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

Peter Bowditch peter at ratbags.com
Thu Sep 10 08:00:36 AEST 2009

Wow! Synchronicity or coincidence? I just happen to be rereading Simon 
Singh's "The Code Book" from 1999 and I'm up to the chapter about the 
possibility of using quantum computing for cryptanalysis.

> Quantum Chip Helps Crack Code
> Experimental chip does part of code-cracking quantum algorithm
> BY ANNE-MARIE CORLEY // IEEE Spectrum, September 2009
> [original
> http://www.spectrum.ieee.org/computing/hardware/chip-does-part-of-codecra
> cking-quantum-algorithm/0with
> 2 photographs]
> 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
> physics."
> 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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Peter Bowditch
The Millenium Project - http://www.ratbags.com/rsoles
Australian Council Against Health Fraud - http://www.acahf.org.au

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