Quantum information is the enabling factor behind a number of emerging technologies that many physicists expect to have a huge impact on society in future: powerful quantum computers, (almost) perfectly secure quantum cryptography and the quantum internet that will distribute these capabilities round the planet.
But there’s a problem with this vision of the quantum future. At the moment, physicists can only send photons carrying quantum information over the length of a single optical fibre.
Guiding the photons into another fibre is a process called routing, which uses a control signal to determine the destination and route of a data signal. A classical router simply reads the data in the control signal and routes the data signal accordingly.
But in the quantum world, reading a control signal also destroys it. So it’s only been possible to route quantum data signals using classical control signals. And although that’s handy, it doesn’t allow the routing process to exploit the full power of quantum information.
Today, Xiuying Chang and a few buddies at Tsinghau University in China announce that they have built and tested the first quantum router to use a quantum control signal to determine the route of a quantum data signal. “We…realize the first proof-of-principle demonstration of a genuine quantum router,” they say.
In this new device, the information is encoded in the polarisation 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 polarisation 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 polarisation states.
The router works by using the polarisation 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 polarisation.