Why photons?
Photons barely interact, move at light speed, and come with convenient dial-a-basis encodings (polarization, time-bin, frequency, path). That’s why modern quantum networks send light between nodes even when computation happens on atoms, ions, or superconducting circuits.
The simplest qubit uses linear polarization. Using \(|H\rangle\) and \(|V\rangle\) as a computational basis, and \(|D\rangle=\tfrac{1}{\sqrt2}(|H\rangle+|V\rangle)\), \(|A\rangle=\tfrac{1}{\sqrt2}(|H\rangle-|V\rangle)\) as the diagonal basis, we can encode, transmit, and measure single-photon qubits.
States, Measurements, and Channels
Ideal transmission is a rotation on the Bloch sphere: a unitary \(U\) from birefringence or wave plates. Real links add loss (photon disappears) and depolarization (randomize the Bloch vector toward the center). A convenient model is the depolarizing channel \[ \mathcal{E}_p(\rho)=(1-p)\rho + p\frac{\mathbb{I}}{2}, \] and amplitude loss with probability \(\ell\) (no click).
Interactive • Polarization QKD-style Link (cinematic + scientific)
Send single photons from Alice to Bob. Choose the send basis and bit. The channel applies loss \(\ell\) and depolarization \(p\). Bob measures in a basis; matching bases produce a sifted key. Watch photons fly across the canvas (cinematic) while statistics update (scientific).
Entanglement Distribution & Repeaters
To link distant quantum processors, we distribute entanglement and then use teleportation. Since loss grows exponentially with distance in fiber, quantum repeaters break the link into segments, purify noisy entanglement locally, and swap it to extend range.
Photons are central at every step: they carry entanglement out of trapped-ion or solid-state emitters, interfere at beam splitters to herald links, and feed back classical messages for error detection.
Applications
QKD (BB84, E91): uses basis mismatch to detect eavesdropping (QBER grows if Eve measures). Quantum teleportation: send unknown states via shared entanglement + two classical bits. Clock networks & sensing: distribute phase-stable light for interferometry and timekeeping.