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The types of detectors used in quantum cryptography are those that are able to distinguish between individual photons. PicoQuant’s vice-president of sales and marketing, Uwe Ortmann, described the detector technology: ‘Photon counting is more sensitive than analogue technology. Analogue measurement needs multiple photons to generate a signal; photon counting needs only a single photon. It’s basically the most sensitive system because it reacts to a single photon. You don’t need hundreds of photons to get a useable system.’ Photomultipliers or avalanche photodiodes are two common detectors that can be used for single photon detection. The principle is


detectors will have absorption peaks at different wavelengths

relatively simple: a photon hitting the active area of the photodetector will be converted into an electron, which is then amplified into a measurable pulse signal.

A photomultiplier has a dynode

structure in which one electron is multiplied leading to a larger, measurable electrical pulse. Photons striking an avalanche photodiode, meanwhile, cause an avalanche effect in a semiconductor material that generates the measureable pulse. For both, Ortmann said: ‘One photon of light hits the photoactive area and under a certain detection efficiency is able to generate an electron. This is amplified inside to create an electrical pulse that could be measured as a single event.’ This means the devices need a high quantum efficiency in order to make the most of the limited light source available. Elliott Chick, a technical sales engineer at Laser Components, explained that the quantum efficiency is dictated by the semiconductor material’s ability | @electrooptics

Avalanche Photodiodes Si APDs

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to absorb a particular wavelength: ‘Essentially, the quantum efficiency is the percentage chance that a photon will be successfully converted into an electron event. The quantum efficiency at different wavelengths is what discerns the wavelength of light that a detector is suited to.’ Different detectors will have absorption peaks at different

wavelengths. However, Chick gave the example of Laser Components’ Count single photon counting module, which would detect an NIR photon even if this is not the peak efficiency of the device. ‘This [the peak efficiency] is normally governed by the structure of the semiconductor and the complex electronics behind the sensor.

You know that a photon will give a certain amount of energy to an electron, so you can tailor your bandgaps to give better efficiency,’ he said.

Keeping dark count down One issue, especially with photon counting detectors in the infrared, is the phenomenon known as the lasercomponent www lasercompo 26.02.2014 11:40:31

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