Quantum Mechanics Latest Visions

Courtesy : Quantum Mechanics


Quantum mechanics is a realm of
weirdness: electrons being linked to each
other even though the vastness of the
universe might separate them, things being
in two places at once, and, of course,
knowledge precluding knowledge. This last
is the standard bearer of quantum oddity:
measuring the momentum of an object
precludes precise knowledge of where that
object is. But I think I have found
something that is stranger than them all.
Researchers have suggested that it might be
possible to make measurements that trick a
photon into thinking it is, in fact, a crowd of
Let’s imagine that we want to introduce a
phase shift to one single photon through a
control photon. A phase shift is basically a
time delay. In traditional optics this delay is
applied through high-intensity light beams:
a high intensity pulse can modify the
refractive index of the medium through
which it propagates. Our signal photon
traveling through that medium will see that
different refractive index and either be
delayed or sped up.
The problem is that we want to do this all
with single photons, and just one photon
does not fit the definition of high intensity.
It seems a bit hopeless, right? However, in
quantum mechanics, things are not all that
they seem. One type of measurement in
particular—called a weak measurement—
can give very strange results. For instance,
if you measure the spin of an electron using
a weak measurement, you can be
reasonably certain that you haven’t
disturbed the spin state of the electron,
but, you might get a strange value.
Electrons only take on spin values of +1/2
or -1/2, but a weak measurement could
return something like 100. So, under the
right circumstances, that single electron can
behave as if it had the spin effect of 200
In our case, we’re using two photons. A
single control photon goes through a beam
splitter where it gets the choice of going
through the medium with a signal photon—
the one we want to phase shift—or go
through a separate channel. These paths
are then recombined at another beam
splitter, but this beam splitter isn’t quite
balanced. In a perfectly balanced splitter,
the control photon will always exit the
beam splitter in the same direction, called
the bright port. In an unbalanced beam
splitter, it’s possible for a photon to
sometimes head off in a different direction,
called the dark port.
When you calculate the possible ways that a
photon could hit a detector looking at the
dark port, one of them is that there are
simply more photons traveling through the
medium with the signal photon than on the
path outside the medium. Even better, the
closer to balanced the detector is, the rarer
the clicks on the detector for the dark port
are. So, to get a click, you need a much
larger number of photons in the medium
with the signal photon . Even if you know
you only send in one photon at a time.
In other words, we are measuring the
number of photons, but getting an answer
that is wrong by several orders of
magnitude. The truly weird thing: nature
believes us rather than reality.
If we make a weak measurement on the
number of photons in the control photon
beam, then a single photon is misreported
as several hundred. And, if everything is set
up correctly—which, in this case, means
that we only look for phase shifts on the
signal photon when the dark port detector
clicks—that lone control photon will have a
much larger effect on the refractive index of
the medium. The end result is that the
phase of the signal photon is shifted by lot
more than would normally be expected.
The catch is that this is a work of theory.
And the phase shifts, even with this
amplification factor, may be really small.
Even so, I can imagine that if you chose
your medium correctly (say an alkali metal
gas), and your wavelengths correctly (right
on the edge of an absorption feature of the
gas), then it might well be possible to
observe the amplification of the phase shift.
Like the Bell inequalities and entanglement,
we will have to wait before this can be
tested. But, unlike some quantum
phenomena, it won’t be decades from
theory to experiment.