Quantum entanglement: A modern perspective

Entanglement as something you can measure, spend, dilute and distill.

This first appeared in Physics Today in April 2003, and the magazine brought it back in its archive series. I read it while starting my work on entanglement purification, and it was the first time the subject felt like one story rather than a list of results. It goes from Schrödinger’s cat and Bell’s inequalities to the modern view, where entanglement is a resource you can count, spend and move around.

The part I keep coming back to is the difference between how many perfect pairs it costs to make a noisy state and how many you can distill back out of it. For mixed states the second can be smaller, and for some states it is zero even though they are entangled, which the authors call the black holes of quantum information. Much of what I now work on, from purification protocols to scheduling in quantum networks, sits in that gap.

Quantum entanglement: A modern perspective, Physics Today.