Light sources also absorb photons

"Counting light particles"

Photo detectors measure light by converting incoming light particles into an electrical signal. However, the light particles are destroyed in the process. A new type of sensor can now not only count individual photons - the particles also survive the measurement unscathed. Tracy Northup from the University of Innsbruck explains how this works in an interview with Welt der Physik.

World of physics: how can light be measured?

Tracy Northup: In normal daylight, countless light particles - also called photons - buzz around. When it gets darker, the number of light particles decreases, even if the energy per photon remains the same. The human eye is an excellent light sensor for measuring light particles. When we have got used to the darkness - after around half an hour without a brighter light source - just a few photons are enough to stimulate the visual cells so that we can recognize a point of light. For some animal species, especially nocturnal animals such as cats, even a single photon is sufficient.

Tracy Northup

How do technical light sensors work?

Technical sensors, so-called photodetectors, work in a similar way: An incident photon is absorbed in matter and releases its energy in the process. This process is amplified electronically or chemically, for example in a camera or on analog film. However, with the usual methods, the photons are lost in the detector. Because they usually disappear when they take part in a chemical or physical process.

Why is that a problem?

If you want to build novel quantum systems - for example, quantum computers or quantum sensors - you want to disturb the states in these systems as little as possible. One can already make visible individual atoms or molecules that are locked in a trap, for example. But light always moves at the speed of light, so it is not so easy to lock it up and count the number of light particles.

You have now developed a method with which light particles can still be counted. How is that possible?

Our experiment works like this: First we send a laser pulse through several strong filters so that ultimately only one photon or only a few photons remain. We direct these into an absolutely dark cavity and let them fly back and forth between two almost perfect mirrors. We now place an electrically charged calcium atom in the middle between the mirrors. We cool this atom down to just above absolute temperature zero and bring it into a special quantum mechanical state with a laser pulse. In this state, the atom reacts to the light without absorbing individual photons. With a second laser pulse we can now query the properties of the atom. In this way we can count the light particles in the cavity without destroying them through the measurement.

Can the process also be used technically?

We work with very complex equipment in our experiments, because on the one hand we have to cool the calcium atom very deeply and we also need an ultra-high vacuum. The effort to achieve these conditions is very high. And in the vast majority of applications, it doesn't matter at all to get the number of photons.

In which areas could your photon counter still be used?

The latest quantum technology, which the European Union is pursuing with the quantum flagship project, should include, in particular, ultra-sensitive quantum sensors and quantum computers. It is important to disturb the quantum states as little as possible. These are interesting uses for non-destructive photo detectors. This could perhaps also improve the quality of gravitational wave observatories that demonstrate the merging of black holes and neutron stars in the depths of space.