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"Asking photons where they've been" -- the DFBV experiment

  1. Aug 12, 2014 #1


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    Here's an experiment using weak measurements on photons in a "nested interferometer." They conclude:

    In conclusion, we have performed direct measurements which shed new light on the question: Where were the photons passing through an interferometer? The main results are presented in Fig. 2B. The photons themselves tell us where they have been. And the story they tell is surprising. The photons do not always follow continuous trajectories. Some of them have been inside the nested interferometer (otherwise they could not have known the frequencies fA, fB), but they never entered and never left the nested interferometer, since otherwise they could not avoid the imprints of frequencies fE and fF of mirrors E and F leading photons into and out of the interferometer. Only the description with both forward and backward evolving quantum states provides a simple and intuitive picture of pre- and postselected quantum particles.


    Here's the abstract:

    Quantum mechanics does not provide a clear answer to the question: What was the past of a photon which went through an interferometer? Various welcher weg measurements, delayed-choice which-path experiments and weak-measurements of photons in interferometers presented the past of a photon as a trajectory or a set of trajectories. We have carried out experimental weak measurements of the paths of photons going through a nested Mach-Zehnder interferometer which show a different picture: the past of a photon is not a set of continuous trajectories. The photons tell us that they have been in the parts of the interferometer which they could not have possibly reached! Our results lead to rejection of a "common sense" approach to the past of a quantum particle. On the other hand, they have a simple explanation within the framework of the two-state vector formalism of quantum theory.
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  3. Aug 12, 2014 #2


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  4. Aug 13, 2014 #3


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    We had this discussion recently in another thread. I'm a bit surprised by this abstract, because people investigating photons should not talk about them in such a misguiding way as if they were comparable to the behavior of classical particles. Particularly the idea of a "path" (or even "trajectory") of a photon is to be taken with a grain of salt (at least). As physicists we should stick to observable facts, and these are interference patterns or their vanishing and the postselection of subensembles making use of entangled photon pairs. Nothing whatsoever is changed retrocausally. The interference pattern or its vanishing is an established fact by storing it from the measurement.

    In the other thread someone pointed to a nice paper about precisely this point:

    Bram Gaasbeek, Demystifying the Delayed Choice Experiments
  5. Aug 13, 2014 #4


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    We certainly think of photons as possessing continuous trajectories through spacetime when we set up experiments or use a TV remote, we "aim" our EM sources towards mirrors and TV's and remove obstacles in their "path." So, why not highlight the fact that experiments can violate the way we think about the exchange of EM energy? Why would we want to squelch intellectual inquiry by requiring physicists to "stick to observable facts?"

    P.S. This experiment does not use "entangled photon pairs." If done one photon at a time, the results should also obtain.
  6. Aug 14, 2014 #5


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    There is some commentary on the formalism of retrodictive quantum states here:

    Quantum instruments as a foundation for both states and observables
    Justin Dressel, Andrew N. Jordan
  7. Aug 16, 2014 #6
    I would have thought that photons, as they travel at the speed of photons (light) have no concept of time, time does not pass for a photon, as such asking it were it was in the 'past' is pointless.
    A photon has never 'been' anywhere, for a photon, it exists and ceases to exist at the same 'instant'.

    (I could be, and am, often completely wrong about these things):)
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