From: aephraim@physics3 (Aephraim M. Steinberg)
Newsgroups: sci.physics
Subject: Re: photon 'detectors' - how reliable?
Date: 21 Jan 1993 19:03:00 GMT
Organization: /etc/organization
NNTP-Posting-Host: physics3.berkeley.edu

In article <wwadge.727584610@csr> wwadge@csr.UVic.CA (Bill  Wadge) writes:
>I was browsing through one of the many 'pop' physics books,
>where they are describing one of the stock quantum mechanics
>experiments, and noticed something along the lines of
>
> .. towards a detector which records every photon ..
>
>Is this possible? Can one really build a device so sensitive that it will
>detect 100% reliably every photon that enters it, and never
>go off by accident?
>
>Seems unlikely, but then I'm not a physicist ...

Does seem unlikely, doesn't it?  But it will happen one day (just about).

Some photodiodes currently quote quantum efficiencies very close to
100%, but not when operating in single-photon detection mode.  That is, they
yield a continuous current proportional to the incoming intensity and
supposedly nearly every incoming photon yields an electron-hole pair.  (I
don't know how-- or whether-- this last fact is confirmed.)

The SINGLE-photon detectors of preference until recently were photomultiplier
tubes, with efficiencies ranging from less than 1% to perhaps 5%.  This is
one of the reasons (along with the geometric nature of the decay process
originally used) that the "detection loophole" was not closed by experiments
like those of Clauser and of Aspect.  In recent years, pmt's have been
superceded by avalanche photodiodes, frequently with quantum efficiencies
of 10 or 20%.  We have one photodiode from EG&G which is about 40% efficient.

VERY recently, however, there has been some progress towards nearly-100%
efficient detectors.  EG&G offers rather expensive "single-photon counting
modules" which we have measured to have efficiencies of approximately 75%
when they are sufficiently overbiased-- the active element in these modules
is still an APD.  On a different tack, Rockwell has been working on 
"solid state photomultipliers" which theoretically should have quantum
efficiencies as high as 90 or 95%, although unlike the APD's, they need to
be cooled to near-liquid Helium temperatures.  We have measured efficiencies
close to 70% for these devices as well, and it is quite likely that there
were other losses present during that measurement, so the actual efficiency
could well be higher.  Needless to say, both companies (and others) are
working towards further improvements.

As far as I know, most people only care in a "linear" fashion about
such efficiencies.  For tests of Bell inequalities, on the other hand, 
there is a sharp cutoff of 83% (ignoring all other experimental imperfections)
for eliminating the detection loophole.  (Although some recent papers
propose ways of reducing this cutoff to around 70%, it is nearly certain
that there will always be some sharp cutoff, and that these schemes will
be less feasible than those which require 83%.)

Anyway, don't trust everything you read.  The same physics book that
mentioned 100% efficient photodetectors could well have mentioned frictionless
pulleys and massless ropes: With many of the former and one of the latter,
one could construct a 100% efficient photodetector simply by amplifying
the "light pressure" acting on a ball attached to one end of the rope!


-- 
Aephraim M. Steinberg                    | "WHY must I treat the measuring
UCB Physics                              | device classically??  What will
aephraim@physics.berkeley.edu            | happen to me if I don't??"
                                         |       -- Eugene Wigner