Wikipedia on
Elitzur-Vaidman bomb-tester:
Start with a Mach-Zehnder interferometer and a light source which emits single photons. When a photon emitted by the light source reaches a half-silvered plane mirror, it has equal chances of passing through or reflecting. On one path, place a bomb for the photon to encounter. If the bomb is working, then the photon is absorbed and triggers the bomb.
On
Mach-Zehnder interferometer:
The Mach-Zehnder interferometer is a device used to determine the phase shift caused by a small sample which is placed in the path of one of two collimated beams (thus having plane wavefronts) from a coherent light source.
On
collimated light:
Collimated light is light whose rays are nearly parallel, and therefore will spread slowly as it propagates. The word is derived from "collinear" and implies light that does not disperse with distance. ... Collimated light is sometimes said to be focused at infinity. Thus as the distance from a point source increases, the spherical wavefronts become flatter and closer to plane waves, which are perfectly collimated.
On
coherence length
In physics, coherence length is the propagation distance from a coherent source to a point where an electromagnetic wave maintains a specified degree of coherence. The significance is that interference will be strong within a coherence length of the source, but not beyond it. ... Helium-neon lasers have a typical coherence length of 20 cm, while semiconductor lasers reach some 100 m. Fiber lasers can have coherence lengths exceeding 100 km.
In addition to an arbitrary use of statistics, the central fallacy of such experiments lies in the fact that photons are 'social particles', which tend to come into existence and travel together in the same state. Wikipedia on
stimulated emission:
In optics, stimulated emission is the process by which, when perturbed by a photon, matter may lose energy resulting in the creation of another photon. The perturbing photon is not destroyed in the process (cf. absorption), and the second photon is created with the same phase, frequency, polarization, and direction of travel as the original.
So already the central premise of this QM thought experiment, namely that
one single photon functions as two collimated beams of coherent light, is an
impossibility. Coherence is a property only of groups of photons and not of single photons. A single photon cannot have different phases, frequencies, polarizations, and directions of travel. (Yes, I know, Heisenberg's authority ...).
On the one hand, it is very astonishing how little research has been done on coherence of light. On the other hand, this is understandable, because the acknowledgement that photons normally appear as coherent groups
undermines even on the theoretic side the
beloved strangeness of the world of quanta. Interference effects which are now assumed to result from interference of photons with themselves can then easily be explained by interference between photons belonging to a same coherence group, but having taken different paths before reuniting again.
So the meaning of this interesting quote (
Wikipedia on Bell test experiments)
Nevertheless, despite all these deficiencies of the actual experiments, one striking fact emerges: the results are, to a very good approximation, what quantum mechanics predicts. If imperfect experiments give us such excellent overlap with quantum predictions, most working quantum physicists would agree with John Bell in expecting that, when a perfect Bell test is done, the Bell inequalities will still be violated.
actually is this (
see also):
We must admit that strictly speaking, these experiments are imperfect (i.e. not good enough in order to decide the question). However, we as working quantum physicists devoutly believe that also perfect experiments would confirm that Bohr is right and Einstein wrong.
Cheers, Wolfgang
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