, 113602 (2009) [4 pages]

We present a method to convert certain single photon sources into devices capable of emitting large strings of photonic cluster state in a controlled and pulsed “on-demand” manner. Such sources would greatly reduce the resources required to achieve linear optical quantum computation. Standard spin errors, such as dephasing, are shown to affect only 1 or 2 of the emitted photons at a time. This allows for the use of standard fault tolerance techniques, and shows that the photonic machine gun can be fired for arbitrarily long times. Using realistic parameters for current quantum dot sources, we conclude high entangled-photon emission rates are achievable, with Pauli-error rates per photon of less than 0.2%. For quantum dot sources, the method has the added advantage of alleviating the problematic issues of obtaining identical photons from independent, nonidentical quantum dots, and of exciton dephasing.

8 page pdf with supplemental information.

Exciton Dephasing

In the paper we only briefly mentioned the spectral dephasing which will occur
while the system is excited. Our intuition contrasted with that of others, namely
we felt that this process would not a¤ect the entanglement of the state - in
particular with respect to the polarization degrees of freedom we are interested
in - but would only lead to the emitted photon wavepackets being in a mixture
of di¤erent frequencies. This in turn would only a¤ect the (small fraction) of
photons which have to go through fusion gates, and such photons can be …ltered
before entering the gates in a way which will only lead to a change in the success
probability of the (non-deterministic) gate. That is, such …ltering need not even
lead to a loss error (as explained below). As such the only e¤ect will be that we
need to use more photons - but the overhead is some constant factor.

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