A sensitive single-photon source (a pumped nonlinear crystal or a quantum dot) emits photons forward down the optical bench toward a downstream component — a lens, connector or detector. Real components are never perfectly anti-reflective: a fraction of the light bounces straight back the way it came. If that reflected light re-enters the source, it can perturb the pump laser's own internal cavity, adding noise and jitter to the emission rate and degrading photon-statistics quality.
An optical isolator exploits the Faraday magneto-optic effect: it rotates the polarization of light passing through it in a way that depends only on the direction of propagation, not on reversing the light and running it backward through the same rotation. Combined with polarizers on each end, forward light passes through with almost no loss, while backward light picks up a net 90° rotation and is extinguished at the entrance polarizer — a genuinely nonreciprocal device, unlike an ordinary lens or mirror.
forward: source → isolator (pass) → downstream
backward: downstream → isolator (BLOCK) → source (isolator ON)
backward: downstream → source directly (isolator OFF)
- Downstream reflectivity — how much light the downstream component sends back; a bare fiber connector reflects a little, an untreated glass interface or misaligned detector reflects a lot.
- Isolator ON/OFF — toggling it removes or restores the one-way gate. Forward photons always get through either way; only the backward path changes.
- Source stability — a live readout that erodes when back-reflected light reaches the source and recovers when it's blocked, with a jittery emission rate as a symptom of a perturbed pump cavity.
Real-world relevance: isolators are near-mandatory in front of any laser or single-photon source feeding a fiber network — without one, connector reflections alone are often enough to destabilize the source's linewidth and output power.