Chanseung Lee

One continuous-wave laser pumps a Si₃N₄ microresonator that turns it into a comb of evenly spaced wavelengths. On the silicon transmitter, a tree of ring-assisted and asymmetric MZI filters sorts the comb lines into lanes, a ring modulator tuned to each line writes its data, and a mirrored tree folds every channel back onto one output fiber.

The animation below follows four of those channels. Each ring modulator writes its color's bits, and at the receiver a matching ring filter drops that color into its own photodetector. Packaging these chiplets next to the driver and receiver electronics keeps the electrical path short, which is what sets energy per bit in chip-to-chip and memory links.

In a high-Q Si₃N₄ microring, a χ⁽³⁾ material, two pump photons can annihilate and create a signal–idler pair on neighbouring resonances (spontaneous four-wave mixing). In a periodically poled lithium niobate waveguide, a χ⁽²⁾ material, one pump photon at twice the frequency splits into a pair (spontaneous parametric down-conversion), and the poling period Λ sets the phase matching. In both, detecting one photon heralds the other.

Driven harder, the same nonlinear processes produce squeezed light: one quadrature's noise falls below the vacuum level. How far below depends on loss, so source purity and low-loss design decide what is usable.

Most quantum memories emit photons at visible or near-infrared wavelengths, where optical fiber is lossy. Quantum frequency conversion shifts a single photon to a telecom wavelength while keeping its quantum state, so it can travel kilometers of fiber.

A strong pump and a χ⁽²⁾ waveguide carry out the shift through difference-frequency generation. The practical targets are high conversion efficiency and low added noise from the pump, both set by the waveguide design.