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Moscow University Physics Bulletin

Light interference and the absence of definite values of measured quantities a priori

Aleksandr V. Belinsky

Published May 1, 2017• 2 citations
Publisher

Abstract

The peculiarities of the behavior of photons in various experimental situations are considered. Variants of double-beam interference of a single photon and the possibility of forming a standing wave by this photon are analyzed; three-beam interference is also discussed. The effects observed in this case prove the absence of particular values of the measured quantities up to the moment of measurement; in the latter case, the number of photons in the field does not a priori have a certain value, in spite of the fact that the system is in the energy state. An experiment is also considered that proves the absence of a certain phase and the phase difference of photons in the Fock state, which makes it possible to treat different types of theories of hidden parameters, including nonlocal ones, more critically.

DOI
10.3103/s0027134917030031

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The peculiarities of the behavior of photons in various experimental situations are considered. Variants of double-beam interference of a single photon and the possibility of forming a standing wave by this photon are analyzed; three-beam interference is also discussed. The effects observed in this case prove the absence of particular values of the measured quantities up to the moment of measurement; in the latter case, the number of photons in the field does not a priori have a certain value, in spite of the fact that the system is in the energy state. An experiment is also considered that proves the absence of a certain phase and the phase difference of photons in the Fock state, which makes it possible to treat different types of theories of hidden parameters, including nonlocal ones, more critically.
Specific features of the behavior of quantum particles in different experimental situations are considered. The variants of two-beam interference of single photon and other quantum particles, as well as their ability to form “standing” and “progressive” waves with the interference minima, i.e., “dead” zones in the propagation path, are studied. We also consider some type of quantum particle teleportation in an unconventional understanding of this word, where elementary particles overcome regions of the space where they cannot be, or rather the borders where the probability of finding them is zero. At these boundaries there is no pulse impact of particles on anything, it is as if they are unobservable. When observing three-beam interference, it turns out that all three modes should be present simultaneously in the light field prior to photodetection. If each mode involves a photon, it contradicts the energy conservation law, it is indicative of the fact that the observed quantity (number of photons in the field) has no any particular value prior to the measurement (a priori) unless the quantum system is in the eigen state of a measured value (Fock state).
The conditions are examined under which interference effects should be observable in the superposition of two light beams derived from independent sources. The quantum-mechanical description of these effects differs from the classical description principally in that it is necessarily based on expectation values of the light intensity at one or more space-time points. It is shown that pure states of the radiation field which are not energy states give rise to interference effects, but in that case the two beams cannot meaningfully be described as statistically independent. For a realistic description mixed states have to be introduced, when the expectation value of the intensity gives no indication of interference effects. On the other hand, the intensity correlation at two space-time points is a periodic function of the separation of the points and indicates the presence of transient interference effects. The effects become readily observable only when the average photon occupation number of each cell of phase space is appreciably greater than 1, and this explains why laser beams were needed for the experimental observations. It is pointed out that, even in these experiments, each photon may be regarded as interfering only with itself.
Abstract This paper reviews the schemes of experiments in which it was possible to prove, based on measured photon cross-correlation suppression effects in a beam splitter and preparing squeezed states, the nonexistence of a definite value of the photon phase difference in Fock states (otherwise, these two effects cannot exist simultaneously). It is shown that this conclusion emphasizes the internal inconsistency of interpreting the quantum mechanics in the spirit of classical nonlocal ‘realism’ — in the sense that measured physical quantities have no definite values a priori , i.e., before the instant of measurement.
The intention of this paper is to underscore that to understand fundamentally new properties of light beams, we must first find the limits of semi classical model to explain optical interference phenomena. We claim that we have not yet reached that limit. Careful analysis of the processes behind detecting fringes indicate that the effect of superposition of multiple optical beams can become manifest only through the mediation of the detecting dipoles. Since the detectors are quantum mechanical, (i) the observed effects are different for different detectors for the same superposed light beams, and further, (ii) they are only capable of registering discrete number of "clicks", whose rate will vary with the incident intensity. A reduced rate of "clicks" at very low intensity does not prove that light consists of indivisible packets of energy. We have also experimentally demonstrated that (i) neither Fourier synthesis, nor, (ii) Fourier decomposition actually model the behavior of EM fields under all possible circumstances. Superposed light beams of different frequencies do not synthesize a new average optical frequency. A pure amplitude modulated pulse does not contain any of the mathematical, Fourier analyzed frequencies. The QED definition of photon being a Fourier mode in the vacuum, it necessarily becomes non-local. Since we have demonstrated that the Fourier theorem has various limitations in classical physics, its indiscriminate use in quantum mechanics should also be critically reviewed.
The photon in the interferometer This chapter introduces many of the ideas of quantum theory by exploring three specific “case studies” of quantum systems. Each is an example of a qubit , a generic name for the simplest type of quantum system. The concepts we develop will be incorporated into a rigorous mathematical framework in the next chapter. Our business here is to provide some intuition about why that mathematical framework is reasonable and appropriate for dealing with the quantum facts of life. Interferometers In Section 1.2 we discussed the two-slit interference experiment with a single photon. In that experiment, the partial waves of probability amplitude were spread throughout the entire region of space beyond the two slits. It is much easier to analyze the situation in an interferometer , an optical apparatus in which the light is restricted to a finite number of discrete beams . The beams may be guided from one point to another, split apart or recombined as needed, and when two beams are recombined into one, the result may show interference effects. At the end of the interferometer, one or more sensors can measure the intensity of various beams. (A beam is just a possible path for the light, so there is nothing paradoxical in talking about a beam of zero intensity.) Figure 2.1 shows the layout of a Mach–Zehnder interferometer , which is an example of this kind of apparatus.