Re: Quantum Computers breaking ciphers

From: Bill Unruh (unruh_at_string.physics.ubc.ca)
Date: 10/09/04 

Date: 9 Oct 2004 06:14:48 GMT

"Roger Schlafly" <rogersc1@mindspring.com> writes:

]"Guy Macon" <http://www.guymacon.com> wrote:
]> From http://www.research.ibm.com/resources/news/20011219_quantum.shtml
]> "The simplest meaningful instance of Shor's Algorithm is finding
]> the factors of the number 15, which requires a seven-qubit quantum
]> computer. IBM chemists designed and made a new molecule that has
]> seven nuclear spins -- the nuclei of five fluorine and two carbon
]> atoms -- which can interact with each other as qubits, be programmed
]> by radio frequency pulses and be detected by nuclear magnetic resonance
]> (NMR) instruments similar to those commonly used in hospitals and
]> chemistry labs.
]> "The IBM scientists controlled a vial of a billion-billion (1018)
]> of these molecules so they executed Shor's algorithm and correctly
]> identified 3 and 5 as the factors of 15. "Although the answer may
]> appear to be trivial, the unprecedented control required over the
]> seven spins during the calculation made this the most complex
]> quantum computation performed to date," Amer said.

]Yes, that's what the IBM PR said, but they did not make a
]7-qubit computer. They did some spin experiments on a
]single molecule. Or rather measured some averages from a
]bunch of those molecules. That's all. It was a big publicity
]stunt. It is not a step towards making some actual quantum
]computer.

The problem with NMR is, as all of the practitioners acknowledge, that the
signal to noise ratio falls exponentially with the number of spins. Ie,
more than about 10 bits, and you cannot see the signal. Plus, the level
spacings get so close together (the number of levels grows exponentially)
that it becomes exceedingly difficult to control them. The exponential fall
in SN is inherent in liquid state NMR. The liquid state is needed to
provide the motional narrowing needed to give the long lifetimes required
for the QM. Ie the technique does not scale.

Techniques which might scale ( condensed matter systems) have managed to
get one or maybe two qubits. Techniques with are closer to realisation but
where scaling is difficult (but not impossible like NMR) like
atomic/optical ones have maybe two or three qubits.