[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 
Brisbane, Australia.

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.

Repeated calculation

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.

Astounding accuracy

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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