(PhysOrg.com) -- "Many people are trying to build a quantum computer," Olivier Pfister tells PhysOrg.com. "One to the problems, though, is that you need hundreds of thousands of qubits. So far, scalability has been something of a problem, since generating that many qubits is difficult."

Pfister is a physics professor at the University of Virginia in Charlottesville. Working with Matthew Pysher, Yoshichika Miwa, Reihaneh Shahrokhshani, and Russell Bloomer, Pfister is using quadripartite cluster entanglement in order to make a breakthrough in the scalability for the number of qubits available for use in a quantum computer. The work is presented in Physical Review Letters: Parallel Generation of Quadripartite Cluster Entanglement in the Optical Frequency Comb.

There are several ways to make qubits with light, Pfister explains. One is to use a resonant mode of a cavity. A single laser cavity has millions of harmonic modes, and if you can design it, your scalability problem is solved.

The team at the University of Virginia made use of an optical frequency comb in their design to emit light fields that to be used as qubits. We excite a great number of them. These are Qmodes, and can be used as qubits. I can control where I put them, and then also entangle them, Pfister says. We use a two-photon emission medium, putting one photon in a given frequency, and the other in another. The Qmodes are well separated in frequency.

Since the set up allows for entanglement, it is possible for Pfister and his colleagues to create a cluster entangled state designed especially for quantum computing. Our design has correlations for all the qubits, and you can do measurements on them and implement quantum gates for one-way quantum computing, Pfister says.

Pfister points out that quantum computers of this sort cannot actually replace classical computers. However, quantum computers can be used for processing some types of information faster. This is an attractive model for experiments that need cluster states. The big deal is that we got all these little quantum registers, and the entanglement is remarkably consistent.

The next step, Pfister says, is to entangle the already-entangled qubits into a bigger register. It requires additional complexity to entangle them all together, and were on our way to this. We have shown that our control of entanglement is pretty good, but we need even better control to make entangled sets bigger than four.

Pfister thinks that the results of this experiment will result in increased interest in Qmodes of light. People will start thinking differently about Qmodes of light, he says. We are driving the field, and hopefully well make them on a single large scale, rather then make many small scale ones. Once that happens we will be ready to start with quantum processing.

There are a lot of tools available right now to make qubits, and this is one of them, Pfister continues. Our experiment shows a great potential for scaling up the number of entangled qubits that can be used in quantum processing. We are another step closer.

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More information: Matthew Pysher, Yoshichika Miwa, Reihaneh Shahrokshahi, Russell Bloomer, and Olivier Pfister, Parallel Generation of Quadripartite Cluster Entanglement in the Optical Frequency Comb, Physical Review Letters (2011). Available online: Matthew Pysher, Yoshichika Miwa, Reihaneh Shahrokshahi, Russell Bloomer, and Olivier Pfister, Parallel Generation of Quadripartite Cluster Entanglement in the Optical Frequency Comb,(2011). Available online: link.aps.org/doi/10.1103/PhysRevLett.107.030505

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