Saturday, June 4, 2011

Double slit measurements

The hot quantum mechanics news of the week was this Nature mag story:
An international group of physicists has found a way of measuring both the position and the momentum of photons passing through the double-slit experiment, upending the idea that it is impossible to measure both properties in the lab at the same time. ... Steinberg stresses that his group's work does not challenge the uncertainty principle, pointing out that the results could, in principle, be predicted with standard quantum mechanics.
This is nonsense, of course. The experiment does not contradict the Uncertainty principle. It is an attempt at weak measurement, but it is questionable whether the concept even makes sense.

Here is another explanation from the guy who wrote a book about telling this stuff to his dog.

The UK BBC story also suggests that the experiment has done the impossible:
Researchers have bent one of the most basic rules of quantum mechanics, a counterintuitive branch of physics that deals with atomic-scale interactions.

Its "complementarity" rule asserts that it is impossible to observe light behaving as both a wave and a particle, though it is strictly both.

In an experiment reported in Science, researchers have now done exactly that.

They say the feat "pulls back the veil" on quantum reality in a way that was thought to be prohibited by theory. ...

While they were able to easily observe the interference pattern indicative of the wave nature of light, they were able also to see from which slits the photons had come, a sure sign of their particle nature.

The trajectories of the photons within the experiment - forbidden in a sense by the laws of physics - have been laid bare.
No, this experiment has not shown the particle nature of photons any more than previous experiments. Here is the conclusion from the actual research paper:
Single-particle trajectories measured in this fashion reproduce those predicted by the Bohm–de Broglie interpretation of quantum mechanics (8), although the reconstruction is in no way dependent on a choice of interpretation.

Controversy surrounding the role of measurement in quantum mechanics is as old as the quantum theory itself, and nowhere have the paradoxes been thrown into such stark relief as in the context of the double-slit experiment. Our experimentally observed trajectories provide an intuitive picture of the way in which a single particle interferes with itself. It is of course impossible to rigorously discuss the trajectory of an individual particle, but in a well-defined operational sense we gain information about the average momentum of the particle at each position within the interferometer, leading to a set of “average trajectories.” The exact interpretation of these observed trajectories will require continued investigation, but theseweak-measurement results can be grounded in experimental measurements that promise to elucidate a broad range of quan tum phenomena (7, 11–13, 15–17). By using the power of weak measurements, we are able to provide a new perspective on the double-slit experiment, which Feynman famously considered to have in it “the heart of quantum mechanics” (27).
This seems to give a boost to the De Broglie–Bohm theory. That theory has had a cult following for decades, but nothing useful has ever come out of as far as I know. The main advantage is that it is supposed to be more intuitive, but I think that it is much stranger than conventional quantum mechanics.

The Double-slit experiment is 200 years old, but continues to cause confusing today. An animated video explaining it is here. This paper uses averages to gain an "intuitive picture" of how light gets from the slits to the detectors, but that's all. It is not clear that it is measuring anything real.

1 comment:

  1. I agree with your skeptical view of QM and recent experiments. The way to understand QM is to develop an experiment that contradicts its predictions. I have been doing that since ~2002 with beam split coincidence tests with gamma-rays and alpha-rays. A photon should go one way or another at a beam splitter, and past experiments see that sort of noise. I managed to show that a singly emitted gamma-ray hf from spontaneous decay of radioisotopes can be split to two detectors to cause coincident detection greatly exceeding chance. QM fails. Please see fqxi essay A Challenge to Quantized Absorption by Experiment and Theory
    The double slit experiment is easily explained by light is emitted at energy hf but then spreads like a wave, its energy loads up at the detector, then the energy reaches threshold hf to make a particle-like detection event. I speak from many successful experiments on this concept.
    Thank you, Eric S Reiter, Aug 17, 2012.