Enlarge / There are no entangled photons in this picture. (credit: Douglas Muth / Flickr)

It has been a while since I wrote about some really fundamental quantum physics. I feel that you, my dear reader, have not suffered enough during that drought. So, quantum physics it is. Even better, we are going to talk about entanglement and the strange case of one-way EPR steering. One-way EPR steering is an idea that has moved from a purely theoretical suggestion to something that might actually work in practice.

Let’s do some physics

The concept of entanglement in quantum mechanics expresses the idea that seemingly separate quantum particles can have correlations that are larger than would be possible in a purely classical world. When combined with superposition, in which particles have an indeterminate mix of two properties, it becomes pretty mind blowing.

Since the research in question used photons, let’s use photons for our examples. Imagine that I have a device that produces pairs of photons that are entangled in their polarization state. (Polarization describes the orientation of the photon’s electric field.) The polarization could involve oscillating in parallel with the lab table, or it could be oscillating vertically. But, whatever polarization one photon has, the entangled one has the opposite. As soon as I measure one, I know the other. So far, so not special.