Simple Mod Turns Diodes Into Photon Counters 118
KentuckyFC writes "The standard way to detect single photons is to use an avalanche photodiode in which a single photon can trigger an avalanche of current. These devices have an important drawback, however. They cannot distinguish the arrival of a single photon from the simultaneous arrival of two or more. But a team of physicists in the UK has found a simple mod that turns avalanche photodiodes into photon counters. They say that in the first instants after the avalanche forms, its current is proportional to the number of photons that have struck. All you have to do is measure it at this early stage. That's like turning a Fiat 500 into a Ferrari. Photon counting is one of the enabling technologies behind 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 (abstract). 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."
So In other words... (Score:4, Informative)
...they treat the earliest stages as if the diode were a unijunction transistor of sorts (using the photon reaction as if it were the gate 'voltage'), and check the waveform? Gah - I'll RTFA(bstract) @ work... :)
Re:Cooled devices? (Score:5, Informative)
Various people, including Shields himself, have come up with complex, cooled devices that can count photons.
It is the current generation of photon counting detectors which typically require high degrees of cooling (usually with LN2, as you suggest). Photodiodes of the type discussed in the article typically don't have such extreme cooling requirements under normal operation, so presumably that's what's so nice about this mod, as well.
Re:Cooled devices? (Score:5, Informative)
There's really no way around cooling the sensor for photon counting, especially if you use near-infrared or lower. If the sensor itself is giving off black-body radiation of the type you're looking for, then it's pretty much worthless to try to count photons because the laws of thermodynamics and quantum mechanics conspire against you. I can imagine visible or UV photon counting with uncooled sensors, but certainly not far-infrared. These thermally-generated photons are what cause the "dark count rate" of a device, and cooling the device can help reduce the dark count rate. Here you are, from wikipedia:
http://en.wikipedia.org/wiki/Single-Photon_Avalanche_Diode [wikipedia.org]
Re:Neutrino measurements here we come... (Score:3, Informative)
Neutrinos are not charged. Therefore they don't give off Cherenkov radiation.
I'm under the impression that it's the "reverse" of n -> p + e + v_e'; n + v_e -> p + e that is the usual reaction that is needed - probably the e then gives off Cherenkov radiation.
Tim.
It's like an eye (cell)! (Score:2, Informative)
Isn't that what those little cells (rods) in our eyes do?
I remember they can detect one single photon, (although we wouldn't perceive it) going all the way up to super bright light.
Although they are more of an analog detector, (stronger signal just looks brighter) rather than digital, it's pretty much the same device.
Re:Ferrari? Not quite (Score:5, Informative)
Up to this point, photon counters were elaborate devices with scintillation media, anticoincidence detctors, veto logic, and complex timing and biasing requirements.
Now you can just apply 9.8V and an instrumentation amp and a couple analog filter/comparator chains, and off you go counting.
Single photon avalanche diodes produce rising edge times well under 1ns. You need to measure the *shape* of that rising edge to use this technique. That is a complex circuit, no matter how you look at it.
The new circuits will be vastly simpler. But they will require a fair bit more than instrument amp and a ballast resistor and a comparator.
Re:Can anyone explain that graph? (Score:3, Informative)
I think you understand the graph, the presentation is just non-intuitive.
What the graph is saying is that if 0 photons come in, the maximum probability is around 4.5 millivolts. If 1 photon comes in, the maximum probability is about 7.5 mV, and the high end tail of the curve for the 1 photon case is negligible over 11 mV. If 2 photons come in the peak is at 12 mV, and the low end tail of the curve for 2 photons is negligible at 7.5 mV.
In which case what is being shown is that the resting output is around 5mV, and that the pulse height depends on the number of simultaneous photons.
Yep.
If the reading is 7.5 mV, you almost certainly received one photon.
If the reading is 12 mV, you almost certainly received two photons.
If the reading is 9.8 mV, you're 50% likely to have received one or two.
The area under the dark curve can be used to get an idea of the probabilities of any given reading, and most readings are going to be outside the range (from, say, 9 to 10.5) where they're ambiguous.
Re:Hopelessly confused about a "single photon" (Score:5, Informative)
Believe it or not, "a single photon" isn't as small of an amount of light as you'd think.
In a study [ucr.edu], 60% of participants were able to correctly identify a pulse of 90 "green" photons. Because only approximately 10% of the light that enters your eye ends up on your retina, that's just 9 photons required to trigger a neural response.
Because your retinas have approx. 350 rods in them, which sense light in a dark environment (and only in black & white), those 9 photons are spread across those 350, which can be interpreted to mean that parts of your eye are indeed responding to single photons.
Considering just how small of an amount of light/energy is contained within a single photon, this result is absolutely astonishing.
For more information regarding single photons, read up on the photoelectric effect [wikipedia.org]. It's quite simple in concept, and its discovery by Einstein in 1905 conclusively confirmed the notion that light exists as a particle.
This paved the way to Quantum Physics, and won Einstein the Nobel prize in 1921.