
Quantum researchers able to split one photon into three - jonbaer
https://phys.org/news/2020-02-quantum-photon.html
======
ahelwer
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...](https://www.newlightphotonics.com/SPDC-Components/405nm-Pumped-SPDC-
Components)

~~~
daxfohl
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.

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

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

~~~
Svoka
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.

~~~
dkersten
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.

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

I assumed that photon is an elementary particle.

~~~
rpz
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
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
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.

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

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

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gentleman11
Very neat physics news in 2020 so far. It isn’t easy to research something at
that scale

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jonbaer
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
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.

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

~~~
nuccy
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
> 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)

