1 · the qubitA photon's polarization
Every polarization is a point on a sphere (the Bloch sphere).
2 · the channelWhat the trip does to it
The sphere of inputs becomes an ellipsoid of outputs: shrunk by noise, turned by drift.
3 · the certificateIs it still quantum?
Above 0.5, no classical relay (measure, then resend) could do it.
4 · what it carriesBits, qubits, secrets
Capacities: the most a channel can carry per use, of each kind.
5 · with a classical channelHeralding and bits
Lost photons are announced over the classical fiber; teleportation needs two bits. Both channels work together.
6 · measure itTomography in the rooms
Send five polarizations, measure three ways, fit the map: P14 with two $10 waveplates.
Terminal: the channel, line by line
Same arithmetic as qll/circuits/qubit_channel.py (tests/test_lab_js.py); the log loads with channel_tomography.read_log.