A light pulse can be sliced into short time bins, like a train into carriages. If two photons are born together and their carriages are tightly linked, the arrival of one in a specific bin guarantees that the other arrives in the paired bin, this connection is called time-bin entanglement. It is not afraid of a dim channel: even when some pulses are lost, the remaining pairs retain their correlation.

In the new work, physicists built a source of such pairs, aiming for simplicity and long-term stable operation. Instead of bulky setups, they used a compact device that emits bright photon pairs with time-bin entanglement. The quality of entanglement was verified with a new method: photons were sent through two nested interferometers (devices where light is split and recombined, accumulating phase differences). If the phases match the predicted pattern, the entanglement is genuine.

Then the same data stream was processed with a new quantum key distribution protocol. Its feature is flexibility: a single set of measurements allows changing the number of time bins and their duration after the experiment, as if you could reschedule trains without sending the train out again. This helps choose settings that maximize the key generation rate for a specific line.

It turned out that under strong noise and loss, it is better to use not two states (arrived/not arrived) but more — three, four, five time bins. In these modes, the key is generated faster than with the usual binary scheme. Thus, for long-distance communication over standard optical fiber, time-bin entanglement could become a practical tool, not just a laboratory curiosity.