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Communications Science

Scientists Build New Type of Photon Gun 90

Posted by Zonk from the pew-pew-pew-for-science dept.
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)."
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Scientists Build New Type of Photon Gun More

Scientists Build New Type of Photon Gun

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  • I thought a photon is a photon....

    • Re: (Score:3, Informative)

      by $RANDOMLUSER ( 804576 )

      I thought a photon is a photon....
      Depends on what color it is. (i.e. wavelength)

      • Re:Identical photons? (Score:5, Informative)

        by gardyloo ( 512791 ) on Tuesday April 01, 2008 @01:22PM (#22932034)
        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: (Score:2, Informative)

          by 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 in
      • "Ahh, photons. I don't know if you're waves or particles, but you sure do go down smooth." --bender
        • Aren't photons wavicles? (A wave with its ends chopped off, thus displaying the properties of a discrete particle in some cases.)

    • Re: (Score:2)

      by Eudial ( 590661 )

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

  • The key question (Score:5, Funny)

    by dkleinsc ( 563838 ) on Tuesday April 01, 2008 @01:07PM (#22931888) Homepage
    Is it possible to correctly attach this photon gun to a shark's head?

  • great, bloody typical. (Score:5, Insightful)

    by apodyopsis ( 1048476 ) on Tuesday April 01, 2008 @01:08PM (#22931896)
    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: (Score:2, Informative)

      by calebt3 ( 1098475 )
      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. (Score:5, Informative)

        by JustinOpinion ( 1246824 ) on Tuesday April 01, 2008 @01:42PM (#22932270)
        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: (Score:2)

          by geekoid ( 135745 )
          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.

          • 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 [findarticles.com], arXiv preprint [arxiv.org]).

            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 [wikipedia.org] and be assured perfect unbreakable encryption.

            • 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 recipien
              • 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]
                • 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 :)
        • the objective is to interfere two photons with each other to generate entangled pairs

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

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

    • Re: (Score:3, Interesting)

      by sm62704 ( 957197 )
      A quantum dot [wikipedia.org] 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 [google.com] 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" [harvard.edu]. The dictionary [reference.com] only lists towns named "Stark" when you search for the term.

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

        • Re: (Score:2)

          by sm62704 ( 957197 )
          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.
    • "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: (Score:3, Insightful)

      by CompMD ( 522020 )
      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.
  • 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!
  • And where's the ammunition coming from?

    • Re: (Score:2)

      by sm62704 ( 957197 )
      And where's the ammunition coming from?

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

      • Re: (Score:3, Funny)

        by 4D6963 ( 933028 )

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

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

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

          • Re: (Score:2)

            by 4D6963 ( 933028 )

            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: (Score:2)

          by sm62704 ( 957197 )
          If they botch my eye operation this Thursday I won't see any photons either.
        • I don't recall seeing any photons there though..

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

          • Re: (Score:2)

            by 4D6963 ( 933028 )

            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.

      • Shop smart! Shop SMart!
  • And nobody yet has made a witty remark about photon torpedoes being next?

  • Poniez outdated? (Score:4, Funny)

    by electrosoccertux ( 874415 ) on Tuesday April 01, 2008 @01:18PM (#22931998)
    OMGPHOTONS!!!

    ?
  • Flashlights are so old. This is the new shit.
  • 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.
  • 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 yo
  • No references to crappy plastic helmets, Bhodi Li, or being tagged by an active CRT?

    I am disappointed in you, slashdot...
  • 1) Invent a photon gun that fires single photons on demand.
    2) ????
    3) Profit!
    • 1) Invent a photon gun that fires single photons on demand.
      2) Mention it's effects on reducing global warming
      3) Profit!
  • Since it emits a "single photon", I guess the resulting photon is always self-identical? What's the difference between a duck?

  • 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.

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

  • Heart attack... (Score:1, Funny)

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

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