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Simple Mod Turns Diodes Into Photon Counters
Posted by
timothy
on Tue Jul 08, 2008 09:11 AM
from the measure-once-vote-often dept.
from the measure-once-vote-often dept.
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."
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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... :)
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So, you are a registered user (you can read the sig). And post AC to flame. Fucking wuss.
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I wouldn't be so sure of it happening soon... some odd thoughts:
The oscilloscopes required to read and reliably analyze such a waveform at the moment cost approximately more than most folks' houses (not counting probes, tips, calibration contracts, etc). We have a few here where I work, and they're treated like newborn children because of the pricetag.
Even in a digital waveform, you've always had rise and falloff, and those could conceivably be read and acted on (though in only a few damned-small niche case
Back in my day (Score:5, Funny)
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You had hands AND a number system? You lucky bastard.
Re:Back in my day (Score:5, Funny)
I had to make photons on my first day and I saw it was good.
Parent
Re:Back in my day (Score:4, Funny)
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Re:Back in my day (Score:4, Funny)
Yet your uid is only 667959! Man, that must make some people round here old!
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You had PAPER? Spoiled kids, in my day we had to use clay tablets.
Now get off my lawn.
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So you didn't have problems with workers stealing your photons?
Still irrelevant (Score:2)
As the physics is _not_ understood well enough to support the outrageous security claims that are being made by these grant-hungry researchers.
My guess is that at some time the possibility to evasdrop on these connections (and there are already some succcesses with that, despite the initial claims of impossibility), will render this technology useless. I hoper there is at least some useful other discoveries alsong the way...
Not Quite (Score:5, Insightful)
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.
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Well, I happen to know some of the people in quantum computing. First, it is not exponential, just a factor of n for most practical computing problems. Then there is the problem of scalability. Most people do not know it, but current CPUs are speed limited by interconnect, i.e. the signal lines of the chip. With quantum computing, the interconnect problem is dramatically worse. I would think that quantum computers will not reach the power of CPUs 20 years ago in our lifetimes, if ever. There may still be so
Is it legal in Florida? (Score:5, Funny)
Re:Is it legal in Florida? (Score:5, Funny)
You see, it depends on the wavelength of the photon.
If they are red they may be counted multiple times
If they are blue, counting is more difficult, and sometimes several pass without being counted
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Neutrino measurements here we come... (Score:5, Insightful)
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 [wikipedia.org])
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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.
A Fiat 500 into a Ferrari? (Score:3, Funny)
Simple Mod? (Score:2)
The guy creates a photon counter out of a little diode and you're still making fun of your moderators?
Bad form.
Ferrari? Not quite (Score:5, Insightful)
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.
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And the commodity price of photo diodes shoots up to match that of photon counters.
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That's no joke. I was going to buy a photon counter for my wife and I to share, but now, I'm going to buy about ten of these for each of us, just in case of a hurricane or something.
Hopefully economies of scale will kick in once these hit the Best Buy shelves.
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.
Parent
Beautiful, Simple, Engineering (Score:5, Interesting)
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This is pretty neat.
I used avalanche photo diodes as cheap photon counters back in university a decade ago. In our case, however, we would quench the diode after each count and allow it to reset. This works fine except that it severly limits the rate at which photons can be counted and doesn't distinguish when multiple photons arrive simultaneously.
So this is a cool extension to that technology.
Digital cameras? (Score:2)
Right. So how long before these simpler devices get into digital cameras? It would certainly help to increase the signal to noise ratio.
I wonder how these things would perform in direct sunlight. You're getting a few more than a couple of photons at once in that case....
Can anyone explain that graph? (Score:2)
If this is the probability of finding particular signal amplitudes (time unspecified), how does it relate to single photon counting?
It seems all back to front. Naively one might expect that multiple photons would result in a larger initial avalanche current, but the graph doesn't seem to relate to this at all. It shows a high probability
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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 5m
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.
Photon Guns? (Score:2)
Isn't that what the Starship Enterprise used to destroy enemy ships?
Hopelessly confused about a "single photon" (Score:3, Interesting)
Can some less physics challenged person enlighten me as to how we can actually manipulate a single photon. That to me seems to be such a small amount of energy as to be undetectable. I mean if I remember correctly, if you bounce one electron down from one high level to a lower level on one atom, then, it would give off one photon... and getting one atom to do that seems rather a tall challenge..
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I'm with you... I'm a CS major, not a physics geek, so I'm still in awe that scientists can not only emit and detect individual photos, but do so reliably in several quantum states.
I thought our use of light and particle streams was still in the "drink from a firehose of similar particles" stage.
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.
Parent
Re: (Score:3, Interesting)
You know how you get that grainy effect in your vision under very low light conditions with well adjusted night vision? That's quantum noise due to individual photons hitting your eye [dspguide.com].
Very interesting. From the link:
"Vision appears very noisy in near darkness, that is, the image appears to be filled with a continually changing grainy pattern. This results from the image signal being very weak, and is not a limitation of the eye. There is so little light entering the eye, the random detection of individual photons can be seen. This is called statistical noise, and is encountered in all low-light imaging, such as military night vision systems."
A Fiat 500 into a Ferrari? (Score:4, Funny)
What a strange analogy.
I wonder if a classic car buff would really want to convert their beloved collector's piece into a modern consumerist status symbol for power and wealth?
In any case, while one is fast, and the other is slow, the Fiat was originally made small so that it could navigate the teeny one-lane streets featured in many Italian cities, whereas the Ferrari needs some serious hubcap room. It could be argued that the analogy should be reversed since the smaller car is more agile and able to deal with small details whereas the other cannot and is in fact primarily focused on flying through as many kilometers as quickly as possible and isn't terribly concerned with counting them off in smaller quantities.
Of course, this kind of observation is the reason why I would be irritating at parties. Carry on. I'm listening.
-FL
Thank you! (Score:3, Funny)
That's like turning a Fiat 500 into a Ferrari
Ha, thank you, dear Slashdot editor, for inserting a necessary car analogy here! As you rightly guessed, it came right at the point in the summary where my feeble mind wondered "huh?!? photon avalanche wtf???".
Thanks to your enlightening analogy my next thoughts were "ha, a Ferrari, of course! So this has nothing to do with the dangers of skiing on beams of light!", at which point I decided to stop reading the summary as these few concise words seemed to synthesise perfectly the very essence of this discovery, and that my mind proceeded to wander about how awesome it would be to be able to ski on waves of light!
Thank you a thousand times, oh most esteemed and wise Slashdot editor!
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.
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Erm.. It doesn't matter what you're using for photon counting. You're going to have to chill it to cut the noise [wikipedia.org] down. Of course, for some applications, the noise might be below an acceptable level.
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Obviously, if you want to measure photons at thermal energies, then cooling is certainly a requirement.
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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]
Parent
Re:Cooled devices? (Score:5, Interesting)
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.
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Re:Down already? (Score:4, Funny)
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Re:Down already? (Score:5, Funny)
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.
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Re:Down already? (Score:5, Funny)
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.
Parent
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Thanks, I never even thought of trying that (sometimes it takes a bit to engage the brain while at work). Seems (with very minimal testing) to have resolved the issue.
Ability to count 1,2,3 photons... (Score:4, Interesting)
Smells like a wonderful technology to implement as part of a camera sensor (when dealing with very low light, such as in astrophotography, nightshots of nature, etc.)
Canon's got the "switchable capacitor well" patent hanging in reserve, and it looks like they're going to have to use it with Nikon's new D700 going ASA 6400 and pushing all the way to 25600; but I wonder just how far you could take a camera's sensitivity if you had *accurate* photon counting... imagine a photodetector that counts photons as they arrive and simply increments a large counter. This would literally be a "digital" sensor, rather than an analog one. Precision light sensors. Mmmmm-good. :-)
I like wide-field astrophotography [flickr.com]. I'd be all over a (relatively) affordable DSLR that could really do low light in a precise manner. Right now, you have to spend about three grand to get a camera body that can go to an honest ISO 6400; if they could get the price in or around that area with something that was effectively counting all the photons... Oy.
Have to do something about the color and IR filters, too. Swing them out of the way or something equally tricky. Maybe some variation on a single-well, filterless approach like the Foveon one [wikipedia.org].
Parent
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Click to give her a greater more pleasure load of k.um!