[Cryptography] Aharonov-Bohm effect

Henry Baker hbaker1 at pipeline.com
Sun Feb 11 14:16:43 EST 2018


At 08:28 PM 2/9/2018, Perry E. Metzger wrote:
>I've thought for a bit, and I've come up with an even finer useless side channel, which is to use the Aharonov-Bohm effect for exfiltration.
>
>This effect has the added advantage of involving quantum mechanics, which, from what I can see of the levels of accuracy surrounding stories on quantum cryptography and quantum computing, is even more magical to reporters than electromagnetism, which they seem to already regard as a form of witchcraft.
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>A quantum phenomenon would yield the added benefit that no one in the news business would report this even vaguely accurately.
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>As most of you are computer scientists or electrical engineers and not physicists, you may be unfamiliar with the Aharonov-Bohm effect, so I'll give a brief explanation.
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>The effect is a way that you can detect the presence of magnetic fields that's non-zero somewhere even from somewhere else where the field is zero.
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>Imagine an infinite solenoid carrying a steady current.
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>Inside the solenoid there's a magnetic field, but outside, although there's a magnetic vector potential, the field is zero.
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>Classically, there's no way to measure that field from the outside.
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>However, it turns out that even though you can't measure the magnetic vector potential's presence in any classical way, if you move an electron in a full 360 degree circle around such a magnetic vector potential, you will invert its phase.
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>Now you might also think you can't measure the phase, which is after all an imaginary number, but you can do that by conducting interference experiments.
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>People have, with really complicated equipment and a great deal of trouble, measured this effect.
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>Thus, it should be possible to build a sensitive experiment to route electrons in a complete circle around your target's computer, conduct interference experiments with them afterwards, and use the indirectly inferred magnetic vector potential to further indirectly infer the presence of magnetic fields within the computer which is otherwise fully shielded.
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>Perhaps you can even build a giant experimental apparatus that completely encircles Ft. Meade and who knows what you might learn! This doubtless can be used for data exfiltration! Instant pressworthy side channel, yes?
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>Sure, actually doing it would be extremely difficult, but so would getting within a meter or two of an important machine with a faraday cage round it in a colo to measure a magnetic field, and that didn't stop interest from our friends at the major industry rags, right?
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>Reporters, do you have a slow news day and you need to report breathlessly on something no one understands?
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>The Aharonov-Bohm effect is here to come to your rescue!

Actually, the Aharonov-Bohm effect is one of the major reasons why building a decent quantum computer is going to be extremely difficult: precisely because it's nearly impossible to *shield* a QM from outside influences which disturb and "collapse" the superposition effects.

Perhaps quantum error-correcting codes are the *only* way to effectively "shield" a QM?



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