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Quantum computers do chemistry

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 computer. 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 digit.

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 White.

"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."

Journal reference: Nature Chemistry, DOI: 10.1038/nchem.483


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Comments 1 | 2

Model Dna

Mon Jan 11 17:11:04 GMT 2010 by Kayvan Walker

imagine if one could model DNA using quantum computing! i had this idea as an undergraduate eight years ago - put the sequence of the genome into a computer model and watch as life begins to evolve.... obviously one would have to supply all the supporting molecules too, but such a model could show how life works at the atomic level and allow us to see the secrets of life within all organisms.... thousands of scientists would be out of a job, of course..... ;)

Model Dna

Mon Jan 11 19:09:46 GMT 2010 by yetihehe

"thousands of scientists would be out of a job, of course..... ;)" Are you joking? Thousands scientists would be very, very busy with all the possibilities this would open.

Model Dna

Tue Jan 12 00:14:58 GMT 2010 by ziphead

"Thousands scientists would be very, very busy with all the possibilities this would open"

Thousands of other quantum machines, you mean?

Once you go quantum, neurons look a mountains of hopeless blob.

Model Dna

Sat Jan 23 14:21:04 GMT 2010 by Mr B

"Once you go quantum, neurons look a mountains of hopeless blob."

I for one welcome our Quantum overlords.

This comment breached our terms of use and has been removed.

This comment breached our terms of use and has been removed.

Model Dna

Tue Jan 12 03:56:07 GMT 2010 by Vin

Yes, seems like it could be very useful (one day) in considering possible scenarios for abiogenesis and evolution on earth and even for other planets and environments

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Model Dna

Sun Jan 24 00:28:03 GMT 2010 by CQuences

Although the fullness of this technology will not be realized for perhaps a generation, the implications from experiments such as this are staggering! Consider, as you hypothesize in the use of these computers for DNA building, using quantum computers to analyze the chemical makeup of distant worlds to see if they are capable of supporting life, and using the percentages of chemicals on that world to determine what forms of life it may have developed. Of course, for such studies of other worlds to be accurate, they would have to model the numerous ecosystems of our own world for comparison. In closing, these computers will push the boundaries of science into frontiers once thought impossible to reach. True artificial intelligences are on their way...

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Computing The Meaning Of Life

Tue Jan 12 23:57:20 GMT 2010 by Stephen Wilson

I think Mr Barras overstates a little the progress made to date in quantum computing. I am pretty sure that Shor's algorithm so far has not in fact "been successfully used to crack encryption schemes".

Shor's algorithm actually calculates factors of a composite number. The traditional inefficiency of factorisation underpins much moden cryptography. A radical shortcut provided by QC would render most of today's cyber security redundant.

The first demonstration of Shor's algorithm in a working QC calculated that 15 is the product of 3 times 5.

We all look forward to further progress. I can't wait to see if a QC can factor 42 (into 6 times 9 of course) thus revealing the meaning of life.

Computing The Meaning Of Life

Wed Jan 13 19:34:04 GMT 2010 by AD

"I can't wait to see if a QC can factor 42 (into 6 times 9 of course) "

i think a simple calculator would do for you, not just a quantum computer !

Computing The Meaning Of Life

Wed Jan 13 23:15:31 GMT 2010 by dave

Not a simple calculator as most work in base ten and would tell you that 6*9=54 what we need is a quantum computer or deep thought

Comments 1 | 2

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