*©Fernando Caracena* 2015

## Background Ideas

Previous posts have described the weird nature of quantum mechanics based on its theory. There are many experiments that verify various details of the quantum theory framework. Now, experimenters are trying to test the fundamental notions behind quantum theory. For a background on some of the issues being experimentally investigated see some of the previous posts from this site. One such post describes "States in Quantum Mechanics". Another, "Further Mathematical treatment of Quantum States", fills in some of the theory.

# The Wave-Particle Daulity Experiment

Recently, researchers released a statement that they did a "Quantum experiment [that] verifies Einstein's 'spooky action at a distance' ".

"According to quantum mechanics, a single particle can be described by a wave function that spreads over arbitrarily large distances, but is never detected in two or more places."

A statement by the experimental team described the problem as follows:

“Einstein’s view was that the detection of the particle only ever at one point could be much better explained by the hypothesis that the particle

isonly ever at one point, without invoking the instantaneous collapse of the wave function to nothing at all other points."“However, rather than simply detecting the presence or absence of the particle, we used homodyne measurements enabling one party to make different measurements and the other, using quantum tomography, to test the effect of those choices.”

Further, the experimentalists said,

“Through these different measurements, you see the wave function collapse in different ways, thus proving its existence and showing that Einstein was wrong.”

Basically, the experiment showed that: 1) a photon cannot be detected at two places at the same time; and 2) while it is in free flight, its motion is described by an extended wave function. These conditions are two fundamental postulates of quantum mechanics, which generated a lot of heated discussion in the early days of quantum theory. Eventually, controversy subsided, and practitioners just "shut up" and calculated, because the theory gave such accurate predictions. Perhaps that is why no one thought of trying to verify the basic postulates of quantum mechanics. Another reason may be that technology was not advanced enough to do that then.