[LINK] Quantum computing

[Richard Chirgwin]

Easy. Do the sum in base 13 ...

RC

Stephen Wilson wrote:
> 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).
>
> Cheers,
>
> Stephen Wilson
> Lockstep
> www.lockstep.com.au
>
>
> 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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