If you shine light on a barrier with two openings, it produces a distinct pattern of light on a distant screen. Measuring that pattern is standard in introductory physics laboratories. (You could even do it at home, but I recommend a very dark room and a bright laser pointer if you hope to see anything at all.) Where things get fun, though, is if you have a light source capable of sending a slow stream of photons — particles of light — through: you still get the interference pattern, but it emerges slowly from individual points of light. In other words, the photons behave as though the entire wave interference pattern is already present, even though they are single particles.

My latest article for Nautilus shows how researchers have taken this classic experiment, but use single photons to manipulate the interference pattern via the phenomenon known as entanglement. The result is a mind-bending experiment known as the “quantum eraser”:

The best way to see the quantum eraser is to couple the double-slit experiment with another fascinating quantum phenomenon: entanglement. In a typical implementation, light from a laser stimulates a certain kind of crystal, which in turn emits two photons with opposite polarization—one could oscillate left-right, while the other oscillates up-down. (You can see how this works by putting one pair of polarized sunglasses in front of another and rotating one pair. At certain angles, the light going through both lenses will fade to almost nothing, a sign that the light is passing through two filters with perpendicular orientations.)

The polarization of each photon is unknown before measurement, but because of how they’re generated, they are entangled, and measuring one can instantly affect each the other. That holds true no matter how far apart the two particles are or when the measurements are taken. [read more….]

Quantum droplets in an ocean of light