[FoRK] Robust quantum computing
Contempt for Meatheads
jbone at place.org
Tue Apr 13 07:09:47 PDT 2004
Sturdy quantum computing demoed
April 7/14, 2004 By Eric Smalley, Technology Research News
The quantum states of atoms and subatomic particles that prototype
quantum computers use to represent the 1s and 0s of computer
information are so fragile that the energy from heat, light and
magnetism ordinarily found in their environments is usually enough to
change them, effectively stuffing out the information they hold.
Rather than fight the odds, many researchers are working with the
environmental noise to create safe havens for quantum bits, or qubits.
Particles like atoms, electrons and photons can be used as qubits
because they can be oriented in one of two directions -- spin up and
spin down. Qubits can also be encoded in the interactions of pairs of
particles. The key to making protected qubits is to encode logical
qubits in multiple physical qubits.
These approaches are central to efforts aimed at making viable quantum
computers, said Jason Ollerenshaw, a researcher at the University of
Toronto in Canada. "Techniques for resisting environmental noise will
be essential in building quantum computers on a practical scale," he
said. Quantum computers hold the promise of solving certain types of
problems like cracking secret codes that are far beyond the reach of
Ollerenshaw and his colleagues at the University of Toronto have built
a prototype quantum computer that can execute a quantum search
algorithm despite environmental noise. "We have experimentally
demonstrated that a quantum computer can be protected from decoherence
-- the detrimental effects of environmental noise -- during the course
of a complete quantum computation," said Ollerenshaw.
The prototype has just four qubits. Practical quantum computers will
require thousands or millions of qubits. "The specific technique we
have demonstrated here may be important for the construction of larger
computers," said Ollerenshaw.
According to the laws of quantum physics, particles can also be viewed
as waves, and interacting pairs of particles have a common waveform. By
tuning the way a pair of particles interact with noise in the
environment, researchers can create waveforms whose shapes are
symmetrical. These symmetrical portions of the waves are unaffected by
These decoherence-free subspaces can be used to make sturdy qubits.
"Think of a chessboard in the middle of a game. Some spaces on the
board are dangerous. If you move a piece there, it will be open to
attack by one of your opponent's pieces. But some spaces are safe; none
of your opponent's pieces can attack there," said Ollerenshaw. "Our
computer protects its information by keeping it in the safe spaces," he
The researchers used decoherence-free subspaces to implement Grover's
quantum search algorithm, which finds items in lists using far fewer
than steps than it would take to check item by item. The researchers
found that the algorithm worked under different levels of noise, and
the algorithm failed in the presence of relatively low levels of noise
when it was run without decoherence-free subspaces.
The researchers also tested the system using the Deutsch-Jozsa
algorithm, a simpler quantum algorithm that performs the equivalent of
looking at both sides of a coin at once.
The researchers' computer used the nuclear spins of two carbon atoms, a
nitrogen atom, and a hydrogen atom that were all part of a glycine
molecule. Millions of copies of the molecule in a test tube were
subjected to magnetic pulses as the computer's input and the
researchers measured the radio waves emitted by the atomic nuclei to
read the computer's output. This is the same nuclear magnetic resonance
method used in medical magnetic resonance imaging systems. The quantum
search algorithm was implemented by a series of carefully timed
Nuclear magnetic resonance is widely considered a dead-end in terms of
building practical quantum computers because the system's
signal-to-noise ratio is unacceptable for more than about 10 qubits.
Nuclear magnetic resonance quantum computers are popular with
researchers as testbeds, however. "NMR is an excellent technique for
testing new ideas in quantum computing because NMR theory and
instrumentation are so well-developed and nuclear spin systems are so
well-behaved," said Ollerenshaw.
A similar experiment carried out at about the same time by another team
of researchers at the University of Toronto demonstrated the
Deutsch-Jozsa algorithm in decoherence-free subspaces implemented in a
prototype optical quantum computer that formed two logical qubits from
four physical qubits.
Decoherence-free subspaces involve a trade-off. They provide sturdy
qubits, but require at least twice as many particles.
The NMR researchers' experiment used artificially induced noise. The
next step is to use the same technique to resist naturally occurring
decoherence, said Ollerenshaw.
It will be a long time before practical applications of quantum
computing are possible, said Ollerenshaw. "Longer than 20 years," he
Ollerenshaw's research colleagues were Daniel Lidar and Lewis Kay. The
work appeared in the November 21, 2003 issue of Physical Review
Letters. The research was funded by The Natural Sciences and
Engineering Research Council of Canada (NSERC) and the Defense Advanced
Research Projects Agency (DARPA).
Timeline: > 20 years
TRN Categories: Quantum Computing and Communications; Physics
Story Type: News
Related Elements: Technical paper, "Magnetic Resonance Realization of
Decoherence-Free Quantum Computation," Physical Review Letters,
November 21, 2003
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