Go further · Chapter 10 · 10a · Real-world case
How does a photonic quantum computer work?
The short answer (tap to open)
A photonic quantum computer makes single photons on a chip, steers them through tiny waveguides, and detects them at the end. Since photons don't interact directly, it builds large entangled webs by "fusing" small clusters with measurements. The main enemy is loss: every component that absorbs even a small share of photons multiplies into a big problem.
What's on a photonic chip?
- Sources that produce single photons, or pairs, on demand or nearly so.
- Waveguides: channels of glass-like material that guide light around the chip, like optical fibre in miniature.
- Beam splitters and phase shifters that split, steer and delay photons (chapter 4's interferometer, shrunk onto a chip).
- Detectors that click when a photon arrives. The best, superconducting nanowire detectors, need cooling to a few degrees above absolute zero.
Why fusion?
Photons pass through each other without interacting, so a two-qubit gate can't rely on them bumping. In 2001 Emanuel Knill, Raymond Laflamme and Gerard Milburn showed that measurements can stand in for interactions, at the cost of only working some of the time. Later designs, including PsiQuantum's "fusion-based" architecture (2021), build big entangled webs from small clusters and compute by measuring the web (chapter 10).
Why is loss the enemy?
Each component loses a small share of photons. Chain hundreds together and the losses multiply. Error-correction designs can tolerate some loss, but only below a threshold, so photonic companies obsess over every fraction of a per cent.
Photons surviving the whole path: 95.1%. Inside the roughly 10% loss one published design can tolerate.
Five components at 99% keep about 95%. Ten would keep only about 90%.
A worked example
Five components each keeping 99%: 0.99 × 0.99 × 0.99 × 0.99 × 0.99 is about 0.95, so about 95% of photons survive. Ten components at 99% keep only about 90%, already near the limit one published design can tolerate.
Where does it stand?
PsiQuantum's Omega chipset (February 2025) reported 99.98% accuracy in preparing and measuring qubits and 99.22% for fusions. Xanadu's Aurora (January 2025) networked 35 chips at room temperature, and in June 2025 Xanadu made error-resistant GKP states on a chip. No large photonic quantum computer exists yet; PsiQuantum broke ground on a big facility in Brisbane in June 2026.
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Light computes through measurement.
Fusion builds the web; every lost photon is the price.