[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  
ordinary computers.

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  
magnetic pulses.

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
Funding:   Government
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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