Re: Science News - article on Quantum Crypto

>The new Science News that showed up in today's mail
>has a short but informative article on quantum
>cryptography.  It explains what quantum crypto is for
>us newbies and gives the history of a recent success
>in Lake Geneva (Switzerland, not Wisconsin) - an
>encrypted message was sent 22.7 km over fiber-optic
>cable successfully.  It's a nice article.

Does anyone know if the new in-line optical amplifiers (not switches!) have
any effect on quantum crypto messages?

Cheers,
Bob Hettinga

-----------------
Robert Hettinga (rah@shipwright.com)
e$, 44 Farquhar Street, Boston, MA 02131 USA
"Reality is not optional." --Thomas Sowell
The e$ Home Page: http://thumper.vmeng.com/pub/rah/

Bob writes:

> Does anyone know if the new in-line optical amplifiers (not switches!) have
> any effect on quantum crypto messages?

Yes, any active devices in your communications path would be unable to
function without making some kind of classical measurement on the
photons involved (e.g. measuring phase relative to a definite test
angle, if phase is what's being modulated), thereby collapsing the
wavefunction and spoiling any special properties afforded by being
able to send photons down the line without "looking at them."  Optical
repeaters have to pass your signal through an intermediate electronic
stage anyway, since we have no purely optical valve/transistor
equivalents (bosons don't interact with each other at all).

Can someone think of a reason why this wouldn't necessarily be so?

> Cheers,
> Bob Hettinga

- alex

> 
> Bob writes:
> 
> > Does anyone know if the new in-line optical amplifiers (not switches!) have
> > any effect on quantum crypto messages?

  Optical
> repeaters have to pass your signal through an intermediate electronic
> stage anyway, since we have no purely optical valve/transistor
> equivalents (bosons don't interact with each other at all).

	This is not true.   There is now a whole technology of optical
amplifiers for fiber communications systems that used Ettrium doped
fibers pumped with strong light from a laser at a slightly shorter
wavelength. These fiber optical amplifiers have gains in the order of
10-12 db in a section of special doped fiber only about 10 feet long.

	The current generation of undersea cables from the US to Europe
use these amplifiers instead of the more traditional regenerating
repeaters that convert the light to electronic signals, reclock the data
stream and convert it back to light with another laser diode.   There is
no conversion from light to digital electronic signals all the way from
Rhode Island to England - the same light pulses that go into the fiber
on one side of the Atlantic come out on the other end without ever
having been converted to electronic form in between.

	 These amplfiers have enourmous bandwidth, and can be used to
amplify several slightly different wavelengths of light allowing
wavelength division multiplexing of multiple streams of light flashes of
slightly different "colors" (all the current technology works at around
1500 nm which is well into the infrared).   This can expand the capacity
of a single fiber to four to six times the 5 Gb/sec that is the current
state of the art.

							Dave Emery
							die@die.com


























                                        

> 
> Can someone think of a reason why this wouldn't necessarily be so?
> 
> > Cheers,
> > Bob Hettinga
> 
> - alex
>

I wrote:

> > Optical
> > repeaters have to pass your signal through an intermediate electronic
> > stage anyway, since we have no purely optical valve/transistor
> > equivalents (bosons don't interact with each other at all).

Dave Emery responds:
> 	This is not true.   There is now a whole technology of optical
> amplifiers for fiber communications systems that used Ettrium doped
> fibers pumped with strong light from a laser at a slightly shorter
> wavelength. These fiber optical amplifiers have gains in the order of
> 10-12 db in a section of special doped fiber only about 10 feet long.

You're right, I do remember reading about these somewhere... didn't
realize they were already in use.

Even so, I still don't think such a repeater would pass quantum-crypto
signals, excepting any photons that happened to just "leak" directly
through.  Your useful quantum state information resides in the
individual photons originally sent, and any even the optical repeaters
you describe achieve gain by by gating in *more* photons under the
incoming signal's control.  In so doing it will collapse the
wavefunctions of these incoming photons.

Not to say repeaters on the line aren't possible, but they'll have to
decode your data using a copy of the "secret" key, then re-encode for
transmission... so this will be a potential break-in point and need
good physical security.

> 	The current generation of undersea cables from the US to Europe
> use these amplifiers instead of the more traditional regenerating
> repeaters that convert the light to electronic signals, reclock the data
> stream and convert it back to light with another laser diode.   There is
> no conversion from light to digital electronic signals all the way from
> Rhode Island to England - the same light pulses that go into the fiber
> on one side of the Atlantic come out on the other end without ever
> having been converted to electronic form in between.

You said power for the amps comes from a high-intensity,
shorter-wavelength beam... can this be superimposed on the original
signal at the point of origin, as with in-line coax-cable amplifiers?

> 	 These amplfiers have enourmous bandwidth, and can be used to
> amplify several slightly different wavelengths of light allowing
> wavelength division multiplexing of multiple streams of light flashes of
> slightly different "colors" (all the current technology works at around
> 1500 nm which is well into the infrared).   This can expand the capacity
> of a single fiber to four to six times the 5 Gb/sec that is the current
> state of the art.

Nice... :)

- alex