Re: Quantum entanglement and information transfer

From: Caroline Thompson (ch.thompson1_at_virgin.net)
Date: 07/25/04 

Date: 25 Jul 2004 09:16:35 -0400

"Joe Rongen" <joe@alpha.to> wrote in message
news:a345818b.0407211703.4801f5f0@posting.google.com...
> "Caroline Thompson" <ch.thompson1@virgin.net> wrote in message
news:<3KZJc.104$wq5.31@newsfe3-gui.ntli.net>...
>
> >There are some very neat sources of "photons" around these
> >days that do seem to naturally support QM, but have they really
> >proved that they were initially of random direction? It's not easy
> >to tell the difference between a mixture of signals, some of one
> >polarisation, some orthogonal, and a genuinely random set.
>
> One can find a practical example here:
> http://xxx.lanl.gov/abs/quant-ph?0404115
>
> "We present an entangled-state quantum cryptography system ...

They don't tell us anything about how they generated the "entangled" pairs
here, though they reference Kwiat's 1995 paper. This is the source with the
overlapping cones of light, one polarised vertically, the other
horizontally, if I remember rightly. The "photons" used are taken from the
points A and B where cones intersect. The QM story is that you have either
an H at A and a V at B or a V A and an H at B.

I'm rather sticking my neck out here, I know, but I've studied quite a lot
of experiments that use this source and am fairly sure that what really
happens is that each of the supposedly single photons is really a
superposition of two signals, one H and one V. The difference between the
emissions at A and B is not really a matter of them being of orthogonal
polarisation directions but of the H and V components having related phase
differences. I'd love to have more info on this!

There are a lot of questions I'd like to ask about just what the role of the
various half-wave plates etc that are used is supposed to be. A half-wave
plate changes the phase difference by 180 deg. It can change the
polarisation of plane polarised light by 90 deg, or reverse the direction of
polarisation of circularly polarised light. What I suspect happens when
Kwiat's source is used in QKD is that small frequency differences from one
pair to the next cause difference in the phase difference between V and H
components, so that some pairs are approximately circularly polarised, some
approximately plane. The choice by Alice of whether to try and measure
plane or circular polarisation (by inserting an extra half-wave plate or
not) results, effectively, in selection of the pairs that are most nearly
perfectly in the selected categories, pairs that otherwise polarised having
much less chance of detection.

Much of the above is guesswork -- and I may have got some of the jargon
wrong -- but I do know that Kwiat never proved conclusively that his photon
pairs *were* entangled since he used the CHSH Bell inequality and so had to
assume "fair sampling". There are real correlations between the emitted
photons, and I think, for various reasons, that the correlation is in a
combination of phase and frequency. If it really was a matter of getting
either H or V then we would *not* have the rotational invariance that QM
assumes, since the axes of the nonlinear crystal source would provide
preferred directions. As it is, we may have rotational invariance (in that
all phase differences may be almost equally possible) but nobody has proved
that the results cannot be explained by ordinary correlations in the manner
I've suggested: the pairs have identical phase differences. Or possibly, as
I suggest in quant-ph/9912082 in relation to Weihs' Bell test experiment,
about half have one phase difference, the other half another, differing by
180 deg. It all depends how accurately frequency is controlled.

In the QKD demonstration, it is possible that the method works by selecting
one of four distinguishable subsets of the emitted signals: circular L or R
and linear +45 deg or -45 deg. The signals received by Alice are always
identical (or always conjugate?) to the ones received by Bob.

Caroline
Caroline H Thompson

ch.thompson1@virgin.net
http://freespace.virgin.net/ch.thompson1/

Relevant Pages

  • Re: Which Polarizer?
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    (rec.photo.digital.slr-systems)
  • Re: Quantum communication might be possible?
    ... Now suppose that Alice can *either* measure polarisation along direction ... they will find that both photons passed through their ... Alice and Bob receive sequences of photons which are correlated as above. ...
    (sci.physics.research)
  • Re: Bras, kets etc.
    ... circular polarisation is the more fundamental of the two, ... Look at the box of beetles through a circular polarisation filter, ... you can't determine if the incident photon was circularly ...
    (sci.physics)
  • Re: Bras, kets etc.
    ... circular polarisation is the more fundamental of the two, ... Look at the box of beetles through a circular polarisation filter, ... you can't determine if the incident photon was circularly ...
    (sci.physics)
  • Re: Quantum Cryptography can not work
    ... As I understand it, you cannot clone photons, even with a ... plus the number of photons already in that state, ... Quantum crypto interception relies on the cloning of single photons, ... half will have the same polarisation at random). ...
    (sci.crypt)