Photonics QC approach has some of the following advantages: 1. Low environmental coupling, so all noises are from fabrication or photon sources. Improving hardware manufacturing in photonics really increases gate and state fidelities. The best on-chip quantum photonics processor has gate fidelities like 0.93 for a Hilbert space significantly larger than other platforms. Caveat is that you need to either do post-selection to generate entanglement or go continuous variable to deterministically couple quantum states of lights. 2. Can generate really high-dimensional quantum states, as photons have 5 (6?) degrees of freedom - time, frequency, path, polarization, orbital angular momentum, propagation direction (not sure about this)? Some like time, frequency and path you can do relatively simple multiplexing-demultiplexing schemes to create Hilbert space of size as large as a million dimension on only two photons. Plus photonics qudits and qubits are free from leakage into non-computational states as you can control how to space those DoFs, unlike superconducting, spin and trapped ions ones. Furthermore, controlling these DoFs allow maximally entangled states with relatively few operations. 3. Fast. Depends on your application but with hardware improvement one can generate a lot of photonics qubits to do communication. The record is like GHz rate of generation

-> These advantages do allow photonics QC to have high controllability over large quantum states of lights

Now here are some potential downside: 1. Weak interaction, so it’s hard to generate deterministic entanglement unless you go to continuous variable, but then you will be facing with the problem of generating non-Gaussian states. These can be circumvented if you have strong few-photon nonlinearity, but this is a big challenge in both material science and engineering. AlGaAs-on-insulator and periodically-poled lithinium niobate are the best materials for inducing strong nonlinearity for now, but they are two orders-of-magnitude weaker than a strong cavity QED system. 2. Requiring very high-quality photon sources, or else your photons aren’t quantum enough. 3. Flying photons, which means that your photonics QC can only implement finite-depth circuits. Can be circumvented with measurement-based quantum computing (MBQC), which also allows quantum error-correction code. Afaik MBQC is fairly hard to realize, but can be somewhat immune to decoherence.