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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

#41

Post by kgwgk »

whatshisface wrote: post #38
kgwgk wrote: post #35

Earlier quoted context omitted.

> The value of c is specific to the medium. c is the speed of light in vacuum, a universal constant that obviously doesn't depend on the medium.

Sometimes people write c with subscripts, so I've seen the notation he's using before. It sounds reasonable enough to me. Edit: To be specific, in this context c was used for the speed of light in the medium and c_0 for the speed of light in vacuum. (To match epsilon and epsilon_0).

Have you seen c being used to refer to something else than the speed of light in vacuum in a discussion about the interaction of photons with matter?

Edit: yes, I know. But using c in that way is absolutely non-standard in physics (v is used instead). Actually he later used c* for the speed of light in a medium, so he knows what c is.

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

#42

Post by kgwgk »

ridgeguy wrote: post #25
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…

Path length differs from velocity. The speed of light in a medium varies according to the refractive index of the medium. [1] Free space (vacuum) has an RI of 1.0. Photons move fastest in free space, about 300,000km/sec. Water has an RI of ~1.33, meaning photons move through water at about 1/1.33, or about 75% of the free space velocity of light. RI of optical fiber is often ~1.4. So while there may be path length in…

> The speed of light in a medium varies according to the refractive index of the medium.

Sure, as he said "Speed of light through glass is ~2e5 km/s"

> Water has an RI of ~1.33, meaning photons move through water at about 1/1.33,

Not really. Something moves at this lower speed, but it's not the photons.

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

#43

Post by kgwgk »

javajosh wrote: post #32
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…

Nope. See https://en.wikipedia.org/wiki/Permittivity . But basically the material resists the formation of an electric field, which means the photons actually travel slower.

No, they don't.

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

#44

Post by kgwgk »

kurthr wrote: post #18
a3n wrote: post #2

> 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 profound…

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 wit…

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

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

#45

Post by mjburgess »

kgwgk wrote: post #29
mjburgess wrote: post #24

Earlier quoted context omitted.

That's not correct, but a widely held view. The speed differs to the properties of the EM field in these materials, not due to absorption.

How do you explain the properties of the EM field in different materials in terms of photons, electrons, etc?

How do you explain the magnetic and dielectric constants of free space?

With great difficulty.

cf. https://en.wikipedia.org/wiki/Permittivity

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

#46

Post by kgwgk »

mjburgess wrote: post #45
kgwgk wrote: post #29

Earlier quoted context omitted.

How do you explain the properties of the EM field in different materials in terms of photons, electrons, etc?

How do you explain the magnetic and dielectric constants of free space? With great difficulty. cf. https://en.wikipedia.org/wiki/Permittivity

Edit: maybe I didn't understand what is the "incorrect, but widely held view" you talked about.

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

#47

Post by mjburgess »

kgwgk wrote: post #46
mjburgess wrote: post #45

Earlier quoted context omitted.

How do you explain the magnetic and dielectric constants of free space? With great difficulty. cf. https://en.wikipedia.org/wiki/Permittivity

Edit: maybe I didn't understand what is the "incorrect, but widely held view" you talked about.

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 vacuum itself resits electromagnetic motion to some degree. I was observing that it isnt the case that light simply has one speed, the field in which it travels itself is damped.

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

#48

Post by kgwgk »

mjburgess wrote: post #47
kgwgk wrote: post #46

Earlier quoted context omitted.

Edit: maybe I didn't understand what is the "incorrect, but widely held view" you talked about.

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".

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

#49

Post by smoyer »

comboy wrote: post #16
smoyer wrote: post #12

Earlier quoted context omitted.

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…

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 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.

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

#50

Post by smoyer »

whatshisface wrote: post #40
smoyer wrote: post #12

Earlier quoted context omitted.

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…

If you know what data you sent, and if the same reflections happen every time, could you subtract them out as you read? (I'm imagining convolving the just-sent data with the recorded reflections of a test pulse.)

Yes ... we could do the equivalent to echo cancellation to a very small degree. The problem is that there will be more than one echo and, with the processing power available at the time, it was too difficult to cancel the reflected signal. I should also have noted that we were able to send analog signals over these devices. Cancelling the reflected signal would have been much easier with a digital signal.
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