This module has assembled a worldview in five steps: computing is throttled by
Axis 1 — analog vs digital. Does the system compute with continuous physical
quantities (field amplitudes, phases), or with restored, discrete logic levels? Analog photonics
inherits interference's effortless linear algebra but pays in noise and precision — recall the
3-dB-per-bit law from the
Axis 2 — compute vs interconnect. Does light transform the data, or transport it? Interconnect asks nothing of light that light doesn't love doing — which is why it is the one photonic product already shipping in enormous volume.
Axis 3 — classical vs quantum. Classical photonics computes with fields — many photons, wave physics, no fragile quantum states. Quantum photonics encodes information in individual photons or squeezed states, and computes with interfering probability amplitudes — a different machine for different problems, sharing the same chips and couplers.
| Family | Quadrant | What light does | Module | Status |
|---|---|---|---|---|
| MZI meshes / optical NNs | analog · compute | matrix–vector products by interference | 4–5 | startups, lab-to-market |
| Fourier / 4f processors | analog · compute | transforms and convolutions via lenses | 6 | venerable, niche |
| Ising machines, reservoirs | analog · compute | physical dynamics as optimisation/inference | 6 | research |
| All-optical logic | digital · compute | nonlinear switching of light by light | 7 | largely abandoned |
| WDM links, co-packaged optics | digital · interconnect | transporting bits between chips and racks | 8 | shipping in volume |
| Photonic quantum computing | quantum · compute | interfering single-photon amplitudes | 9 | heavily funded research |
| Neuromorphic / emerging | analog · compute | spiking dynamics, phase-change weights, combs | 10 | research |
Two readings of the map are worth memorising. Reading one: maturity increases toward the interconnect row — the less you ask light to decide, the sooner you can buy the product. Reading two: every viable compute entry is linear-algebra-shaped — meshes, Fourier engines, even quantum interference are all "propagate and superpose", never "switch mid-flight". The map is this module's one-sentence physics, drawn as geography.
One more instrument belongs in the toolkit before the course descends into hardware. Every
photonic compute system on the map is an accelerator bolted to an electronic host — and
accelerators are governed by
A workload that is 90% matrix multiplication caps at 10×, even with infinitely fast photonics — and the data must still cross the electronic–photonic border, at conversion costs the energy lesson priced. This single formula disciplines every glossy claim in Modules 5, 6 and 11:
Note the shape: pushing
No machine on this map — not one — is a computer in the sense of your laptop. Every real photonic
system is a photonic organ inside an electronic body: electronics holds the
memory (light cannot be parked, as the photon's zero rest mass decreed), runs the control flow,
makes the nonlinear decisions, and converts data across the border in both directions. This is
not a temporary embarrassment to be engineered away; it follows from the physics of Module 1 and
the history of the 1990s. So when a headline says a photonic chip "runs a neural network", the
precise claim is that it performs the linear layers — and Amdahl's law plus the border
tax decide how much that is worth. The practitioners' term of art is hybrid, and the
sooner "photonic computing" is read everywhere as "electronic–photonic computing", the fewer
disappointed investors the field will produce. The course's systems module returns to this with
Follow the money and the map lights up unevenly. Optical transceivers for datacenters — the digital-interconnect corner — are a market worth tens of billions of dollars a year, growing ferociously with AI clusters, and every hyperscaler buys them by the container-load. That corner funds the fabs, the packaging lines, the design tools and the trained engineers that every other quadrant borrows. The analog-compute startups have raised billions in venture capital but ship in small volumes; quantum photonics runs on strategic and venture funding with revenues far in the future. The historical rhyme is exact: as telecom incubated the components that enabled the 2010s return, today's interconnect boom is incubating the platform — cheap lasers, dense modulators, co-packaging — on which the compute quadrants will stand or fall. In photonics, the boring quadrant has always been the patron of the exciting ones.
Module 1's work is done: you can argue from carrier physics, particle properties, energy ledgers
and history to a defensible map of the field. The course now walks the map bottom-up.
Module 2 puts light on a chip —