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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

#31

Post by evanb »

gus_massa wrote: post #7
amelius wrote: post #4

Does this have implications for Maxwell's equations?

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 photo…

It's also only valid classically. Quantum-mechanically we expect to generate photon-photon interaction terms (the leading piece is light-by-light). Essentially, two photons have an interaction moderated by the virtual charged particles in the vacuum.

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

#32

Post by javajosh »

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…

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.

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

#33

Post by O_H_E »

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, that's annoying

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

#34

Post by hpcjoe »

Jeff_Brown wrote: post #27

"A photon’s phase indicates its frequency of oscillation." That can't be right. Phase and frequency are orthogonal.

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.

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

#35

Post by kgwgk »

hpcjoe wrote: post #28
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…

Well ... no. The value of c is specific to the medium. In "empty" space, c is the constant we know and love, and is related to the permittivity and dielectric constant of the vacuum. Specifically with a classical E-M model, when you are in a material different than vacuum, the differing dielectric and permittivity of the material are (in a macroscopic sense) what lead to the reduced c*. This is encapsulated in the fo…

> 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.

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

#36

Post by 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’.

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

#37

Post by whatshisface »

bawana wrote: post #36

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 i…

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

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

#38

Post by whatshisface »

kgwgk wrote: post #35
hpcjoe wrote: post #28

Earlier quoted context omitted.

Well ... no. The value of c is specific to the medium. In "empty" space, c is the constant we know and love, and is related to the permittivity and dielectric constant of the vacuum. Specifically with a classical E-M model, when you are in a material different than vacuum, the differing dielectric and permittivity of the material are (in a macroscopic sense) what lead to the reduced c*. This is encapsulated in the fo…

> 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).

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

#39

Post by kgwgk »

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…

It seems your opinion is not very popular around here, looking at how many classical counter-arguments are given to your (necessarily imperfect) quantum mechanics description.

https://en.wikipedia.org/wiki/Photon#Photons_in_matter

Light that travels through transparent matter does so at a lower speed than c, the speed of light in a vacuum. [...] The factor by which the speed is decreased is called the refractive index of the material.

In a classical wave picture, the slowing can be explained by the light inducing electric polarization in the matter, the polarized matter radiating new light, and that new light interfering with the original light wave to form a delayed wave.

In a particle picture, the slowing can instead be described as a blending of the photon with quantum excitations of the matter to produce quasi-particles known as polariton (other quasi-particles are phonons and excitons); this polariton has a nonzero effective mass, which means that it cannot travel at c.

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

#40

Post by whatshisface »

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…

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.)
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