The Clarendon Ghost-Bridge Gambit

Entanglement swapping across a relay node

Mission 7: Building the Quantum Internet. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

Right then, recruit, listen close: we’ve a bit of a sticky wicket with our quantum data, and the only way to bridge the gap between Oxford and Tokyo without alerting the Echo Network is a rather dashing manoeuvre we call "Entanglement Swapping." Node A at Oxford already shares a Bell pair with relay qubit R1, and node B in Tokyo shares one with R2, yet the two far endpoints have never so much as met. Press "Bell-measure relay" and read R1 and R2 out together in a single stroke. The moment you do, A and B are entangled with each other, genuinely, having never met. But look at the two classical bits the relay just spat out, because they matter as much here as they did in Module 6. Those two bits say which of the four possible bonds A and B ended up sharing, and until they've travelled down an ordinary line at ordinary speed, neither end knows which link it's holding. The entanglement is routed the instant you measure. The usable link arrives at the speed of the phone call. Not a single qubit was copied, old bean, but nothing outran light either.