
Newly observed optical state could enable quantum computing with photons - charlysl
http://news.mit.edu/2018/physicists-create-new-form-light-0215
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ghusbands
The paper is "Observation of three-photon bound states in a quantum nonlinear
medium"; details here:
[http://science.sciencemag.org/content/359/6377/783](http://science.sciencemag.org/content/359/6377/783)

~~~
danielam
Discussion here:
[https://news.ycombinator.com/item?id=16392892](https://news.ycombinator.com/item?id=16392892)

Edit: Previous submission, not discussion.

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dhimes
There's actually no discussion there.

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gmueckl
Is anyone else annoyed by the fact that while MIT news is good at creating
interest in their stories, they are annoyingly watered down and never link
back to the source paper they are paraphrasing? In this instance, they never
mention the paper title even once.

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smoyer
And they tend to over-generalize - "While photons normally have no mass and
travel at 300,000 kilometers per second (the speed of light)". The maximum
speed of a photon is (currently?) around 300,000 kps but they often travel
slower than this. Our fiber optics systems are coursing with photons that
travel approximately 200,000 kps. The key results of this experiment is that
the photons are interacting with _each other_ ... and that they acquire mass.

The flashlight analogy is actually one that I not only described to people,
but built a unique product around in the '90s. CyberFiber (yeah ... it was an
tongue-in-cheek name for an engineering project that escaped into the
marketing and sales department) was a duplex communication platform that used
the same wave-length of light for both directions. It was a great marketing
tool but required that your optical connections had very low reflectivity (and
nobody likes to put coupling gel in their connectors).

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comboy
Photons travelling through fiber are still traveling at 300,000 km/s. It's
just that they bounce of walls making their path longer and much more
importantly than that, the medium is denser. Speed of light through glass is
~2e5 km/s, but photons are still travelling at 3e5 km/s. It's just that they
hit atoms, energy gets absorbed and re emitted and so on. But photons always
move at 3e5 km/s. The speed of light, that is the speed of a photon, is a
constant.

~~~
smoyer
You might want to look into the theory behind fiber optics a bit ... I left
out the deviation due to path length (bouncing) as that obviously has a lot of
variations (including wavelength). I've also ignored SBS and other unrelated
sources of scatter.

It's pretty common to hear people refer to the "speed-of-light in a vacuum".
If the speed of light were a constant, why would you need to specify the
vacuum? It's also worth noting that electrons don't move through conductors at
the speed-of-light either - in the cable television industry, the best hard-
line coaxial cable was about 0.85.

~~~
kgwgk
> electrons don't move through conductors at the speed-of-light either

The drift velocity of electrons through conductors is many orders of magnitude
lower than the speed of light (~mm/s).

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a3n
> While photons normally have no mass and travel at 300,000 kilometers per
> second (the speed of light), the researchers found that the bound photons
> actually acquired a fraction of an electron’s mass. These newly weighed-down
> light particles were also relatively sluggish, traveling about 100,000 times
> slower than normal noninteracting photons.
    
    
      300,000 / 100,000 = 3
    

"Relatively sluggish?" That seems more like profoundly sluggish, for light.

~~~
kurthr
An index of refraction of 3 is relatively high (like silicon carbide). Glass
and a lot of optically transparent materials are only ~1.5.

But, if you look at IR transparent materials like silicon, gallium arsenide,
or germanium, you can get up to and over 4.

More to the point, diode pumped solid state lasers are a common thing these
days so photons interacting just isn't news... (edit) however if (and I can't
tell without reading the original paper) it is actually the effect of photon
mass (eg due to light density) on the photons itself, then it's a significant
technical achievement! Just not how I'd have phrased it to a technical
audience.

~~~
kgwgk
This is more like an index of refraction of 100000, isn't it?

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bawana
Please clarify the model that describe the mechanism of the observations. Is
it A) a photon travels through a medium and changes the state of an atom in
some way. Another photon travels through the medium and interacts with the
same ‘excited’ atom. The second photon is then changed in a way directly
related to the first photon

Or

B) a photon interacts with an atom but instead of being absorbed and
retransmitted, it is just absorbed and captured. Only when another photon is
captured does the atom release them both.

In A the information from the first photon is stored in the atom (flipped spin
of an electron or proton for example) but the total energy of the atom is not
changed much.

In B, the energy of the photon is contained in the atom (jump of an electron
to a higher energy orbital) and the atom is truly in an excited state- this
would be the mechanism of laser. An incident photon would then push the
excited atom to quantum orgasm where it would spew out its photons.

I tend to favor B because it might make me famous as the inventor of the
‘quantum orgasm’.

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whatshisface
B is called stimulated emission and is the principle behind lasers.

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Jeff_Brown
"A photon’s phase indicates its frequency of oscillation."

That can't be right. Phase and frequency are orthogonal.

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hpcjoe
Yeah, there were some serious faulty elements of the article. That was just
one. Apart from that, I'm not aware of experiments that measure absolute
phase, only relative phase differences.

But I've been out of physics for 20+ years, so mebbe they've gone and done it.

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crispycrafter
Just trying to wrap my head around this. Photon energies are distributed
across quantised states. We can obtain the single photon energy for a given
wave-length by the Planck-Einstein relation. We then assume the photons become
"entangled", and decrease their speed. Is this to conserve momentum/energy,
assuming the frequency is unaltered. I assume this frequency stays the same on
the premise of constructive interference, and hence not altering the phase or
frequency.

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amelius
Does this have implications for Maxwell's equations?

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gus_massa
No, no changes to Maxell's equation, but I think it's easy to be mislead by
the bad press coverage and title.

They only made a weird material where photons interact in a strange way, the
photons "like" to be bound in triplets in that material.

There are other unusual materials, for example one special crystal can split a
green photon into two infrared photons. Or the inverse, pick two photons and
create a green photon with them.
[https://en.wikipedia.org/wiki/Spontaneous_parametric_down-
co...](https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion)

The process in the article is different because it's temporal and it's only
inside the material, but the split/unsplit make new photons than can live
forever outside the material. (There are other differences ...)

Maxwell's Equation is only valid in vacuum, or when the material doesn't do
anything interesting. The technical term is probably "linear materials". In
nonlinear materials, weird things can happened and you can no long use the
Maxwell's Equations.

~~~
John_KZ
I came here to say that. I'm no expert in optics, much less quantum optics,
but they seem to have created some kind of fancy material with unusual optical
properties.

I'm very disappointed that MIT News themselves are so misleading. If I
couldn't pirate the paper, I'd be left with the impression that they said they
broke some kind of fundamental physics rule. They should leave heavy
editorialization and sensationalisation for the news websites further down the
chain.

~~~
ridgeguy
Agreed.

This seems like just another instance of "slow light", which has been a thing
since 1998 [1].

Speed of light in tailored materials has been as low as zero. As in, stopped.
And then restarted on command.

[1]
[https://en.wikipedia.org/wiki/Slow_light](https://en.wikipedia.org/wiki/Slow_light)

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jadavies
I really, really hoped that they'd at last confirmed the existance of octarine
:)

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Koshkin
TL;DR: entanglement of photons

