Photomultipliers, developed during the early part of the 20th century, are ultrasensitive light detectors. They are still in widespread use, and can detect extremely low levels of light - even single photons. See : Wikipedia

As with all scientific measurement devices, there is an inherent and unavoidable low level random 'noise floor' in this case called the 'Dark Rate'.

It was verified in 2010 that the 'dark rate' noise increases as the photomultiplier is cooled down at very low temperatures (from about 250K to ~0K). This is contrary to all other electronic systems - and physical systems in general - where the noise always decreases as temperature is lowered.

There is currently no explanation for this effect.

The properties of cryogenic emission are inconsistent with any of the spontaneous emission processes known, such as thermal emission, field emission, radioactivity, or penetrating radiation, including cosmic rays. At this time, regrettably, a quantitative explanation of the observed characteristics of cryogenic emission in terms of known physics is still eluding us.

See : Spontaneous electron emission from a cold surface Europhysics Letters, Volume 89, Number 5 ( a full copy may be found here)