archiv~1: RE: SETI Quantum Communications?

RE: SETI Quantum Communications?

jbrown2 ( jbrown2@san.rr.com )
Mon, 2 Nov 1998 16:16:33 -0800

Jacques - Please clear something up for me. When a physicist, such as your
self, says that you 'measure the state of a particle' do you mean that
literally or figuratively? If literally - what kinds of instruments are
used that can differentiate the state of one particle from another. Or do
you mean that you measure the state from inference or a mathematical
understanding of the action of the particles?

This sort of question is always occurring to me as I read about science
being done in an area that I have no basic understanding.

-----Original Message-----
From: Jacques.Tempere [SMTP:jtempere@uia.ua.ac.be]
Sent: Monday, November 02, 1998 11:32 AM
To: seti@sni.net
Subject: Re: SETI Quantum Communications?

Hi,

As I'm new to this list, I'd like to quickly introduce myself before I
join in on discussions. I'm a physics ph.d. student at Antwerp university,
Belgium. I do theoretical research on the properties of ultracold atoms
and atom lasers, a field in the crossover between solid state physics and
quantum optics. I got interested in SETI after the report of fossilised
bacteria in a meteorite alledgedly from Mars, and after the BBC report on
EQ Pegasi I decided to join a discussion list to find out what's going on.
Which I did the last few days ;-)

> Well, I could see it possibly providing a secure channel between
> extra-terrestrial civilizations when used with some other method of
> communication (laser or microwave), but I can't see it providing an FTL
> link, only a method of encryption.

As I understand it, that's right. The heart of any quantum teleportation
experiment is a pair of 'twinned' particles (not necessarily photons).
When the unmeasured state of one of the twins is altered, the
unmeasured state of the other twin is altered instantaneously in the
reference frame of the particle pair.

Due to the probabilistic nature of any quantum measurement, all that you
see when you measure on one of the twins is a statistical distribution,
like numbers drawn from the lottery.

But when you compare the statistical distribution of one of the twins with
that of the other twin, you see that there are correlations. But this you
can only notice when you compare the measurements on both twins, and to do
this comparison you have to communicate, light-speed limited, the results.

To summarise, even if you have a member of the twin pair, your measurement
results don't reveal that there is another twin somewhere else - only
comparison of measurements on both show that there was some 'link'. And
this is the basic reason why you can't use quantum teleportation to send
information FTL, as I see it.

Regards,

Jacques Tempere