There's a lot of talk about quantum entanglement these days, where a particle influences its 'entangled' counterpart when any sort of measurement is attempted on it. The particles are entangled in a way that their spatial separation doesn't matter when it comes to one particle influencing the other.
[My references to 'particles' favour the omnipresent, multi-purpose photon of course :-) - Two entangled photons can be created by a process called 'parametic down conversion', where a photon spends a romantic evening with an atom from a beta-borium-borate crystal, and nine picoseconds later, said atom decays and pops out twins, or rather two entangled photons].
Einstein and two of his students, Podolsky and Rosen - in that order (E.P.R), if you want the best google results - conceived a famous argument to question the completeness of quantum theory. A salient component of this argument had Einstein considering the hypothetical example of gunpowder that was intrinsically unstable (i.e. could explode as a result of forces/reactions from within the gunpowder system). He applied Schrödinger's equation to it to determine the state of the gunpowder after a year and determined that the equation would give him garbage (of course, Einstein put this result across very politely to Schrödinger).
The culture of physics in these quantum echelons is such that Schrödinger could respond to Einstein with his cat experiment (strikingly similar to Einstein's gunpowder) and though the result gave us little concrete understanding, we still applaud the response.
In these rarefied clouds of opinions and philosophies on the nature of particles, it's refreshing to see these theories actually put to use in real-life:
One such application of quantum entanglement is in cryptography. The parties involved are Alice and Bob. Each of them have two distinct bit-measuring machines. Alice sends Bob the entangled dual of a piece of information and both of them proceed to measure the bits comprising the information. The trick here is that they each choose a random machine to measure individual bits.
The key to this encryption: Alice and Bob then share with each other information about which machine they used to measure which bit. This information can be shared across a public channel and it won't help an eavesdropper. Instances where different machines were used are dropped, because the results, even if identical, do not confirm entanglement. The remaining bits are condensed and of these, Bob and Alice again compare, publicly, a random sample to ensure that the information they have is identical bit-for-bit. If yes, keep, if no, discard.
Simplistically, the probability is high that an eavesdropper would alter the information while trying to spy on the communication. The more information that the eavesdropper gathers about the key, the higher the likelihood that Alice and Bob will realise they're being snooped upon and will try the communication anew.
Possibly the most foolproof cipher :)