[LINK] Uni of Bristol reports use of photon-based quantum computer chip (4 qubits) solving long division

Phillip Musumeci pmusumeci at gmail.com
Thu Sep 10 07:11:12 AEST 2009

Quantum Chip Helps Crack Code

Experimental chip does part of code-cracking quantum algorithm

BY ANNE-MARIE CORLEY // IEEE Spectrum, September 2009

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3 September 2009-Modern cryptography relies on the extreme difficulty
computers have in factoring huge numbers, but an algorithm that works only
on a quantum computer finds factors easily. Today in Science, researchers at
the University of Bristol, in England, report the first factoring using this
method-called Shor's algorithm-on a chip-scale quantum computer, bringing
the field a tiny step closer to realizing practical quantum computation and
code cracking.

Quantum computers are based on the quantum bit, or qubit. A bit in an
ordinary computer can be either a 1 or a 0, but a qubit can be 1, 0, or a
"superposition" of both at the same time. That makes solving certain
problems-like factoring-exponentially faster, because it lets the computer
try many more solutions at once. The race is on to find the ideal quantum
computer architecture, with qubit contenders that include ions, electrons,
superconducting circuits, and in the University of Bristol's case, photons.

MIT professor Seth Lloyd, who has been researching quantum computing and
communication systems since the early 1990s, says that "optical methods
[using photons] have a long way to go before being useful." But, Lloyd adds,
the Bristol experiment demonstrates that the components for optical quantum
computing can be squeezed onto a chip, which is an important step forward.

Shor's algorithm was first demonstrated in a computing system based on
nuclear magnetic resonance-manipulating molecules in a solution with strong
magnetic fields. It was later demonstrated with quantum optical methods but
with the use of bulk components like mirrors and beam splitters that take up
an unwieldy area of several square meters.

Last year, the Bristol researchers showed they could miniaturize this
optical setup, building a quantum photonic circuit on a silicon chip mere
millimeters square. They replaced mirrors and beam splitters with waveguides
that weave their way around the chip and interact to split, reflect, and
transmit light through the circuit. They then injected photons into the
waveguides to act as their qubits.

Now they've put their circuit to work: Using four photons that pass through
a sequence of quantum logic gates, the optical circuit helped find the prime
factors of the number 15. While the researchers showed that it was possible
to solve for the factors, the chip itself didn't just spit out 5 and 3.
Instead, it came up with an answer to the "order-finding routine," the
"computationally hard" part of Shor's algorithm that requires a quantum
calculation to solve the problem in a reasonable amount of time, according
to Jeremy O'Brien, a professor of physics and electrical engineering at the
University of Bristol. The researchers then finished the computation using
an ordinary computer to finally come up with the correct factors.

Of course, says O'Brien, "a smart schoolkid could tell you [the answer] in a
few seconds." To be really useful, he says, "what we'd need is a quantum
computer that has millions of qubits, to solve problems that are genuinely
hard to solve otherwise."

That quantum factoring machine is decades away, but in the meantime
chip-scale optical architectures like those of the Bristol team could help
in applications like quantum key distribution, which guarantees secure
communication based on the laws of quantum mechanics rather than on the
mathematical difficulty of factoring. Or they could be used to simulate
quantum systems in physics experiments, which might require just hundreds of
qubits instead of thousands or millions.

"We know 3 times 5 is 15," says University of Maryland quantum computing
expert Christopher Monroe, but this experiment "has promise for developing
something that could tell us the answer to something we don't know."

MIT's Lloyd is not convinced that the technology is scalable. The real
trick, he says, will be to develop a self-contained method that measures the
photons, reads the results, and finds the factors of huge numbers without
dipping back into classical computation or knowing the answer ahead of time.
That's the "tough technological problem that no one has any idea how to
solve," Lloyd says, although he believes it's "not against the laws of

O'Brien, however, says that only the hard part needs to be done on a quantum
computer, which will likely be a highly sophisticated device and in much
demand. "You wouldn't waste its time with classical computations," O'Brien
says. "If the other bits are easy, why do them on a quantum computer?"

The Bristol group next aims to build larger, more sophisticated quantum
optical circuits, with more waveguides packed on the chip, in addition to
more-efficient single-photon generators and detectors. That will push them
toward a scaled-up system that might, decades hence, break math-based
encryption codes using millions of qubits.

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