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