Quantum researchers able to split one photon into three | Hacker News Reader

Quantum researchers able to split one photon into three

phys.org

115 pointsjonbaer53 comments

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...

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.

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.

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?

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.

It's not enough to produce entangled photons, you also have to be able to detect them individually. That's the expensive part.

Could you get a similar effect shooting electrons at something? Like is there a known path to do that.

If they get back together, then do they join to create the old photon? What if 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.

Wouldn't interaction with the particles that modified that portion of spacetime destroy the entanglement?

Photons don't experience time dilation as they don't even experience time (since they're moving at the speed of light).

Ack, can't believe I missed that! Still, the same question could be asked for other particles that can be split in the same manner ... if such particles exist.

They do blue or red shift in response to gravitational gradients and the like.

Yes, but from your point of view, not theirs

Why do we associate the speed of time with the speed of light? Is this even valid assumption?

It is not an assumption, in layman terms: "speed of light" is a bad name. It is actually speed on interaction. Top speed of any interaction. Everything which isn't slowed down by mass has this speed, like light.

I’ve heard it explained as the speed of causality (that is, the maximum speed at which cause and effect can occur) and, like you say, anything without mass to slow it down, like light, will travel at that speed. It’s the propagation delay for quantum states or whatever.

Yes.

A better name for "the speed of light" might actually be "the speed of time" or "the speed of information transfer".

It's a mathematical limit and you can derive it using pretty much anything, not just light.

As an object goes faster, it experiences time dilation, length contraction, etc. The "Speed of Light" C is basically the point at which all of these things reach either zero or infinity.

It also "just so happens" that electromagnetic waves (such as light) travel at exactly this speed in a vacuum. They travel that fast because they travel at the maximum possible speed, and that happens to be the maximum possible speed.

It's like if you always traveled at the speed limit because you didn't want to break the law, and we called it "triplesex_ speed", but really it's just the maximum speed that anyone would go if they didn't want to break the law.

Except in this case it's not just a law- it is (to the best of our knowledge) physically impossible to go faster. It's not just that we haven't seen anything go that fast- it's that going faster than that doesn't even really make sense theoretically - for example, it would result in time-travel (this has to do with the fact that the order in which events happen and the speed at which time passes is different to different observers depending on the speed at which you're traveling).

I have never though about it but that makes absolute sense.

Thanks for pointing that out.

> do they join to create the old photon?

Fundamental particles don't have unique characteristics so they don't have identity. It doesn't make much sense to ask if it's the old photon. It's a photon just like any other photon.

I think I am trying to save locality by giving this split, entangled particle an identity. To the split particle, it's the rest of the world that's split and entangled. So when it collapses, there is no faster than light travel from its perspective. Basically extending the concept of locality to allow for being in multiple places at the same time, relative to your perception of the world.

Basically, trying out the thought experiment where, say the universe is two photons, and one splits, can we model it so that each photon thinks it is the other one who split? The same way general relativity did for rotating bodies.

But, entanglement isn't always just a single particle that splits. An electron can be entangled with a proton and such too. So probably this approach is silly. And physicists seem to be fine with ftl correlation so long as no communication takes place, so probably there is no problem to begin with.

What's the underlying physical mechanism? The photon is absorbed and reemitted as two?

I guess one could draw the Feynman diagram for this interaction, but I don't think it makes a whole lot of sense to talk about a physical "mechanism". If the quantum numbers are conserved (energy, momentum, angular momentum, etc.) then it can just happen.

Confirmed, Its magic!

Can't say about the physics, but with my recent approximate knowledge of algebraic structures I think it has to mean that QM is an OK division algebra, meaning something is invertible. If the invertible thing is a matrix then it might have nice computational implications.

Do they really split a single photon or just absorb one and generate 3 new one?

I assumed that photon is an elementary particle.

> 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.)

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.

If you produce 2 entangled photons, if you "split" it into 3, do the the 3 new photons still entangled to the first one?

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.

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.

The photon is split into multiple photons with lower frequencies. Energy conserved

> 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.

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?

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-co... )

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.

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

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.

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?

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.

Not even wrong

Lol downvote me all you want folks.

https://img1.wsimg.com/blobby/go/e266e5e7-739c-437c-89c3-eed...

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

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.

Entanglement doesn't necessarily always produce opposites, even with spin. Measurements just become correlated in some way.

Photons have a spin of 1.

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

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

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.

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

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

> 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

Theres a Greg Egan book - Schilds ladder which has something like this, very quantumy.

If a super nova doesn’t glitch out the universe I think we’re safe for a while.

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