Scientists Build New Type of Photon Gun

KentuckyFC writes "Single photons are surprisingly difficult to generate. But since they are crucial for quantum communication, a number of research groups are working on photon guns that fire single photons on demand. The problem they have come up against is that making the photons identical is proving harder than expected. Now a group in Cambridge, UK, has cracked the problem using a quantum dot on a transistor to emit single photons that are essentially identical. In the process, the group has developed an entirely new technique to trigger photon emission (abstract on the physics arxiv)."

Comments

Identical photons?

[RandoX]

I thought a photon is a photon....

Re:Identical photons?

[$RANDOMLUSER]

I thought a photon is a photon....

Depends on what color it is. (i.e. wavelength)

Re:Identical photons?

[gardyloo]

Also (much more difficult to control) what the "phase" of it is. Lasers achieve tremendous frequency-uniformity, which is quite nice, but the amazing thing is that their photons are essentially mostly phase-locked, so each is identical to the last. It means that one can get tremendous power a large distance with them. But lasers are inherently producers of large populations of photons (in a sense, you need a lot of photons to control the mechanisms which produce more photons) at the same time. The ability to produce single photons of a given frequency and random phase is relatively easy; producing single frequencies and single phases is much more difficult.

Re:Identical photons?

[Eudial]

I thought a photon is a photon....

They've got plenty of characteristics: You've got phase, amplitude, wavelength, direction and polarization.

Anyways, this has that april fools funky science written all over it. The ability to emit controlled, discrete photons goes against (my understanding of) quantum physics.

Re:Identical photons?

[alexj33]

Right, just like in the academy.

Re:Identical photons?

[Anonymous Coward]

Because of the Heisenberg Uncertainty relations between photon number and phase, being able to produce exactly one photon (which is what they do in this case) means that the photons have and undefined phase. However, this is not important for the applications that they use. If you were to send such a photon into an interferometer, you would still see interference as the photon interferes with itself and a relative phase can develop between the two modes/arms of the interferometer. Almost all classical interference is just single photon interference scaled up. Here the challenge is to ensure that each of the successive photons emitted are indistinguishable. The photons must have the same wavelength, there wavefunctions the same shape, and there emission time must be short enough to make sure that dephasing mechanisms within the quantum dot do not affect when the photons are emitted (low jitter). With an ideal source of indistinguishable photons (which is very different from a laser source), it is possible to combine the photons and obtain higher order quantum interference effects where two different photons interfere with one another as opposed to just themselves. This effect can be used to create simple logic gates used in quantum computing.

Re:Identical photons?

[Chris Mattern]

Of course not! You can have overloaded photons, or proximity photons, or...

Re:Identical photons?

[TooMuchToDo]

"Ahh, photons. I don't know if you're waves or particles, but you sure do go down smooth." --bender

Re:Identical photons?

[Anonymous Coward]

take a look at any form of fractal geometry and you will realize that "nearly identical" on that scale is not worth a crock of shit.

Re:Identical photons?

[electrictroy]

Aren't photons wavicles? (A wave with its ends chopped off, thus displaying the properties of a discrete particle in some cases.)

The key question

[dkleinsc]

Is it possible to correctly attach this photon gun to a shark's head?

Re:The key question

[mapsjanhere]

Since the gun operates at 4 K, you currently have an issue with the shark getting rapidly encased in a growing block of ice. So the shark mounted version will have to wait until someone breeds a low temperature shark.

Re:The key question

[Anonymous Coward]

As long as your sharks don't cross the streams, why not?

Re:The key question

[syousef]

A cold shark huh? Well I knew the politicians would come in handy for something someday.

Re:The key question

[Anonymous Coward]

"the shark mounted version will have to wait until someone breeds a low temperature shark."

My ex-wife is both qualified and available for it.

Sorry, too obvious, but I had to get it off my chest.

Now we are making progress!

[Anonymous Coward]

Phasers on stun gentlemen.

great, bloody typical.

[apodyopsis]

great, bloody typical. another /. story to make me feel stupid. FTFA:

"We generate indistinguishable photons from a semiconductor diode containing a InAs/GaAs quantum dot. Using an all-electrical technique to populate and control a single-photon emitting state we filter-out dephasing by Stark-shifting the emission energy on timescales below the dephasing time of the state. Mixing consecutive photons on a beam-splitter we observe two-photon interference with a visibility of 64%"

snooze. snooze. snort. no mention of stun, kill, slicing, death ray, x-ray specs or photonic propulsion, so there is nothing there for me.

if some obliging, and more informed /. reader could supply us all with an explanation why this is a big deal then on behalf of the others I'll be thanking you in advance.....

Re:great, bloody typical.

[calebt3]

Right in the first sentence:

Single photons are surprisingly difficult to generate. But since they are crucial for quantum communication... As for why it is crucial, I am not sure.

Re:great, bloody typical.

[Arcturax]

Someday, this will lead to the important invention of the Photon Torpedo. You'll see!

Re:great, bloody typical.

[JustinOpinion]

One model for secure communication uses quantum cryptography to exchange a key that is actually pairs of entangled photons. In rough terms, you have a source that generates entangled photon pairs, and you keep one and send the other whoever you're trying to communicate with. You use this stream of photons to generate cryptographic keys, with the added quantum bonus that you can detect whether someone else has intercepted the key exchange (because, if so, the entanglement will be broken so the correlations between the two sets of photons will be "wrong").

For this to work, you need a way to reliably generate single photons or single photon pairs, and a way to transmit these photons without them losing their entanglement. This paper helps address the first part, by generating single photons on demand. Better yet, they generate 'indistinguishable' photons, which is necessary because the objective is to interfere two photons with each other to generate entangled pairs.

Re:great, bloody typical.

[sm62704]

A quantum dot is a semiconductor whose excitons are confined in all three spatial dimensions (links to other terminology you may or may not understand from the link provided)

From Wikipedia: "There is no page titled "Stark-shifting." However, a Google search of the term brings up lots of interesting links, like "Effect of pressure-dependent quantum interference on the ac Stark shifting of two-photon resonances". The dictionary only lists towns named "Stark" when you search for the term.

snooze. snooze. snort. no mention of stun, kill, slicing, death ray, x-ray specs or photonic propulsion, so there is nothing there for me

Well then you should read my journals. Lots of sex, drunken debauchery, and violence. No lasers, but there's mention of taking a "hydrogen bomb" to junior high school. See, slashdot's got something for everybody!

Re:great, bloody typical.

[markbark]

"Using an all-electrical technique to populate and control a single-photon emitting state we filter-out dephasing by Stark-shifting the emission energy on timescales below the dephasing time of the state."

Is it just me, or does that sound like Geordi LaForge telling Captain Picard that all he has to do is "demodulate the phase coils on the main deflector dish and then reverse the polarity on the Heisenberg compensators?"

--MAB

Re:great, bloody typical.

[CompMD]

I can't believe someone modded you insightful. Don't complain if you don't understand semiconductors and quantum physics, its annoying to those of us who do understand them.

Re:great, bloody typical.

[Anonymous Coward]

because they are a quantum particle that isn't affected by cosmic radiation.

see if you want to be able to use quantum data transfer not having that data change states in transit is pretty important weather you're going a few nanometers or 2000 kilometers.

also optics are fairly well understood, optical processors have been developed in labs, and now with the ability to create specific photons easily, quantum optical computing becomes slightly more feasible...

optical processors haven't been reasonable replacements for transistors because of the large number of lasers and high power and cooling requirements... currently this technique requires super cooling, but if they work at it more they may well get it to work at room temperature, or ideally at room temp on up to 100 C

if they can do that then we're one step closer to making a real, scalable quantum computer based on specific photon generation and optical processing. even a super cooled quantum computer could be very useful for cryptographic work (eg: you could make a single quantum computer chip that could break 1024-bit encryption OVERNIGHT) although we are a Long way from even a quantum computer designed to break normal computing cryptography..

Re:great, bloody typical.

[geekoid]

Given enough power, someday they might be able to move one side of the pair a dozen feet!

You know, if we have quantum computers, then cracking any key becomes a trivial matter.

Re:great, bloody typical.

[Chris Mattern]

Isn't Stark-shifting just when you get into one of your *old* power armors?

Re:great, bloody typical.

[JustinOpinion]

Given enough power, someday they might be able to move one side of the pair a dozen feet! The current record for transmitting single photons seems to be 150 km (press release, arXiv preprint).

You know, if we have quantum computers, then cracking any key becomes a trivial matter. I don't think that's true. Some cryptographic systems are vulnerable to quantum computer cracking, but others are not (or, at least, no one yet knows of a way). And importantly, the whole point of using quantum cryptography for your communication is a secure way to generate keys. If we had this working, we could reliably exchange one-time pads and be assured perfect unbreakable encryption.

Re:great, bloody typical.

[sm62704]

I have no clue, I'll have to actually read some of the links in Google, and most likely look up most of the terms to even have a glimmer of understanding.

Especially today, I didn't sleep well last night.

Re:great, bloody typical.

[Anonymous Coward]

Well then you should read my journals. Lots of sex, drunken debauchery, and violence. No lasers, but there's mention of taking a "hydrogen bomb" to junior high school. See, slashdot's got something for everybody!

*wank wank wank wank*

Re:great, bloody typical.

[Pinky's Brain]

You would still have to exchange an initial shared secret to kickstart the process (without a shared secret a Man in the Middle attack is always possible).

PS. is there a method with QC to get the equivalent of the Web of Trust? The ability to authenticate someone else's public key without him having to expose his private key is essential to that ... and I don't immediately see an equivalent in QC. If QC requires complete trust in third parties to avoid having to exchange secrets with every intended recipient then it can't really replace PK crypto.

Re:great, bloody typical.

[AgentPhunk]

the objective is to interfere two photons with each other to generate entangled pairs

'Scuse me Egon? You said crossing the streams was bad!

Re:great, bloody typical.

[BytePusher]

As I understand it; Quantum encryption allows one to flawlessly detect an unintended observer, so that you know your data is being compromised and you stop sending or take some other security measure before sending more than 1 or 2 bits of data.

"An important and unique property of quantum cryptography is the ability of the two communicating users to detect the presence of any third party trying to gain knowledge of the key." [http://en.wikipedia.org/wiki/Quantum_cryptography]

Re:great, bloody typical.

[ParanoidJanitor]

Almost, but not exactly. You mentioned two different quantum cryptographic schemes. Greatly simplified they are as follows:

The first scheme involves sending single polarized photons through an optical medium (such as a fiber optic cable) to a recipient who will then measure the polarization of the photon. There are two possible ways to send and measure the photon (X scheme and + scheme.) If the same scheme that was used to send the photon is used to measure it, the recipient will (ignoring errors introduced by the optical medium) always measure the correct result. If the recipient uses the wrong measurement scheme, they will have a 50/50 chance of measuring the correct result. When enough photons have been exchanged, the sender and recipient discuss over a public channel what measurement schemes they used. Any time that they did not use the same measurement scheme, they both discard whatever bit was measured. Through error checking and disclosing a few bits (which are later discarded), they end up with a perfectly secure, random sequence of bits. Eavesdroppers usually introduce a large error to this process, and are easy to detect. This is the one that the device in TFA is useful for.

I'm not well versed in the second scheme, but it has a dual measurement setup similar to the first scheme. Essentially, two entangled photons are created by the sender (or third party.) The sender keeps one photon and another is sent to a recipient. They both use one of two schemes to measure the photons. If they both use the same scheme, their results are highly (but not perfectly) correlated. AFAIK, the device in TFA cannot produce entangled photons, since it only produces one at a time, and is useless for this scheme.

If you really want to know the details, I'd suggest reading Quantum Cryptography and Secret Key Distillation by Gilles Van Assche

Not Possible!

[PC and Sony Fanboy]

Photons cannot be replicated, duplicated or pirated; it isn't simply the state of the photon that determines its configuration, but also the placement, and the receipt of said photon!

Re:Not Possible!

[um_atrain]

pirated?
Arr! give me all your photons!

Sure, placement and receipt are a huge factor, but this is still a huge step forward.

Re:Not Possible!

[pandrijeczko]

Photons cannot be replicated, duplicated or pirated

Jeez! That DRM gets everywhere!

When do we get the Photon Torpedos?

[CitznFish]

Seriously. I've been waiting for years.

Re:That's Nothing to be Proud Of

[Otter]

IIRC, individual photons are actually visible to darkness-adapted eyes...

Photon gun?

[4D6963]

And where's the ammunition coming from?

Re:Photon gun?

[sm62704]

And where's the ammunition coming from?

Um, Wal Mart has a good selection of ammo...

Re:Photon gun?

[4D6963]

Um, Wal Mart has a good selection of ammo...

I don't recall seeing any photons there though..

Re:Photon gun?

[Chris Mattern]

If there weren't any photons, how were you able to see anything?

Re:Photon gun?

[sm62704]

If they botch my eye operation this Thursday I won't see any photons either.

Re:Photon gun?

[4D6963]

If there weren't any photons, how were you able to see anything?

Well at least people who didn't understand the joke will get it now ;-)

Re:Photon gun?

[ArsonSmith]

I don't recall seeing any photons there though..

I almost can't tell if you meant for that to be hilarious?

Re:Photon gun?

[4D6963]

I almost can't tell if you meant for that to be hilarious?

I did, unfortunately it was almost too subtle, enough for people to feel the need to point out the reason why it's funny. Be a bit too subtle and people will be too busy being confused to laugh.

Re:Photon gun?

[Codifex Maximus]

Shop smart! Shop SMart!

The Science of Star Trek

[nervouscat]

And nobody yet has made a witty remark about photon torpedoes being next?

Re:The Science of Star Trek

[calebt3]

Two posts before yours.
http://tech.slashdot.org/comments.pl?sid=507192&cid=22931914

Re:The Science of Star Trek

[Suicidal Gir]

He said witty.

Re:That's Nothing to be Proud Of

[ArcherB]

I have a photon gun that shoots out lots and lots of photons! You mean a flashlight?

Poniez outdated?

[electrosoccertux]

OMGPHOTONS!!!

?

Re:Poniez outdated?

[geoff43230]

1. Scientists with guns
2. ???
3. Ponies!

Yes!

[LinusMartensson]

Flashlights are so old. This is the new shit.

it's [not] funny, [don't] laugh

[fred fleenblat]

So far today no joke stories.
is this supposed to be a reverse-meta-april-fool's joke?
cause if it is, i'm laughing so hard i pooped my adult diapers.

Re:it's [not] funny, [don't] laugh

[sconeu]

I think that's the April Fools joke -- there are no April Fools joke stories.

Though, I want my ZOMG!!! PONIES!!!! theme back for one day.

Re:it's [not] funny, [don't] laugh

[sm62704]

So far today no joke stories.

Apparently they were all voted down.

AND?

[lorenlal]

So... How long till they can ramp this up to some real heat and mount it on the sharks?

Single photon or high probability?

[caudex]

Before saying anything furthur, IANAQP. All the single photon guns that I've dealt with have dealt not with shooting out single photons, but by increasing the probability that when a photon does get shot out, its a single photon. For example, the machine may shoot out 1/1000th of a photon (wierd concept, I know) per pulse, with perhaps 1000 pulses per second or so. One could say that this gun shoots a photon per second, but as is usually encountered with quantum physics, its hard to be sure about what you're doing. The machines I've worked with had the ability to shoot out 1/1 of a photon, but statistically speaking you're far more likely to end up with more photons than you want with that method. This looks like a very cool development and might make quantum encryption a bit more viable. Though it doesnt help the fact that most high QP single photon DETECTORS need to work at 4 kelvins...

Re:That's Nothing to be Proud Of

[E1v!$]

Naw, that's more like a photon sprinkler.

I can't beleive I'm the only one...

[geminidomino]

No references to crappy plastic helmets, Bhodi Li, or being tagged by an active CRT?

I am disappointed in you, slashdot...

Re:I can't beleive I'm the only one...

[teh lauar]

Slashdot prefers Lazer Tag.

Re:That's Nothing to be Proud Of

[sm62704]

Well, imagine if it were shooting out REALLY REALLY BIG photons one at a time. Wouldn't that be a photon torpedo?

"Aye, Cap'n. It was inwented in Russia."

Re:That's Nothing to be Proud Of

[spikedvodka]

ob [need citation] but still... I though that the eye wouldn't trigger on fewer than 8 photons in 1/25 seconds

Huh? Someone invented a flashlight?

[Anonymous Coward]

This patent thing is really getting out of hand. Then again, I might just invent the wheel today.

a new idea...

[EdtheFox]

1) Invent a photon gun that fires single photons on demand.
2) ????
3) Profit!

Re:a new idea...

[ArsonSmith]

1) Invent a photon gun that fires single photons on demand.
2) Mention it's effects on reducing global warming
3) Profit!

Identical to what?

[aminorex]

Since it emits a "single photon", I guess the resulting photon is always self-identical? What's the difference between a duck?

Re:Identical to what?

[gammygator]

The difference between a duck is that one leg and both the same.

Very nice.

[Animats]

Very nice. One photon, emitted in a useful direction, no less. Unfortunately, the current approach has to be done at liquid helium temperatures, but maybe someone will make progress on that.

Reading stuff like this makes one realize how good we're getting at quantum mechanics. It's not just statistical any more.

Re:That's Nothing to be Proud Of

[Lost Penguin]

"photon torpedo"
I came here to say that...
You beat me to it.

Dirty Experimentalists

[Bootle]

PDF the only download on the arXiv? How 'bout you get your heads out of Bill Gates' ass and write papers like men!

Re:Dirty Experimentalists

[Anonymous Coward]

Chill, dude. PDF is an Adobe standard, not a Microsoft standard.

Re:Dirty Experimentalists

[Anonymous Coward]

That PDF was exported by Office, look at the poor kerning, font selection. No beautiful latex there boy!

Heart attack...

[Anonymous Coward]

I kid you not, I read that as "Scientologists Build New Type of Photon Gun."

That would be a whole new battle for Anonymous...

Re:That's Nothing to be Proud Of

[wattrlz]

The figure I had quoted to me was that you have a 50% chance of seeing a single photon in an otherwise completely dark room, unfortunately the guy who said it was a grad student and can't be cited. Here's a link, though. http://www.accessexcellence.org/AE/AEC/CC/vision_background.html paragraph ten.

Just like Utah?

[TooMad]

Single photons are a lot like single women in Utah. Most of them are essentially the same and inherently unstable.

Re:That's Nothing to be Proud Of

[martinQblank]

So, is there a shark-mounted application for this or are these guys just wasting time?

Could be worse...

[Anonymous Coward]

"Scientists Build New Type of Potato Gun"

QC is not magic

[Pinky's Brain]

A Man in the Middle is not an observer, he is a complete replacement of the other user to both of the original users. The QC circuits he makes are completely valid and untapped. I see the Wikipedia page reflects this fact nowadays, so if you are interested ... read it :)

They killed my stories, too.

[Anonymous Coward]

Yeah, they killed all of my April Fool's Day stories, too :(

Pity, because I made up a bunch of good ones and it seems like no one will read them now.

Re:They killed my stories, too.

[sm62704]

I'll read them, and if they're good I'll link from my journal. There must be at least one wierdo reading my journal...

Re:That's Nothing to be Proud Of

[AdamHaun]

According to the Usenet Physics FAQ, one photon will trigger the retina, but the brain filters out the signal unless it gets several in a small period if time:

http://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html

Pauli Exclusion Priniciple

[Channing]

What do they mean by 'identical'? I am not a physicist but I thought that the Pauli Exclusion Principle (http://en.wikipedia.org/wiki/Pauli_exclusion_principle) would apply here.

Re:That's Nothing to be Proud Of

[jwo7777777]

Photon fire hose. It puts out dark fires.