From: mkagalen@lynx.dac.northeastern.edu (michael kagalenko)
Newsgroups: sci.physics
Subject: Re: photon 'detectors' - how reliable?
Date: 25 Jan 93 22:36:10 GMT
Organization: Northeastern University, Boston, MA. 02115, USA

In article <1k1nn0$b8d@agate.berkeley.edu> aephraim@physics3 (Aephraim M. Steinberg) writes:
>In article <1993Jan23.224608.16959@lynx.dac.northeastern.edu> mkagalen@lynx.dac.northeastern.edu (michael kagalenko) writes:
>>In article <1jms14$ibn@agate.berkeley.edu> aephraim@physics3 (Aephraim M. Steinberg) writes:
>>>In article <wwadge.727584610@csr> wwadge@csr.UVic.CA (Bill  Wadge) writes:
>>>>I was browsing through one of the many 'pop' physics books,
>>>>
>>>> .. 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).
>>>
>>No, it won't. It's general property of QM that if you have inelastic
>>scattering (photon-detector interaction is inelastic, of  course), there
>>always exists non-zero elastic part in the cross-section (see Landau's
>>QM I, chapter about non-elastic scattering). 
>
>Thank you for the correction.  I haven't looked up the reference yet,
>but I can believe the result; by adding "(just about)" I mean to acknowledge
>that no process is 100% efficient, but that one could get arbitrarily
>close in principle.
>
>Nonetheless, I am curious about a particular inelastic process which seems
>to me to be 100% efficient (in principle, of course), and I wonder whether
>(i) Some idealization I assume is not even possible in principle, OR
>(ii) Your statement holds not for cross-sections but rather for scattering
>amplitudes, implying that interference effects could be arranged to make
>the total elastic cross-section go to zero.
>
>The instance I have in mind is a superconducting micromaser cavity.  If
>an atom in an excited state, whose excitation is resonant with the cavity,
>is sent through the maser, it has some chance of decaying.  As the atom  
>propagates through the cavity, it undergoes Rabi oscillations, emitting
>a photon, reabsorbing it, and so on.  Walther's group in Munich has
>shown that by correctly selecting the atom's velocity, it is possible to
>cause it to decay most of the time.  Now, if the cavity is lossless,
>it seems to me that there exist incident k-vectors for which the probability
>is exactly 100%.  True, in a real experiment, there is a superposition of
>different velocities, and the cavity isn't ENTIRELY lossless, but are these
>issues of fundamental importance, or is it perhaps unfair to neglect edge 
>effects when the atom enters or exits the cavity, or is my picture correct IN
>PRINCIPLE, and the theorem somehow evaded in this case?
>
Now I'm intruding into the area I'm not really fluent in. I don't think 
one can really say that resistance of superconductor is 0 ; it's
possible to prove that it's less than $\epsilon $(what is the current
value of it, BTW ?) You can show than terms up to the sertain order in 
resistance are zero; but the possibility for nasty high-oreder effect 
which will cause dissipation still remains.
 Corrections ?