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From: "Robert J. Bradbury"

*> > New Light-Based Computer Runs At Quantum Speeds:
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*> > http://www.sciencedaily.com/releases/2001/05/010515075526.htm
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*> Does anyone know if this is any different from the work of
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*> Bucksbaum?
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Yes, because they wrote:

" The caveat is that one should be careful of comparing

an inefficient classical algorithm with an efficient quantum one.

The experimental implementations of the Grover search to date

have not in fact implemented a search of an unsorted database,

but rather a database with one single marked item.

Therefore experimental evidence for the claimed speedup has

yet not been achieved, since a sorted database can be

searched classically in log 2 N steps, which is

identical to the readout limit for a quantum computer.

The superiority of the quantum search algorithm becomes

apparent only when one examines carefully the notion of a

query. It is evident that there is no information gain in the sort

of oracle query described above, which involves only unitary

transformations. It is in the encoding of such oracles, which

must be done by a quantum computer, that the real speedup

occurs. Once a properly encoded oracle is available, the number

of steps required to perform the information processing is

then limited by the final register readout to O(log 2 N).This

is exactly the same as for a classical information processor,

which shows that it is the realization of actual information

by the readout, as opposed to predictive information that is

contained in the quantum state, that is the ultimate limiting

procedure in quantum information processing.

We have shown here that if the readout of the register is

performed by particle counting then there exists a one-to-one

correspondence of single-particle quantum interference and

classical interference. Therefore we conclude that any enhancements

in processing power that can be ascribed to quantum

interference can also be found in classical wave processors,

and this includes systems based on modal entanglement.

Multi-particle entanglement, on the other hand, may provide

enhancements that cannot be efficiently transcribed to classical

interferometers, even when particle counting is used to

realize the output information. "

And then: ". As a consequence, computers

based on classical waves are as efficient as those

based on single quantum particles."

S. Wallentowitz, I.A. Walmsley, J.H. Eberly

http://arxiv.org/abs/quant-ph/0009069

"How big is a quantum computer?"

Accounting for resources is the central issue in computational

efficiency. We point out physical constraints implicit in

information readout that have been overlooked in classical

computing. The basic particle-counting mode of read-out sets a

lower bound on the resources needed to implement a quantum

computer. As a consequence, computers based on classical waves

are as efficient as those based on single quantum particles

Konrad Banaszek, Alfred B. U'Ren, Ian A. Walmsley

http://arxiv.org/abs/quant-ph/0103026

"Generation of correlated photons in controlled spatial modes

by down-conversion in nonlinear waveguides"

We report the observation of correlated photon pairs generated

by spontaneous parametric down-conversion in a quasi-phase

matched KTiOPO4 nonlinear waveguide. The highest ratio of

coincidence to single photon count rates observed in the 830 nm

wavelength region exceeds 18%. This makes nonlinear waveguides

a promising source of correlated photons for metrology and

quantum information processing applications. We also discuss

possibilities of controlling the spatial characteristics of the

down-converted photons produced in multimode waveguide structures.

**Next message:**Natasha Vita-More: "Re: Extropic interplanetary Art"**Previous message:**John Marlow: "Re: PHYSICS/SPACE: Legitimate time traval proposal"**In reply to:**Robert J. Bradbury: "Re: New Light-Based Computer Runs At Quantum Speeds"**Next in thread:**John Clark: "Re: New Light-Based Computer Runs At Quantum Speeds"**Reply:**John Clark: "Re: New Light-Based Computer Runs At Quantum Speeds"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

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