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Jack Woehr

Dr. Dobb's Bloggers

Quantum Computing -- Standing on the Shoulders of Giants

July 19, 2009

Schroedinger's Cat by http://commons.wikimedia.org/wiki/User:DhatfieldOver the years I have slowly imbibed a tiny sip of quantum mechanics in pursuit of quantum computing. It's cumulative and it's starting to make sense. I'm now going back and reading early classic papers on the subject by the originators of QM.

At a certain point in my (recently intense) studies (I'm on a roll) I had that "D'oh!" moment. I had read over and over again at this and that QM web page about the thought experiment "Schroedinger's Cat" without - d'oh - searching the web for the original paper. Turns out it's right here: Schroedinger: "The Present Situation in Quantum Mechanics" (1935). And wouldn't you know it? The original author is ever so much clearer than his disciples and interpreters.

Schroedingers' analogy is offered as an expression of the incredulity one might easily feel at that which his and his peers' research was compelling science to accept. He wrote:

One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.

Point on superposition, so useful a characteristic of the qubit, taken.

Another cogent urdokument of QM whose literary spriteliness brightens the obscurity of its subject matter is the famous "EPR" paper, Einstein, A., Podolsky, B., and Rosen, N., 1935: "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?", again, readily available on that most entangled of networks, the worldwide web. "EPR" is wherein Einstein and his collaborators exhibit one last shudder of revolt before surrendering to the applicability of a probablistic approach to the subatomic world. They wrote:

[S]ince either one or the other, but not both simultaneously, of the quantities P and Q can be predicted, they are not simultaneously real. This makes the reality of P and Q dependent on the process of measurement carried out on the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this.

We will continue to study that most unreasonable yet practical definition of reality in the shadow of the prometheuses of yore and with the assistance of the experts of today (thanks, readers!).

More to follow.

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