[HN Gopher] Quantum researchers able to split one photon into three
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Quantum researchers able to split one photon into three
Author : jonbaer
Score : 32 points
Date : 2020-02-28 08:46 UTC (2 days ago)
(HTM) web link (phys.org)
(TXT) w3m dump (phys.org)
| phkahler wrote:
| 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 wrote:
| Entanglement doesn't necessarily always produce opposites, even
| with spin. Measurements just become correlated in some way.
| gentleman11 wrote:
| Very neat physics news in 2020 so far. It isn't easy to research
| something at that scale
| seqizz wrote:
| I sometimes think one day in a lab we'll trigger a bug of the
| universe and whole thing will dismantle..
| [deleted]
| nuccy wrote:
| 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
| tempay wrote:
| > 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
| eveningcoffee wrote:
| Do they really split a single photon or just absorb one and
| generate 3 new one?
|
| I assumed that photon is an elementary particle.
| athaht wrote:
| The photon is split into multiple photons with lower
| frequencies. Energy conserved
| lisper wrote:
| > 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 wrote:
| 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?
| bvinc wrote:
| 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
| analog31 wrote:
| 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.
| JadeNB wrote:
| > 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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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.
| ahelwer wrote:
| 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-
| Pump...
| anfractuosity wrote:
| 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 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 wrote:
| 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 wrote:
| 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?
| fsh wrote:
| Most quantum optics experiments are done by simply
| attenuating laser pulses until they have on average much
| less than one photon per pulse. This way the probability
| of having more than one photon is very low (at the cost
| of reduced count rates since most pulses don't contain
| any photons). These pulses can then be used to create
| entangled photon pairs using SPDC.
|
| There are also several ways of creating pulses that
| contain exactly one photon, but this is way more
| complicated.
| lisper wrote:
| It's not enough to produce entangled photons, you also have
| to be able to detect them individually. That's the expensive
| part.
| sriku wrote:
| What's the underlying physical mechanism? The photon is
| absorbed and reemitted as two?
| ianai wrote:
| Could you get a similar effect shooting electrons at something?
| Like is there a known path to do that.
| [deleted]
| daxfohl wrote:
| If they get back together, then do they join to create the old
| photon? What you stick one in a stronger gravitational field
| for a while and then later you stick them back together but
| maybe the phases of something are out of sync due to time
| dilation (or maybe they're in sync but just one has experience
| an extra second of life)? Does the resulting photon become
| entangled with the second-ago rendition of _itself_? Are we
| able to control an entangled photon well enough to run a
| process like this?
|
| Sorry for the interested bystander questions.
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