There have been a bunch of stories recently talking about quantum effects at room temperature– one, about coherent transport in photosynthesis , even escaped the science blogosphere. They’ve mostly said similar things, but Thursday’s ArxivBlog entry had a particular description of a paper about entanglement effects that is worth unpacking:
Entanglement is a strange and fragile thing. Sneeze and it vanishes. The problem is that entanglement is destroyed by any interaction with the environment and these interactions are hard to prevent. So physicists have only ever been able to study and exploit entanglement in systems that do not interact easily with the environment, such as photons, or at temperatures close to absolute zero where the environment becomes more benign.
In fact, physicists believe that there is a fundamental limit to the thermal energies at which entanglement can be usefully exploited. And this limit is tiny, comparable to very lowest temperatures.
(That’s cut-and-pasted– there are obviously words missing from the last sentence, but I don’t know what they are.)
Is this saying that entanglement does not happen at higher temperatures? No, it’s not. What’s going on here is a little subtle, so it’s worth talking a bit about (even though I know this will invite comments from crazy people).
The generic sort of entanglement experiment looks like this:
In the center, you have a black box that emits photons whose polarizations are entangled. That is, the two photons are guaranteed to have the same polarization, when they’re measured, but that polarization is equally likely to be either horizontal or vertical. Those photons travel out from the box to two detectors consisting of a polarizing beamsplitter that sends horizontally polarized photons to one detector (where a photon is registered as a 1), and vertically polarized photons to another (registered as a 0).
If the photons are entangled, when you look back over the results of many repetitions of the experiment, you will always find pairs of identical numbers. If one detector gets a 0, the other also gets a 0 (I’ll call this “00” to keep things compact); if the one gets a 1, the other also gets a 1 (“11”). When you look over a long list of results of the experiment, you’ll see roughly equal numbers of “00” and “11” results, but never a “10” or a “01” (assuming ideal detectors, etc.).
Of course, this has left out the possibility of interaction with the wider world, so let’s think about what happens if we do something to one of those photons en route to the detector. Imagine that a tiny demon, bored with sorting hot and cold molecules comes along and sticks a polarization rotator in the path of one of the two photons, like this:
What happens then?
Continue reading “Entanglement Happens”