Quantum
measurement
73
Some
fUll
with
photons
Before we leave this chapter,
it
is
worth taking a quick look at some
of
lhe
curious
observations that can be
made
with
photons,
observations
that we must
attempt
to interpret in terms
of
quantum
theory. These
will
serve
to
whet the appetite for the fun which is
to
follow
in
the remaining
chapters,
Thomas
Young explained his observation
of
double-slit interference
using a wave theory
of
light. A light beam
of
moderately high intensity
incident
on
two
closely spaced, narrow apertures produces an interfer-
ence
pattern
consisting
of
bright and dark fringes. Now imagine that
we
reduce the light intensity
of
the source so that only
one
photon
passes
through
the double-slit
apparatus
at a time, to impinge
on
some photo-
graphic film.
Such experiments can,
and
have, been performed in the
laboratory, After a significant number
of
photons
have passed through,
we
find
that
the interference pattern
is
clearly visible (the equivalent
experiment with electrons
was described in
Chapter
I,
see
Fig_
1.3).
jfwe
assume that an individual
photon
must pass through
one-and
only
one-slit,
we should be able to repeat the experiment using a detec-
tor
to discover which one, However, when
sLlch
an experiment
is
done,
we
find
that
the
interference
pattern
is replaced with a completely dif-
ferent
pattern
corresponding
to
the diffraction
of
light through the
remaining open sliL
The
act
of
removing
the
detector
and
unblocking
the second slit restores the interference
pattern.
We conclude that
if
a
photon
does pass
through
one
slit,
it
must
be somehow affected by the
second, even
though
it
cannot
'know' in advance that the second slit
is
open.
We
have seen
that
a calcite crystal can be used
to
decompose left cir·
cularlY pOlarized light
into
vertical
and
horizontal components.
If
we
take an identical crystal,
and
orient
it
in the opposite sense,
we
can use
it
to
recombine the vertical
and
llOrizontal components
and
reconstitute
the
left circularly polarized light (see Fig. 2.6)_
That
such a reconstitution
can be achieved has been proved in careful
laboratory
experiments,
Now
suppose
that
an
individullllclt..circularly polarized photon passes
through
the first crystal
and
emerges from the vertical channel.
The
photon
enters the vertical channel
of
the second crystal. At first glance
there seems
to
be no way
of
obtaining a left circularly polarized
photon
out
of
this,
and
yet this
is
exactly
what
is obtained as
the
light intensity
passing
through
the arrangement is reduced
to
very low levels. A detector
can be used
to
check
that
the
photon
passes through
one-and
only
one-channel
of
the first crystal. The
photon
therefore appears
10
be
'aware'
of
the existence
of
the open horizontal channel, and
is
affected
by it. Close the horizontal channel by inserting a
SlOP
between the