The closest encounter I had with superconductors was when I was helping a
friend with some measurements on some uranium-based ceramics. Was both
brief and nice, and I lost fear of liquid nitrogen there.
Rational fear of LN is a good thing, though. Minor splashes aren't bad, but enough can cause serious burns.
I also worked with uranium in ceramics, though they were not "uranium-based" (though sometimes we thought they were!).
Jamming is grossly less efficient than detection. If you want an explanation, let me know and I'll spend 10 minutes writing a small piece on it. But first, think deeply about why this is so. Think especially about recovering signals from noise.
Had my brush, though only theoretical, with integrating repeating signals
back at school, when I was learning how to interpret NMR spectrums and how
they work. (Good old times, it feels like yesterday.)
Sorry, hadn't specified I am not talking about RFID tags anymore; was thinking about at least partially alleviating/sidestepping the problems with shielding of standard desktop computers.
But will be definitely interested in the minilecture.
Sounds like you already have the gist. There are many good ways to pull weak signals out of noise, either by direct integration over time or by "chopper" techniques (e.g., only looking in narrow time intervals, via gated integrators and boxcar averagers).
And if the RF ID tag is sending out a signal over a couple of different frequencies, using some pseudorandom sequence for the frequency-hopping, then the "noise gain" can be enormous. That is, an attacker trying to jam a spread-spectrum (Direct Sequence Spread Spectrum, DSSS, typically) signal will have to match and greatly exceed the frequences and times.
Even better, pulse systems which send out ultrawideband signals at various coded time points (so-called Gold codes, or Kosami codes, for example) are even more difficult to jam.
You mention that your point was about jamming intercepts from insufficiently-shielded computers, a la TEMPEST, which, by the way, is not an acronym ("To Ensure More Private Eavesdropping-Safe Telecommunications"--NOT).
This is similar to trying to hide phone bugs by running a background noise source, like a shower or a television set. But as with those attempts, a skilled eavesdropper can "strip out" nonrandom noise sources like music or television, thus improving S/N ratios. Or the quasi-random noise of a shower just adds to the baseline of noise already present. (And multiple detectors can help in various ways, much the way noise-cancellation headphones work...off the shelf consumer technology, so imagine what the spooks have.)
More dB of eavesdropping attenuation is gotten by reducing the signal than by increasing the noise, short of the equivalent of jet engines. Better to whisper than to speak normally but turn on "cover noise" sources elsewhere in a room.
Measuring leakage at a distance of a few centimeters is easy to do. And if a leakage signal is very, very small at a few centimeters, the usual inverse-square falloff will make it truly tiny at 100 meters or so. (Where a van might be parked outside one's flat, for example.)
I'm not saying RF emissions are not an issue. Much was written about this some years ago, even here on this list, when "Van Eyck Radiation" (just the RF) was being studied. Ross Anderson at Cambridge and his group have been doing lots of work on this.
Truly sensitive communications may be best done on laptops, even laptops in metal mesh bags. (Either with one's head poked into the bag, or a bag big enough to enclose the user and laptop, etc.)
There are also heads-up LCD displays now costing less than $600, which can be used with handheld computers and the like. Besides (likely, but don't quote me) low emissions from the start, a mesh hood would be very easy to construct, thus knocking probably another 30 dB off the already low emissions.
Note that the inverse-square law falloff and the vast number of communications is probably why the Osama Bin Laden deputy, Sheik Mohammed, wasn't caught because of RF emissions from his laptop, but instead because of an informant (as I understand things).
I would strongly bet on quietness of computers winning out over increased RF detection capabilities. (Needless to say, detection goes as the square of the antenna size, so even really large antennas don't have that many dB of extra capture capability, compared to quietness at the source.)
--Tim May
"The only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others. His own good, either physical or moral, is not a sufficient warrant." --John Stuart Mill
