Quantum Fundamentals

What are Coincidence & Synchronisation Systems?

Coincidence detection and precision time synchronisation are the backbone of multi-photon quantum experiments — enabling correlated photon pair measurement, entanglement verification, and distributed quantum network coordination with sub-nanosecond accuracy.

Detecting Quantum Correlations with Precision Timing

Quantum optical experiments fundamentally depend on the ability to detect and correlate photon arrival events across multiple detectors with extreme temporal resolution. A coincidence counter identifies when two or more photons arrive within a defined time window — an essential measurement for proving entanglement, verifying QKD sources, and studying multi-photon interference.

At the heart of every Bell test, Hong-Ou-Mandel experiment, or entanglement swapping protocol is a Coincidence Counting Unit (CCU) — a high-speed logic system that compares time-stamped detector pulses and records coincident detection events. The resolution of this system directly limits the signal-to-noise ratio of the entanglement measurement.

Equally critical in distributed quantum networks is Precision Time Protocol (PTP) synchronisation — ensuring that geographically separated nodes share a common time reference with sub-microsecond (and sub-nanosecond) accuracy, enabling coherent quantum state manipulation across network links.

Multi-photon

8-Fold Coincidence Detection

Arquis CCU supports up to 8-channel inputs with configurable coincidence folding, enabling complex multi-photon correlation measurements in a single unit.

2.5 ns Resolution

Fine Timing Windows

Configurable coincidence time windows down to 2.5 ns suppress accidental coincidences while preserving true correlated events from entangled photon pairs.

Network Sync

PTP IEEE 1588 Compliant

Hardware-timestamped PTP synchronisation locks distributed quantum network nodes to a shared time reference, enabling coherent multi-node quantum operations.

Essential Questions About Coincidence & Sync Systems

Technical answers on how coincidence counting and precision synchronisation work in quantum optical systems.

In spontaneous parametric down-conversion (SPDC), entangled photon pairs are generated simultaneously within a coherence time of femtoseconds to picoseconds. However, detector jitter, fibre dispersion, and electronic noise spread arrival times to nanosecond scales. If the coincidence window is too wide, accidental coincidences (uncorrelated photons from different pairs) accumulate and degrade the signal-to-noise ratio (signal/noise ∝ 1/√τ for a given pair rate). Narrowing the window to 2.5 ns maximally suppresses accidentals while capturing true coincidences, directly improving measured entanglement fidelity and Bell parameter violations.

Precision Time Protocol (IEEE 1588 PTP) is a network timing standard that synchronises clocks across distributed nodes using hardware timestamping of network packets, achieving sub-microsecond accuracy over standard Ethernet. In quantum networks, Alice and Bob must share identical timing references to correctly assign photon detection events to the same qubit transmission window. Without precise synchronisation, the quantum bit error rate (QBER) rises dramatically — classical NTP (millisecond accuracy) is entirely insufficient for nanosecond-scale quantum coincidence detection.

Greenberger-Horne-Zeilinger (GHZ) states require the simultaneous detection of 3 or more entangled photons across distinct polarization analysis paths. An 8-channel CCU enables arbitrary multi-fold coincidence logic — for example, detecting 4-fold coincidences between H/V analysis paths on Alice, Bob, Charlie, and Dave simultaneously. The coincidence logic is fully programmable, allowing real-time switching between 2-fold, 3-fold, and higher-order coincidence measurements without hardware changes.

The Coincidence System operates at TRL-9 (fully operational system proven in operational environments), meaning it has been deployed and validated in real quantum laboratories and QKD testbeds worldwide. The PTP Synchronisation module is at TRL-3 (experimental proof of concept validated in a controlled laboratory environment), indicating active research and development toward full network deployment. Arquis provides clear TRL labelling so customers can accurately assess integration risk and production readiness for their specific use case.

Arquis Coincidence & Synchronisation Products

Precision quantum timing and detection systems from Arquis for your laboratory and quantum network deployments.

Multi-channel Coincidence System

Inhouse 8-Channel CCU with up to 8-fold coincidence folding, 2.5 ns timing resolution, and real-time configurable coincidence logic for quantum optics experiments.

PTP Synchronisation (TRL-3)

Precision Time Protocol based time synchronisation for distributed quantum networks, providing IEEE 1588 hardware-timestamped sub-microsecond clock alignment across network nodes.