[LINK] Quantum computing
stephen at melbpc.org.au
stephen at melbpc.org.au
Tue Jan 12 10:21:28 AEDT 2010
Quantum computers do chemistry
11th January 2010 by Colin Barras
A team of quantum physicists has taken the first steps towards using a
quantum computer to predict how a chemical reaction will take place.
Even the most powerful classical computers struggle when trying to
calculate how molecules will interact in a chemical reaction. That's
partly because the complexity of such systems doubles with the addition
of every atom, as each atom is entangled with all the others.
Such escalating complexity is far easier for a quantum computer to deal
with, because quantum computers exhibit similar properties: adding just
one extra quantum bit or "qubit" doubles computational power.
"There is a natural match between quantum computers and modelling
chemistry," says Andrew White at the University of Queensland in
In 2005 Alán Aspuru-Guzik at Harvard University and his team proposed an
algorithm to carry out quantum chemistry calculations on a quantum
Now White, Aspuru-Guzik and colleagues have implemented the algorithm on
state-of-the-art two-qubit photonic quantum computing hardware.
Their "iterative phase estimation algorithm" is a variation on existing
quantum algorithms such as Shor's algorithm, which has been successfully
used to crack encryption schemes. It is run several times in succession,
with the output from each run forming the input to the next.
"You send two things into the algorithm: a single control qubit and a
register of qubits pre-encoded with some digital information related to
the chemical system you're looking at," says White.
"The control qubit entangles all the qubits in the register so that the
output value a 0 or 1 gives you information about the energy of the
chemical system." Each further run through the algorithm adds an extra
The data passes through the algorithm 20 times to give a very precise
energy value. "It's like going to the 20th decimal place," White says.
Errors in the system can mean that occasionally a 0 will be confused with
a 1, so to check the result the 20-step process is repeated 30 times.
The team used this process to calculate the energy of a hydrogen molecule
as a function of its distance from adjacent molecules.
The results were astounding, says White.
The energy levels they computed agreed so precisely with model
predictions to within 6 parts in a million that when White first saw
the results he thought he was looking at theoretical calculations. "They
just looked so good."
Though cryptography is often cited as the most likely first application
for quantum computing, chemistry looks to be more promising area in the
short term, Aspuru-Guzik says.
A system with 128 qubits "would be able to outperform classical
computers" as a tool for chemistry, he says. Cryptography quantum
algorithms would require many thousands of qubits to be as useful, says
"The model of hydrogen we used was a simple first-year undergraduate
quantum model, where almost all the complexity has been removed," White
says. "But it turns out we can do more complicated models in principle.
It just comes down to using a system with many more qubits."
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