Macro quantum mechanics

Imagine if this could be used for communication (probably not, but you never know). That means you could have communication between two mechanical entities at the opposite sides of the earth without worrying about pesky radio wave things like line of sight and increased signal strength requirements over a distance.

But a more appropriate answer is that there probably could be some sort of application, but unless we start building stuff we won't know what we're capable of and will be unable to engineer cool gizmos that use it.

Secret communication channel? I don't know how easy to intercept quantum entanglement. I seem to remember the quantum state would collapse if it's eavesdropped.

how about sharing a private key? Share 256 quantum bits, both 'realize' the bits, now you share a private key.

The experiments are cool for two reasons:

First, they helps us learn about and control decoherence, which is the primary obstacle to building a quantum computer. Decoherence is the reason everything doesn't behave quantum mechanically all the time, and its strength generally grows exponentially with the size of the system being studied. Building a useful quantum computer will require suppressing decoherence for order 100 qubits.

Second, these experiments test quantum mechanics in new physical regimes where it might break (to be superseded by new physics). Just like one will find a discrepancy with Newtonian mechanics if one starts doing experiments near the speed of light, the suspicion is that quantum mechanics might breakdown when applied to large objects. People who are unsatisfied with the interpretation of quantum mechanics are especially interested in this possibility.

I believe the idea is that the photons are entangled with a certain vibrational mode in the diamonds. When you do an interference experiment with the photons, it demonstrates that the photons are in a coherent superposition. If the vibrational mode in the diamonds had decohered, the photons would have automatically been decohered as well (since they were entangled with the diamonds).

It's hard to tell for sure without being able to read the journal article, though.

Also, it's important to note that the center-of-mass position of the diamonds has not been put in a superposition, which is what people usually mean when they say "macroscopic superposition". This experiment only shows that a certain vibrational mode in the diamond is superposed. (For all I know, that vibrational mode might just involve a few atoms.) Unfortunately, this type of link-bait is quite common in the quantum information community.

OK, I've taken a look at the actual article and it was basically what I suspected. First, they entangle the two diamond modes by beam-splitting a photon and directing the two possible photon paths towards the diamonds. (The photon scattered off the diamonds is recombined with a beam splitter and then detected, to ensure that the scattering occurred.) Then, they hit each diamond with a lazer pulse which, if the diamond's mode was excited, produced a second photon. They were able to coherently interfere the second photon, which was in a super position of originating from the first diamond and from the second diamond.