Photons are the natural medium for quantum communication: they travel at the speed of light in fiber, maintain coherence for long times at room temperature, and barely interact with their environment. Integrated quantum photonics aims to shrink the tabletop optical setups of quantum optics experiments onto silicon chips — manufacturable, stable, and scalable.
## Components on a Chip
An integrated quantum photonic chip combines:
**Photon sources**: spontaneous parametric down-conversion (SPDC) sources produce entangled photon pairs; quantum dots (InAs/GaAs) emit deterministic single photons with high purity.
**Waveguides and interferometers**: silicon, silicon nitride (Si₃N₄), or indium phosphide (InP) waveguides guide photons; Mach-Zehnder interferometers implement quantum gate operations.
**Single-photon detectors**: superconducting nanowire single-photon detectors (SNSPDs) achieve detection efficiencies exceeding 98% with picosecond timing resolution — the leading technology, requiring cooling below 4K.
**Phase modulators**: electro-optic or thermo-optic phase shifters allow reconfigurable quantum circuits.
## Companies and Research Groups
**PsiQuantum** is betting on photonic silicon chips manufactured in CMOS foundries, targeting million-qubit quantum computers.
**QuiX Quantum** (Netherlands) commercially offers a 20-mode quantum photonic processor for research applications.
**NTT** (Japan) has long-term programs in integrated quantum photonics and photon-based quantum network interfaces.
**Intel and imec** are developing quantum photonic interconnects to link cryogenic quantum processors with room-temperature control electronics.
## Role as a Quantum Network Interface
Photons can travel kilometers in fiber at room temperature, making integrated photonics the natural interconnect layer between quantum computing nodes — a quantum bus linking superconducting processors (cryogenic) to other quantum network nodes. Quantum transducers that convert between photonic and microwave frequencies are an active research frontier. See [Nature Physics review](https://www.nature.com/articles/s41567-023-02129-w) and [Quantum Internet](https://sunqi.org/quantum-internet-en/).
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