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Quantum Thought-Experiments with Light |
D. Branning
Rose-Hulman Institute of Technology
Many of the founders of quantum mechanics were troubled by what they had created. The theory had finally allowed them to accurately describe the elusive behavior of atomic and subatomic particles, but only at the expense of some basic "necessary" features of physical laws. To gain a better understanding of these strange new ideas, Einstein, Bohr, and their contemporaries often discussed "thought experiments," in which they imagined very delicate measurements on single electrons, atoms, or photons that might reveal some flaw in the elegant theory they had constructed. But could they have imagined that, one day, it would be possible to carry out such gedankenexperiments in the real world?
Today, because of the advent of the laser, and the subsequent advances in optics and electronics, it is possible to test nature against even the most bizarre predictions of quantum theory. A technique called "parametric downconversion," in which single photons are split into pairs of photons, has become particularly effective at demonstrating the beautiful and mysterious principles of quantum mechanics. I will discuss one experiment in which information about one of these photons can force the other to behave either as a wave or as a particle, and a second experiment related to the "Einstein-Podolsky-Rosen paradox," in which the photons do not have definite polarizations until they are measured. I will also show how these photons can be used to send encrypted messages whose complete security is guaranteed by the laws of quantum mechanics. |
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Thursday, March 3rd 2005, McCook 102, 3:00 pm, Refreshments 2:45 pm |