Counting photons is a tricky business. They’re slippery beasts that arrive silently, often and in packs, in ways that are almost impossible to count.
One of the most widely used of devices that can spot the arrival of a single photon is the avalanche photodiode. These cheap and easy to use devices rely on the ability of diodes to allow the flow of electrons when the voltage across them is in one direction but prevent that flow when the bias is reversed. But if the reverse bias is increased beyond a specific threshold then a breakdown occurs and a reverse current suddenly starts to flow.
Choose the right material for your photodiodes and this breakdown can be triggered by a the arrival of a single photon smashing into an electron which goes on to hit other electrons causing a chain reaction. The result is an avalanche of current that signals the arrival of your photon.
Avalanche photodiodes are widely use to detect single photons but have an important limitation: they cannot distinguish between the arrival of a single photon and the arrival of two or more photon’s simultaneously.
But that is set to change. Today, our old friend Andrew Shields, at Toshiba’s research labs in Cambridge UK, explains how to soup up a bog-standard avalanche photodiode so that it can count photons as they arrive. That’s like turning a Fiat 500 into a Ferrari.
He says that the trick is to measure the characteristics of the avalanche current in the very first instants that it forms. At this early stage, say Shields and friends, the avalanche current is proportional to the number photons that have struck.
Simple really but with enormous potential. The ability to count photons is one of the key enabling technologies for optical quantum computing. A number of schemes are known in which it is necessary to count the arrival of 0,1 or 2 photons at specific detectors.
Various people, including Shields himself, have come up with complex, cooled devices that can count photons. But this is a major step forward. Avalanche photodiodes are cheap, widely available and easy to use. With such a cheap detector now available (as well as decent photon guns), we could see dramatic progress in this field in the coming months.
If you haven’t quite seen the significance of this, imagine overclocking your calculator and matching the performance of a workstation. Or polishing up the 3 inch reflector in your attic and outclassing Hubble with your images.
Ref: arxiv.org/abs/0807.0330: An Avalanche-Photodiode-Based Photon-Number-Resolving Detector