FTL by Down-converting (Revised)

From: Horace Heffner (hheffner_at_mtaonline.net)
Date: 10/27/04 

Date: Wed, 27 Oct 2004 14:02:54 -0800

                     FTL by Down-converting (Revised)

A method is proposed here to achieve faster than light (FTL) communication
by the use of down-converters. A down-converter splits a photon into two
photons each having half the energy of the original photon.

Suppose we have a sender Alice, a receiver Bob, and an intermediary
facilitator Charlie. Charlie uses a beam splitter to create two beams of
laser light: L the left beam and R, the right beam. Charlie then
down-converts the L beam to create beams L1 and L2, and similarly creates
beams R1 and R2 from the beam R. Beams R2 and L2 are normal path or
"signal" photons through the down-converter, while beams R1 and L1 are
called "idler" photons. "Beam" here means a flow of individually
detectable photons sent in very short intervals so as to provide a useful
rate of communication. Charlie directs beams L1 and R1 to Alice and beams
R2 and L2 to Bob. The corresponding photons arrive at both Bob and Alice
at nearly the same time, but here assume Alice receives hers first, but
just barely before Bob.

Bob directs beams R2 and L2 such that they can create an interference
pattern in a set of detectors arranged so it is feasible to rapidly and
with high probability determine whether an interference pattern is present
or not. The signal photon beams R2 and L2 can create such an interference
pattern because they are the two paths from a beam splitter.

Alice can direct her idler beams L1 and R1 at will, in a co-linear
fashion, to opposing sides of a half silvered mirror, but at an angle of
45 degrees. Fig.1, which requires fixed font (e.g courrier) to view,
shows this configuration. Half of L1 and half of R1 then goes to a
detector DL. Similarly, half of L1 and half of R1 then goes to a detector
DR. The beams emerging from both sides of the mirror are thus fully mixed,
and the which-path information for all photons is lost. In this case Bob
must see an interference pattern. If Alice then diverts her beams
directly to detectors, the which-way information is then restored to 100
percent available, and Bob must see a bimodal distribution.

                       Full Mirror
  R1--->-------------\
                     |
                     | Half Mirror
  L1---->------------\----------------------DL
                     |
                     |
                     DR

    Fig. 1 - Alice's which-path scrambler

Bob will in fact see such an interference pattern provided the which-path
information is lost for idler beams R1 and L1.[1] If Alice does place
detectors directly in both her idler beams, then this is equivalent to
knowing which path each of Bob's photons have traveled, and thus Bob can
observe no interference pattern. This known-path-no-interference result
has been characteristic of numerous versions of the two slit or two path
interference experiments.[2] If Alice detects directly and sees an idler
she knows which path the corresponding signal photon took to Bob, and the
interference wavefunction instantly collapses. Bob, when his photons
arrive shortly after Alice's corresponding photons, knows the current
state of Alice's detectors by whether he sees an interference pattern or
not.

Since Alice and Bob could be light years away from each other, and since
Alice thus might have years from the time Charlie released the photons to
make the choice to detect or not detect her photons, faster than light
communication from Alice to Bob is clearly a possible result. It might be
said that the communication can not be verified for years, but such
verification is in this case not necessary. Bob does not require
verification or comparison to Alice's results to know the immediate state
of Alice's detectors, or to immediately detect a change of state of those
detectors, with sufficient speed and reliability to establish a practical
communication channel. Further, a similar channel can be established from
Bob to Alice, thus permitting immediate error detection and correction or
retransmission.

Assuming that beams adequate for fast communication can be generated and
the resulting interference detected sufficiently fast, achieving high data
rate FTL communication at short range then primarily boils down to how
fast Alice can switch from a detecting mode to a non-detecting mode. This
might be as simple as her redirecting beams R1 and/or L1, or by switching
on and off the information from her detectors. This experiment then, in
addition to achieving FTL communication, may be useful for determining
exactly of what an observation consists.

An experiment requiring the simplest possible message would involve
sending a data bit (actually only a change of state) via a one-way FTL
communication channel and returning it via a second one-way return FTL
communication channel, and repeating this process to establish an
oscillation. A fiber pair from Charlie to Bob and Charlie to Alice could
be used, if desired, to create a single FTL communication channel. A
similar set of fiber pairs would be used for the return channel. To
demonstrate FTL communication it is then necessary to transmit over a
sufficient distance D that the oscillation frequency, f, is faster than
the oscillation frequency F = c/D that can be achieved by light. A 10 km
communication link (each way) need only cycle faster than about 15 kHz to
break the light speed barrier. Assuming a sample of 100 photons to be
sufficient for determining interference, a photon transmission and
detection rate of 1.5 million photons per second is required. However, it
is not known what precisely constitutes an observation. It may be that
individual photon detection is not even necessary, but rather mere beam
intensity determination is sufficient.

References:

[1] Kim et al, Phys. Rev. Lett., Vol 84, no. 1, pp 1-5
[2] Brian Green, *The Fabric of the Cosmos*, (New York, Alfred A Knopf,
2004), pp 193-197

Regards,
                                 
Horace Heffner

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