photon counting (original) (raw)

Definition: photodetection where single photon absorption events are counted

Alternative term: single photon counting

Categories: photonic devices, light detection and characterization, optoelectronics, quantum photonics

Related: photonsphotodetectorssingle-photon detectorselectronics for photonics

Page views in 12 months: 2134

DOI: 10.61835/6rq Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

Content quality and neutrality are maintained according to our editorial policy.

📦 For purchasing photon counting, use the RP Photonics Buyer's Guide — an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.

Contents

What is Photon Counting?

Photon Counts Instead of Analog Signals

Temporal Aspects and Statistical Processing

Counting Photons in Multi-photon Quantum States

Energy Resolution

Inappropriate Nomenclature

Detector Technologies

Features of Photon Counters

Practical Pitfalls and Good Practice

Applications of Photon Counting

Frequently Asked Questions

Summary:

This article provides a comprehensive introduction to photon counting, the technique of detecting and counting individual photons. It explains the advantages of this digital approach over analog signal detection, particularly the elimination of multiplication noise, and discusses limiting factors like shot noise and dark counts.

Key advanced methods are explored, including time-correlated single-photon counting (TCSPC) for measuring lifetimes and photon statistics for analyzing the nature of light. The article clarifies the important distinction between simple single-photon detection and true photon-number resolving, detailing the specialized detectors required for the latter.

Finally, it surveys the various detector technologies used, such as SPADs, PMTs, and SNSPDs, and highlights the wide range of applications in fields like quantum science, microscopy, medical imaging, and astronomy.

(This summary was generated with AI based on the article content and has been reviewed by the article’s author.)

What is Photon Counting?

Some kinds of photodetectors are so sensitive that they allow the detection of single photons of light (and of electromagnetic radiation in other spectral regions up to the X-ray regime). This is explained in the article on single-photon detectors.

If single photons can be detected with reasonable probability, counting them, i.e., determining their number, seems like a trivial extension: Combine a single-photon detector with a fast electronic counter. The same idea extends to spatially resolved counting with detector arrays and matching counter arrays — e.g. silicon SPADs. However, the concept of photon counting encompasses subtler ideas as explained in the following sections.

Some detector types such as CCDs (charge-coupled devices) accumulate electronic charges, which are related to photon numbers. Nevertheless, they are generally not considered to be photon counting, as they are usually not able to respond to single photons due to too high noise.

Photon Counts Instead of Analog Signals

Counting the registered photons in a time interval provides a discrete alternative to measuring analog photocurrent. This can be advantageous for the signal-to-noise ratio (SNR), particularly when analog multiplication noise would dominate — for example, with photomultipliers or linear-mode APDs where excess noise factors degrade SNR. In Geiger-mode devices (SPADs) including silicon photomultipliers, each detection is normalized to one “click”, so multiplication noise effectively becomes irrelevant. On the other hand, some amount of dark counts occurs.

Some practical notes:

• Shot noise limit: A limiting factor for the SNR is still shot noise, related to the discreteness of detection events. How that impact compares with the case of analog detection depends on the quantum efficiency of the detectors: higher is better.
• Dark counts: Photon counting may substantially suffer from dark counts, i.e., registered events not triggered by light, but e.g. by thermal fluctuations in the detector or in the used electronics.
• Dynamic range: Counting starts becoming nonlinear at high flux where dead time and pile-up come into play. That limits the dynamic range.

Temporal Aspects and Statistical Processing

Beyond totals, temporal aspects can carry vital information:

• Time-correlated single-photon counting (TCSPC): Following a trigger (e.g., involving optical pumping), photon arrival times can be used to create histograms. With those, one can extract lifetimes, more general rise/decay kinetics or transport times.
• Photon statistics: Counting allows the measurement of fluctuations beyond the mean, e.g., a full photon number histogram, the Fano factor or Mandel ($Q$) parameter, revealing Poissonian, sub-Poissonian (nonclassical) or super-Poissonian light.
• Correlations: By counting coincidences between detectors — for example, in a Hanbury Brown–Twiss setup — one measures the second-order correlation function ($g^{(2)}(\tau)$), diagnosing bunching/antibunching and single-photon purity.

Therefore, modern systems often include special electronics, going beyond simple counting:

• There are time-tagging electronics that record a timestamp for every event.
• Higher-level processing then provides histograms and correlation functions.
• There are constant-fraction discriminators which reduce time-walk, and time-to-digital converters (TDCs) providing temporal bins on picosecond time scales.

At high count fractions per cycle, early events are over-represented, while later ones suffer from pile-up. Operating well below one detection per excitation period (typically <1–5%) or using multi-stop TCSPC and statistical corrections can be used to avoid such bias.

Counting Photons in Multi-photon Quantum States

In quantum optics and quantum photonics, it is sometimes of interest to measure the number of photons in a multi-photon quantum state — for example, in a Fock state or in a short light pulse. Unfortunately, however, most single-photon detectors can not distinguish events with different numbers of photons within a short time interval. That means that such “binary detectors” are not suitable for photon counting in that sense. One requires a photon-number-resolving detector, which can be realized in two ways:

• A few types of single-photon detectors have that ability intrinsically. For example, one may use superconducting transition-edge sensors (TES), microwave kinetic inductance detectors (MKIDs), or visible-light photon counters (VLPCs). Note that TESs and MKIDs actually sense photon energies, so that two higher-energy photons might have the same effect like three lower-energy photons, for example.
• Alternatively, one may use multiplexing: Split the field over time bins (time-division) or over many spatial channels (with multi-pixel arrays or fiber trees) and sum the independent clicks.

In many applications, however, only single-photon events are relevant, so that no photon-number-resolving detectors are required.

Energy Resolution

Some photon-counting systems also resolve photon energy (“spectral photon counting”). That works particularly in the X-ray regime for medical imaging: Direct-conversion semiconductors (e.g., CdTe/CZT, Si) with multiple energy thresholds or full spectroscopic readout enable energy-discriminating X-ray CT and radiography. This can improve the image quality while reducing the required irradiation dose.

In the optical/IR regime, TES and MKID devices measure photon energy directly, but it is hard to combine photon number and energy resolution.

Inappropriate Nomenclature

Unfortunately, the terms photon counting and photon-counting detectors are often inappropriately used, where counting (= determining numbers) is not really involved and relevant for the measurement outcomes. (This happened already in the early scientific literature.) Better terms are then single-photon detection and single-photon detectors. A detector which only registers photons without counting them should not be called photon counting even though one might add a counter to it.

Where a detector can resolve photon numbers (at least partially), it can be called a photon-number-resolving detector, which is far more clear than photon counting detector.

Detector Technologies

Photon counting can use different types of single-photon detectors. A brief overview:

• SPADs (Geiger-mode APDs). Silicon SPADs (visible/NIR) and InGaAs/InP SPADs (telecom) are compact, offer time gating, and can be made as large detector arrays. Key issues are dead time, afterpulsing and dark count rate; cooling and active quenching mitigate these. Imaging SPAD arrays are available, particularly based on silicon technology for the near-infrared.
• PMTs and MCP-PMTs. High gain and large area; MCP variants offer excellent timing (tens of ps) and are common in fast timing/TOF applications. Sensitive to magnetic fields; analog multiplication noise is irrelevant when operated in counting mode with discriminators.
• SNSPDs (superconducting nanowire single-photon detectors). These have exceptional efficiency, low dark counts, and timing jitter as low as a few picoseconds. Arrays exist; intrinsic PNR is limited, but parallel nanowires/pixels and multiplexing provide quasi-PNR.
• TES / MKID / VLPC. These cryogenic devices have true energy or number resolution. They are slower, but are uniquely capable for spectroscopy and absolute calibration.
• Intensified and EMCCD/sCMOS cameras. With appropriate thresholding (and often image intensifiers), they provide photon-counting imaging over many pixels. The timing resolution is modest compared to SPAD arrays.
• X-ray direct-conversion sensors. Pixelated CdTe/CZT/Si with per-pixel counters and energy thresholds are used for spectral computed tomography (CT) and radiography.

For more details, see the article on single-photon detectors.

Features of Photon Counters

Properties of single-photon detectors are already explained in that article. Briefly, these include the quantum efficiency, dead time, maximum detection rate, and dark count rate. Possibly also relevant properties can be timing jitter, latency and active area, apart from required operating conditions. Also, there are imaging detectors, providing spatial resolution. Most of these properties are also relevant for photon counters.

In addition, there are specific features photon counters:

• Some have a built-in digital display for the number of counted photons, while others deliver a digital signal output. Of course, both may be combined.
• With a reset signal, one can reset the counter to zero.
• There may also be a gate input for suppressing the counting during certain times.

Practical Pitfalls and Good Practice

For avoiding common problems, some good practice should be applied:

• Saturation: Considering that the used photodetectors are mostly not capable of detecting multiple photons at a time, one should work in a regime with low enough photon flux where saturation related to detection dead time is unlikely.
• Background light should of course be blocked as far as possible, e.g. using tubes with black coatings for shielding.
• Afterpulsing and crosstalk: The effects of such phenomena on the measurement results should be checked carefully. For multi-pixel systems, also consider crosstalk, variations of pixel efficiency, timing and dead time.
• For PMTs, consider their magnetic field sensitivity.

Applications of Photon Counting

Photon counting equipment is used in various areas of science and technology:

• Fundamental quantum science often requires the accurate detection and timing analysis of photon events. For example, photon statistics need to be measured, and coincidences are counted to estimate correlation functions.
• Astronomy utilizes photon counting detection to reach shot-noise-limited imaging.
• Fluorescence lifetime imaging microscopy (FLIM) uses time-correlated single-photon counting (TCSPC), building histograms of photon arrival times to extract fluorescence lifetimes.
• Medical imaging: X-ray diagnostics like computed tomography (CT) and positron emission tomography (PET) count photons — partly with energy resolution for higher image resolution and reduced irradiation dose.
• Radiometry and calibration: Absolute photon counting is used to calibrate light sources at very low flux (few-photon metrology).
• Time and frequency standards: Counting photons from atomic transitions is used for precisely measuring decay rates and linewidths.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is photon counting?

Photon counting is the process of detecting and counting individual photons using a highly sensitive photodetector and a fast electronic counter. It provides a digital measurement of light intensity, as opposed to measuring an analog photocurrent.

How does photon counting improve the signal-to-noise ratio?

By registering each detected photon as a normalized digital pulse or 'click', photon counting eliminates the multiplication noise that degrades the signal-to-noise ratio in analog detectors like photomultipliers or linear-mode avalanche photodiodes.

What is time-correlated single-photon counting (TCSPC)?

Time-correlated single-photon counting (TCSPC) is a technique where the arrival times of single photons are recorded relative to a repeating trigger signal. By creating histograms of these times, one can precisely measure phenomena like fluorescence lifetimes or decay kinetics.

Can standard single-photon detectors count multiple photons arriving at once?

No, most single-photon detectors are binary, meaning they register an event if one or more photons arrive but cannot distinguish between a single-photon and a multi-photon event. For that, a photon-number-resolving detector is required.

What are photon-number-resolving detectors?

Photon-number-resolving detectors are special devices that can determine the exact number of photons in a light pulse. This can be an intrinsic property of detectors like transition-edge sensors (TES) or achieved by multiplexing many binary detectors.

What are common detector technologies for photon counting?

Common technologies include single-photon avalanche diodes (SPADs), photomultipliers (PMTs), superconducting nanowire single-photon detectors (SNSPDs), and specialized cryogenic devices like transition-edge sensors (TES) for photon-number resolving.

What are the main applications of photon counting?

Key applications include fundamental quantum science, shot-noise-limited imaging in astronomy, fluorescence lifetime imaging microscopy (FLIM), and medical imaging techniques like positron emission tomography (PET) and spectral X-ray computed tomography (CT).

Suppliers

Bibliography

(Suggest additional literature!)

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.