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The EPR gedanken experiment was invented by Einstein Podolsky and Rosen in 1935. It involved positions and momenta. In 1957, Bohm revised this gedanken experiment into one involving spins, or polarizations. In 1964, Bell analyzed this experiment and showed that it implied nonlocality (or defeated hidden variables) by a finite margin. Soon thereafter, the EPRBB version of the experiment was actually performed many times, with results supporting QM.

However, the original EPR experiment involving position and momentum remains an unperformed gedanken experiment, as far as I know. I can't even find a reference to a failed attempt to perform it. Why is this? Will we ever be able to perform the original EPR experiment?

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2 Answers

In the original EPR gedanken experiment, they assumed two particles that have perfect correlations in position, i.e., they are described by a delta function. That does not pose a problem for a thought experiment but cannot be performed in a lab because such a state cannot be normalized and is therefore unphysical.

However, in quantum optics, many experiments were done with so-called squeezed states that are an approximation of this. These states can be described by two quadrature operators with continuous spectra which in the limit of (again unphysical) infinite squeezing coincide with the original EPR state.

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+1, but this doesn't really answer the question completely. In the EPR experiment, the particles can't have perfect correlations in position, but theoretically they certainly can have large enough correlations in position to demonstrate the validity of the original thought experiment. I assume that performing this experiment with large enough precision in position and momentum would be difficult, but we need an experimental physicist to tell us why. –  Peter Shor Apr 17 '13 at 15:54
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I agree with answer 1. Position and momentum have an infinite number of states while a spin is the simplest quantum system with only two states. Also spin can be much more easily manipulated than position and momentum, using photons (and polarizers) or particles with spin (and Stern Gerlach filters).

EPRB is therefore an easier experiment to do and easier to treat theoretically.

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