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Photonic machine gun for quantum computing

Colin Barras

There is a simple rule of computing that holds true even in the weird quantum world: increase the number of units of information available and you boost computing power. Raising the number of quantum bits, or qubits, carries an even greater reward – every additional qubit doubles the computing power.

But 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.

Existing methods of producing entangled photons tend to spew out photons more or less randomly, says Terry Rudolph at Imperial College London. "People have produced six entangled photons but they come out at times we cannot control," he says. "Ultimately that isn't going to help us build a proper quantum computer."

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" (Physical Review Letters, DOI: 10.1103/physrevlett.103.113602).

At the heart of the gadget is a quantum dot - a nanoscale crystal within a semiconducting device - chilled to a low temperature. When a short, strong pulse of light hits the dot, one of the electrons inside is raised to an excited state. As it "relaxes" back to its resting energy state it throws out a photon.

"We can manipulate the electron in such a way that it is entangled with the photon," Rudolph says. Excite the same electron again and it spews out a second photon that is also entangled with the electron, and therefore with the first photon too. Repeat the process many times and a string of entangled photons emerges, ready to inject into a quantum computer.

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.

The quantum machine-gun idea has generated a buzz among researchers. "It's a superb piece of work," says Andrew White at the University of Queensland in Brisbane, Australia. "I think this is one of the most exciting theoretical proposals I've read in five years - it will be a revolutionary advance for photonic quantum computing."

Image: Dan Talson/Rex Features


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