X-Message-Number: 13714
From: 
Date: Fri, 12 May 2000 07:40:58 EDT
Subject: Mirrors for quantum nondemolition interferometers.

Mirrors for QND interferometers

Here I start to look at the technology exploited to build a brain reader:

An X-ray interferometer presents a difficult requirement: All its componments 
must be positionned with a precision far better than an atom diameter. Some 
years ago, that was seen as an impossibility. Now even stringent requirements 
are found in interferometric gravitational wave detectors. A small revolution 
in itself.

Another problem comes from the mirrors. Ordinary surfaces reflect X-rays only 
at grazing incidence, so many of them must be used to send back a beam after 
a 180 degrees rotation. Multicoated mirrors do a better job, with molecular 
epitaxy and one atom tick deposition a mirror can send back an X-ray beam. 
That technology has been exploited in H-bombs for nearly forty years and 
falls now in the civilian domain. Such mirrors have a reflectivity in the 
90-99 percent range, this is good for an "ordinary" interferometer, not for a 
QND system where the  beam must make up to one million trips in the aparatus.

Here, the only solution is the active mirror. This is a device where the beam 
is absorbed and each photon is regenerated. In fact it is an amplifier (a 
laser) with a gain tunned to one. The problem here is that the new photon 
must "know" the history of the old one: its polarization, how many trips it 
has done in the interferometer and so on. The quantum state of the old photon 
must be teleported to the new particle. Such mirrors are highly complex 
quantum system with an X-ray laser, correlated photons and "entangler". This 
is a far cry from a simple silvered smooth surface. The interferometer uses 
three mirrors, there must be as many X lasers.


Yvan Bozzonetti.

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