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Simple Mod Turns Diodes Into Photon Counters

timothy posted about 6 years ago | from the measure-once-vote-often dept.

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."

cancel ×

DrLudicrous (607375) | about 6 years ago | (#24099673)

Instantaneously slashdotted site?

courteaudotbiz (1191083) | about 6 years ago | (#24100113)

2. Post on slashdot that the site has been slashdotted ONLY if your internet connection was working on step one

torkus (1133985) | about 6 years ago | (#24100365)

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.

courteaudotbiz (1191083) | about 6 years ago | (#24100455)

Yes, and my post was exactly a joke... maybe not a good one since I had to explain it...

CDMA_Demo (841347) | about 6 years ago | (#24101687)

I hope the same simple mods mod your joke up

Mathness (145187) | about 6 years ago | (#24107397)

Oh, I thought /. supported RFC 1149 [ietf.org] .

jddj (1085169) | about 6 years ago | (#24101375)

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.

nigger post

Ability to count 1,2,3 photons... (3, Interesting)

fyngyrz (762201) | about 6 years ago | (#24102493)

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] .

Re:Ability to count 1,2,3 photons... (0, Offtopic)

fyngyrz (762201) | about 6 years ago | (#24103825)

You gotta love slashdot's moderation. Of course a post about how you might use the precise technology brought up by an article is off-topic. Of course it is.

This is why letting any idiot moderate is a bad idea. Inevitably, you get idiots moderating. :-) This is also why you have to read slashdot at -1; moderation as a filter is pathologically unfit for use by sensible readers.

Re:Ability to count 1,2,3 photons... (1)

thePowerOfGrayskull (905905) | about 6 years ago | (#24107667)

AC said:

nigger post

You said:

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.)

Get it now?

Slashdot is dying a prolonged death (-1, Offtopic)

Anonymous Coward | about 6 years ago | (#24099743)

Netcraft confirms it!

Do old nerd sites go to heaven when death comes, or is it a drop off the face of the earth into nothingness thing?

I need to know!

Re:Slashdot is dying a prolonged death (0)

Anonymous Coward | about 6 years ago | (#24099817)

Yes, on both

bitheaven

heard of it a lot

never seen it myself

WTF is a Fiat 500? (-1, Offtopic)

Anonymous Coward | about 6 years ago | (#24099783)

And if it's that bad, is the Fiat 400 even worse?

Re:WTF is a Fiat 500? (0)

Anonymous Coward | about 6 years ago | (#24099863)

F found
i in
a alleyway
t torched

worth more dead than alive, junker from italy (ferrari is all name; construction is middling; also italy)

Going compact (-1, Offtopic)

smitty_one_each (243267) | about 6 years ago | (#24099785)

Felling the photons
By fives, sevens, fives again
Is like Burma Shave

So In other words... (3, Informative)

Penguinisto (415985) | about 6 years ago | (#24099799)

...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... :)

/P

Re:So In other words... (-1, Troll)

Anonymous Coward | about 6 years ago | (#24099859)

Looking at your sig, you must be religious. So do you expect Jesus to count photons for you? Get out of the way.

Re:So In other words... (1)

MyLongNickName (822545) | about 6 years ago | (#24099921)

So, you are a registered user (you can read the sig). And post AC to flame. Fucking wuss.

Re:So In other words... (1)

slmdmd (769525) | about 6 years ago | (#24103089)

I think this will usher in a new era where logic will be built around multiple states as opposed to binary(current 1/0 state of diode). The implications are huge. If else programming will be cave man tool. To my understanding this will make us create unimaginably complex/intelligent machines. In other words equal to humans if not better. The dream of matrix will come true. Consider if 1 then.. if 2 if 3 if 4 then... wow

Re:So In other words... (1)

Penguinisto (415985) | about 6 years ago | (#24103697)

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 cases). You've also always had analog circuits and components, though in programming you read the output as a float value, the shunt off the desired threshold(s) as the breakpoint(s) in your if/else (or case) statements.

Truly analog programming would require that we pretty much cast out nearly everything we're using now, or at best modify the hell out of a given language (if you think C gives you the shakes now... :) ).

Otherwise I agree perfectly - it would be pretty cool.

/P

huh? (0)

Anonymous Coward | about 6 years ago | (#24099811)

Goldberg photon state machines have been able to do this since the 70s.

Re:huh? (0)

Anonymous Coward | about 6 years ago | (#24100777)

Comic books don't count.

Back in my day (5, Funny)

UnknowingFool (672806) | about 6 years ago | (#24099821)

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!

Re:Back in my day (3, Funny)

Anonymous Coward | about 6 years ago | (#24100145)

You had hands AND a number system? You lucky bastard.

Re:Back in my day (5, Funny)

JustOK (667959) | about 6 years ago | (#24100171)

I had to make photons on my first day and I saw it was good.

Re:Back in my day (3, Funny)

A beautiful mind (821714) | about 6 years ago | (#24100905)

Oh, hello Big Bang, nice to meet you!

Re:Back in my day (3, Funny)

h4rm0ny (722443) | about 6 years ago | (#24100919)

Yet your uid is only 667959! Man, that must make some people round here old! ;) :)

Re:Back in my day (1)

sm62704 (957197) | about 6 years ago | (#24101131)

You had PAPER? Spoiled kids, in my day we had to use clay tablets.

Now get off my lawn.

Re:Back in my day (1)

digitrev (989335) | about 6 years ago | (#24105163)

You had clay? We scratched the count in diamond using nothing but our fingernails. And thanked our parents for the bloody stumps of fingers we had at the end.

Re:Back in my day (2, Funny)

m.ducharme (1082683) | about 6 years ago | (#24101353)

So you didn't have problems with workers stealing your photons?

We all knew it (0)

Anonymous Coward | about 6 years ago | (#24099825)

Simple Slashdot Mod Turns Diodes Into Photon Counters

Now it is official.

Cooled devices? (1, Interesting)

EmagGeek (574360) | about 6 years ago | (#24099877)

Cheap, cooled devices...

They don't go into a whole lot of detail on what they mean by "cooled devices." This smells an awful lot like "you have to spend a ton of money to save a tiny bit of money."

If these things have to be cooled with some liquefied gas, then I suppose any cost savings on the material or approach would be quickly eaten up by energy used to cool them... just a hunch...

Re:Cooled devices? (4, Informative)

ZombieWomble (893157) | about 6 years ago | (#24099933)

I think you misread the article. Specifically where it says:

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? (1)

EmagGeek (574360) | about 6 years ago | (#24100283)

You're right, I misread it. That's what I get for letting my work distract me from my /. reading.

Hah...

Re:Cooled devices? (1)

zippthorne (748122) | about 6 years ago | (#24101491)

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.

Re:Cooled devices? (1)

ZombieWomble (893157) | about 6 years ago | (#24102247)

The noise from "real" photons (that is, those directly produced by black body radiation) is really rather small - vanishingly so, at visible and higher frequencies for an emitter at room temperature. Cooling in detectors at such energies is primarily needed for electrical noise caused in the system by events at thermal energies, something which is significantly less of a problem in avalanche photodiodes.

Obviously, if you want to measure photons at thermal energies, then cooling is certainly a requirement.

Re:Cooled devices? (1)

digitrev (989335) | about 6 years ago | (#24105295)

You're close. The blackbody radiation is often the signal that's getting drowned out by all the noise. For example, we have to cool our bolometers down to 300 mK to get any meaningful data. And that's just for testing. When the detector finally gets launched, it's going to have a huge tank of liquid Helium-3 to keep it cold (via a complicated pump system).

Re:Cooled devices? (5, Informative)

Robotbeat (461248) | about 6 years ago | (#24100121)

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:Cooled devices? (4, Interesting)

mapsjanhere (1130359) | about 6 years ago | (#24100413)

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.

Did anyone else read this as... (-1, Redundant)

Anonymous Coward | about 6 years ago | (#24099901)

Simple Mod Turns Dilithium into Photon Torpedoes"?

No they didn't... (0)

Anonymous Coward | about 6 years ago | (#24102887)

...and you're just being a dork.

Still irrelevant (1)

gweihir (88907) | about 6 years ago | (#24099919)

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...

Re:Still irrelevant (0)

Anonymous Coward | about 6 years ago | (#24100593)

What scum bags! I can't believe they almost got away with it. Thanks for explaining how their methodology is wrong because they need grant money.

Not Quite (4, Insightful)

ruin20 (1242396) | about 6 years ago | (#24100731)

Although I agree with you on the aspect of quantum encryption, that's not the only application for this technology. Quantum computing [wikipedia.org] is a means to increase the maximum processor speed exponentially. [caltech.edu] (see the "The Potential and Power of Quantum Computing" for a good explanation on how)

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.

Re:Not Quite (1)

gweihir (88907) | about 6 years ago | (#24104515)

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 some interesting side results, but the main, stated goals of quantum computing are basicallu impossible for all we know. Many of the researchers in this area admit that off the record without even putting up a small fight.

In my T-16 back home (0)

192939495969798999 (58312) | about 6 years ago | (#24099935)

"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 nothing, I used to turn avalanche photodiodes into photon counters in my T-16 back home.

Is it legal in Florida? (4, Funny)

140Mandak262Jamuna (970587) | about 6 years ago | (#24099939)

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.

Re:Is it legal in Florida? (5, Funny)

JamesP (688957) | about 6 years ago | (#24100671)

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

Re:Is it legal in Florida? (1)

EchaniDrgn (1039374) | about 6 years ago | (#24102883)

Well, since all these photons are causing changes in the waveform all you really get are dimpled chads so none of them really get counted. Now if we could only get a photon of infinite energy...

To burn some karma (-1, Offtopic)

ericrost (1049312) | about 6 years ago | (#24099967)

What is the deal with the new tree system since the theme change? I constantly have issues with deeply nested comments that I click on a reply to a comment I'm reading and end up viewing a completely unrelated comment. I just don't think this is a successful "upgrade" to the discussion system. I end up having to click on the post numbers and navigating that way, which is annoying!

Re:To burn some karma (-1, Offtopic)

Finallyjoined!!! (1158431) | about 6 years ago | (#24100077)

Agree. Also the stupid advert on the front page (have to use Exploder 7 at work) completely covers the article, rendering it unreadable. Bah.

Re:To burn some karma (1)

thomasdz (178114) | about 6 years ago | (#24100389)

yeah, I just read with Javascript off because of this problem. Also, when I'm on a Mac, Safari frequently crashes while browsing Slashdot when Javascript is on for the past month or so

Re:To burn some karma (1)

ericrost (1049312) | about 6 years ago | (#24100425)

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.

Neutrino measurements here we come... (5, Insightful)

H+FTW (1264808) | about 6 years ago | (#24099995)

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] )

Re:Neutrino measurements here we come... (2, Informative)

locofungus (179280) | about 6 years ago | (#24100543)

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.

Re:Neutrino measurements here we come... (1)

H+FTW (1264808) | about 6 years ago | (#24100743)

given enough time the v_e- will interact to give off an e- that causes the cherenkov radiation

you're not

Re:Neutrino measurements here we come... (0)

Anonymous Coward | about 6 years ago | (#24101733)

Yes, this is one possible reaction. However, the \nu can also simply scatter at an electron, via a weak neutral current (a virtual Z-Boson is exchanged).

This is quite interesting, because inducing a reverse beta-decay is only possible with a \nu_e, whereas scattering at an electron is possible with any flavour of neutrino. Therefore, you can directly see whether the "missing" solar electron neutrinos are really missing, or just changed their flavour.

Turns out they changed their flavour => the Neutrino flavour-eigenstates have different masses => Neutrinos have masses > 0 => the Standard Model is not correct => we get to build LHC => we'll produce lots of reeaally sloooow micro black holes => the end is nigh!

SCNR

A Fiat 500 into a Ferrari? (3, Funny)

FlyingSquidStudios (1031284) | about 6 years ago | (#24100061)

So it turns something cheap and unreliable into something most people can't afford that doesn't perform well in crashes?

Simple Mod? (1)

eepok (545733) | about 6 years ago | (#24100081)

The guy creates a photon counter out of a little diode and you're still making fun of your moderators?

Ferrari? Not quite (4, Insightful)

bughunter (10093) | about 6 years ago | (#24100159)

FTFA:

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.

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.

Re:Ferrari? Not quite (1)

mikael (484) | about 6 years ago | (#24101607)

And the commodity price of photo diodes shoots up to match that of photon counters.

Re:Ferrari? Not quite (2, Funny)

Bluesman (104513) | about 6 years ago | (#24102731)

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 (0)

Anonymous Coward | about 6 years ago | (#24101873)

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.

Not so.

The photon counter was a device _inside_ the complicated setup you just described, e.g. a Photon-Multiplier (PM), or an Avalanche Photo Diode (APD)

What they actually do is take an expensive photon counter, and make it distinguish between one..two..n photon arriving within such a short time that the photon counter would normally miss all but the first.

So: this is not about sensitivity (the APDs used here are single-photon sensitive already!), but about pile-up.

Still, it's a HUGE development: up until now, you had to decide _before_ the measurement, whether you'd be expecting one or two photons or so (setting up the thing in Geiger mode so as to not miss a single photon), or whether you wanted to measure how many photons were streaming in (setting the thing into proportional mode)

Re:Ferrari? Not quite (5, Informative)

evanbd (210358) | about 6 years ago | (#24101929)

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:Ferrari? Not quite (0, Flamebait)

bughunter (10093) | about 6 years ago | (#24106407)

That's what "analog filter/comparator chains" do... so you are correct.

Re:Ferrari? Not quite (1)

Virmal (1281900) | about 6 years ago | (#24105463)

No wonder everyone was gawking at my "photon enhanced" Fiat 500 the other day...

Beautiful, Simple, Engineering (5, Interesting)

Dr.Pete (1021137) | about 6 years ago | (#24100197)

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 [stanford.edu] 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.

Re:Beautiful, Simple, Engineering (2, Interesting)

spike_gran (219938) | about 6 years ago | (#24101043)

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.

Make every one count. (1)

Taibhsear (1286214) | about 6 years ago | (#24100205)

They say that in the first instants after the avalanche forms... All you have to do is measure it at this early stage... like turning a Fiat 500 into a Ferrari.

Summary sounds like a v1agra spam ad...

Re:Make every one count. (1)

spazdor (902907) | about 6 years ago | (#24103087)

Click to give her a greater more pleasure load of k.um!

It seems to me... (0)

Anonymous Coward | about 6 years ago | (#24100327)

...that they want to take an avalanche photodiode and get some readings before the avalanching behaviour kicks in. Wouldn't it be easier to use a PIN photodiode without avalanching?

Anonymous Coward | about 6 years ago | (#24100383)

simple mods turn this comment into +5 Insightful?

ahem (1)

brotheralien (961574) | about 6 years ago | (#24100399)

Simple Mod Turns Diodes Into Photon Counters

and yet he still couldn't get his Lambretta to start...

Digital cameras? (1)

gringer (252588) | about 6 years ago | (#24100601)

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? (1)

Kupfernigk (1190345) | about 6 years ago | (#24100647)

The Y axis is probability, the x axis is millivolts. The output of an avalanche photodiode is current. So what is being measured here and how does it relate to the article?

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 that the output at no photon will be around 5mV, whatever that is supposed to mean. Is that the dark current of the diode at a given bias, before it triggers?

I suspect that what we are seeing here is a diode biased with some low voltage from a limited current source, and that the pulse amplitude results from the self-discharge of the diode capacitance into the measuring resistor. 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. If so, this could have been shown quite clearly on a conventional graph with the output pulse height represented on the Y axis, the number of photons on the X axis, and the probabilities shown by the shape of the distribution.

So, either I don't get it (always a possibility) or someone needs to go and (a) read up statistics again and (b) read Tufte.

Re:Can anyone explain that graph? (0)

Anonymous Coward | about 6 years ago | (#24101287)

The Y axis is probability, the x axis is millivolts. The output of an avalanche photodiode is current. So what is being measured here and how does it relate to the article?

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 that the output at no photon will be around 5mV, whatever that is supposed to mean. Is that the dark current of the diode at a given bias, before it triggers?

I suspect that what we are seeing here is a diode biased with some low voltage from a limited current source, and that the pulse amplitude results from the self-discharge of the diode capacitance into the measuring resistor. 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. If so, this could have been shown quite clearly on a conventional graph with the output pulse height represented on the Y axis, the number of photons on the X axis, and the probabilities shown by the shape of the distribution.

So, either I don't get it (always a possibility) or someone needs to go and (a) read up statistics again and (b) read Tufte.

You are correct, you don't get it. Now, go and do your reading homework.

Re:Can anyone explain that graph? (2, Informative)

argent (18001) | about 6 years ago | (#24102325)

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.

It's like an eye (cell)! (2, Informative)

yoinkityboinkity (957937) | about 6 years ago | (#24100957)

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? (1)

LM741N (258038) | about 6 years ago | (#24101083)

Isn't that what the Starship Enterprise used to destroy enemy ships?

Re:Photon Guns? (1)

Tsujiku (902045) | about 6 years ago | (#24102263)

No, those were Photon Torpedoes.

Torpedo stength? (1)

Cur8or (1220818) | about 6 years ago | (#24101237)

Finally we can settle whether a Galaxy class star ship can defeat the Deathstar. How many photons in a photon torpedo?

Hopelessly confused about a "single photon" (2, Interesting)

tjstork (137384) | about 6 years ago | (#24103119)

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..

Re:Hopelessly confused about a "single photon" (1)

richardtallent (309050) | about 6 years ago | (#24103265)

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" (4, Informative)

moosesocks (264553) | about 6 years ago | (#24104551)

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.

Re:Hopelessly confused about a "single photon" (1)

ozbird (127571) | about 6 years ago | (#24106575)

Believe it or not, "a single photon" isn't as small of an amount of light as you'd think.

Oh? Is there a smaller amount of light than a single photon?

Re:Hopelessly confused about a "single photon" (1)

postmodernistic (1183285) | about 6 years ago | (#24106703)

Rods and Cones The retina uses special cells called rods and cones to process light. Just how many rods and cones does your retina have? How about 120 million rods and 7 million cones â" in each eye! http://kidshealth.org/kid/htbw/eyes.html [kidshealth.org]

Re:Hopelessly confused about a "single photon" (2, Interesting)

Anonymous Coward | about 6 years ago | (#24107057)

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] .

Re:Hopelessly confused about a "single photon" (2, Interesting)

Raenex (947668) | about 6 years ago | (#24107855)

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] .

"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."

Re:Hopelessly confused about a "single photon" (1)

oojah (113006) | about 6 years ago | (#24106987)

Generating small numbers of photons is quite commonplace. Fluorescence imaging is often done as single photon or two photon imaging - there's lots if you search for that in google (and pictures too!). It's exactly the kind of thing you describe - excite the atom with your incoming light pulse and then get a different wavelength out when the electons decays to its previous state. You'll likely be looking at using femto-second lasers (ie. pulses a fs long) to do it. I confess this is an area that I'm not particularly familiar with. Don't forget that you might be spreading a reasonable few photons over a large area as well, so although the total number of photons could be relatively high, the number per pixel detector could still be low.

Avalanche photodiodes (at least the ones that I'm working with) are simply silicon diodes biased past their breakdown voltage. This means that there is a strong field across the junction and essentially any charge crossing the junction will cause it to go into breakdown, ie. pass a lot of current and reduce the voltage across the diode dramatically. This is exactly what happens when a photon is aborbed and creates an electron-hole pair. Similarly, it also happens due to thermally generated electrons. The really nice thing about the voltage signal coming out of the diode is that it's essentially just a digital edge, so there's no need for analogue-to-digital conversion - you just know that a photon has appeared. The problem, as this article mentions, is that when your diode is in breakdown, there's (normally) no way to detect any further photons because the diode hasn't recovered.

What gets me still is the wave-particle duality of light. Working with photodiodes, I treat light exclusively as photons and everything else just makes my head hurt :)

1up (0)

Anonymous Coward | about 6 years ago | (#24103231)

photonic dildo huh?

oh wait...

A Fiat 500 into a Ferrari? (3, Funny)

What a strange analogy.

[. . .] the original Fiat 500, launched in 1957, still holds a place in the heart of most Italians similar to the original Mini for the Brits, the Citroën Deux Chevaux for the French, or the VW Beetle for the Germans (and many North Americans, too).

Cinquecento was really the car that put Italy on wheels. In short, it's a cultural icon.

[. . .]
The original's 500 name came from the displacement of the air-cooled, rear-mounted two-cylinder engine. Slow? Oh my dear God yes.

But cute as a bunny and tough as nails. You still see these collector's items in every Italian city. I'd have one in a heartbeat.

--Some car magazine. [wheels.ca]

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

Re:A Fiat 500 into a Ferrari? (1)

TinBromide (921574) | about 6 years ago | (#24107983)

Would be irritating at parties? I suppose that we need to apply the scientific principle. We have the hypothesis (irritating at parties), lets test it and revise the hypothesis! (you crash a frat bash and see what happens)

Re:A Fiat 500 into a Ferrari? (1)

Goldsmith (561202) | about 6 years ago | (#24108371)

The opposite would be better:

Where as previously, you needed a Ferrari to do single photon measurements, now a Fiat will do just as well.

A handful of photons are bouncing off the moon. (1)

boombasticman (1232962) | about 6 years ago | (#24105041)

You could then measure the distance to the moon with a 10 Watts laser, a telescope, a very precise clock and the newly build photon counter. On the moon are placed some special reflection spots just for exactly this purpose.

Re:A handful of photons are bouncing off the moon. (1)

4D6963 (933028) | about 6 years ago | (#24107435)

You could then measure the distance to the moon with a 10 Watts laser, a telescope, a very precise clock and the newly build photon counter. On the moon are placed some special reflection spots just for exactly this purpose.

Utter bullcrap. The signal you get back is quadrillion times (I'm not making that up) weaker than the signal you send, to the point that scientists who do that use very powerful lasers (forgot how much, but it's a lot, look it up) and do it for about 10 minutes (again, if I recall correctly) so that they can distinguish their signal from noise. Besides I believe there's already CCDs out there that are noiseless and can count single photons (not in commercial cameras of course).

Thank you! (2, Funny)

4D6963 (933028) | about 6 years ago | (#24107583)

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!

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