Comment by amirmc | Hacker News Reader

Comment by amirmc | Hacker News Reader

Parent Full thread

My layman's reading of this.

We used a crystal that is normally opaque (not see-through) and fired laser beams at it because, well... it seemed like fun! Apparently the laser beam affected the crystal and made it see-through but only for a narrow range of light frequencies (e.g. think of a filter that only allows one colour of light through). Then we thought Hey, we know how to make this here crystal see-through for a particular wavelength of light (think 'colour' if wavelength doesn't work for you). What if we (1) made it see-through with the laser, (2) shone some of that colour light in and then (3) turned off the laser so that it's no longer see-through. I wonder what happens? <time passes> zOMG! We trapped light!

In other words, I do think it's possible to get the message across without needing to refer to 'quantum' anything. However, this article is from NewScientist and the intended audience is likely to be vaguely familiar with concepts of superposition via stories of Schrödinger's cat [1]

[1] http://en.wikipedia.org/wiki/Schrödinger's_cat

Your understanding is actually pretty good (lines up with mine, at least), and I liked your explanation except for the needlessly conversational tone. Basically by only cutting things out:

> We used a crystal that is normally opaque (not see-through) and fired laser beams at it [which] made it see-through but only for a narrow range of light frequencies (e.g. a filter that only allows one colour of light through). [Then] we made [the crystal] see-through with the laser, shone some of that colour light in and turned off the laser so that it's no longer see-through. We trapped light!

> "... except for the needlessly conversational tone."

Yes, I agree it's not necessary to get the basic information across but it was definitely a lot more fun to write that way!

If I were really writing piece for an article or blog post I'd take a lot more care and probably consider using better analogies.

> Yes, I agree it's not necessary to get the basic information across but it was definitely a lot more fun to write that way!

I agree; for me, "conversational tone" is usually much more fun to write, and much more fun to read. I find it a sign of a good understanding of the topic if one can phrase his explenation in a conversational tone. You have to know your stuff well to be able to correctly express complicated ideas in simple terms.

'Tone' may have been the wrong word. Obviously he understands, he did explain it well. I was just trying to point out that he added a bunch of noise under the guise of being conversational. Cutting out the noise makes it concise will remaining accessible - I don't think my edits actually changed the tone.

Less is more.

> http://www.amazon.com/Writing-Works-Communicate-Effectively-...

This may be a dumb question...

but how are they able to validate that the light was stopped for one minute?

They switched the crystal to opaque... and time passed... until the light exited the crystal??? How much time? How did they validate the light left the crystal? Did they have to do anything to induce the light to leave? If they did, why couldn't this storage crystal hold the light indefinitely... until we trigger the release state?

Sorry if these are dumb questions, just can't really figure that part of it out. I understand the crystal going from opaque, to color filter, to opaque... that is essentially the "writer". How does the "Reader" work?

This is based on my fuzzy knowledge of quantum physics:

- electrons exist at varying levels of excitement "around" a molecule. These states are discrete; an electron can only have certain values of energy

- photons can "collide" with electrons and impart more energy, provided the photon's energy (correlated to the wavelength of light) plus the electron's existing energy puts it into an allowable state

- when an electron falls back from a high energy state to a low energy state it will emit a photon

- crystals have low electron mobility; it's hard for an electron with a lot of energy to escape from a molecule. The molecules also don't move within the lattice.

- in this experiment, the first laser raises the electrons to a higher energy state, where this new state + the wavelength of the light is also an allowable state

- the second laser excites the electrons into an even higher state. These electrons can't escape from their position (they're fixed in space), but they encode the energy of the second laser. When the first laser goes away, the distance the electrons now have to fall back to their natural (ground) state is very large, so it takes some time. This is what limits how long you can keep the light "stopped" for.

-the 'Reader' is just a photodiode (or a hemisphere of them) which emits electricity when hit by a photon. So you pulse laser #2, then wait for the diodes to 'light up', which means they've been hit by light. The 1 minute figure is the time between turning laser #2 on and seeing the diodes turn on.

I don't have enough background knowledge to ascertain if this is the correct understanding of the problem, but this explanation reads very well, and is the best 'mechanical' explanation I've seen in this thread so far.

Thank you.

That makes it sound to me like the light was not stopped, merely the light's energy was stored. Which is quite a different thing?

The important thing is that whatever information was encoded in the light was retained instantly. The only other way to do this is expensive photonics kit to convert light to electricity, then back to light. We're basically developing tools to achieve with light what we now do with electricity.

Also, light gets absorbed and retransmitted all the time in real life. The particular photons involved aren't really important in the typical notion of light.

Edit: to clarify, light in a medium is absorbed and reemitted, which is why light in atmosphere is slower than light in a vacuum. Between atoms it's just as fast, but there's latency in colliding, collapsing and reemitting.

There is no other way to do this, in this case they're maintaining information about the quantum properties of the beam (entanglement, etc.), which get destroyed if you convert it to electricity. When talking about quantum cryptography or, in general, quantum computing (as in "computations that are performed using the fundamental quantum interactions between particles"), the particular photons are indeed very important :)

Just to clarify (I'm pretty sure, but you seem knowledgeable), the quantum state is maintained when photons are absorbed/re-emitted by matter, right? Otherwise it seems like there would be a lot more vacuums involved. That was what I meant by 'the particular photons'; to my mind if you transmit your energy to an electron and are re-emitted you become a 'different' photon. It's a pretty silly point to argue about, though.

They're pretty sure it's the same light because (in other experiments with stopped light, not sure about this paper in particular) it maintained the same quantum information as the input light. Stopped light maintains superpositions etc.

I didn't read it all yet, but I imagine they made it transparent again and saw the beam come out. It likely couldn't ever hold it indefinitely as thermal entropy would likely scramble it to undetectable levels again. But this could be a really cool way to start building some digital optical delay lines depending on how it works.

This part implies some kind of answer, right? "They also used the trap to store and then retrieve an image consisting of three stripes".

I'm not sure that answers exactly how the "Reader" work, though.

I liked and agreed with your description and particularly the transparent reading of your `conversational tone'. There's a career for you editing mathematics journals my friend.

So, the light trapped in the crystal is stationary, and not just "bouncing around" inside, where we can observe it?

I guess the question could be phrased - is the light really stopped, or just confined to a REALLY small area (but the light still vibrates, wobbles, spins, or whatever else light does when trapped in a dark place)?

That's where the above analogy doesn't really work. Without reading the paper, I'd guess that the crystal enters a different energy state. A bit like a jet of water that's been 'absorbed' by a sponge, rather than sloshing around in a hollow ball [1].

[1] I know that's a bad analogy but I can't think of anything better off the top of my head right now.

The light is transformed into a "spin wave", an excitation of a certain atomic angular momentum describable by your good old e^i*theta.

http://en.wikipedia.org/wiki/Spin_wave

For intended purposes like the "Quantum repeater" I believe it does not matter if the photons really stopped or wobbling in some very small space. For the repeater they need to time shift photons. (IMHO, I have no idea what I'm talking about :) )

If they are not halted and then let go, but instead bouncing back and forth, there's a chance that when you open the Cristal it may exit by the other side.

Also the imprinted image may be lost or fuzzier.

It's never proposed as solving the repeater problem in full.

I'm certainly misunderstanding something (considering they did it), but what I don't understand here is how the change of state of the crystal could have spread faster than light could escape the crystal. Is the information spreading faster than the speed of light in that case?

Maybe they had multiple lasers arrayed around the outside to cause the condition of the crystal, and stopped them all at the same time while the light was in or approaching the center of the crystal (allowing for time for change to begin). Wouldn't the state change start at the outsides and move in?

I really have no idea whether you can actually reason about this stuff like that...

I'm not a physicist and the paper is paywalled so I'm speculating, but the speed of light in this crystal is probably much lower than the speed of light in a vacuum, and it's the latter that puts an uppper limit on the rate at which information can propagate (excluding quantum entanglement).

> excluding quantum entanglement

IANAP either, but AFAIK (ah, acronyms :)) quantum entanglement can not transfer information FTL.

I'm also not a physicist, but as I understand it entangled particles can affect each other instantaneously regardless of distance, but cannot transfer information due to random instabilities.

IIRC it's not due to random instabilities, but rather due to the no cloning theorem: http://en.wikipedia.org/wiki/No-cloning_theorem

Maybe because speed of light in the crystal is slower than speed of light in the vacuum (the latter being the theoretical limit for speed of information transfer)?

AFAIR light traveling through material is not just flying through; it's constantly absorbed and reemited - thus "slowing down".

I'm a biochemist, not a physicist, but I have had some quantum chemistry. Here's my (probably incorrect) thinking:

Light passing through the "open state" crystal may travel significantly slower due to interaction/absorbance with the crystal atoms along the beam path. Energy injection from the lasers is probably necessary for maintenance of the "open state", and as soon as the laser is shut off there may remain photons that are being absorbed/emitted by the atoms in the crystal lattice. When the crystal is in the "closed state", the photons will be "trapped" (continuous absorption or non-emission or something?).

It's all about quantized energy levels. At certain energies, the atoms will interact differently.

Someone please correct my thinking here.

A question for you since you seem to know about light in media: my understanding is that the speed of light is also the speed events propagate through spacetime. So, considering a galaxy made entirely out of pure crystalline diamond, would events propagate slower in that galaxy? In other words, does time itself slow down under those circumstances, does it not change?

It could also be that the speed of light is constrained by the speed events propagate through spacetime and not the other way around, but I am most probably thoroughly confused.

> So, considering a galaxy made entirely out of pure crystalline diamond, would events propagate slower in that galaxy? In other words, does time itself slow down under those circumstances, does it not change?

It doesn't change. See e.g. https://en.wikipedia.org/wiki/Cherenkov_radiation where electrons travel faster than the speed of light in a specific medium.

Reply on news.ycombinator.com