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Quantum researchers able to split one photon into three

115 points by jonbaer 7y ago

eveningcoffee 7y ago
Do they really split a single photon or just absorb one and generate 3 new one? I assumed that photon is an elementary particle.
- JadeNB 7y ago
  > Do they really split a single photon or just absorb one and generate 3 new one? I think that there is no meaningful difference between these two descriptions. (IANAP.)
  - lisper 7y ago
    You have it exactly right. Photons are fungible. Splitting a photon is just like splitting a bitcoin. It makes no sense to say whether the output coins are "the same" as the input coins.
    - y04nn 7y ago
      If you produce 2 entangled photons, if you "split" it into 3, do the the 3 new photons still entangled to the first one?
      - lisper 7y ago
        I presume you meant to ask: If you produce 2 entangled photons, and then split one of them again, do you end up with three mutually-entangled photons? The answer to that is: yes. The next obvious question then is: what then is the big deal about producing three mutually entangled photons? And the answer to that is that spontaneous parametric down conversion is very sensitive to the wavelength/frequency of the input light. You can't just throw in any old photon and split it. It has to be a very specific wavelength for the process to work, so as a practical matter it is nearly impossible to do the two-step process that you've asked about.
- contravariant 7y ago
  With the way quantum mechanics works it might not be possible to meaningfully distinguish between the two. Any attempt to verify which of the two is happening is likely to change the result.
- athaht 7y ago
  The photon is split into multiple photons with lower frequencies. Energy conserved
  - lisper 7y ago
    > Energy conserved As well as linear and angular momentum. The latter is the most important quantities for quantum optics because that is what produces polarization correlations.
  - eveningcoffee 7y ago
    This is what I presumed that the output is photons with lower frequencies but is it splitting in sense of really separating parts of the photon or just transfer to lower energy state that for conservation of energy produces two new photons while consuming the first one?
    - gus_massa 7y ago
      The photons have no inner parts. This is more similar to destroying the photon and creating three new photons. (I don't like "absorbing". They use probably a non linear crystal, and the transformation is probably not in a single spot. I'm not sure about the details, but probably the photon slowly disappear while the photon is traveling inside the crystal, and at the same time the other three photons slowly apear. This must be interpreted as probabilities of seen the original photons or the new three photons at the other end of the crystal. You can't have half a photon, only a 50% probability of seeing a whole photon. This explanation has too many weird things and handwaving ... just use "destroying" instead of "absorbing". More info https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion https://en.wikipedia.org/wiki/Spontaneous_parametric_down-co... )
- analog31 7y ago
  There are processes that do the latter, such as fluorescence. There are features of the two processes that let you figure out which is which. In fluorescence, there is a time delay that might be long enough to observe. Also, the generated photons will come out with energies characteristic of the material, rather than of the incoming photon. And there are differences related to polarization and the angular distributions of the photons.
- bvinc 7y ago
  There are many different ways of looking at quantum mechanics, so here is one way of answering your confusion. Probably the standard way of looking at quantum mechanics right now is quantum field theory. In quantum field theory, you give up the notion of "particles" and instead view them as vibrations in fields. Once you do that, it makes more sense that these "particles" can turn into other particles, be absorbed or emitted by other particles, or split. Here's a video of Sean Carroll talking about it, roughly 28:00-33:00. https://youtu.be/gEKSpZPByD0?t=1680 https://youtu.be/gEKSpZPByD0?t=1680
- rpz 7y ago
  I doubt the photon is truly fundamental. I've mentioned this recently in another thread but I'm pretty sure E=hf is hiding information and the units of plancks constant are wrong. I think the equation should be E=htf where h is in Joules/oscillation (quanta of energy) t is measure time and f is osillations/second. Check out Juliana H.J Mortensen's work. The upshot is that in this model, every oscillation of light carries the same energy regardless of frequency. This supports wave particle duality even more so than the current model, and takes the uncertainty out of the uncertainty principle.
  - vermilingua 7y ago
    Forgive the layperson question, but: if all light carries the same energy regardless of frequency, would this experiment (where one photon is split into three with the same sum frequency) not violate conservation of energy?
    - rpz 7y ago
      The idea here is that what people generally regard as a "photon" today is actually one second's worth of oscillations of light at a particular frequency. Imagine each oscillation of light as the true elementary particle instead. There are plenty of those elementary particles within the classical "photon" that can be split out into smaller "photons" of a different frequency.
  - fsh 7y ago
    Not even wrong
  - rpz 7y ago
    Lol downvote me all you want folks. https://img1.wsimg.com/blobby/go/e266e5e7-739c-437c-89c3-eed60d3a9f3f/downloads/1cfqec4a8_58871.pdf https://img1.wsimg.com/blobby/go/e266e5e7-739c-437c-89c3-eed...
ahelwer 7y ago
Basic primer on spontaneous parametric down-conversion: it's an optical phenomenon where a photon of a given frequency (energy) is split into two entangled photons whose frequencies add up to the original photon. The resulting frequencies differ so you can separate the photons spatially by refracting them. Has long been a workhorse for experiments requiring a pair of entangled particles (Bell tests and such). You can buy SPDC crystals online for about $500 these days[0]. I guess they managed to SPDC a photon into three photons instead of two. [0] https://www.newlightphotonics.com/SPDC-Components/405nm-Pumped-SPDC-Components https://www.newlightphotonics.com/SPDC-Components/405nm-Pump...
- anfractuosity 7y ago
  Wow, that seems really interesting that you might not need a very expensive setup to demonstrate entanglement then. I'm just reading more about the bell test. Edit: I just found this talk https://www.youtube.com/watch?v=tn1sEaw1K2k https://www.youtube.com/watch?v=tn1sEaw1K2k which looks really interesting 'Shanni Prutchi Construction of an Entangled Photon Source'. It looks like she's using SPADs for detection of the photons.
  - ahelwer 7y ago
    Yeah, the expensive/hard part is generating & measuring only a single photon at a time. Of course if you fire a laser through an SPDC crystal a bunch of photon pairs will come out entangled, but that isn't terribly useful for most entanglement experiments.
    - anfractuosity 7y ago
      Ah Gotcha, I'm intrigued wiki says "SPDC allows for the creation of optical fields containing (to a good approximation) a single photon. As of 2005, this is the predominant mechanism for an experimenter to create single photons", do you know of a simple explanation on how single photons are generated using the crystal + laser?
      - 7y ago
phkahler 7y ago
If you split one photon into 3 entangled ones of 1/3 the energy, what is the expected outcome of measurements? They cant all be mutually opposite, though photons dont have spin.
- ahelwer 7y ago
  Entanglement doesn't necessarily always produce opposites, even with spin. Measurements just become correlated in some way.
- fsh 7y ago
  Photons have a spin of 1.
seqizz 7y ago
I sometimes think one day in a lab we'll trigger a bug of the universe and whole thing will dismantle..
- deleted 7y ago
  [deleted]
- nuccy 7y ago
  Actually something like that may indeed happen, but spontaneously, see about vacuum decay here [1]. Roughly speaking, the vacuum we know is meta-stable and eventually it can reach its zero state, but such change in any point in space will trigger a neighboring point to do the same and so on, so there will be a bubble expanding with a speed of light. The biggest problem is that the laws of physics of such vacuum are different and what is stable now will be unstable then, so atoms/molecules/anything made of them can no more exist. The worst is if this theory is correct, then such a bubble may already started to expand somewhere in the Universe and we will not know about it until it actually reaches us. [1] https://en.wikipedia.org/wiki/False_vacuum https://en.wikipedia.org/wiki/False_vacuum
  - tempay 7y ago
    > the vacuum we know is meta-stable It's not fully known if this is the case or not. If the standard model is accurate (which is a big if) then it's probably metastable but there is some still room for debate. See page 52 of https://arxiv.org/pdf/1707.08124.pdf https://arxiv.org/pdf/1707.08124.pdf
- chadcmulligan 7y ago
  Theres a Greg Egan book - Schilds ladder which has something like this, very quantumy.
- dokem 7y ago
  If a super nova doesn’t glitch out the universe I think we’re safe for a while.
gentleman11 7y ago
Very neat physics news in 2020 so far. It isn’t easy to research something at that scale
jonbaer 7y ago
Does this have any repercussions into cloning @ all ie https://en.wikipedia.org/wiki/No-cloning_theorem https://en.wikipedia.org/wiki/No-cloning_theorem
- evanb 7y ago
  No. The no-cloning theorem is a real theorem of quantum mechanics. To discover a way around it is to find evidence of a new paradigm.
napsy 7y ago
This is so mind-blowing for me. Is there a "explain like I'm five" for this?

115 points by jonbaer 7y ago

Do they really split a single photon or just absorb one and generate 3 new one? I assumed that photon is an elementary particle.

JadeNB 7y ago
> Do they really split a single photon or just absorb one and generate 3 new one? I think that there is no meaningful difference between these two descriptions. (IANAP.)
- lisper 7y ago
  You have it exactly right. Photons are fungible. Splitting a photon is just like splitting a bitcoin. It makes no sense to say whether the output coins are "the same" as the input coins.
  - y04nn 7y ago
    If you produce 2 entangled photons, if you "split" it into 3, do the the 3 new photons still entangled to the first one?
    - lisper 7y ago
      I presume you meant to ask: If you produce 2 entangled photons, and then split one of them again, do you end up with three mutually-entangled photons? The answer to that is: yes. The next obvious question then is: what then is the big deal about producing three mutually entangled photons? And the answer to that is that spontaneous parametric down conversion is very sensitive to the wavelength/frequency of the input light. You can't just throw in any old photon and split it. It has to be a very specific wavelength for the process to work, so as a practical matter it is nearly impossible to do the two-step process that you've asked about.
contravariant 7y ago
With the way quantum mechanics works it might not be possible to meaningfully distinguish between the two. Any attempt to verify which of the two is happening is likely to change the result.
athaht 7y ago
The photon is split into multiple photons with lower frequencies. Energy conserved
- lisper 7y ago
  > Energy conserved As well as linear and angular momentum. The latter is the most important quantities for quantum optics because that is what produces polarization correlations.
- eveningcoffee 7y ago
  This is what I presumed that the output is photons with lower frequencies but is it splitting in sense of really separating parts of the photon or just transfer to lower energy state that for conservation of energy produces two new photons while consuming the first one?
  - gus_massa 7y ago
    The photons have no inner parts. This is more similar to destroying the photon and creating three new photons. (I don't like "absorbing". They use probably a non linear crystal, and the transformation is probably not in a single spot. I'm not sure about the details, but probably the photon slowly disappear while the photon is traveling inside the crystal, and at the same time the other three photons slowly apear. This must be interpreted as probabilities of seen the original photons or the new three photons at the other end of the crystal. You can't have half a photon, only a 50% probability of seeing a whole photon. This explanation has too many weird things and handwaving ... just use "destroying" instead of "absorbing". More info https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion https://en.wikipedia.org/wiki/Spontaneous_parametric_down-co... )
analog31 7y ago
There are processes that do the latter, such as fluorescence. There are features of the two processes that let you figure out which is which. In fluorescence, there is a time delay that might be long enough to observe. Also, the generated photons will come out with energies characteristic of the material, rather than of the incoming photon. And there are differences related to polarization and the angular distributions of the photons.
bvinc 7y ago
There are many different ways of looking at quantum mechanics, so here is one way of answering your confusion. Probably the standard way of looking at quantum mechanics right now is quantum field theory. In quantum field theory, you give up the notion of "particles" and instead view them as vibrations in fields. Once you do that, it makes more sense that these "particles" can turn into other particles, be absorbed or emitted by other particles, or split. Here's a video of Sean Carroll talking about it, roughly 28:00-33:00. https://youtu.be/gEKSpZPByD0?t=1680 https://youtu.be/gEKSpZPByD0?t=1680
rpz 7y ago
I doubt the photon is truly fundamental. I've mentioned this recently in another thread but I'm pretty sure E=hf is hiding information and the units of plancks constant are wrong. I think the equation should be E=htf where h is in Joules/oscillation (quanta of energy) t is measure time and f is osillations/second. Check out Juliana H.J Mortensen's work. The upshot is that in this model, every oscillation of light carries the same energy regardless of frequency. This supports wave particle duality even more so than the current model, and takes the uncertainty out of the uncertainty principle.
- vermilingua 7y ago
  Forgive the layperson question, but: if all light carries the same energy regardless of frequency, would this experiment (where one photon is split into three with the same sum frequency) not violate conservation of energy?
  - rpz 7y ago
    The idea here is that what people generally regard as a "photon" today is actually one second's worth of oscillations of light at a particular frequency. Imagine each oscillation of light as the true elementary particle instead. There are plenty of those elementary particles within the classical "photon" that can be split out into smaller "photons" of a different frequency.
- fsh 7y ago
  Not even wrong
- rpz 7y ago
  Lol downvote me all you want folks. https://img1.wsimg.com/blobby/go/e266e5e7-739c-437c-89c3-eed60d3a9f3f/downloads/1cfqec4a8_58871.pdf https://img1.wsimg.com/blobby/go/e266e5e7-739c-437c-89c3-eed...

ahelwer 7y ago

Basic primer on spontaneous parametric down-conversion: it's an optical phenomenon where a photon of a given frequency (energy) is split into two entangled photons whose frequencies add up to the original photon. The resulting frequencies differ so you can separate the photons spatially by refracting them. Has long been a workhorse for experiments requiring a pair of entangled particles (Bell tests and such). You can buy SPDC crystals online for about $500 these days[0]. I guess they managed to SPDC a photon into three photons instead of two. [0] https://www.newlightphotonics.com/SPDC-Components/405nm-Pumped-SPDC-Components https://www.newlightphotonics.com/SPDC-Components/405nm-Pump...

anfractuosity 7y ago
Wow, that seems really interesting that you might not need a very expensive setup to demonstrate entanglement then. I'm just reading more about the bell test. Edit: I just found this talk https://www.youtube.com/watch?v=tn1sEaw1K2k https://www.youtube.com/watch?v=tn1sEaw1K2k which looks really interesting 'Shanni Prutchi Construction of an Entangled Photon Source'. It looks like she's using SPADs for detection of the photons.
- ahelwer 7y ago
  Yeah, the expensive/hard part is generating & measuring only a single photon at a time. Of course if you fire a laser through an SPDC crystal a bunch of photon pairs will come out entangled, but that isn't terribly useful for most entanglement experiments.
  - anfractuosity 7y ago
    Ah Gotcha, I'm intrigued wiki says "SPDC allows for the creation of optical fields containing (to a good approximation) a single photon. As of 2005, this is the predominant mechanism for an experimenter to create single photons", do you know of a simple explanation on how single photons are generated using the crystal + laser?
    - 7y ago

phkahler 7y ago

If you split one photon into 3 entangled ones of 1/3 the energy, what is the expected outcome of measurements? They cant all be mutually opposite, though photons dont have spin.

ahelwer 7y ago
Entanglement doesn't necessarily always produce opposites, even with spin. Measurements just become correlated in some way.
fsh 7y ago
Photons have a spin of 1.

seqizz 7y ago

I sometimes think one day in a lab we'll trigger a bug of the universe and whole thing will dismantle..

deleted 7y ago
[deleted]
nuccy 7y ago
Actually something like that may indeed happen, but spontaneously, see about vacuum decay here [1]. Roughly speaking, the vacuum we know is meta-stable and eventually it can reach its zero state, but such change in any point in space will trigger a neighboring point to do the same and so on, so there will be a bubble expanding with a speed of light. The biggest problem is that the laws of physics of such vacuum are different and what is stable now will be unstable then, so atoms/molecules/anything made of them can no more exist. The worst is if this theory is correct, then such a bubble may already started to expand somewhere in the Universe and we will not know about it until it actually reaches us. [1] https://en.wikipedia.org/wiki/False_vacuum https://en.wikipedia.org/wiki/False_vacuum
- tempay 7y ago
  > the vacuum we know is meta-stable It's not fully known if this is the case or not. If the standard model is accurate (which is a big if) then it's probably metastable but there is some still room for debate. See page 52 of https://arxiv.org/pdf/1707.08124.pdf https://arxiv.org/pdf/1707.08124.pdf
chadcmulligan 7y ago
Theres a Greg Egan book - Schilds ladder which has something like this, very quantumy.
dokem 7y ago
If a super nova doesn’t glitch out the universe I think we’re safe for a while.

gentleman11 7y ago

Very neat physics news in 2020 so far. It isn’t easy to research something at that scale

jonbaer 7y ago

Does this have any repercussions into cloning @ all ie https://en.wikipedia.org/wiki/No-cloning_theorem https://en.wikipedia.org/wiki/No-cloning_theorem

evanb 7y ago
No. The no-cloning theorem is a real theorem of quantum mechanics. To discover a way around it is to find evidence of a new paradigm.

napsy 7y ago

This is so mind-blowing for me. Is there a "explain like I'm five" for this?