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Newly observed optical state could enable quantum computing with photons

news.mit.edu

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Re: Newly observed optical state could enable quantum computing with photons

#51

Post by kgwgk »

smoyer wrote: post #49
comboy wrote: post #16

Earlier quoted context omitted.

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 i…

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…

Speed of light != Speed of photons (in general)

Speed of a photon = Speed of light in vacuum

Re: Newly observed optical state could enable quantum computing with photons

#52

Post by kgwgk »

smoyer wrote: post #49
comboy wrote: post #16

Earlier quoted context omitted.

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 i…

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…

> 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).

Re: Newly observed optical state could enable quantum computing with photons

#53

Post by acqq »

smoyer wrote: post #12
gmueckl wrote: post #3

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.

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…

The Feynman Lectures on Physics, Vol I, 31–1 The index of refraction:

http://www.feynmanlectures.caltech.edu/I_31.html

"We have said before that light goes slower in water than in air, and slower, slightly, in air than in vacuum. This effect is described by the index of refraction $n$. Now we would like to understand how such a slower velocity could come about. In particular, we should try to see what the relation is to some physical assumptions, or statements, we made earlier, which were the following:

That the total electric field in any physical circumstance can always be represented by the sum of the fields from all the charges in the universe.

That the field from a single charge is given by its acceleration evaluated with a retardation at the speed $c$, always (for the radiation field).

But, for a piece of glass, you might think: “Oh, no, you should modify all this. You should say it is retarded at the speed $c/n$.” That, however, is not right, and we have to understand why it is not."

Re: Newly observed optical state could enable quantum computing with photons

#54

Post by 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.

Re: Newly observed optical state could enable quantum computing with photons

#55

Post by comboy »

kgwgk wrote: post #48
mjburgess wrote: post #47

Earlier quoted context omitted.

Yes, the electronic structure of a crystalline material. That is one dimension of the "optical dielectric function" which is to do with the directionality of variation. Light is the motion of magnetic and electric fields, the "ease" of this motion is affected by a variety of factors. The speed of light depends not only on electric, but also magnetic field formation. And of each kind, there are a variety factors. The…

Ok, probably you will agree with the "Speed of light through glass is ~2e5 km/s, but photons are still travelling at 3e5 km/s." I guess you object to the "The speed of light, that is the speed of a photon, is a constant" part, where he identifies "light" with "photons".

Wow, I've been afk for a day and you answered so many comments, thank you, I've learned something. And yes, I should have left the speed of light out of the last sentence, my bad.

Re: Newly observed optical state could enable quantum computing with photons

#56

Post by crispycrafter »

mcnamaratw wrote: post #15
gmueckl wrote: post #3

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.

Yeah, 'photons interacting' means nonlinear optics. That's not a new field, to put it mildly. MIT's PR people are not helping science (or the institute) gain credibility.

Yeah that's about it. The refractive index of ant material can be described by a real and complex component. The real component takes into account the opacity, permissivity etc. whereas the complex component is dependent on the electromagnetic properties of the medium, such as the polarisarisability, and in a sense the interaction with quantum effects. (Have a look at the Kramers-Krönig relations)

Now the interesting part is the temperature at which the experiment was performed. I anticipate that the decreased temperature would impact the gas clouds electromagnetic field, meaning less polirisation. Perhaps the gasses electrons occupy a lower quantum state, allowing the photons to occupy the, now un-occupied valence shells, en hence the lensing effect.

Remarkable, if I'm right that is

Re: Newly observed optical state could enable quantum computing with photons

#57

Post by kgwgk »

comboy wrote: post #55
kgwgk wrote: post #48

Earlier quoted context omitted.

Ok, probably you will agree with the "Speed of light through glass is ~2e5 km/s, but photons are still travelling at 3e5 km/s." I guess you object to the "The speed of light, that is the speed of a photon, is a constant" part, where he identifies "light" with "photons".

Wow, I've been afk for a day and you answered so many comments, thank you, I've learned something. And yes, I should have left the speed of light out of the last sentence, my bad.

To be fair, the speed of a free photon is c but when the photon is interacting it may not make sense to keep talking about its intrinsic speed.

In any case, I feel objections to what you said should come from a “better” QED description and not from a “lower-level” classical description.

Re: Newly observed optical state could enable quantum computing with photons

#58

Post by TomMarius »

ghusbands wrote: post #14
gutnor wrote: post #8

Earlier quoted context omitted.

Is "relatively sluggish" the proper wording for "5 order of magnitude slower" ? The overnight backup was relatively sluggish last night, instead of 1 minute we expect it to take a little over 2 months.

Relatively is a modifier that here means "compared to something else". When someone says "relatively X", they are saying it is not actually "X" but it seems so in comparison. So they are saying that the combined particle isn't sluggish (because 3km/s is very much faster than a slug), but in comparison to light it _is_ sluggish. Hence, relatively sluggish.

Why is this downvoted?

Re: Newly observed optical state could enable quantum computing with photons

#59

Post by [deleted] »

superkuh wrote: post #20
mcnamaratw wrote: post #15

Earlier quoted context omitted.

Yeah, 'photons interacting' means nonlinear optics. That's not a new field, to put it mildly. MIT's PR people are not helping science (or the institute) gain credibility.

The bury the real interaction (mediated via the mass in atoms not photon-photon) but the paragraph is in there, >If another photon is simultaneously traveling through the cloud, it can also spend some time on a rubidium atom, forming a polariton — a hybrid that is part photon, part atom. Then two polaritons can interact with each other via their atomic component. At the edge of the cloud, the atoms remain where they…

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