From:
c.h.thompson
To: ajp@kzoo.edu
CC: Prof
Holbrow and others
Subject:
Beamsplitters: do they really deal with whole "photons"?
Date:
Thursday, April 18, 2002 6:26 PM
The Editor
The
American Journal of Physics
F.A.O. Prof
Holbrow
Re: C H Holbrow,
E Galvez and M E Parks, "Photon quantum mechanics and beam
splitters", Am. J. Phys. 70 (3), 260-265 (2002)
Dear Sir
Might I
make a suggestion re the education of the next generation of physicists? It is entirely possible that within their
lifetimes there will be a reversal of the current attitude towards the quantum
world. Could they therefore be set as
an exercise the investigation of alternative explanations for each of your
demonstrations? This would be
absolutely invaluable to them in the future, giving them insights that will
stand them in good stead if and when the bubble of "quantum
computing" bursts. (Nano-scale
computing is one thing; computing that depends on entanglement of separated
particles quite another. It is the
latter that is not on secure foundations.)
The
evidence for the existence of photons is by no means as clearcut as is
generally believed. The evidence for
entanglement of separated quantum particles is even less so, and, quite apart
from any practical implications, in view of the impact that this phenomenon has
on our beliefs about the nature of reality it is only right that students
should be informed of this fact. I have
made a careful study of the relevant experiments and found (as indeed have
others) that ordinary semiclassical, "local realist" explanations
have not been ruled out. I have
corresponded with many of the professionals concerned, and discovered that they
are aware of the "loopholes".
They have faith that these will eventually be blocked and quantum theory's
unbroken record of success continued, but students should surely be prepared
for the possibility that they are wrong.
The kind of
physics you present via your classroom experiments is an idealised version,
using mathematical models invented before the computer age. It is possible, indeed likely, that the
limitations of this method will soon become obvious. Models using unitary matrices etc. are satisfactory so long as it
can be assumed that the detectors are "perfect", but in order to
model real ones, which do NOT obey exact square laws, I think it will be found
that computer simulation assuming something much more general is needed. It is not enough to assume that a detector
can be fully characterised by its "quantum efficiency".
Students
could investigate the above claim! They
could conduct your basic experiment to show the existence of photons, then
conduct further experiments to show that this conclusion is false. All that is needed is to increase the
sensitivity of the detectors[*].
As you say,
modern avalanche photomultipliers "can register single photons with
efficiencies as high as 80%". This
is certainly what is claimed, but ask the students what this really means. How does anyone measure absolute
efficiency? If such a photomultiplier
is used in an experiment in which the results are supposed to reproduce Malus'
Law, I would predict failure. Peaks and
troughs would be in the right places but I would not expect to see a sine
curve. I could be wrong, but students
could think about the alternative explanation that I have found necessary in
order to give local realist explanations for the Bell test experiments. In this picture, the production of an output
is not a matter of the transfer of energy from one complete "photon"
to one "electron". The
detector output occurs when the intensity of the input combined with local
"noise" (and, possibly,
chance occurrence of favourable "phase relationship") combine to
cause a sudden change in the photocathode, conventionally assumed to be the
emission of an electron. The
relationship between input and output thus depends on the shape of the noise
distribution. It can only be at best
approximately linear, so a sinusoidal variation of input will be distorted on
output.
I am
assuming that light is an electromagnetic wave that, even if it comes in small
pulses, has an intensity that varies continuously as those pulses spread. When light passes through a polariser,
however weak it is, part of the energy goes one way, part the other. It is possible that even with a
"50-50" beamsplitter, the division of energy for any given pulse is
not even. It may be affected by local
factors -- by the relative phase of the input to that of some pre-existing
oscillations in the material of the splitter, for instance. The appearance of "photons" going
one way or the other is no more than an artifact caused by the use of detectors
that are unable to detect the weaker part.
(I am assuming also that your statement that there are no coincidences
is a slight exaggeration: there are very few, not none at all.)
My own
approach to these matters is almost entirely intuitive. For a mathematical version that is for the
most part equivalent see Stochastic Electrodynamics (SED) -- a theory that
reproduces much of QED but without the photon concept.
Further
information on local realist explanations of the Bell tests is to be found on
my web site, http://users.aber.ac.uk/cat/
, and in the quantum physics archive, e.g.:
"Subtraction
of ‘accidentals’ and the validity of
Bell tests", http://arxiv.org/abs/quant-ph/9903066
For more on
SED see http://homepages.tesco.net/~trevor.marshall
and archive papers such as:
"The
Myth of the Photon": http://arxiv.org/abs/quant-ph/9711046
Yours
sincerely
Caroline H
Thompson
Email:
c.h.thompson@pgen.net
Web site:
http://www.aber.ac.uk/~cat
[*] (Added 20:04:02)
Holbrow’s
experiment consists of using a nonlinear crystal to produce pairs of “photons”,
one of which is used to monitor the occurrence of a “down-conversion”, the other
passed through a beamsplitter. The
“proof” that we are dealing with photons consists of observing that there are
no coincidences for detectors placed at the two outputs. However (a) are there truly none, or does he
mean merely that there are very few, and (b) do we start to get coincidences if
we improve the efficiency of the detectors?