Dance of the Photons : From Einstein to Quantum Teleportation (9781429963794) by Zeilinger Anton

Author:Zeilinger, Anton
Language: eng
Format: epub
Publisher: Macmillan
Published: 2011-08-05T00:00:00+00:00

LOOPHOLES

After a few moments of thought, Professor Quantinger continues. “Actually, there were three loopholes in the early experiments. You learned a little about two of them already. One is the communication loophole. This is the question of whether the two measurement stations are somehow able to communicate with each other and thus make sure that the measurement results correspond to the quantum mechanical predictions. This loophole is excluded by the Innsbruck experiment.

“There is another very important loophole. It has already been pointed out by John Bell that the individual choice of which measurement is performed at both measurement stations must be completely free. This means that it should not be determined by any earlier event. Clearly, such a possibility cannot in principle be excluded definitely, because there might be unknown information that influences both measurement choices. But it is possible to exclude some rather reasonable assumptions about such possible influences.

One such assumption would be to say that the hidden information influencing the settings of the random number generators was created together with the photon pairs at the moment they were emitted by the source. Such an explanation would in principle be possible even for the Innsbruck experiment, because it took the photons a while to travel in the glass fibers from the source to the respective measurement stations. Such an explanation was excluded in a recent experiment done by Thomas Scheidl and other members of the Vienna group. What they did was decide which parameter to measure using a random number generator such that it was located a distance away, and create a random number at the same instant when the photon pair was created in the source. That way no signal starting from the source could have influenced the random number generator. The output of each random number generator was then sent to its measurement station and in this way the specific measurement of the photon was set. So in that experiment, two loopholes, the communication loophole and the so-called freedom of choice loophole, were both closed at the same time.

“But there are still two possible improvements for these kinds of experiments. One is to use human experimentalists who decide at the last instant which polarization of the photon will be measured. Doing such an experiment, we assume that humans have free will. You should know that there is a broad discussion presently among psychologists and people doing brain research whether we really have free will or not. But in any case, in such an experiment we would have to have two experimentalists separated by a very large distance. This is because from neurophysiology, we know that it takes at least one-tenth of a second to make a decision. A tenth of a second corresponds to the time it takes to travel a distance of 30,000 kilometers at the speed of light. So, such an experiment would be most conveniently done using one measurement station on Earth and the other one on the Moon with the source on a satellite in between.

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