Has FTL communication really never been tested in this way?

[PostReplies]

http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx

I know most physicists reject the idea of FTL communication but I'm
surprised the experiment described in this article wasn't carried out
years ago or has it? Wouldn't the existence or absence of an
interference pattern created by entangled beams of photons be a way to
receive a signal instantaneously?
I'm not a physicist, just an interested lay person.

 

[Uncle Al]

PostReplies wrote:
>
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.[/color]

Chirping a wave packet does not convey information superluminally.
Ditto diddling group vs. phase velocities. The
Einstein-Podolsky-Rosen paradox is *instantaneous* wavepacket collapse
into an observable across arbitrarily large distances and volumes
(throughout the entire universe if so configured). No information
exists until sender and recipient compare data - and that is limited
to lightspeed.

The universe is strictly causal. There is no superluminal conveyance
of information. Conceivably one could cleverly pull a Star Trek or
whatever and get from here to there in violation of Special
Relativity. One doubts there would remain opportunity to do it a
second time.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2

 

[Dirk Bruere at NeoPax]

PostReplies wrote:
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.
>[/color]

Isn't this a type of Wheeler Paradox necessitating a Quantum Eraser type
measurement on the short time-of-flight beam?

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
Remote Viewing classes in London

 

[Marc Millis]

In <7jhah3p1vo77562fircdttj8gvuq1lgsm3@4ax.com> PostReplies wrote:
>=20
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx
>=20
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.
>=20[/color]

Right now, experiments are being conducted at the Univ. of Washington in=20
Seattle, by Dr. John Cramer, to directly attack the causality issues of=20
quantum entanglement and "nonlocality" in communication. I suspect data=20
will be forthcoming later in 2008. Even then the results will be=20
difficult to comprehend.

The difficulty with this and even more-so with prior experiments is to=20
clearly define the connections between the parts of the system and=20
causal relations (cause precedes effect). In short, it is hard to=20
follow the chain of logic and facts in these experiments, and to be able=20
to distinguish facts from interpretations. Our notion of time and=20
causal connections is so innately ingrained in us, that it makes it very=20
hard to break from these notions and just to look at the physics in its=20
raw form, and then to try and decipher what is really going on. It is=20
confusing for physicists too, which is reflected in the ongoing=20
publications. Even the terminology varies from one approach to another (
e.g. the terminology used in quantum tunneling, electrical engineering,=20
retarded potentials... ).

Regarding FTL 'travel,' the situation is still theoretical, not=20
experimental. To get passed the light-speed limit of spacetime,=20
concepts of "warp drives" (Alcubierre) and "wormholes" (Visser) toy with=20
the idea of manipulating 'spacetime' itself to circumvent the limits. =20
In short, even these FTL concepts appear to evoke "closed time-like=20
curves" which means possible causality violations (Arrive back from your=20
trip before you departed to stop yourself from taking the trip...). =20
Given how little is known about spacetime itself and other unknowns=20
dubbed "Dark Matter" "Dark Energy" etc, it is safe to say we all have a=20
lot more learning to do.

In short, we are beginning to get smart enough to ask the right=20
questions on such deep unknowns, but it is likely to take a while before=20
we fully comprehend how nature works - if ever. It is fun to work on it=20
and to ponder the possibilities.

Regrettably, I do not have good books to recommend to you on this=20
subject that are deliberately written for the non-scientist.

Marc

 

[Ian Parker]

On 17 Oct, 21:48, PostReplies <nom...@pleasepost.net> wrote:
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.[/color]

We can look at this from a number of viewpoints. Looking at this
mathematically I can write down Maxwell's equations, I can then
perform a Lorenz transformation and I will still have Maxwell's
equations. The Universe still looks the same. I may write
Schroedinger's equation as

H = E an equation which is invariant under Lorenz transformation.
Quite clearly from this FTL communication is impossible.

How then does this seeming paradox arise? Basically because if
something were a million LY distant it would

a) Take a million years to set the experiment up.
b) H = E we will have 2 eigenvalues of H which differ by 1/
(10^6*365.25*24*3600) Hz. Hence although we have (instantly) changed
state the uncertainty principle prevents us from seeing it.

This question of the UP to me at any rate resolves the paradox.

- Ian Parker

 

[scerir]

"PostReplies":
> I know most physicists reject the idea of
> FTL communication but I'm surprised the experiment
> described in this article wasn't carried out
> years ago or has it?[/color]

Something similar many times, but not exactly
that one. I.e. see this one below
http://www.arxiv.org/abs/quant-ph/0106078

Next step would be (imo) to remove the coincidence
circuit and to have a "100% clean" source of
entangled pairs, i.e. something which emits
entangled pairs *only*, and with the *right* timing
:-)

(I do not know if prof. Cramer is trying something
like that, maybe he is interested in the 'bilking'
effect).

> Wouldn't the existence or absence of an
> interference pattern created by entangled
> beams of photons be a way to receive a signal
> instantaneously?[/color]

FTL signal? Uncontrollable FTL signal?
FTL causation? FTL influences?
It is a bit obscure (to me) what is allowed
by SR, and what is not.

Anyway standard QM seems to forbid that.
But there are (more or less) interesting papers
saying something different (are those theorems
circular?).

-J. B. Kennedy. On the empirical foundations
of the quantum no-signalling proofs.
Philosophy of Science 62, 543-560 (1995).

see also this one
http://www.arxiv.org/abs/quant-ph/9906036

 

[Knud Soerensen]

Maybe you should take a look at
www.cheniere.org/references/G-COM 3.pdf

PostReplies wrote:
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.
>[/color]

 

[Dirk Bruere at NeoPax]

Knud Soerensen wrote:
> Maybe you should take a look at
> www.cheniere.org/references/G-COM 3.pdf
>[/color]

Never mind the PR BS, where's the circuit diagrams so we can test this
for ourselves? Or is this another inventor who wants $1billion up front
before he reveals the 'secrets'? Clearly a Nobel prize isn't a big
enough reward.

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
Remote Viewing classes in London

 

[Dirk Bruere at NeoPax]

Knud Soerensen wrote:
> Maybe you should take a look at
> www.cheniere.org/references/G-COM 3.pdf
>[/color]

Never mind the PR BS, where's the circuit diagrams so we can test this
for ourselves? Or is this another inventor who wants $1billion up front
before he reveals the 'secrets'? Clearly a Nobel prize isn't a big
enough reward.

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
Remote Viewing classes in London

 

[Dirk Bruere at NeoPax]

PostReplies wrote:
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.
>[/color]

BTW, isn't this expt similar to something Sarfatti suggested years ago?
Not saying he originated it but I seem to recall reading about him
mentioning it.

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
Remote Viewing classes in London

 

[PostReplies]

On Sat, 20 Oct 2007 10:09:25 +0000 (UTC), "scerir" <scerir@libero.it>
wrote:
>
>Next step would be (imo) to remove the coincidence
>circuit and to have a "100% clean" source of
>entangled pairs, i.e. something which emits
>entangled pairs *only*, and with the *right* timing
>:-)[/color]

That's what Cramer is doing. Here's another page I found explaining
his experiment and a very interesting earlier experiment which was
encouraging:

http://www.paulfriedlander.com/text/timetravel/experiment.htm

 

[Dirk Bruere at NeoPax]

PostReplies wrote:
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.
>[/color]

BTW, isn't this expt similar to something Sarfatti suggested years ago?
Not saying he originated it but I seem to recall reading about him
mentioning it.

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
Remote Viewing classes in London

 

[PostReplies]

On Sat, 20 Oct 2007 10:09:25 +0000 (UTC), "scerir" <scerir@libero.it>
wrote:
>
>Next step would be (imo) to remove the coincidence
>circuit and to have a "100% clean" source of
>entangled pairs, i.e. something which emits
>entangled pairs *only*, and with the *right* timing
>:-)[/color]

That's what Cramer is doing. Here's another page I found explaining
his experiment and a very interesting earlier experiment which was
encouraging:

http://www.paulfriedlander.com/text/timetravel/experiment.htm

 

[scerir]

"PostReplies":
> Here's another page I found explaining
> his experiment and a very interesting
> earlier experiment which was encouraging:
> http://www.paulfriedlander.com/text/timetravel/experiment.htm[/color]

There is an interesting quote,
about Dopfer experiment, from that link.

"The important conclusion is that,
while individual events just happen,
their physical interpretation
in terms of wave or particle
might depend on the future"
[A.Zeilinger]

Now it seems to me that the key word
is "interpretation". Meaning that
from a *single* spot, on the two-slit
screen, produced by one photon passing
through the interferometer, one cannot
realize if that *single* spot is part
of an inteferential pattern *or* part of
a smooth pattern. One can realize precidely
that only after he measures, in the future,
the position *or* the momentum of the other
photon entangled with the first one.

(The third possibility, that one doesn't measure,
in the future, the position or the momentum of
the entangled photon is interesting but it is not
relevant here).

But the problem I see (since long time) is this.
Imagine there is not a *single* spot (produced
by one photon) on the two-slit screen, but 1000
distinct spots (produced by 1000 distinct photons).
Can we still say that nobody can realize whether
these 1000 spots form an interferential pattern *or*
a smooth pattern, until one measures,
in the future, the position *or* the momentum
of all the other distinct 1000 photons, each one
of them entangled with a photon producing the spot?

s.

 

[Gerry Quinn]

In article <7jhah3p1vo77562fircdttj8gvuq1lgsm3@4ax.com>,
nomail@pleasepost.net says...
>
> http://cosmiclog.msnbc.msn.com/archive/2007/07/17/274531.aspx
>
> I know most physicists reject the idea of FTL communication but I'm
> surprised the experiment described in this article wasn't carried out
> years ago or has it? Wouldn't the existence or absence of an
> interference pattern created by entangled beams of photons be a way to
> receive a signal instantaneously?
> I'm not a physicist, just an interested lay person.[/color]

<QUOTE>
So what happens when the beams go their separate ways, and you conduct
a wave-vs.-particle measurement on one beam? When someone else checks
the other beam, the same measurement should yield the same result. In
fact, you could visualize using the wave-vs.-particle toggle as a means
for communicating information, sort of like Morse code.
</QUOTE>

It's an interesting experiment. The key is that he is using beams
rather than single entangled pairs of photons. With single entangled
pairs it clearly won't work, because the determination between wave-
like and particle-like properties depends on the statistics of multiple
measurements.

So, what precisely is the measurement protocol he is using to make wave
or particle like measurements on the particles constituting each beam?
Has anyone got a link, because the article doesn't say, and googling
failed to find anything? If we knew that, we could write down the sort
of statistics we would expect from a successful 'bilking' experiment,
and perhaps that would considerably elucidate the situation (most
likely by proving that bilking is actually undetectable).

- Gerry Quinn

 

[scerir]

"Gerry Quinn"
> So, what precisely is the measurement protocol he is using to make wave
> or particle like measurements on the particles constituting each beam?
> Has anyone got a link, because the article doesn't say, and googling
> failed to find anything? If we knew that, we could write down the sort
> of statistics we would expect from a successful 'bilking' experiment,
> and perhaps that would considerably elucidate the situation (most
> likely by proving that bilking is actually undetectable).[/color]

Only this one
http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf

As far as I remember (?) you need few photons (something
like 50 or less) to realize if it is a diffraction
pattern or an interferential one. But you also need
a very clean source of entangled photons imo.

There is an interesting paper here
http://www.arxiv.org/abs/quant-ph/0506027
about that 'bilking' or, to say it better,
once the future has unfolded, it cannot change
the past.

s.

"In the early 1950s, my mentor Fritz Bopp
pondered the question as to why the probabilities
in quantum mechanics appear as absolute squares
of complex amplitudes. This led to a series
of papers with titles such as "Dice Games Whose
Tokens Move Quantum Mechanically". In 1953, I had
the great chance to spend a year in Copenhagen.
One day Niels Bohr came to me saying: "I received
again a manuscript by Professor Bopp. I do not
understand why people occupy themselves with
questions which have been clarified for decades
while there are so many unsolved interesting new
problems around." My imprudent answer: "Maybe things
are not so clear", prompted a series of discussions."
- Rudolf Haag

 

[Gerry Quinn]

In article <XcHUi.153401$%k.295389@twister2.libero.it>,
scerir@libero.it says...
> "Gerry Quinn"
> > So, what precisely is the measurement protocol he is using to make wave
> > or particle like measurements on the particles constituting each beam?
> > Has anyone got a link, because the article doesn't say, and googling
> > failed to find anything? If we knew that, we could write down the sort
> > of statistics we would expect from a successful 'bilking' experiment,
> > and perhaps that would considerably elucidate the situation (most
> > likely by proving that bilking is actually undetectable).[/color]
>
> Only this one
> http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf[/color]

Thanks!

> As far as I remember (?) you need few photons (something
> like 50 or less) to realize if it is a diffraction
> pattern or an interferential one. But you also need
> a very clean source of entangled photons imo.
>
> There is an interesting paper here
> http://www.arxiv.org/abs/quant-ph/0506027
> about that 'bilking' or, to say it better,
> once the future has unfolded, it cannot change
> the past.[/color]

Yes. I find the argument quite convincing. It does raise some
interesting questions with regard to quantum computation.

The authors propose that you cannot go back in a time machine and shoot
your father because, essentially, the observation of your own existence
demands that a chain of events, no matter how unlikely, led up to it.
If you try to do it, you will perhaps shoot someone you think is your
father but actually isn't, or your gun will misfire, or some such
possibility.

>From now on I will revert to the traditional experiment of shooting[/color]
your grandfather, as it somehow seems less brutal than simple
patricide. Anyway, I propose the Grandfather Computer (TM),
constructed using a grandfather, a time machine, pen and paper, and a
deadly weapon of some kind.

You travel back in time, locate your grandfather, and demand that, on
pain of death, he write down the answer to some difficult computation,
say the factors of a large number (incidentally, you don't need to tell
him the number). If you have so arranged matters that this is the only
remotely possible way he could survive, it seems like a way to leverage
the evolution of the wave function of the universe to carry out this
computation; in short, it is a form of quantum computer.

Probably the flaw in the this scheme is the difficulty of actually
removing all feasible alternative courses of events that will lead to
your grandfather's, and thus your, survival. In reality it will not be
possible to remove all low-probability events (weapon malfunction,
mistaken identity, a random police visit, etc.) that will still be much
more likely than his correctly guessing the answer to the computation.

I wonder if this argument can be extended to investigate the
possibility of quantum computation in general. While I know that
devices have been built that are technically working quantum computers,
there are none that can do general computations that are not feasible
by other means. For example, the system using caffeine molecules needs
a lot of molecules, and does not perform better in practice than a
classical molecular computer based on simple parallel computation.

My feeling is that quantum computation as usually advertised is also a
sort of 'bilking' attempt, albeit not exactly in the same sense as
Cramer's experiment. It might be that both forms of bilking are
impossible, and that the reasons are related. Ways in which you might
be born despite your grandfather not solving the computation are the
equivalent of errors in typical quantum computation schemes. Both can
be seen as resistance on the part of the wave function of the universe
to being squeezed into a very narrow range of selected outcomes after a
considerable amount of complex unitary evolution.

- Gerry Quinn

 

[Dirk Bruere at NeoPax]

Gerry Quinn wrote:
>
>> http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf[/color]
>
> Thanks!
>
>> As far as I remember (?) you need few photons (something
>> like 50 or less) to realize if it is a diffraction
>> pattern or an interferential one. But you also need
>> a very clean source of entangled photons imo.
>>
>> There is an interesting paper here
>> http://www.arxiv.org/abs/quant-ph/0506027
>> about that 'bilking' or, to say it better,
>> once the future has unfolded, it cannot change
>> the past.[/color]
>
> Yes. I find the argument quite convincing. It does raise some
> interesting questions with regard to quantum computation.
>
> The authors propose that you cannot go back in a time machine and shoot
> your father because, essentially, the observation of your own existence
> demands that a chain of events, no matter how unlikely, led up to it.
> If you try to do it, you will perhaps shoot someone you think is your
> father but actually isn't, or your gun will misfire, or some such
> possibility.
>
>>From now on I will revert to the traditional experiment of shooting[/color]
> your grandfather, as it somehow seems less brutal than simple
> patricide. Anyway, I propose the Grandfather Computer (TM),
> constructed using a grandfather, a time machine, pen and paper, and a
> deadly weapon of some kind.
>
> You travel back in time, locate your grandfather, and demand that, on
> pain of death, he write down the answer to some difficult computation,
> say the factors of a large number (incidentally, you don't need to tell
> him the number). If you have so arranged matters that this is the only
> remotely possible way he could survive, it seems like a way to leverage
> the evolution of the wave function of the universe to carry out this
> computation; in short, it is a form of quantum computer.
>
> Probably the flaw in the this scheme is the difficulty of actually[/color]

One obvious flaw in the scheme is that it should be the grandmother:-)
Also, isn't the above quantum computer just a form of random number
generator plus quantum suicide?

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
Remote Viewing classes in London

 

[Andreas Most]

scerir wrote:
> "PostReplies":
>> Here's another page I found explaining
>> his experiment and a very interesting
>> earlier experiment which was encouraging:
>> http://www.paulfriedlander.com/text/timetravel/experiment.htm[/color]
>
> There is an interesting quote,
> about Dopfer experiment, from that link.
>
> "The important conclusion is that,
> while individual events just happen,
> their physical interpretation
> in terms of wave or particle
> might depend on the future"
> [A.Zeilinger]
>
> Now it seems to me that the key word
> is "interpretation". Meaning that
> from a *single* spot, on the two-slit
> screen, produced by one photon passing
> through the interferometer, one cannot
> realize if that *single* spot is part
> of an inteferential pattern *or* part of
> a smooth pattern. One can realize precidely
> that only after he measures, in the future,
> the position *or* the momentum of the other
> photon entangled with the first one.
>
> (The third possibility, that one doesn't measure,
> in the future, the position or the momentum of
> the entangled photon is interesting but it is not
> relevant here).
>
> But the problem I see (since long time) is this.
> Imagine there is not a *single* spot (produced
> by one photon) on the two-slit screen, but 1000
> distinct spots (produced by 1000 distinct photons).
> Can we still say that nobody can realize whether
> these 1000 spots form an interferential pattern *or*
> a smooth pattern, until one measures,
> in the future, the position *or* the momentum
> of all the other distinct 1000 photons, each one
> of them entangled with a photon producing the spot?[/color]

Yes, we can say this. Without any coincidence unit you will
not be able to see an interference pattern. Actually, by
involving the correlation with the entangled partner you choose
about 500 of the 1000 initial photons. The way you perform the
selection, either momentum or position measurement, determines
whether there is an interference pattern or not. What is actually
disturbing is that it does not depend on when you perform the
measurement on the entangled partner. But that is the way it is in
quantum mechanics.

I think Cramer is wrong in assuming he could send a signal backwards
in time because the interference pattern occurs when the correlation
between the two entangled photons has been established (and of course
roughly 500 events have been rejected) and not the moment the photons
hit the screen.

Andreas.

 

[Dirk Bruere at NeoPax]

Andreas Most wrote:

>>
>> But the problem I see (since long time) is this.
>> Imagine there is not a *single* spot (produced
>> by one photon) on the two-slit screen, but 1000
>> distinct spots (produced by 1000 distinct photons).
>> Can we still say that nobody can realize whether
>> these 1000 spots form an interferential pattern *or*
>> a smooth pattern, until one measures,
>> in the future, the position *or* the momentum
>> of all the other distinct 1000 photons, each one
>> of them entangled with a photon producing the spot?[/color]
>
> Yes, we can say this. Without any coincidence unit you will
> not be able to see an interference pattern. Actually, by
> involving the correlation with the entangled partner you choose
> about 500 of the 1000 initial photons. The way you perform the
> selection, either momentum or position measurement, determines
> whether there is an interference pattern or not. What is actually
> disturbing is that it does not depend on when you perform the
> measurement on the entangled partner. But that is the way it is in
> quantum mechanics.
>
> I think Cramer is wrong in assuming he could send a signal backwards
> in time because the interference pattern occurs when the correlation
> between the two entangled photons has been established (and of course
> roughly 500 events have been rejected) and not the moment the photons
> hit the screen.[/color]

And at what point is that?
Precisely *when* does the interference pattern exist?

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
Remote Viewing classes in London

 

[Andreas Most]

Dirk Bruere at NeoPax wrote:

> And at what point is that?
> Precisely *when* does the interference pattern exist?[/color]

The point is that you can only decide on whether you see
an interference pattern when you have established the correlation
of the measurements on the two entangled photons which means
after the measurements have been performed. Quantum mechanics
tells us something about the probabilities of the outcomes of a
measurement. It does not tell us anything about when an outcome
exists.
"Interference Pattern" is not a hermitian operator but you could
modify the experiment into a Bell-type experiment. From the
violation of Bell's inequalities it is then clear that the
interference pattern could not "have existed" before the second photon
is measured and the correlation between the entangled partners
is established.

Andreas.

 

[PostReplies]

On Fri, 2 Nov 2007 06:06:07 +0000 (UTC), Andreas Most
<Andreas.Most@nospam.de> wrote:

>Quantum mechanics
>tells us something about the probabilities of the outcomes of a
>measurement. It does not tell us anything about when an outcome
>exists.[/color]

I don't follow that. On the sender side of the communication channel
you have dual slits and a "which slit" detector that can be turned on
or off. On the receiver side of the channel (the other beam of
photons) you have dual slits but no which way detector. The sender
leaves his detector off. The receiver should be seeing an interference
pattern. The sender turns on his "which slit" detector. His beam and
the entangled receiver beam should no longer exhibit an interference
pattern until he turnes his detector back off. In other words, the
timing is based on the sender turning his detector on and off. That's
a simplification but I think is pretty much the crux of the experiment
in FTL communication. If that works (which would violate relativity so
likely won't) Cramer will then lengthen the sender side so the sender
side is lagging the receiver side in order to check for backward
causality--again likely won't work but I have no idea what the mode of
failure will be.

 

[Ben Rudiak-Gould]

PostReplies wrote:
> That's what Cramer is doing. Here's another page I found explaining
> his experiment and a very interesting earlier experiment which was
> encouraging:
>
> http://www.paulfriedlander.com/text/timetravel/experiment.htm[/color]

There is no nonlocal communication here. It's critical to understand that
the four graphs (around halfway down the page) do not represent images
recorded on a photographic plate or CCD. Rather, they represent hit rates on
a yes-or-no detector (think Geiger counter) as it's physically moved across
the detection field while the other detector is held fixed, and only the
cases where both detectors register a particle are counted. This makes a big
difference! You will get completely different results this way than with
photographic film. To see why, consider a simplified experiment in which
each detector can be moved to four different locations (D1 in locations
11,12,13,14 and D2 in locations 21,22,23,24). Suppose our light source is
such that all the light beams it generates pass through locations whose sum
is even -- for example, it will generate beam pairs going through 11 and 21,
but never through 11 and 22. The possible combinations are marked with "X"
below.

21 22 23 24
11 X X
12 X X
13 X X
14 X X

Now suppose D1 is held fixed (at any position) while D2 is moved, and
simultaneous clicks of D1 and D2 are recorded. Regardless of the fixed value
of D1, you will get a bright, dim, bright, dim pattern, which is our
simplified discrete version of an interference pattern. But if you consider
only the data from D2, without the coincidence counter, there will be no
interference pattern, just an equal distribution over all four locations.
Similarly, if you replace the detectors with photographic plates, there will
be no interference pattern on either plate.

Now in front of D1 insert a scrambling device that perturbs each incoming
photon so that, regardless of where it was originally headed, it's now
equally likely to go to any of the locations 11,12,13,14. Now, when you
again hold D1 fixed while varying D2 and counting coincidences, you will no
longer see an interference pattern. But the raw data from D2 has not changed
at all -- all that has changed is which raw detection events we subsequently
threw away at the coincidence counter.

This is what's going on in Dopfer's experiment (as both Dopfer and Zeilinger
realize).

-- Ben

 

[Ben Rudiak-Gould]

I've now read Cramer's three-page description of his experiment, found here:

http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf

One of his coauthors is an undergraduate and the other claims to have only a
faint understanding of the work:

http://seattlepi.nwsource.com/local/292378_timeguy15.html

so I'm going to treat this report as Cramer's alone.

Um. Cramer apparently thinks that the coincidence counter in experiments
like Dopfer's is just an engineering detail, and that by throwing it away
you can get a superluminal communication device. Cramer's ideas have always
seemed a bit odd to me, but this is the first time I've seen what appears to
be clear evidence that he's incompetent as a theorist. I can't tell whether
he thinks quantum mechanics predicts superluminality in this experiment or
whether he thinks previous work fully consistent with quantum mechanics
somehow points the way to a result violating quantum mechanics -- but either
way his understanding of QM appears to match Herbert Dingle's understanding
of special relativity and Tom Van Flandern's understanding of classical
field theory. Seriously, what the heck?

The P-I article quotes Cramer saying that "there's no obvious explanation
why this won't work" and that "even if it doesn't work, we should be able to
learn something new about quantum mechanics by trying it." I can't imagine
what he expects to learn.

-- Ben

 

[Andreas Most]

PostReplies wrote:
> On Fri, 2 Nov 2007 06:06:07 +0000 (UTC), Andreas Most
> <Andreas.Most@nospam.de> wrote:
>
>> Quantum mechanics
>> tells us something about the probabilities of the outcomes of a
>> measurement. It does not tell us anything about when an outcome
>> exists.[/color]
>
> I don't follow that. On the sender side of the communication channel
> you have dual slits and a "which slit" detector that can be turned on
> or off. On the receiver side of the channel (the other beam of
> photons) you have dual slits but no which way detector. The sender
> leaves his detector off. The receiver should be seeing an interference
> pattern. The sender turns on his "which slit" detector. His beam and
> the entangled receiver beam should no longer exhibit an interference
> pattern until he turnes his detector back off.[/color]

No. Nothing you do on the sender side will change the outcome of the
measurement on the receiver side. See
http://en.wikipedia.org/wiki/No-communication_theorem

You need to know the correlation additionally (i.e. coincidence).
This piece of information can only be transmitted classically, i.e.
at best with speed of light.

The "spooky action at a distance" is about correlations of measurements
and not about single measurements. People tend to forget this sometimes.

> In other words, the
> timing is based on the sender turning his detector on and off. That's
> a simplification but I think is pretty much the crux of the experiment
> in FTL communication. If that works (which would violate relativity so
> likely won't) Cramer will then lengthen the sender side so the sender
> side is lagging the receiver side in order to check for backward
> causality--again likely won't work but I have no idea what the mode of
> failure will be.[/color]

Definitely, he will see no interference pattern whatever he does on the
sender side. Only if he correlates his actions on the sender side with
the measurements on the receiver side he is able to extract an
interference pattern.

Andreas.

 

[Gerry Quinn]

In article <fgku7u$nv1$1@gemini.csx.cam.ac.uk>, br276deleteme@cam.ac.uk
says...
> PostReplies wrote:
> > That's what Cramer is doing. Here's another page I found explaining
> > his experiment and a very interesting earlier experiment which was
> > encouraging:
> >
> > http://www.paulfriedlander.com/text/timetravel/experiment.htm[/color]
>
> There is no nonlocal communication here. It's critical to understand that
> the four graphs (around halfway down the page) do not represent images
> recorded on a photographic plate or CCD. Rather, they represent hit rates on
> a yes-or-no detector (think Geiger counter) as it's physically moved across
> the detection field while the other detector is held fixed, and only the
> cases where both detectors register a particle are counted. This makes a big
> difference! You will get completely different results this way than with
> photographic film. To see why, consider a simplified experiment in which
> each detector can be moved to four different locations (D1 in locations
> 11,12,13,14 and D2 in locations 21,22,23,24). Suppose our light source is
> such that all the light beams it generates pass through locations whose sum
> is even -- for example, it will generate beam pairs going through 11 and 21,
> but never through 11 and 22. The possible combinations are marked with "X"
> below.
>
> 21 22 23 24
> 11 X X
> 12 X X
> 13 X X
> 14 X X
>
> Now suppose D1 is held fixed (at any position) while D2 is moved, and
> simultaneous clicks of D1 and D2 are recorded. Regardless of the fixed value
> of D1, you will get a bright, dim, bright, dim pattern, which is our
> simplified discrete version of an interference pattern. But if you consider
> only the data from D2, without the coincidence counter, there will be no
> interference pattern, just an equal distribution over all four locations.
> Similarly, if you replace the detectors with photographic plates, there will
> be no interference pattern on either plate.[/color]

I'm not convinced your 'simplified' version of the experiment is
actually the same experiment at all!

The primary function of the coincidence counter, as described in the
linked URL, is to separate valid pairs of entangled photons. In
principle, surely, a system could generate a stream of entangled
photon-pairs at predictable intervals. In that case, the primary
function of the coincidence counter would not apply.

However, the coincidence detector also has an another important
function, which is closer to what you are saying, but not quite the
same. The two-slit portion of the apparatus only passes a small
percentage of the photons that hit it. The coincidence counter allows
only photons that passed the slits, and their entangled partners, to be
considered.

Given that most photons fail to pass the slits, this particular
apparatus won't do anything very amazing without a coincidence
detector. The question becomes: could we increase the percentage of
photons passing the two slits (or a different obstacle), so as to
achieve statistically interesting results without deploying a
coincidence detector?

The important point here is whether the coincidence detector is
actually the key to generating the alternative patterns, or whether it
could in principle be dispensed with. While you may well be right that
the former is correct, I don't think your 'simplified version' does
anything to demonstrate it.

- Gerry Quinn

 

[Gerry Quinn]

In article <5oke6hFng1diU9@mid.individual.net>, dirk.bruere@gmail.com
says...
> Gerry Quinn wrote:
> > [Scerir wrote][/color][/color]

> >> There is an interesting paper here
> >> http://www.arxiv.org/abs/quant-ph/0506027
> >> about that 'bilking' or, to say it better,
> >> once the future has unfolded, it cannot change
> >> the past.[/color]
> >
> > Yes. I find the argument quite convincing. It does raise some
> > interesting questions with regard to quantum computation.
> >
> > The authors propose that you cannot go back in a time machine and shoot
> > your father because, essentially, the observation of your own existence
> > demands that a chain of events, no matter how unlikely, led up to it.
> > If you try to do it, you will perhaps shoot someone you think is your
> > father but actually isn't, or your gun will misfire, or some such
> > possibility.
> >
> >>From now on I will revert to the traditional experiment of shooting[/color]
> > your grandfather, as it somehow seems less brutal than simple
> > patricide. Anyway, I propose the Grandfather Computer (TM),
> > constructed using a grandfather, a time machine, pen and paper, and a
> > deadly weapon of some kind.
> >
> > You travel back in time, locate your grandfather, and demand that, on
> > pain of death, he write down the answer to some difficult computation,
> > say the factors of a large number (incidentally, you don't need to tell
> > him the number). If you have so arranged matters that this is the only
> > remotely possible way he could survive, it seems like a way to leverage
> > the evolution of the wave function of the universe to carry out this
> > computation; in short, it is a form of quantum computer.
> >
> > Probably the flaw in the this scheme is the difficulty of actually[/color]
>
> One obvious flaw in the scheme is that it should be the grandmother:-)
> Also, isn't the above quantum computer just a form of random number
> generator plus quantum suicide?[/color]

No, it might look similar in some respects but it's actually quite
different. The big difference is that there is no need to ignore all
outcomes that do not involve the survival of a particular observer.
You can see this by looking at it from the point of view of a third
observer:

In the quantum suicide case, he sees (almost always) just a pointless
death.

In the time-machine version, he always sees exactly what the machine
operator sees; a bilking attempt that fails in some physically
consistent manner. [Conceivably some outcomes would involve the death
of the operator, but these not be numerous and there is no reason for
them to be important.]

So in the latter, there is a always a physical system equivalent to a
quantum computer that can be examined to see how it succeeded or
failed. There is no need to hypothesise the existence of other
universes with a different outcome, or to assert that any hypothetical
alternative universe has a status equivalent to the observed universe.

- Gerry Quinn

 

[scerir]

Andreas Most:
> The "spooky action at a distance" is about correlations
> of measurements and not about single measurements.
> People tend to forget this sometimes.[/color]

There is a general agreement that it is incorrect to say
that the experimenter=92s arbitrary choice of measuring position
or momentum of the idler photon on the right side of the apparatus
somehow CAUSED a specific collapse of the signal photon wavepacket
on the left side.

Only nonlocal, instantaneous, UNCAUSED correlations-at-a-distance
are predicted by quantum theory. Clearly, the collapse phenomenon
is nonlocal and NONCAUSAL in nature.

However whether or not fringes, in coincidence detection, show up
on the left side of the apparatus, DEPENDS ON THE ARBITRARY
CHOICE by the experimenter of measuring - even in the future
(delayed choice) - on the right side of the apparatus, position
or momentum of the idler photon.

Now, what (imo) Cramer is trying to do is to *study* that apparent
contradiction between 'UNCAUSED/NONCAUSAL' and 'DEPENDS ON THE
ARBITRARY CHOICE ...'.

For this purpose he needs a clean source of entangled photons,
to remove the coincidence detection unit, to perform the *same*
measurement on (say) 1000 idler photons, and see what happens
on the other side.

Is that correct?

s.

 

[Gerry Quinn]

In article <fgl9fi$igr$1@gemini.csx.cam.ac.uk>, br276deleteme@cam.ac.uk
says...

> Um. Cramer apparently thinks that the coincidence counter in experiments
> like Dopfer's is just an engineering detail, and that by throwing it away
> you can get a superluminal communication device.[/color]

Well, on the face of it, you might! The question is whether it *can*
be thrown away. Its role is important, but not quite so central as you
make out in your other post.

> Cramer's ideas have always
> seemed a bit odd to me, but this is the first time I've seen what appears to
> be clear evidence that he's incompetent as a theorist. I can't tell whether
> he thinks quantum mechanics predicts superluminality in this experiment or
> whether he thinks previous work fully consistent with quantum mechanics
> somehow points the way to a result violating quantum mechanics[/color]

A middle ground is to expect that quantum mechanics doesn't predict
superluminal communication, but that the way in which superluminal
communication fails is due to just a little bit too much error-
correction being needed in any scheme that might seem like it could
generate superluminal communication.

That's what I believe; what Cramer believes I obviously can't say. He
hasn't stated that it definitely will create superluminal
communication, though. He has observed that according to a certain
argument - no claim is made that every possible ramification of quantum
theory is fully taken into account - it appears as if it might. That
is nothing out of the ordinary in physics, in my opinion.

- Gerry Quinn

 

[scerir]

Gerry Quinn:
> That's what I believe; what Cramer believes
> I obviously can't say. He hasn't stated that
> it definitely will create superluminal
> communication, though. He has observed that
> according to a certain argument - no claim
> is made that every possible ramification of quantum
> theory is fully taken into account - it appears
> as if it might. That is nothing out of
> the ordinary in physics, in my opinion.[/color]

Since QM does not "explain" those correlations,
that is to say it does not explain them
in terms of space (see nonseparability)
and in terms of time (see noncausality,
see delayed choices, etc.) Cramer is trying
to fix an ontology (in terms of advanced
and retarded actions, via the source).
Of course his ontology is not new:
Huw Price, Costa de Beauregard, Klyshko,
and many more, wrote papers about it.

See his transactional interpretation
of Dopfer's experiment on the slides here
http://www.kathryncramer.com/kathryn_cramer/2006/09/retrocausality.html
and also
http://www.analogsf.com/0612/altview.shtml

 

[Andreas Most]

scerir wrote:
> Andreas Most:
>> The "spooky action at a distance" is about correlations
>> of measurements and not about single measurements.
>> People tend to forget this sometimes.[/color]
>
> There is a general agreement that it is incorrect to say
> that the experimenter=92s arbitrary choice of measuring position
> or momentum of the idler photon on the right side of the apparatus
> somehow CAUSED a specific collapse of the signal photon wavepacket
> on the left side.
>
> Only nonlocal, instantaneous, UNCAUSED correlations-at-a-distance
> are predicted by quantum theory. Clearly, the collapse phenomenon
> is nonlocal and NONCAUSAL in nature.[/color]

"Collapse" is actually nothing physical. It is only a notion we have
introduced to explain when we know "what" about the physical state.
E.g., you cannot assign a time stamp to the collapse. By choosing
an appropriate reference frame one could come to the conclusion that
the collapse took place right after the creation of the entangled pair.

> However whether or not fringes, in coincidence detection, show up
> on the left side of the apparatus, DEPENDS ON THE ARBITRARY
> CHOICE by the experimenter of measuring - even in the future
> (delayed choice) - on the right side of the apparatus, position
> or momentum of the idler photon.
>
> Now, what (imo) Cramer is trying to do is to *study* that apparent
> contradiction between 'UNCAUSED/NONCAUSAL' and 'DEPENDS ON THE
> ARBITRARY CHOICE ...'.
>
> For this purpose he needs a clean source of entangled photons,
> to remove the coincidence detection unit, to perform the *same*
> measurement on (say) 1000 idler photons, and see what happens
> on the other side.
>
> Is that correct?[/color]

You cannot remove the coincidence unit. This has nothing to do
about whether you have a "clean source" of entangled photons.
The measurement on the idler so-to-say selects the valid pairs.
There is no way to settle this in advance.
(Someone once came up, that it is a type of book keeping, that
the idler photon determines which photons on the screen are
taken into account. I don't like this type of picture, because
it silently implies that the outcome of a measurement is fixed
since the creation of the pair, which is disproved by the violation
of Bell's inequality. But it is maybe not so bad as a guideline...)

The only way Cramer could be right would be quantum mechanics being
wrong in this respect. From my point of view, however, this would
violate observations in quantum statistics that are well described
by quantum mechanics.

Andreas.

 

[PostReplies]

On Mon, 5 Nov 2007 19:04:01 +0000 (UTC), Andreas Most
<Andreas.Most@nospam.de> wrote:

>Definitely, he will see no interference pattern whatever he does on the
>sender side. Only if he correlates his actions on the sender side with
>the measurements on the receiver side he is able to extract an
>interference pattern.
>[/color]

I thought I understood the classical two slit experiment but I must
not (I'm not a physicist). In the classical experiment if you rig the
experiment such that only photons that went through the slits can
possibly hit the back wall of the apparatus you should see a definite
interference pattern on the back wall. If you then put a detector on
the slits to record their location information the pattern should
disappear. The detector is causing a collapse of the wave function and
therefore each photon no longer has wave characteristics so cannot
interfere with itself. You can even slow the experiment down to firing
one photon a second if you want to convince yourself that different
photons aren't interferring with each other causing the pattern.

I thought that the EPR paradox was a thought experiment in which if
you take that experiment but add a second beam of entangled photons
then when you collapse the wave function for a photon in beam A it
would in theory collapse the wave function for the entangled photon in
beam B. I thought according to QM that happens instantaneously no
matter how far apart the two photons are. That is "spooky action at a
distance" so Einstein considered it a paradox.

I know that in theory FTL communication cannot happen. The "why" part
confuses me. If the photons in the entangled beam actually do take on
particle-like properties then wouldn't that beam no longer be able to
produce an interference pattern?

 

[Ben Rudiak-Gould]

Gerry Quinn wrote:
> I'm not convinced your 'simplified' version of the experiment is
> actually the same experiment at all![/color]

It's not intended to be the same, just to illustrate the importance of
the coincidence counter. It's pretty similar though, aside from being
discrete and classical and omitting the slits.

> The primary function of the coincidence counter, as described in the
> linked URL, is to separate valid pairs of entangled photons.[/color]

That's wrong; it's a misunderstanding by the guy who wrote that page,
and it's presumably the cause of all his other misunderstandings. The
two detectors have very narrow detection cross sections, and
deliberately miss most of the photons that pass them by. If the
coincidence counting had the purpose you suggest, it would make sense to
replace the fixed detector by one with a much wider cross section, since
this would give you a much larger data set. In reality this would
destroy the signal: the better the detector at D2, the less difference
there will be between the two graphs labeled "Measurement at D1". These
graphs do not show measurements at D1. They show a slice through the
parameter space of the nonseparable function f(x1,x2) that relates
detector position to coincidence count. With a wider detector at D2
you'd instead get integral f(x1,x2) dx2, which would look completely
different.

-- Ben

 

[Andreas Most]

PostReplies wrote:
> On Mon, 5 Nov 2007 19:04:01 +0000 (UTC), Andreas Most
> <Andreas.Most@nospam.de> wrote:
>
>> Definitely, he will see no interference pattern whatever he does on the
>> sender side. Only if he correlates his actions on the sender side with
>> the measurements on the receiver side he is able to extract an
>> interference pattern.
>>[/color]
>
> I thought I understood the classical two slit experiment but I must
> not (I'm not a physicist). In the classical experiment if you rig the
> experiment such that only photons that went through the slits can
> possibly hit the back wall of the apparatus you should see a definite
> interference pattern on the back wall. If you then put a detector on
> the slits to record their location information the pattern should
> disappear. The detector is causing a collapse of the wave function and
> therefore each photon no longer has wave characteristics so cannot
> interfere with itself. You can even slow the experiment down to firing
> one photon a second if you want to convince yourself that different
> photons aren't interferring with each other causing the pattern.
>
> I thought that the EPR paradox was a thought experiment in which if
> you take that experiment but add a second beam of entangled photons
> then when you collapse the wave function for a photon in beam A it
> would in theory collapse the wave function for the entangled photon in
> beam B. I thought according to QM that happens instantaneously no
> matter how far apart the two photons are. That is "spooky action at a
> distance" so Einstein considered it a paradox.[/color]

The old Kopenhagen interpretation considered the so called collapse
of the wave function as a physical process. This implies a lot of
paradoxes not to mention that nobody has ever been able to prove that
there is actually a collapse happening.
Nowadays most physicists avoid the term collapse. Quantum mechanics
does not actually describe a collapse but it describes the probabilities
of possible measurement outcomes. This is called the minimal
interpretation.
An entangled pair cannot be considered as two separate objects
like you would do in classical physics. It is a superposition
of states or loosely spoken a superposition of possible measurement
results.

It is actually worth noting about what you said about EPR, that the term
instantaneously immediately raises the question:
In which reference frame? In a certain frame the measurement on particle
B might have been the first. So, which measurement actually collapses
the wavefunction of the other particle?
Going a bit beyond the minimal interpretation you could consider the
wave function being the information content about a quantum system.
This goes back to Heisenberg, I think already in the 1930s.

> I know that in theory FTL communication cannot happen. The "why" part
> confuses me. If the photons in the entangled beam actually do take on
> particle-like properties then wouldn't that beam no longer be able to
> produce an interference pattern?[/color]

In an quantum eraser experiment the signaling photons do not make
up an interference pattern. Only if you apply a certain measurement
on the idler photon and set up the appropriate coincidence between
the two photons (thereby selecting a subset) you will be able to
see an interference pattern.
The point is that you cannot control the measurement outcome of your
idler photon. That is, you need to know the result in order to select
or deselect the corresponding signaling photon for the interference
pattern. By doing so you need to communicate classical information
about the measurement on the idler which is not FTL.

By no means Cramer will be able to circumvent the necessity of a
coincidence unit.

Andreas.

 

[scerir]

Andreas Most:
> You cannot remove the coincidence unit.
> This has nothing to do about whether you have
> a "clean source" of entangled photons.
> The measurement on the idler so-to-say
> selects the valid pairs. There is no way
> to settle this in advance.[/color]

If you want to measure the two-photon interference,
you have to observe, and register, the behaviour
of both entangled photons, the idler photon at its
wing, the signal photon at its wing.

This has much to do with Wheeler's (and Bohr's) saying:
'No elementary phenomenon is a phenomenon until it is
a registered phenomenon.'

Of course, because we are performing a two-photon
interference experiment, we must use an appropriate
source of entangled photons (this source must have
a 'large' size).

Now we can also ask something like this.

Imagine we want to observe what happens at the signal
photon wing only. That is to say: we want to see if there
is some visible interference pattern, made by signal
photons, on the two-slit interferometer screen,
without looking at the other side (where somebody
else is measuring the idler photons).

(We do not use the coincidence detection unit here,
because we are interested in whether there is some
visible interference pattern, made by signal photons,
on the two-slit interferometer screen, without looking
at the other side, where idler photons are measured).

And imagine we cannot see any interference pattern
on that screen. (Apparently this is very strange
because we have a two-slit interferometer and a beam
of photons, and we do not see any interference
pattern).

What is the reason?

The possible reason seems (to me) this one. Signal
photons cannot cause their interference pattern on
the screen because their momentum uncertainty is large.
And their momentum uncertainty is large because the
source of entangled photons we (must) use to perform
a two-photon interference experiments has a 'large'
size (divergence of the beam).

 

[Andreas Most]

scerir wrote:

> And imagine we cannot see any interference pattern
> on that screen. (Apparently this is very strange
> because we have a two-slit interferometer and a beam
> of photons, and we do not see any interference
> pattern).
>
> What is the reason?
>
> The possible reason seems (to me) this one. Signal
> photons cannot cause their interference pattern on
> the screen because their momentum uncertainty is large.
> And their momentum uncertainty is large because the
> source of entangled photons we (must) use to perform
> a two-photon interference experiments has a 'large'
> size (divergence of the beam).[/color]

This has nothing to do with our poor set up of the experiment.
Imagine a quantum eraser experiment with two slits where the
left slit is covered with a horizontal polarization filter and
the right slit is covered with a vertical one. If you shoot linearly
polarized photons (at an angle of 45°) through it there will be no
interference pattern on the screen because left and right linearly
polarized electromagnetic waves do not interfere.
Now, If you chose to measure circular polarization on the idler photon
and select those events where the idler photon has e.g. right circular
polarization then you would see the interference pattern for the
selected signaling photons on the screen.
(By choosing the left circularly polarized idler photons you would
see a shifted interference pattern)

You could argue now that it is possible to use circularly polarized
photons from the very beginning in which case you would see an
interference pattern on the screen without the need of any
coincidence unit. The question is then, what is the use of the idler
photon if not to decide on the type of measurement. And, does the type
of measurement actually change the interference pattern?
The answer by QM and experiment is definitely: No.

Andreas.

 

[scerir]

Andreas wote:

> > Signal photons cannot cause their interference pattern
> > on the screen because their momentum uncertainty is large.
> > And their momentum uncertainty is large because the
> > source of entangled photons we (must) use to perform
> > a two-photon interference experiments has a 'large'
> > size (divergence of the beam).[/color][/color]

> This has nothing to do with our poor set up of the
> experiment.[/color]

Since we were talking about the possibility of 'signaling',
at a distance, using a two-photon interference set-up,
having removed the coincidence detection unit, I was only
pointing out that the usual set-up, i.e. the usual SPDC
source itself, might not allow any single-photon interference
pattern, at the signal wing, for essential reasons (divergence
of the beam). More below.

> Imagine a quantum eraser experiment with two slits where the
> left slit is covered with a horizontal polarization filter and
> the right slit is covered with a vertical one. If you shoot linearly
> polarized photons (at an angle of 45°) through it there will be no
> interference pattern on the screen because left and right linearly
> polarized electromagnetic waves do not interfere.
> Now, If you chose to measure circular polarization on the idler photon
> and select those events where the idler photon has e.g. right circular
> polarization then you would see the interference pattern for the
> selected signaling photons on the screen.
> (By choosing the left circularly polarized idler photons you would
> see a shifted interference pattern).[/color]

Yes, I know these interesting experiments.
http://www.arxiv.org/abs/quant-ph/0106078
http://icpr.snu.ac.kr/resource/wop.pdf/J01/1998/033/R04/J011998033R040383.pd
f

> You could argue now that it is possible to use circularly polarized
> photons from the very beginning in which case you would see an
> interference pattern on the screen without the need of any
> coincidence unit. The question is then, what is the use of the idler
> photon if not to decide on the type of measurement. And, does the type
> of measurement actually change the interference pattern?
> The answer by QM and experiment is definitely: No.[/color]

Again, an important distinction is to be made.
Two-photon interference and one-photon interference
are obviously different phenomena. In the first case
you need a coincidence detection unit of some sort
(two clocks at least). In the second you do not need
any coincidence device.

It seems to me (I may be wrong of course) that
these position/momentum correlated photons
'signaling' machines are based on a sort of ... fusion :-)
of the one-photon and the two-photon interference
phenomena (you perform a specific measurement on the idler
photons and, at a distance, without checking the coincidences,
an interference pattern would appear, or disappear,
at the signal wing).

Now it is known, since long time, there is a weird
'complementarity' principle between the one-photon
and the two-photon interference. In the sense that
the more you can see the first interference, the less
you can see the second interference, and viceversa.

See, i.e., these papers:

M.A.Horne, A.Shimony, A.Zeilinger, 'Two-Particle Interferometry',
Phys.Rev.Lett. 62, 2209 (1989).

M.A.Horne, A.Shimony, A.Zeilinger,
'Two-Particle Interferometry', Nature, 347, 429 (1990).

D.M. Greenberger, M.A. Horne and A. Zeilinger,
'Multiparticle Interferometry and the Superposition Principle',
Physics Today 46 8, (1993).

and these specific experiments ...
http://www.arxiv.org/abs/quant-ph/0112065
http://josab.osa.org/abstract.cfm?id=35389

Since the 'complementarity' principles, in general,
presuppose a 'smooth' transition from the visibility
of a phenomenon to the visibility of the other,
here we can also expect (imo) a smooth transition
from the visibility of a single-photon interference
to the visibility of a two-photon interference,
and viceversa. If there is an intermediate situation
in which both interferences are (badly) visible,
and if - in this intermediate situation - it is possible
to perform 'signaling' (in principle) experiments,
I cannot say.

Summing up. I think WE CAN AGREE that in the two-photon
interference we need a coincidence detection unit,
and in the usual single-photon interference we do not
need such a device. I am pointing out that it is not
just about the use of the coincidence unit, or the use
of specific detectors. There is much more physics beyond,
there are many essential principles involved here.

Regards,
s.

 

[PostReplies]

On Wed, 14 Nov 2007 19:14:10 +0000 (UTC), "scerir" <scerir@libero.it>
wrote:
>
>The possible reason seems (to me) this one. Signal
>photons cannot cause their interference pattern on
>the screen because their momentum uncertainty is large.
>And their momentum uncertainty is large because the
>source of entangled photons we (must) use to perform
>a two-photon interference experiments has a 'large'
>size (divergence of the beam).
>[/color]

Reading all the posts I still don't see why there wouldn't be an
interference pattern. What causes the pattern is each photon
interferring with itself. Since each photon that registers on the back
of the apparatus had to go through a slit in order to even register,
and since we don't know which slit, then according to QM it seemingly
goes through both slits and interferes with itself. Isn't that the
crux of the dual slit experiment? That the source of the photons just
so happened to generate another entangled beam of photons I don't see
how that affects the dual slit experiment.

As far as I can see, polarization doesn't matter in Cramer's
experiment since he's only using the particle vs wave
complimentariness in the experiment--will a particle-like pattern show
up or an interference patttern.

Also, I don't think his experiment has anything to do with quantum
erasers. If you make a which-slit detection and then send the photon
through another 'which path did it take?' choice then you lose the
intereference pattern--not too surprising given what is known about
how QM works. But I don't see what that has to do with Cramer's
experiment. Neither do I see what any kind of delayed choice
experiment has to do with it since the experiment can be performed
without that.

I expect that the sender can at will turn on and off the interference
pattern on his side in the same way as the classical dual slit
experiment. Apparently, that will have no effect on the receiver side
since that would violate Relativity. Perhaps once the results of the
exxperiment are published it will be more understandable to me about
why it failed.

 

[Andreas Most]

PostReplies wrote:

> Reading all the posts I still don't see why there wouldn't be an
> interference pattern. What causes the pattern is each photon
> interferring with itself. Since each photon that registers on the back
> of the apparatus had to go through a slit in order to even register,
> and since we don't know which slit, then according to QM it seemingly
> goes through both slits and interferes with itself.[/color]

That is not quite true. It doesn't matter whether we know or we do not
know which slit the photon has taken. If we could possibly know which
path the photon has taken there will be no interference pattern.
That is a subtle but important difference and it is actually the reason
why the coincidence unit in the quantum eraser experiment is mandatory.

> Isn't that the
> crux of the dual slit experiment? That the source of the photons just
> so happened to generate another entangled beam of photons I don't see
> how that affects the dual slit experiment.
>
> As far as I can see, polarization doesn't matter in Cramer's
> experiment since he's only using the particle vs wave
> complimentariness in the experiment--will a particle-like pattern show
> up or an interference patttern.[/color]

"Polarization" is here a tool to tag the photons path. There are other
ways of doing that but polarization is probably the easiest to accomplish.

> Also, I don't think his experiment has anything to do with quantum
> erasers. If you make a which-slit detection and then send the photon
> through another 'which path did it take?' choice then you lose the
> intereference pattern--not too surprising given what is known about
> how QM works. But I don't see what that has to do with Cramer's
> experiment. Neither do I see what any kind of delayed choice
> experiment has to do with it since the experiment can be performed
> without that.[/color]

IMHO the quantum eraser experiment exhibits all the features to
understand what is going on in a delayed choice experiment and why
Cramers experiment will fail. Actually, what Cramer sets up is a
quantum eraser without a coincidence unit. Since the outcome is
already known it is clear that Cramer will fail.

> I expect that the sender can at will turn on and off the interference
> pattern on his side in the same way as the classical dual slit
> experiment. Apparently, that will have no effect on the receiver side
> since that would violate Relativity. Perhaps once the results of the
> exxperiment are published it will be more understandable to me about
> why it failed.[/color]

Andreas.

 

[Andrew Ray]

In post 23, Ben Rudiak-Gould said

... if you replace the detectors with photographic plates, there will
be no interference pattern on either plate.

Putting aside the question of FTL communication and Cramer's experiment, this appears to be an important part of the argument that coincidence detection is an essential part of Dopfer's experiment (or, at least, a show-stopping consequence of not having it).

Ben, by "either plate", do you mean 'a plate at either position of D1' (i.e. positions f and 2f beyond the Heisenberg lens), or are you saying that furthermore, there will be no fringes recorded at D2 (i.e. on the far side of the slits), regardless of where the film on the lens leg is? If the former, then do you predict there will always be fringes on the plate at D2?