[LINK] Quantum computing

Stephen Wilson swilson at lockstep.com.au
Wed Jan 13 10:48:47 AEDT 2010

It's pretty cool to think that my grandkids might have QCs on their 
desks, as the Apple IIs of the 2040s.

But I am pretty that Shor's algorithm has *not* "been successfully used 
to crack encryption schemes". So far, Shor's algorithm in a working QC 
has factored 15 into 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).


Stephen Wilson

stephen at melbpc.org.au wrote:
> Quantum computers do chemistry 
> 11th January 2010 by Colin Barras 
> www.newscientist.com/article/dn18365-quantum-computers-do-chemistry.html
> 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."
> --
> Cheers
> Stephen
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