Physicists Demonstrate Quantum Router

Diggester tips news that physicists from Tsinghua University in China have published "the first proof-of-principle demonstration of a genuine quantum router." The group's paper (PDF) is available at the arXiv. MIT's Technology Review describes it thus: "In this new device, the information is encoded in the polarization of photons, either horizontal or vertical. The Chinese group begin by creating a single photon that is in a superposition of both horizontal and vertical polarization states. They then convert this single photon into a pair of lower energy photons that are entangled, a process called parametric down conversion. Both of these photons are also in a superposition of polarization states. The router works by using the polarization of one of these photons as the control signal to determine the route of the other, the data signal. The device is simple, little more than a collection of half mirrors for guiding photons and waveplates for rotating their polarization."

Re:So...

[ByteSlicer]

Please, someone correct me if im wrong, but my understanding is its like having a red, and blue ball in seperate bags. You throw one ball (in its bag) across the room, then open the other bag. The open bag is blue, you now know the red one is across the room.

It's more like having two balls, both half-red half-blue, in separate bags. The balls contain a magnet that causes them to align: when one is red-up, the other is blue-up.

Then you shake the bags, open one, and pick out the ball. If it's red-up, you'll know the other ball will be blue-up, and vice versa.

At first glance this may look the same as your model with the full red or blue ball (a so called hidden variable model), but the statistics differ from real entanglement (which includes superposition of states).

In the first (classical) model FTL information transfer is impossible because nothing is actually transferred. The ball was blue or red to begin with, and opening the bag doesn't change that.

In the second (quantum) model FTL information transfer actually does occur (with entanglement, the magnet alignment in the model occurs slower than light obviously). But the observer can't choose what this information will be: picking a ball out of a bag will result in a random color. And the other bag will then contain a ball in the opposite but equally random color. You can't pick the color, so you can't choose the message/information.

Statistics from experiments have shown that the second model is the correct one, and that entanglement and the FTL state transfer are real.