Quantum Fundamentals

What are Photon Sources?

Photon sources are the cornerstone of quantum communication and cryptography systems — generating single photons or entangled photon pairs with precise control, enabling secure quantum key distribution and fundamental quantum experiments.

The Quantum Optical Heart of QKD Systems

Quantum communication protocols rely entirely on the precise generation and manipulation of photons — the fundamental carriers of quantum information. A photon source is not merely a light emitter; it is a highly engineered optical system designed to produce photons with specific, controlled quantum properties.

In quantum key distribution (QKD) and other quantum information protocols, two fundamentally different photon source types are employed: Weak Coherent Sources (WCS), which produce attenuated laser pulses approximating single-photon behaviour, and Entangled Photon Sources (EPS), which generate photon pairs whose quantum states are correlated regardless of separation distance.

Arquis engineers both source types to laboratory and deployment-grade specifications, enabling everything from fundamental Bell inequality tests to full fibre-based QKD networks operating at TRL-7 and beyond.

Quantum Encoding

Polarization Control

Sources encode quantum information via photon polarization states — the basis of BB84 and E91 protocols — with high-fidelity beam splitting and wave-plate alignment.

High Fidelity

Entanglement Quality

PPKTP and BBO crystal sources achieve entanglement fidelities exceeding 97%, validated through two-photon interference and quantum state tomography.

Deployment Ready

Plug & Play Integration

Ready-to-deploy form factors support both free-space and fibre-coupled operation for seamless integration into existing quantum lab infrastructure.

Essential Questions About Photon Sources

Technical answers explaining the differences, use cases, and practical considerations for quantum photon sources in QKD and research.

An Entangled Photon Source (EPS) generates pairs of photons whose quantum states are intrinsically correlated via spontaneous parametric down-conversion (SPDC) in nonlinear crystals (Type-1 BBO or Type-0 PPKTP). A Weak Coherent Source (WCS) uses an attenuated laser to produce pulses with a Poissonian photon number distribution, approximating single-photon behaviour at very low mean photon numbers (<0.1 photons/pulse). EPS provides true quantum entanglement for device-independent QKD, while WCS is preferred for high-speed, practical BB84 implementations.

Periodically poled KTP (PPKTP) achieves quasi-phase-matching through periodic domain inversion, allowing much higher conversion efficiencies than bulk BBO via birefringent phase-matching. PPKTP operates at room temperature without angle tuning, produces co-propagating photon pairs with excellent spectral brightness, and exhibits lower group velocity mismatch — resulting in narrower bandwidth entangled pairs ideal for interfacing with atomic quantum memories.

In BB84, Alice must randomly switch between two complementary polarization bases for each qubit transmission. If basis selection uses a deterministic or predictable source (even a hardware TRNG), an adversary who can bias or predict the switching pattern can mount powerful intercept-and-resend attacks or photon-number-splitting attacks. Integrating a QRNG for true random basis selection ensures that eavesdroppers gain zero advantage from any pattern analysis, providing information-theoretic security for the basis choice itself.

Technology Readiness Level 7 (TRL-7) signifies that the system has been demonstrated in an operational environment — meaning the photon source has been successfully integrated and validated in real-world quantum communication testbeds, not just laboratory conditions. Arquis TRL-7 sources have been demonstrated in quantum network deployments, verifying stable long-term operation, reproducible entanglement visibility, and plug-and-play compatibility with standard fibre and free-space optical infrastructure.

Arquis Photon Source Products

Explore the precision photon source systems available from Arquis for your quantum lab or QKD deployment.

Weak Coherent & Entangled Photon Sources

High-brightness, ready-to-deploy EPS (TRL-7) and 800 MHz pulsed Weak Coherent Sources with QRNG-driven basis selection for BB84 QKD.

High-brightness PPKTP Sources

Ultra-bright entangled photon pairs via Type-0 PPKTP crystal with exceptional fidelity, visibility, and spectral brightness for advanced quantum experiments.