The Clarendon Cascade

Deploying repeaters to fight signal loss

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

Right then, pay attention, recruit. We are currently mid-heist, attempting to hold a live entangled link from the Clarendon Lab all the way to Tokyo, and the fibre-optic line is a thirsty beast, drinking up our photons as if it were a student on a bender. Over any real distance a single fibre loses almost everything, and the secret key rate sits stubbornly at zero. Our answer is a chain of quantum repeaters that hand the entanglement along in short, survivable hops.

Drag the repeaters slider up from nothing and watch the secret key rate climb off the floor: each extra relay shortens every hop, so far more photons make it through. But there is no free lunch. Now pull the memory lifetime down and watch that hard-won rate collapse again: a repeater has to HOLD its half of the link in a quantum memory while it waits for the next hop, and if that memory forgets faster than the photons arrive, the whole chain decoheres into classical garbage. It's a delicate balance of distance against memory, so mind the two sliders and find where the key rate actually peaks. Chop chop!