The lids lifted up when I approached. If I stood in front of one, it took a guess at my gender and lifted up the seat as well.
The BBC on how toilets are going to solve the classic "seat down" dilemma, improve our lives, and also bring world peace.
2008-11-30
Technology to Solve One of Woman's Oldest Problems?
The BBC on how toilets are going to solve the classic "seat down" dilemma, improve our lives, and also bring world peace.
2008-11-28
Mumbai
That is the Taj Mahal Hotel about two and a half years ago when I visited. I never thought a photo with an overcast sky could be beautiful, but now I think it will be for a while.
It is very strange seeing places I've actually been in the news as terrorist targets. New York City was a distant land during the World Trade Center attacks in 2001. But I walked through the Taj Mahal Hotel when I was in Mumbai. My hopes and thoughts are with all of the victims.
Europeans: No to Pie
Now that the election season is finally over (okay, well, not in MN), it is time to look forward to future candidates. With the recent murmurs of a pie run for President, I though I'd talk a bit about European views on pie. Europeans prefer desserts that can take the heat, and though pie shows promise its distasteful actions leave Europeans hungry for something else.
First, most Europeans consider pie too inexperienced to lead. Do you know how old most desserts are? Modern pudding, for example, is a highly refined beast. Pudding started as a custard in Roman times, experimented with sausage meat in the 17th and 18th century century, replaced its easily-spoiled egg components with modern chemicals during the great push west, and finally became the sweet dish we know today just as pie came onto the scene. Chocolate, too, has also undergone a defining journey. As a drink chocolate was too wishy-washy on issues, and has now solidified its stance against quaffing as a primary means of consumption. Looking at these histories, Europeans ask themselves, "Where is pie's history? How do we know that it isn't just the crust that is flaky"?
Thsee long histories have also given these recipes a chance to work across boundaries. The international crisis in Iceland gave British pie a chance to rise on the international scene. What do pie do? Pie used terrorism laws to keep Icelandic food in the British Isles! Pie's clumsy maneuvers have allowed blini to further its questionable motives. Europeans know that chocolate would never take such a stance; Nordic milk helps create some of the finest chocolate.
Blini has already shown up on the cover of an Icelandic recipe book. Or is that a dinosaur in the right photo? I can't really tell, I need glasses.
Finally, Europeans fear pie's strong ties to the dark, corporate side of America. Pie and corn starch's on-going attempts at alignment show that pie isn't the grass roots hero it claims to be. Furthermore, pie's relationships with cloying sugar and cream cause grave concern in the European community. Europeans often look at an American pie and exclaim, "Is there any healthy ingredient in that abomination? Why does pie drown out the fruit with sugar? No wonder Americans are so fat".
While pie faces strong opposition overseas, I don't think this will affect American opinion in the next election. With over a billion dollars spent in the last campaign, pie's ability to raise funds could lead to a decisive advantage. At least with the kitchen demographic.
First, most Europeans consider pie too inexperienced to lead. Do you know how old most desserts are? Modern pudding, for example, is a highly refined beast. Pudding started as a custard in Roman times, experimented with sausage meat in the 17th and 18th century century, replaced its easily-spoiled egg components with modern chemicals during the great push west, and finally became the sweet dish we know today just as pie came onto the scene. Chocolate, too, has also undergone a defining journey. As a drink chocolate was too wishy-washy on issues, and has now solidified its stance against quaffing as a primary means of consumption. Looking at these histories, Europeans ask themselves, "Where is pie's history? How do we know that it isn't just the crust that is flaky"?
Thsee long histories have also given these recipes a chance to work across boundaries. The international crisis in Iceland gave British pie a chance to rise on the international scene. What do pie do? Pie used terrorism laws to keep Icelandic food in the British Isles! Pie's clumsy maneuvers have allowed blini to further its questionable motives. Europeans know that chocolate would never take such a stance; Nordic milk helps create some of the finest chocolate.
Blini has already shown up on the cover of an Icelandic recipe book. Or is that a dinosaur in the right photo? I can't really tell, I need glasses.Finally, Europeans fear pie's strong ties to the dark, corporate side of America. Pie and corn starch's on-going attempts at alignment show that pie isn't the grass roots hero it claims to be. Furthermore, pie's relationships with cloying sugar and cream cause grave concern in the European community. Europeans often look at an American pie and exclaim, "Is there any healthy ingredient in that abomination? Why does pie drown out the fruit with sugar? No wonder Americans are so fat".
While pie faces strong opposition overseas, I don't think this will affect American opinion in the next election. With over a billion dollars spent in the last campaign, pie's ability to raise funds could lead to a decisive advantage. At least with the kitchen demographic.
2008-11-27
Happy Thanksgiving
Nobody really celebrates Thanksgiving over here, but I decided to find something American that I could be thankful for. And find something I did.
America, thank you for exporting your original, canned hot dog recipe. Tonight I'm going to have a TV dinner with these guys and some Coke, eating them while watching the Simpsons episode where they reverse the toilet flow in Australia. And I'm going to try not to cry, but I don't think I'm going to succeed.
God bless America.
America, thank you for exporting your original, canned hot dog recipe. Tonight I'm going to have a TV dinner with these guys and some Coke, eating them while watching the Simpsons episode where they reverse the toilet flow in Australia. And I'm going to try not to cry, but I don't think I'm going to succeed.
God bless America.
2008-11-23
2008-11-22
On the Rationality of Contracts
If a punishment of breaking a contract was death, would a rational person ever enter into such a contract? I've been thinking about this recently because an Islamic tenet is that apostasy's punishment is death.
The US social contract has death as an available punishment; certain criminal actions in Texas could lead to your death. All US citizens live under this contract. One could argue, however, that this punishment is a feature of the natural contract between all people and further restrictions on the right of life is not rational. I mean Locke's "state of nature" contract when I say natural contract.
The US social contract has death as an available punishment; certain criminal actions in Texas could lead to your death. All US citizens live under this contract. One could argue, however, that this punishment is a feature of the natural contract between all people and further restrictions on the right of life is not rational. I mean Locke's "state of nature" contract when I say natural contract.
Civics Quiz
I got 31 out of the 33 questions correct. I missed the origin of the "wall of separation" quote, and the definition of the Puritans. Supposedly elected officials and Americans average under 50% correct. I actually don't think elected officials would miss so many questions - I'll believe it when I see it with my own eyes.
2008-11-21
Follow-up to Charging by Weight
Flying in Canada, you can get another seat for free...but you have to qualify as obese.
In a previous post I guessed about how much people pay for fuel. I estimated that about 5% of a ticket's cost on long-distance, crowded flights is the weight of a person and his/her luggage.
In case the previous post wasn't clear, I think that airlines should be allowed to charge a passenger for his/her weight and baggage's weight. Thus I disagree with the Canadian ruling, though I can think of a few arguments for it.
In a previous post I guessed about how much people pay for fuel. I estimated that about 5% of a ticket's cost on long-distance, crowded flights is the weight of a person and his/her luggage.
In case the previous post wasn't clear, I think that airlines should be allowed to charge a passenger for his/her weight and baggage's weight. Thus I disagree with the Canadian ruling, though I can think of a few arguments for it.
Knowledge Export
As a kennismigrant (knowledge immigrant), I appreciate some pieces of knowledge more than others. Such as the European style of eating a kiwi. In these busy days I wouldn't normally have time to eat a kiwi, much less prepare a blog post. Luckily, however, Europeans love solving multiple problems with a single solution. Looking for a solution that is easy, classy, filling, delicious, *and* blog-postable? Carve out a kiwi, yo.
Several kiwis were harmed in the creation of these photographs. Feel free to direct any complaints here.
Several kiwis were harmed in the creation of these photographs. Feel free to direct any complaints here.
2008-11-14
Noise in SPADs
Today I'm going to continue my post from two weeks ago and talk about noise in SPADs.
As I discussed in my previous post, a SPAD consists of a fast, positive feedback system in conjunction with a slow, negative feedback system. The positive feedback system detects generated electrons, whether they're created by tunneling electrons, traps, thermal electrons, or photons. Since we're focusing on photons, the other types of generated electrons are noise.
If you don't know much about silicon, you probably don't know what a trap is. Standard silicon consists of a repeating pattern of atoms. We need to implant ions into this repeating pattern so we create the properties we need - raise the voltage here and it goes down over there, etc. However, implanting these ions can damage silicon's structure so that the atoms no longer form a neat pattern. When atoms don't form a neat pattern, electrons interact differently with the damaged part of the lattice. Sometimes electrons get stuck in these damaged parts for a bit of time, hence the term "trap". When the electrons finally leave the trap, they can cause an avalanche.
The most prevalent location of traps is at the interface between the silicon and the insulating silicon oxide. Every modern SPAD I've seen buries the region in which the avalanche occurs, the multiplication region. Burying the multiplication region keeps it away from most of the traps and "trap-generated" electrons.
Another source of noise is tunneling. Tunneling is a phenomenon that requires knowledge of quantum mechanics, and I don't think I can explain it well in a paragraph or two. A good one sentence description of tunneling might be, "The laws of nature do not stop electrons from jumping over barriers of any size, although the odds decrease with different barriers." The main way to control tunneling is to change the properties of the barriers between different regions of the chip, specifically the doping of the different implants we add into the silicon.
There is another source of noise I didn't mention called crosstalk. Crosstalk is ostensibly named because improperly-wired phone circuits can pick up pieces of other phone conversations, or leak portions of your phone conversations. SPAD crosstalk is the phenomenon of one detector's firing causing the unwanted firing of another detector. Crosstalk can be standard electrical crosstalk, which is uncommon in SPADs, or optical. Optical crosstalk is pretty cool - it turns out that stray electrons can create photons much as photons create electrons. During the middle of an avalanche a SPAD gives off a lot of photons, and these photons might accidentally cause another detector's firing. Optical crosstalk isn't a severe issue in most SPADs, but it varies from environment to environment.
Though optical crosstalk can increase the noise from SPADs, the created photons do have one advantage. Since an avalanching SPAD gives off photons in the multiplication region, taking a long exposure picture of a SPAD allows us to see whether the multiplication region has the shape we expect. If our silicon implants go poorly, we won't see a uniform multiplication region.
I hope you enjoyed a discussion of the source of noise in SPADs. If this seems boring, keep in mind that we're detecting single photons here! Over the next few weeks I'll start to discuss some additional properties of SPADs, and eventually I'll get to the point where I start comparing them to CCD or CMOS image sensors.
PS Blogspot's dictionary claims that avalanching isn't a word, but it doesn't contain truthiness either. Very suspect.
As I discussed in my previous post, a SPAD consists of a fast, positive feedback system in conjunction with a slow, negative feedback system. The positive feedback system detects generated electrons, whether they're created by tunneling electrons, traps, thermal electrons, or photons. Since we're focusing on photons, the other types of generated electrons are noise.
If you don't know much about silicon, you probably don't know what a trap is. Standard silicon consists of a repeating pattern of atoms. We need to implant ions into this repeating pattern so we create the properties we need - raise the voltage here and it goes down over there, etc. However, implanting these ions can damage silicon's structure so that the atoms no longer form a neat pattern. When atoms don't form a neat pattern, electrons interact differently with the damaged part of the lattice. Sometimes electrons get stuck in these damaged parts for a bit of time, hence the term "trap". When the electrons finally leave the trap, they can cause an avalanche.
The most prevalent location of traps is at the interface between the silicon and the insulating silicon oxide. Every modern SPAD I've seen buries the region in which the avalanche occurs, the multiplication region. Burying the multiplication region keeps it away from most of the traps and "trap-generated" electrons.
Another source of noise is tunneling. Tunneling is a phenomenon that requires knowledge of quantum mechanics, and I don't think I can explain it well in a paragraph or two. A good one sentence description of tunneling might be, "The laws of nature do not stop electrons from jumping over barriers of any size, although the odds decrease with different barriers." The main way to control tunneling is to change the properties of the barriers between different regions of the chip, specifically the doping of the different implants we add into the silicon.
There is another source of noise I didn't mention called crosstalk. Crosstalk is ostensibly named because improperly-wired phone circuits can pick up pieces of other phone conversations, or leak portions of your phone conversations. SPAD crosstalk is the phenomenon of one detector's firing causing the unwanted firing of another detector. Crosstalk can be standard electrical crosstalk, which is uncommon in SPADs, or optical. Optical crosstalk is pretty cool - it turns out that stray electrons can create photons much as photons create electrons. During the middle of an avalanche a SPAD gives off a lot of photons, and these photons might accidentally cause another detector's firing. Optical crosstalk isn't a severe issue in most SPADs, but it varies from environment to environment.
Though optical crosstalk can increase the noise from SPADs, the created photons do have one advantage. Since an avalanching SPAD gives off photons in the multiplication region, taking a long exposure picture of a SPAD allows us to see whether the multiplication region has the shape we expect. If our silicon implants go poorly, we won't see a uniform multiplication region.
I hope you enjoyed a discussion of the source of noise in SPADs. If this seems boring, keep in mind that we're detecting single photons here! Over the next few weeks I'll start to discuss some additional properties of SPADs, and eventually I'll get to the point where I start comparing them to CCD or CMOS image sensors.
PS Blogspot's dictionary claims that avalanching isn't a word, but it doesn't contain truthiness either. Very suspect.
2008-11-13
What do Connecticut, Rhode Island, New York and Iowa all have in common?
The federal government owns less than 1% of land in each state.
2008-11-12
Geographical Variation in Sensitivity to Blue Light
We have a blue laser in one of our setup rooms, and my Japanese colleague's eyes appear to be more sensitive to blue light than my own. We pointed the laser at the wall and pointed to where we thought the light ended, and the two European descendants (Swiss and American) both saw less blue light than the non-Europeans (Iranian and Japanese). Has anyone heard of cross-regional studies in sensitivity to blue light? Neither I nor my Swiss colleague had heard this before, but my Japanese colleague said this was (somewhat) common knowledge.
My head is spinning thinking that people from different regions see all of art differently. I wonder if this has an effect on designing web logos? Would we expect to see different shades in different regions?
EDIT: Since laser light contains only specific frequencies, someone suggested that we might be most sensitive to different frequencies of light. This is known to occur - there are variations in what frequencies people are most sensitive to. Some studies even suggest that a faction of women have four different kinds of cones, rather than three.
My head is spinning thinking that people from different regions see all of art differently. I wonder if this has an effect on designing web logos? Would we expect to see different shades in different regions?
EDIT: Since laser light contains only specific frequencies, someone suggested that we might be most sensitive to different frequencies of light. This is known to occur - there are variations in what frequencies people are most sensitive to. Some studies even suggest that a faction of women have four different kinds of cones, rather than three.
2008-11-09
So Long, First Web-site
I think the first, useful web-site I completely created was a political web-site for Swati Dandekar. Swati won a seat in the 2002 Iowa state house of representatives election. This year she ran for an Iowa state senate seat and won. Congratulations Swati!
Unfortunately they had to do away with my 1st symphony for an updated web page that is clearly *not* the work of a high-school student. Maybe they could have kept the animated state logo and "Excellence for Office" gif? That was genius. Seriously. How many web-sites can pull off an animated gif? I should have gone into advertising.
(I'd like to point out that the way-back machine doesn't cache the roll-overs, so those image roll-overs were not only fashionable at the time, they were also working.)
Unfortunately they had to do away with my 1st symphony for an updated web page that is clearly *not* the work of a high-school student. Maybe they could have kept the animated state logo and "Excellence for Office" gif? That was genius. Seriously. How many web-sites can pull off an animated gif? I should have gone into advertising.
(I'd like to point out that the way-back machine doesn't cache the roll-overs, so those image roll-overs were not only fashionable at the time, they were also working.)
2008-11-06
...where I don't write about government...
I was going to write about government for my regular Friday post, but with all the politics lately I decided this would be a bad idea. So instead I'm going to share a lighter subject, which is a little family history about how my two paternal grandparents met. I know I've told a few "Oog Robot" readers this story, but this is my favorite family story and deserves a repeat or two.
My grandmother was a city girl from Pennsylvania. She was a chemical engineer who applied to the Heinz company for an internship over one of her summers. The Heinz company decided she was some combo of awesome and fearsome, because they decided to send her to be a chemical engineer in one of their ketchup factories. If she didn't like the acid levels in the ketchup, the plant's ketchup wasn't going out the door. Except the ketchup factory was in Muscatine, Iowa (i.e. middle-of-nowhere-small-town, middle-of-nowhere-state).
My grandpa was home for a summer from the University of Iowa. He met my grandma and worked up the courage to ask her to a movie, but she turned him down with something like, "I've got to wash my hair." Anyways, my grandpa decided to go to the movies anyways with a few of his buddies...and he ran into my future grandmother, with another boy of course.
After the movie, things must not have gone too well with the other boy because my grandma was waiting for the bus or a taxi when my grandpa ran into her. He offered her a ride home, and the rest is history. I don't know how my grandpa convinced her to move from Pennsylvania to Muscatine, IA. But I appreciate this now.
Grandpa and Grandma were two of the coolest people I know. My grandpa was a farmer and a lawyer! He'd get up before dawn, feed some cows, and then go work out some intricate legal arguments at work. He believed people should always have two jobs, in case one didn't work out. My grandma is probably responsible for sending me to MIT. She gave me a subscription to MIT's Technology Review, which sparked my interest in the school. I might have discovered MIT later, but who knows? I think I probably would have ended up at Iowa State University if she had not given me the TR subscription, and I would be a very different person.
Anyways, my grandpa passed away when I was in 5th grade, and my grandma died when I was a freshmen in high school. I wish I'd been able to get to know them better.
My grandmother was a city girl from Pennsylvania. She was a chemical engineer who applied to the Heinz company for an internship over one of her summers. The Heinz company decided she was some combo of awesome and fearsome, because they decided to send her to be a chemical engineer in one of their ketchup factories. If she didn't like the acid levels in the ketchup, the plant's ketchup wasn't going out the door. Except the ketchup factory was in Muscatine, Iowa (i.e. middle-of-nowhere-small-town, middle-of-nowhere-state).
My grandpa was home for a summer from the University of Iowa. He met my grandma and worked up the courage to ask her to a movie, but she turned him down with something like, "I've got to wash my hair." Anyways, my grandpa decided to go to the movies anyways with a few of his buddies...and he ran into my future grandmother, with another boy of course.
After the movie, things must not have gone too well with the other boy because my grandma was waiting for the bus or a taxi when my grandpa ran into her. He offered her a ride home, and the rest is history. I don't know how my grandpa convinced her to move from Pennsylvania to Muscatine, IA. But I appreciate this now.
Grandpa and Grandma were two of the coolest people I know. My grandpa was a farmer and a lawyer! He'd get up before dawn, feed some cows, and then go work out some intricate legal arguments at work. He believed people should always have two jobs, in case one didn't work out. My grandma is probably responsible for sending me to MIT. She gave me a subscription to MIT's Technology Review, which sparked my interest in the school. I might have discovered MIT later, but who knows? I think I probably would have ended up at Iowa State University if she had not given me the TR subscription, and I would be a very different person.
Anyways, my grandpa passed away when I was in 5th grade, and my grandma died when I was a freshmen in high school. I wish I'd been able to get to know them better.
2008-11-05
Tie in Iowa County
Iowa County, a county in (unsurprisingly) Iowa, had as many blue votes as red votes.
Obama: 4,173
McCain: 4,173
"Other": 125
I thought this would be a rare event until I messed around with a few numbers. If you assume that voters are independent and equally likely to vote for one of two candidates, the chance of an 8,000 person county being equally divided is almost 1%. With fewer people in a county, the odds increase. Assuming 100 counties with fewer than 8,000 people, one expects at least one county in the country to evenly split the vote.
Those are some pretty big assumptions, though. The odds are very sensitive to how split the voters are, so if one candidate has odds of 51% and the other 49%, then the probability for such a county drops to around 0.1%. If you assume 52% odds for one candidate, then we're down to 0.001%. I used the handy binomial calculator from Vassar to crunch the numbers. Also, I do not think that county size and vote color are independent; I would guess that small counties tend to vote red.
A quick, non-exhaustive search shows there were a few other counties that came close:
KY's Bath County: 24
MN's Murray County: 25
MN's Aitkin County: 10
MO's Washington County: 9 (EDIT: added later)
I couldn't find systematic county data to look at, so I might have missed counties that were closer or even tied. The data above (having a few counties in the sub-25 range) suggests around a 20% chance of a tie during an election in which a democrat is slightly ahead.
2008-11-04
Wikipedia Antics
Head over to the front page of Wikipedia. Reload the page a few times (done by hitting f5 in IE or Firefox). Way to be unbiased, Wikipedia. Now if only the sides of the pictures flipped, too.
(this post will probably only work today)
(this post will probably only work today)
2008-11-03
American Elections
I cannot wait until I don't have to answer questions about American elections. Mostly I clarify confusion about why such a large percentage of Americans support John McCain. People are very confused when I explain that I would have voted for McCain in 2000 if I had the chance. I was under 18 at the time, and I'm still confused why 16 year-olds are not allowed to vote.
2008-11-01
Basics of Single Photon Avalanche Diodes
As you may or may not know, I work with single photon avalanche diodes (SPADs), sometimes referred to as detectors instead of diodes. I'm going to give a high level overview of SPADs in this post - why we care and how they work.
First, why do we care about single photons? One reason you should care is quantum computing. Most computing today relies on classical electronics, and uses assumptions which are starting to be not so valid. For example, you might know that CPU makers are no longer focusing on speeding up their processors much more, and instead are focusing on placing more and more "cores" onto chips or lowering power consumption. In the past, shrinking the device size has allowed CPU makers to speed up their processors. Everyone agrees that to speed up processors, we're going to need to find other ways of performing computing. Another way is quantum computing, which does not rely on the same set of assumptions as classical computing, but requires different technology. One applicable technology is accurate detection of single photons. My group works with other applications of these detectors, including radiation hardened detectors, distance sensors, and some mysterious stuff I can't talk about.
The distance sensor application is particularly cool. By measuring the flight time from the laser to the environment to the photon detector we can actually get distance accurate to less than a mm in a second. Keep in mind that the speed of light is 3 * 10^8 meters per second, or 300,000,000,000 mm/s, meaning we need to have accuracy in the picoseconds. The current accuracy is in the 100s of picoseconds, but multiple measurement within the allotted second increase accuracy. If you're really interested in the topic you can look for the 2008 ISSCC paper "A 128x128 Single-Photon Imager with on-Chip Column-Level 97ps 10bit Time-to-Digital-Converter Array".
Short aside: a 4 GHz computer has a clock period of 250 picoseconds. Light and information only travels about 7.5 cm in 250 picoseconds! With all the random stuff in your processor slowing down signals, you cannot route signals across the entire processor and have this information communicated in one clock cycle.
Anyways, back to SPADs. I'm going to explain the notion of an avalanche first, and then I'll explain how an avalanche applies to SPADs. Free electrons are just bouncing around in the air. When we apply an electric field, the fields interacts with the electrics and causes them to move faster in the field's direction. If we make the field strong enough, an electron might gain enough speed to strip an electron or two off an atom during a collision. The original electron and the stripped atoms again start to acquire speed, and when these electrons hit another set of atoms we expect to have more than four electrons. Another round and we expect to have more than eight electrons (see footnote 1 for details). This process is exponential, so in a short amount of time we'll expect to have millions of electrons that are moving very fast. When an event causes more of the same type of event to occur, this is known as positive feedback.
The basic idea of a SPAD couples the avalanche's fast positive feedback system with a slow negative feedback system to quench the avalanche. The key component to an avalanche diode is, surprise, the electrical diode. I don't want to go to much into how a diode works, but the important point is that putting a large, negative voltage across most electrical diodes (not a standard operating condition) causes a very strong electrical field to build up in part of the diode. If the voltage increases enough, we can get avalanches. When a photon interacts with an atom in the diode, the photon can strip free an electron. Thus a single photon creates a single electron, which can cause an avalanche, which would start to move a lot of current. We currently have about a 30% probability that a photon causes an avalanche (see footnote 1).
I can hear a few people protest, "But wait! There is ambient thermal energy which can also create electrons - don't these electrons cause avalanches?" Avalanches occur in the absence of light. Aside from thermal electrons, tunneling electrons also cause photon-less avalanches. These rate of these non-photon avalanches is the dark count rate (DCR) of an avalanche diode. The DCR obscures the signal, and we usually want to keep the DCR as low as possible. (see footnote 2)
Now that we have a lot of electrons moving (and electrical current flowing), how do we quench the avalanche? And what about detection? Placing a component called a resistor in series with the diode causes the current to flow through the resistor, raising the voltage across the resistor, lowering the voltage / electric field across the diode, and quenching the avalanche. We can observe the voltage at the resistor for spikes to detect avalanches. This is the slower negative feedback system I discussed.
There are lots of details I've omitted, but I hope you found my explanation useful. If you have questions, please leave them in the comments - I'll make sure I answer them.
Footnote 1: The interactions of the electrons with the atoms are random, so sometimes an electron only picks up a little speed before it hits an atom, sometime an electron picks up a lot of speed. Thus we can only talk about avalanches in the expected sense, saying "we expect to see X".
Footnote 2: Some people actually like a high DCR. It turns out truly random information is difficult to create. Dark avalanches, to the best of our knowledge, are truly random event (based on quantum interactions). Some people are working to create "random information generators" based on these detectors.
First, why do we care about single photons? One reason you should care is quantum computing. Most computing today relies on classical electronics, and uses assumptions which are starting to be not so valid. For example, you might know that CPU makers are no longer focusing on speeding up their processors much more, and instead are focusing on placing more and more "cores" onto chips or lowering power consumption. In the past, shrinking the device size has allowed CPU makers to speed up their processors. Everyone agrees that to speed up processors, we're going to need to find other ways of performing computing. Another way is quantum computing, which does not rely on the same set of assumptions as classical computing, but requires different technology. One applicable technology is accurate detection of single photons. My group works with other applications of these detectors, including radiation hardened detectors, distance sensors, and some mysterious stuff I can't talk about.
The distance sensor application is particularly cool. By measuring the flight time from the laser to the environment to the photon detector we can actually get distance accurate to less than a mm in a second. Keep in mind that the speed of light is 3 * 10^8 meters per second, or 300,000,000,000 mm/s, meaning we need to have accuracy in the picoseconds. The current accuracy is in the 100s of picoseconds, but multiple measurement within the allotted second increase accuracy. If you're really interested in the topic you can look for the 2008 ISSCC paper "A 128x128 Single-Photon Imager with on-Chip Column-Level 97ps 10bit Time-to-Digital-Converter Array".
Short aside: a 4 GHz computer has a clock period of 250 picoseconds. Light and information only travels about 7.5 cm in 250 picoseconds! With all the random stuff in your processor slowing down signals, you cannot route signals across the entire processor and have this information communicated in one clock cycle.
Anyways, back to SPADs. I'm going to explain the notion of an avalanche first, and then I'll explain how an avalanche applies to SPADs. Free electrons are just bouncing around in the air. When we apply an electric field, the fields interacts with the electrics and causes them to move faster in the field's direction. If we make the field strong enough, an electron might gain enough speed to strip an electron or two off an atom during a collision. The original electron and the stripped atoms again start to acquire speed, and when these electrons hit another set of atoms we expect to have more than four electrons. Another round and we expect to have more than eight electrons (see footnote 1 for details). This process is exponential, so in a short amount of time we'll expect to have millions of electrons that are moving very fast. When an event causes more of the same type of event to occur, this is known as positive feedback.
The basic idea of a SPAD couples the avalanche's fast positive feedback system with a slow negative feedback system to quench the avalanche. The key component to an avalanche diode is, surprise, the electrical diode. I don't want to go to much into how a diode works, but the important point is that putting a large, negative voltage across most electrical diodes (not a standard operating condition) causes a very strong electrical field to build up in part of the diode. If the voltage increases enough, we can get avalanches. When a photon interacts with an atom in the diode, the photon can strip free an electron. Thus a single photon creates a single electron, which can cause an avalanche, which would start to move a lot of current. We currently have about a 30% probability that a photon causes an avalanche (see footnote 1).
I can hear a few people protest, "But wait! There is ambient thermal energy which can also create electrons - don't these electrons cause avalanches?" Avalanches occur in the absence of light. Aside from thermal electrons, tunneling electrons also cause photon-less avalanches. These rate of these non-photon avalanches is the dark count rate (DCR) of an avalanche diode. The DCR obscures the signal, and we usually want to keep the DCR as low as possible. (see footnote 2)
Now that we have a lot of electrons moving (and electrical current flowing), how do we quench the avalanche? And what about detection? Placing a component called a resistor in series with the diode causes the current to flow through the resistor, raising the voltage across the resistor, lowering the voltage / electric field across the diode, and quenching the avalanche. We can observe the voltage at the resistor for spikes to detect avalanches. This is the slower negative feedback system I discussed.
There are lots of details I've omitted, but I hope you found my explanation useful. If you have questions, please leave them in the comments - I'll make sure I answer them.
Footnote 1: The interactions of the electrons with the atoms are random, so sometimes an electron only picks up a little speed before it hits an atom, sometime an electron picks up a lot of speed. Thus we can only talk about avalanches in the expected sense, saying "we expect to see X".
Footnote 2: Some people actually like a high DCR. It turns out truly random information is difficult to create. Dark avalanches, to the best of our knowledge, are truly random event (based on quantum interactions). Some people are working to create "random information generators" based on these detectors.
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