How to Detect Things Without Interacting With Them, and Other Things to Bend Your Mind Around (Also, Study Tips)

I’ve recently been thinking a bit about a really cool idea that may have important applications to quantum computing and other things. The idea goes by the name “quantum interrogation” or occasionally “interaction-free measurement”. Basically, you use the fact that quantum particles can interfere with themselves as they travel unobserved since (to simplify immensely) particles probe the space around them rather like waves do when they are not interacting strongly with other things.

So, you set up a situation where the particle splits along two tracks. Quantum mechanically we can think of the particle as going along both at the same time. At some later point, the particle rejoins itself and is detected. The trick is to now place an obstacle in one of the paths so that the particle can no longer “rejoin” with itself—i.e. you block its ability to interfere with itself. So even if the particle ends up truly going on the other track (the one that the object is not obstructing) you will actually get a different result if you set up your detectors correctly. There is a way to bootstrap this procedure to essentially ensure with arbitrarily high probability that the particle never ends up taking the track that is obstructed, and yet, you can detect the presence of the object precisely because of the lack of particle self-interference.

The idea is Elitzur and Vaidman’s and has been experimentally implemented by Kwiat and collaborators. Take a look at Kwiat’s explanation of these ideas. For a very nice cartoon explanation that gets at the heart of it (it involves adorable puppies), look at Sean Carrol’s explanation on Discover’s Cosmic Variance.

Continuing along our quantum theme, there was a relatively nice article in the New York Times Science section about quantum computation recently.

And in my quantum particle-like walk around the internet, I came across a Scientific American article about plants using quantum entanglement to transport energy around. This is really neat since it is a clear example of biological systems exploiting quantum effects in an explicit manner—I mean, covalent bonding of atoms is a quantum effect, but as you go up the ladder of distance scales, truly weird quantum stuff becomes harder to achieve and more washed out. Things just start to average into classicality usually. So the fact that some plants use entanglement in a way that is essential for energy transport is really cool.

Also, lots of theories that aim at answering fundamental questions about the universe appear to be yielding different versions of the idea that our universe may be a lot more varied than we currently observe. Some like to refer to these types of notion as the “multiverse” concept. Anyway, listen to Brian Greene on Fresh Air talk about these ideas and his new book.

Speaking of alternate worlds, here’s a really amusing short video about the very strange world of Ms. Wind and Mr. Ug. Can you figure out what’s going on here? (thanks to Cosmic Variance for this!)

And that reminds me—I probably posted this a while back, but it’s really worth posting again. These are videos that act as tutorials for understanding various amazing mathematical concepts about geometry (including higher dimensions). Note that you can navigate to different videos by using the arrows that appear when you wobble the mouse over the video itself.

And, to mark the return to teaching Frontiers of Science this semester, enjoy the latest in our attempts to understand the best techniques for learning stuff. Here’s a helpful write-up in the New York Times.

(Mind you, I think a little too much is made about the testing angle of this. Perhaps that is important, but after chatting about this with my wife, I bet that the key here is getting the learners to actively recreate what they are supposed to be learning without reference to the source material. My guess is that this forces the learner to “own” the material themselves. I think it’s part of the reason that you become much better at a subject when you are forced to give lectures on it.)


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