[LINK] Quantum computing
kim at holburn.net
Wed Jan 13 12:45:07 EST 2010
I always remember this theoretical proof that if you had a quantum
computer, you wouldn't even have to turn it on to get results.
I suppose eventually you wouldn't even need the Quantum computer
itself, just a Schrödinger half computer, a possible computer to get
On 2010/Jan/13, at 10:48 AM, 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
> 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
> factor 42 (into 6 times 9).
> Stephen Wilson
> stephen at melbpc.org.au wrote:
>> 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
>> of every atom, as each atom is entangled with all the others.
>> Such escalating complexity is far easier for a quantum computer to
>> with, because quantum computers exhibit similar properties: adding
>> 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
>> quantum algorithms such as Shor's algorithm, which has been
>> used to crack encryption schemes. It is run several times in
>> 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
>> output value – a 0 or 1 – gives you information about the energy of
>> chemical system." Each further run through the algorithm adds an
>> 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
>> 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
>> 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.
>> just looked so good."
>> Though cryptography is often cited as the most likely first
>> for quantum computing, chemistry looks to be more promising area in
>> 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,
>> "The model of hydrogen we used was a simple first-year undergraduate
>> quantum model, where almost all the complexity has been removed,"
>> says. "But it turns out we can do more complicated models in
>> It just comes down to using a system with many more qubits."
>> Link mailing list
>> Link at mailman.anu.edu.au
> Link mailing list
> Link at mailman.anu.edu.au
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