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FEATURE SINGLE PHOTON DETECTION


Keeping count of light


Highly sensitive detectors are enabling techniques like quantum cryptography and forms of fluorescence spectroscopy. Tom Eddershaw investigates the technology for detecting individual photons


O


n 24 April, a team of researchers from Toshiba Research Europe, British


Telecom, ADVA Optical Networking and the UK National Physical Laboratory announced it had transmitted both a quantum encryption key and useable data


down the same fibre cable. The trial is one of the latest advances in Quantum Key Distribution (QKD), a form of data encryption based on quantum mechanics that is much more secure than existing encryption methods. Quantum cryptography, while still in its infancy, is gathering a


lot of interest – the CLEO 2014 event in San Jose, USA will hold a number of talks on the topic. It is a technology that relies, to a certain extent, on the sensitivity of detectors to observe the stream of randomly polarised photons used in QKD. Andreas Bülter, applications specialist at PicoQuant, said: ‘A couple of companies sell working [quantum cryptography] devices, but they are still only used for very specific purposes – for general use the data throughput is still low and you have to solve the problem that the current systems need dedicated point-to-point networks.’ The low data throughput means


the technology currently takes a long time to send a message. Bülter noted that one way to improve this would be to have detectors with higher detection efficiencies, lower dark counts and higher temporal resolution. The photon counting electronics are not really the limiting factor. ‘These days you can get electronics that have temporal resolutions of around a picosecond; so, they are effectively extremely fast stopwatches,’ he said.


He added though that going faster is not really necessary because the best detectors operate at around 20ps – the timing electronics are often faster than the detectors.


One of the sensitive multi-pixel photon counters from


Hamamatsu, which is capable of detecting single photons


16 ELECTRO OPTICS l JUNE 2014


@electrooptics | www.electrooptics.com


Hamamatsu


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