Single-photon detectors are crucial in quantum optics, enabling the detection of individual light particles. These devices convert photons into measurable electrical signals through various mechanisms, including the photoelectric effect, avalanche multiplication, and superconducting transitions.

Different types of detectors, such as photomultiplier tubes, avalanche photodiodes, and superconducting nanowire detectors, offer unique advantages. Key performance characteristics include detection efficiency, dark count rate, timing resolution, and spectral range, with trade-offs between these factors influencing detector selection for specific applications.

Single-photon detector mechanisms

Photon detection principles

• Single-photon detectors are devices that can detect individual photons with high sensitivity and low noise
• Detectors operate by converting photons into measurable electrical signals through various physical processes
• Common detection mechanisms include the photoelectric effect, avalanche multiplication, and superconducting transitions

Types of single-photon detectors

• Photomultiplier tubes (PMTs) convert photons into electrons through the photoelectric effect and amplify the electron signal using a series of dynodes
• Avalanche photodiodes (APDs) operate in Geiger mode, where a high reverse bias voltage creates an avalanche of electrons triggered by a single photon absorption
• Superconducting nanowire single-photon detectors (SNSPDs) consist of a thin superconducting nanowire that becomes resistive when a photon is absorbed, creating a measurable voltage pulse
  • SNSPDs require cryogenic cooling to maintain their superconducting state and achieve high detection efficiency
• Transition edge sensors (TESs) operate near the superconducting-to-normal transition temperature, where a small change in temperature due to photon absorption results in a measurable change in resistance
• Quantum dot single-photon detectors exploit the discrete energy levels in semiconductor quantum dots to detect single photons through changes in their electronic properties

Performance characteristics of single-photon detectors

Detection efficiency and dark count rate

• Detection efficiency is the probability of detecting a photon that arrives at the detector and varies among different types of single-photon detectors
  • SNSPDs and TESs can achieve detection efficiencies above 90% for certain wavelengths, while PMTs and APDs typically have lower efficiencies
• Dark count rate is the rate of false detections in the absence of input photons, which can limit the sensitivity of single-photon detectors
  • SNSPDs and TESs generally have lower dark count rates compared to PMTs and APDs

Timing resolution and spectral range

• Timing resolution is the ability to precisely determine the arrival time of a photon, which is crucial for time-correlated single-photon counting (TCSPC) and quantum communication applications
  • SNSPDs and APDs offer the best timing resolution, typically in the range of tens of picoseconds, while PMTs and TESs have lower timing resolution
• Spectral range is the range of wavelengths over which a single-photon detector is sensitive and varies depending on the detector material and design
• Photon number resolution is the ability to distinguish the number of photons in a single detection event, which is possible with TESs and some advanced APD designs

Trade-offs in single-photon detectors

Balancing performance and practicality

• The choice of a single-photon detector involves trade-offs between detection efficiency, dark count rate, timing resolution, and operating conditions
• PMTs and APDs are more readily available and cost-effective but have lower detection efficiency and higher dark count rates compared to SNSPDs and TESs
• SNSPDs and TESs require cryogenic cooling, which adds complexity and cost to the experimental setup, but they offer superior performance in terms of detection efficiency and low dark count rates

Limitations and considerations

• The spectral range of the detector must match the wavelength of the photons being detected, which can limit the choice of detectors for specific applications
• The maximum count rate of a single-photon detector can limit the data acquisition speed in experiments with high photon flux
• The afterpulsing effect in APDs, where a single detection event can trigger multiple false detections, can introduce errors in photon counting experiments

Selecting single-photon detectors for applications

Matching detector characteristics to experimental requirements

• For applications requiring high detection efficiency and low dark count rates (quantum key distribution and quantum sensing), SNSPDs or TESs are preferred
• In experiments demanding high timing resolution (quantum communication protocols and time-resolved spectroscopy), SNSPDs or APDs with optimized timing performance are suitable
• For wide-field imaging applications (single-molecule fluorescence microscopy), PMTs or APDs with spatial resolution capabilities are commonly used

Cost and wavelength considerations

• In applications with limited budget or less stringent performance requirements, PMTs or APDs can provide a cost-effective solution
• The operating wavelength range of the experiment dictates the choice of detector material and design
  • Silicon-based APDs are suitable for visible light detection
  • Superconducting detectors (SNSPDs and TESs) are often used for infrared photons
• For experiments requiring photon number resolution, TESs or advanced APD designs with photon number resolving capabilities are necessary