LECTURES AND TEACHING
Author: A. Zelinger, A.C. Elitzur
Schrödinger’s Cat: Quantum versus classical world?
Europäisches Forum Alpbach, Alpbach, Austria, August 15-21, 2003.
“The more success the quantum theory has,” Einstein once grumbled (notwithstanding that he was himself one of the theory’s founders), “the sillier it looks.” Nearly a century after quantum mechanics’ inception, its success keeps soaring, but its estrangement from ordinary commons sense increases too. The seminar “Schrödinger’s Cat: Quantum versus Classical World” by Anton Zeilinger (University of Vienna) and Avshalom Elitzur (Bar-Ilan University) addressed this twofold nature of quantum theory. We went into great length in order to present QM in a non-technical manner, while giving an accurate picture of its most intriguing theoretical and technological aspects.
A workshop on quantum mechanics, Alpbach 2003.
Clockwise: E. Schrödinger (dead), A. Elitzur (alive), A. Zeilinger (alive), and Schrödinger’s cat (superposed).
We began with recounting the development of quantum mechanics from classical physics, following a few growing difficulties in the otherwise-beautiful old physics that have made the quantum revolution inevitable. The discoveries of Planck, Einstein, Bohr, Heisenberg and Schrödinger were introduced in a historically minded manner, alongside with the human stories and anecdotes that accompanied this Odyssey.
This introduction served us as a background for presenting the theory’s two so different characteristics. On the one hand, it allows very strange interactions that are entirely impossible according to classical physics – thereby expanding mankind’s technological capabilities to an unimaginable degree – but on the other hand its effects are so bizarre that they seem to defy the very foundations of rational thinking.
We began with the simple yet odd double-slit experiment, where light’s nature enforces the (in)famous “wave-particle duality,” which we later extended to the behavior of all known particles. Then we presented the Mach-Zehnder interferometer, another relatively simple device that allows even more precise interference experiments. Here, Zeilinger described some single-neutron interference experiments performed in his laboratory. This effect, whereby a single neutron seems to simultaneously traverse both distant paths within the same device, provides a very vivid visual demonstration of QM’s counter-intuitive nature.
To emphasize this result we described the Elitzur-Vaidman bomb-testing device, proposed by Elitzur and Vaidman in 1991 and later improved and carried out by Zeilinger. Here, a particle seems to somehow “sense” the presence of an object on one path even though, by everyday logic, it must have traversed another path.
Next we described the EPR paradox. Two particles that have originated from the same source travel far away from one another. Upon being measured, each particle seems to “know” which measurement was randomly performed its twin. They therefore give results that indicate “non-locality,” an effect that clashes with the spirit (though not with the letter) of relativity theory’s prohibition on effects faster than light. Here, Elitzur added a few novel variants of this experiment that he recently devised with Shahar Dolev.
In the early 1990’s, Greenberger, Horne and Zeilinger devised a new version of the EPR experiment that makes quantum non-locality even more obvious – and more mind-boggling. This experiment was explained by Zeilinger with a simple simile of a tyrant, a fortune-teller and three conspirators. With the aid of QM, the tyrant, despite his cunningness, has been finally overthrown.
These non-local effects are the basis of the science-fiction technology called “teleportation,” whereby a particle is “beamed” from one location to another instantly. Here too, Zeilinger presented very advanced experiments performed in his lab by which particles were successfully transferred between two distant locations, apparently without traversing the way between them. Elitzur objected to the very term “teleportation,” arguing that, in some reference frames, the particle has been “received” in its destination even before it has been “sent” by the source, to which Zeilinger gleefully commented that it makes teleportation all the more fun.
And of course there is Schrödinger’s cat, the paradox that involves a quantum trigger of a macroscopic process. If QM is correct, then such processes should present macroscopic superposition, e.g., a dead-and-alive cat. Why does QM not show up at the macroscopic level? Here different hypotheses were mentioned and critically discussed. Zeilinger’s recent experiments shed new light on the possibility of pushing the quantum-classical limit upwards: He succeeded to demonstrate interference effects on larger and larger objects, managing to do that even with the large “Bucky Ball” (C60). At this point Elitzur cautioned the audience that, unless someone stops Zeilinger, he will eventually try to have an innocent human thrown at a wall with two distant windows.
We next discussed quantum computation, cryptography and superconductivity as some of the theory’s future applications. New variants of the above experiments and some intriguing hybrids of them were also described.
Because we differ in our philosophical world-views, we did not refrain from arguing in the classroom about the experimental results we presented. Occasionally the debate became heated, much to the students’ delight. We find this method, namely a joint-seminar given by two teachers with diametrically opposed philosophical views, a very fertile one (following the experience of our illustrious predecessors), and strongly recommend it to everyone who wishes to better understand quantum mechanics.