Chinese researchers have demonstrated a working quantum router that, for the first time, uses quantum control signals to route the quantum data, according to MIT's Technology Review blog.
Historically, routing quantum information has been very difficult because of the peculiar nature of the quantum world. A classical router uses control signals to work out where data needs to be sent. But in quantum mechanics, the act of reading the control signal also destroys it. This fact makes quantum signals proof against most eavesdropping attempts: great for secure communications, less so for getting the data to where you want it to go.
Until now, this issue has been dealt with by using classical control signals, but now the Chinese researchers report a way of using quantum control signals instead.
The technique takes advantage of superposition — the ability of a quantum particle to exist simultaneously in more than one state — and entanglement.
Entanglement is a weird property of quantum particles, which Einstein called "spooky action at a distance". Essentially, two entangled particles are linked in a way that has no equivalent in the classical world. No matter how far apart they might be, entangled particles retain a link, so that an action performed on one produces a result on the other.
According to Technology Review, the device demonstrated by the Chinese researchers takes advantage of this property.
The routing information is contained in the polarisation of a photon — either horizontal or vertical. The researchers start with a single photon that is polarised both horizontally and vertically. This is superposition. They then convert this single photon to a pair of entangled photons, each of which is in the same superposition of states.
The router reads the polarisation of one of the pair to determine the route the other will take.
Xiuying Chang and colleagues at Tsinghau University in China describe the breakthrough in their 31 July paper as, "The first proof-of-principle demonstration of a genuine quantum router".
However — and there is always a however with quantum mechanics — the router will not scale.
As the MIT blog explains: "The new router has significant limitations. The most significant of these is that it can handle only one quantum bit or qubit at a time. And because the process of parametric down conversion cannot handle more qubits, it cannot be scaled to more qubits.
"That's a significant drawback. It means that this is a proof-of-principle device but not one that will ever form the basis of a future quantum internet."