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Simple Mod Turns Diodes Into Photon Counters
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."
You kids and your fancy diodes. Back in my day we counted photon by hand. Some people used paper to record the counts. We called them amateurs. Now get off my lawn!
Well, there's spam egg sausage and spam, that's not got much spam in it.
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.
I mean if multiple photons arrive at the same time at the detector should they be counted as a single vote or multiple votes? Whatever you say someone or the other would object and eventually it will be decided in the Supreme Court. Counting is quite weird in Florida.
sed -e 's/Chuck Norris/Rajnikant/g' joke > fact
Something that hasn't been pointed out is how useful this will be in high energy physics. The basic way of measuring a lot of particles is to look for the photons emitted when they interact with materials.
This should help reduce the cost of certain detectors. Especially for measuring neutrinos that can only be spotted by the cherenkov radiation they give off as they pass through massive detectors (look here http://en.wikipedia.org/wiki/Cherenkov_radiation)
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
I'd say it's more like finding out your "workstation" is an overpriced, overcomplicated Rube Goldberg device that in reality has the same performance as a Razor scooter.
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.
I can see the fnords!
This kind of gorgeous tweaking gives me warm feelings inside. Shields has taken a common device used in the field and, through a deep understanding of the physics of its operations, has increased it's functionality without much additional complexity. From the paper he says he cools the device thermo-electrically to -30 deg. C. Thermo-electric cooling is far nicer than cryogenic cooling (typ. using liquid gasses for heat exchange) used in other devices for photon number counting. Further, the method only introduces electronic capacitance subtraction of the photodiode response which is relatively simple compared to other methods (e.g. http://www.stanford.edu/group/fejer/fejerpubs/2005/Langrock_OL_2005.pdf which uses the nonlinear response of a crystal and a massive amount of supporting optics and electronics). This subtraction gives orders of magnitude greater sensitivity and allows the time response of the initial avalanche to be extracted from which photon numbers can be counted. One of those wonderful, "why didn't I think of that", insights. Very nice.
While I can get to the site...I have to question your logic.
Unless /. has some new magics you generally can't post HERE without an active internet connection.
You can get rich if you own a politician, but you have to be rich to buy one in the first place.
You can buy a single photon counter optimized at 680 nm that works at RT. Unfortunately, they are $5k a pop. This way of using avalanche diodes for counting enables a lot of new technologies.
Years ago we played with detecting high energy particles in a grid of scintillating fibers, but for a high precision array you just couldn't afford the detectors. Now I guess I can revisit that if the technology pans out.
I'm aging rapidly, I bought a new game and had no idea if my machine was good for it.
a lot of the applications for "security" actually is the defeat of cryptanalysis systems as these computers could crack keys in a reasonable amount of time. This would start to drive key length to very large values in order to keep data safe.
Essentially the value in quantum computing is you can set up a logical relationship between all the qbits and then preform an operation on any number of them and they instantaneously effect the remaining qbits. This saves the computation time for preforming operations on all the other qbits. The question on making this feasible is can you make the read/write time for each of the qbits reasonable and the technology affordable to do so. This seems to be a huge step in the right direction for the latter.
Oh honey look... How cute... an angry slashdotter!
Actually, your attempt to read the site caused its superposition to decay into a site that's Slashdotted.
About 50% of visitors from Slashdot will see the non-Slashdotted site.
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.
Believe it or not, "a single photon" isn't as small of an amount of light as you'd think.
In a study, 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. 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.
-- If you try to fail and succeed, which have you done? - Uli's moose