Re: Clock synch. with entangled photons??

From: Tom Roberts (tjroberts_at_lucent.com)
Date: 10/09/04 

Date: Sat, 09 Oct 2004 00:51:57 GMT

This message includes responses to multiple posts in this thread.

shevek wrote:
> I'm trying to go through this paper:
> http://arxiv.org/abs/quant-ph/0407204
>
> which was mentioned last week in science news:
> "Physicists have demonstrated a new technique for bringing distant
> clocks into closer synchronization by means of entangled photons whose
> quantum properties are mysteriously correlated." [...]
>
> Am I right in reading that this is a novel measurement technique, and
> is not a real change in the nature of relativistic coordinate systems?

Correct. As they state in their opening remarks, the resolution of pulse
comparison techniques is the current limit on the ability to synchronize
clocks. In essence what they have done is created a pulse generator of
correlated photons with a smaller uncertainty in the time between the
two photons.

In short, this is an engineering improvement for conventional Einstein
synchronization. The actual synchronization relies on the propagation of
two light signals from a common source to two spatially-separated
detectors+clocks.

Note also that while they claim the time difference between the two
photons is predicted to have a sigma of 800fs, their actual measurement
had a sigma of 750ps, almost a 1000 times less precise than the
theoretical time difference. So while there is a THEORETICAL
improvement, their proof-of-principle experiment did not actually
demonstrate the improvement (resolutions more than ten times better than
750ps are straightforward).

Bill Hobba wrote:
> But from what I can see no fundamental physical principles would seem to be
> violated. It seems to be based on standard theory whose basis is still
> intact - eg it is not possible to send information faster than light.

Yes.

Greysky wrote:
> it remains to be asked how will you obtain the
> correlation with a remote system, like satellites?

Presumably by sending the photons through the atmosphere and space. This
is indeed a major criticism for using such a technique for space-based
clocks, as the propagation through the atmosphere is known to have
uncertainties far larger than standard pulse-comparison techniques -- so
this improvement would be wasted on such systems.

Daniel Weston wrote:
> If separated clocks can be sync.ed with entanglement, would this not
> permit the testing of OWLS?

You should actually read the paper, instead of responding to a sound
bite. Had you actually read the paper you would know that this is merely
a technique to improve the accuracy of Einstein synchronization. The
entanglement is not used to perform the synchronization, it is used to
generate two photons that are tightly correlated in time of emission, so
the uncertainty in the time difference of the two synchronization pulses
is reduced.

bernard.chaverondier write:
> However, this would need an instantaneous communication
> thanks to EPR effect.

You, too, need to actually read the paper. This has nothing whatsoever
to do with the EPR effect, except that the word "entanglement" was used
-- but the entanglement is used in a completely different way than in
the EPR effect. See above.

Tom Roberts tjroberts@lucent.com

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