Single-photon detectors turn quantum light into measurable events.

A single-photon detector registers individual photons and outputs an electrical signal that can be counted, timestamped, and processed. Detectors are the measurement side of quantum photonics.

Two major detector families dominate many quantum-photonics discussions: single-photon avalanche diodes, or SPADs, and superconducting nanowire single-photon detectors, or SNSPDs. SPADs are semiconductor detectors often used for photon counting and timing. SNSPDs use superconducting nanowires and can offer high efficiency, low dark counts, and excellent timing, but usually require cryogenic operation.

A single-photon detector is sensitive enough to register one photon.

Ordinary optical detectors measure light intensity when many photons arrive. Single-photon detectors operate near the quantum limit, where one photon can trigger a measurable electrical response.

In many quantum systems, the detector does not measure a smooth optical wave. It produces a click: an electrical pulse indicating that a detection event happened. That click can be associated with a time, channel, polarization, path, frequency, or other measurement outcome.

Detector quality controls the reliability of quantum measurement.

If photons are lost, detected late, counted falsely, or confused with noise, the quantum protocol suffers. In QKD, detector imperfections can affect key rates and implementation security. In entanglement experiments, detector efficiency affects coincidence rates. In photonic quantum computing, detector performance shapes scalability.

Different detectors turn a photon into an electrical signal in different ways.

Most single-photon detectors work by converting the energy of an absorbed photon into a measurable electronic response. The physical mechanism depends on the detector type.

Single-photon avalanche diodes use avalanche gain to detect photons.

A SPAD is a semiconductor avalanche photodiode designed to operate in Geiger mode. When a photon creates an electron-hole pair, the high electric field can trigger an avalanche of carriers, producing a measurable pulse.

SPADs are important because they can be compact, manufacturable, and compatible with imaging, LiDAR, time-resolved measurement, and many quantum-optics setups. Silicon SPADs are common for visible and near-infrared wavelengths, while InGaAs/InP SPADs are often used around telecom wavelengths.

Superconducting nanowire detectors use cryogenic nanowires to detect photons.

An SNSPD uses a thin superconducting nanowire biased near its critical current. When a photon is absorbed, it locally disrupts superconductivity, creating a resistive hotspot and an electrical pulse.

SNSPDs are often favored in demanding quantum photonics because they can offer high system detection efficiency, low dark counts, fast recovery, and excellent timing performance. The trade-off is that they typically require cryogenic cooling.

Detector performance is measured by more than sensitivity.

A detector must detect useful photons while avoiding false clicks, timing uncertainty, and saturation. The right detector depends on the wavelength, count rate, timing requirement, noise tolerance, and operating environment.

Some systems need to know how many photons arrived, not just whether one arrived.

Many detectors are click/no-click devices: they indicate that at least one photon was detected. But some quantum photonic protocols benefit from photon-number-resolving detection, where the detector can distinguish one photon from two or more photons.

Photon-number resolution can be achieved through specialized detector physics, detector arrays, multiplexing, or transition-edge sensors, depending on the system requirements.

Single-photon detectors are used wherever quantum light is measured.

Detectors are not just support hardware. They define whether a quantum optical experiment or system can produce reliable data.

Integrated detectors move measurement onto photonic chips.

For scalable quantum photonics, detectors need to be integrated with sources, waveguides, switches, interferometers, filters, and control electronics. Integrated SNSPDs and SPADs are active areas of development because they can reduce coupling loss, improve compactness, and support larger circuits.

The challenge is compatibility. Photonic material platforms, detector materials, cryogenic packaging, electrical readout, heat load, and fabrication processes all need to work together.

The perfect single-photon detector does not exist yet.

An ideal detector would have 100% efficiency, zero dark counts, zero timing jitter, zero dead time, photon-number resolution, broad wavelength coverage, room-temperature operation, low cost, easy integration, and high reliability. Real detectors involve trade-offs.

Single-photon detectors are moving from lab instruments to integrated system components.

The next generation of quantum photonic systems will require better detector integration, higher efficiency, lower noise, improved timing, scalable arrays, photon-number resolution, and compatibility with practical packaging.

Once sources and detectors are understood, the next QCLS quantum page should move into **Photonic Quantum Computing Explained**, because scalable photonic computing depends on high-quality sources, low-loss circuits, entanglement, interference, and measurement.

Single-photon detectors, explained clearly.