Not so good news for Quantum Computers

[ruveyn ben yosef 0]

Please see http://physicsworld.com/cws/article/news/36735

Here is the excerpt:

Nov 20, 2008

Is entanglement always good for quantum computers?

The entanglement of quantum bits (or qubits) is what should allow quantum computers to perform certain calculations much faster than the computers we use today. But now, physicists in Germany and Canada are saying that most qubits could be “too entangled” to be of any use in quantum computers.

At the heart of any quantum computer qubits that are entangled — which means that they that have a relationship that is much stronger than that allowed for in classical mechanics. In qubits that are photons, for example, “1” and “0” could be represented by two different polarizations states. If two photons are entangled a measurement of the polarization of one of the photons would reveal the polarization of the other — no matter how far apart the photons are.*

It is this phenomenon that can be used to perform certain calculations much faster than conventional computers.

Is more entanglement better?

The conventional wisdom is the greater the entanglement the better — but an important question facing anyone trying to build a quantum computer is whether any entangled state could be used to perform quantum calculations.

For most states, no such trick exists David Gross, University of Braunschweig

If it doesn’t matter, one could choose the entangled state that is technologically easiest to work with to create a computer. But, if only a few suitable states exist, the challenge becomes how realize these specific states in a given physical system.

Any useful quantum computation must ultimately involve measuring the values of the quantum states. However, if one measures the state of an individual object, the statistical nature of quantum mechanics means that the result will be random — and completely useless for doing quantum computations.

Correlations are key?

All is not lost however, because the outcomes of measurements on several entangled objects are correlated — and it is these correlations that could be used to do calculation. Physicists have proposed specific schemes for measurement-based quantum computation (MBQC) in which such correlations could be used to perform calculations. However, it is not clear whether there exists a universal approach that would work with any system in an entangled state.

Now, David Gross of the University of Braunschweig and colleagues at the University of Potsdam and the Perimeter Institute have show that “for most states, no such trick exists”. The team came to this conclusion by studying a general system of highly-entangled qubits used to perform a certain mathematical calculation. They were able to prove that the number of states that could actually be used to perform the calculation was incredibly small – meaning that entanglement would not speed up the calculation at all (arXiv:0810.4331).

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From the paper itself it is shown that the probability of achieving a useful state configuration for an N q-bit machine is of the order of exp (-N^2). In short there are two chances of getting any computation leverage from an N q-bit machine -- slim and none.

The arxiv paper itself is at http://arxiv.org/abs/0810.4331

As the late Robert A. Heinlein famously said:

TANSTAAFL (There ain't no such thing as a free lunch).

ruveyn

*Entanglement is for real. All of the double-delayed correlation experiments following those done by Aspect and Clauser indicate that Bell's Inequalities are violated and the amount of correlation of entangled states is correctly predicted by quantum theory. The experiments have been getting tighter and tighter and the results very strongly imply that reality is not local (for local reality the Bell Inequalities would hold, as Bell demonstrated).

Dr. Avner Shimony, the man who got the physicists to test out the EPR proposal for real showed me when we had lunch together back in 2001, that there is no way of getting anything useful out of entangled states: no faster than light Morse Code Lamps or any other way of exploiting entanglement. He was prescient as the paper referred to above indicates.

[Betsy Speicher 0]

*Entanglement is for real. All of the double-delayed correlation experiments following those done by Aspect and Clauser indicate that Bell's Inequalities are violated and the amount of correlation of entangled states is correctly predicted by quantum theory. The experiments have been getting tighter and tighter and the results very strongly imply that reality is not local (for local reality the Bell Inequalities would hold, as Bell demonstrated).

I'm no physicist, but my late husband was and showed why non-locality = non-causality. That means, non-locality can't be true no matter what arguments or assumptions are made about it.

[ruveyn ben yosef 0]

*Entanglement is for real. All of the double-delayed correlation experiments following those done by Aspect and Clauser indicate that Bell's Inequalities are violated and the amount of correlation of entangled states is correctly predicted by quantum theory. The experiments have been getting tighter and tighter and the results very strongly imply that reality is not local (for local reality the Bell Inequalities would hold, as Bell demonstrated).

I'm no physicist, but my late husband was and showed why non-locality = non-causality. That means, non-locality can't be true no matter what arguments or assumptions are made about it.

Be that as it may, no one has been able (as yet) to fault J.S.Bell's derivation of the inequalities based on locality. Furthermore no one (as yet) has been able to fault the experiments that show Bell's Inequalities do not hold. Quantum Theory predicts the correlations between spins and polarizations in the double delayed experiments. This is the situation so far. Hidden variables (so-called) do not account for the correlations.

In any case, it looks like quantum computers are likely to be a bust.

ruveyn