6 ms·

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! Appa

by amirmc 13y ago

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 http://en.wikipedia.org/wiki/Schrödinger's_cat

alanctgardner2 13y ago
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!
- amirmc 13y ago
  > "... 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.
  - TeMPOraL 13y ago
    > 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.
    - alanctgardner2 13y ago
      '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.
      - rangibaby 13y ago
        Less is more. > http://www.amazon.com/Writing-Works-Communicate-Effectively-Business/dp/0060956437 http://www.amazon.com/Writing-Works-Communicate-Effectively-...
- bilbo0s 13y ago
  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?
  - alanctgardner2 13y ago
    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.
    - netshade 13y ago
      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.
- e3pi 13y ago
  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.
AlisdairSH 13y ago
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)?
- amirmc 13y ago
  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.
- selimthegrim 13y ago
  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 http://en.wikipedia.org/wiki/Spin_wave
- fsniper 13y ago
  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 :) )
  - gcb0 13y ago
    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.
    - fsniper 13y ago
      It's never proposed as solving the repeater problem in full.
guizzy 13y ago
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?
- kbenson 13y ago
  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...
- john_b 13y ago
  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).
  - TeMPOraL 13y ago
    > excluding quantum entanglement IANAP either, but AFAIK (ah, acronyms :)) quantum entanglement can not transfer information FTL.
    - alcari 13y ago
      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.
      - nbouscal 13y ago
        IIRC it's not due to random instabilities, but rather due to the no cloning theorem: http://en.wikipedia.org/wiki/No-cloning_theorem http://en.wikipedia.org/wiki/No-cloning_theorem

by amirmc 13y ago

alanctgardner2 13y ago

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!

amirmc 13y ago
> "... 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.
- TeMPOraL 13y ago
  > 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.
  - alanctgardner2 13y ago
    '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.
    - rangibaby 13y ago
      Less is more. > http://www.amazon.com/Writing-Works-Communicate-Effectively-Business/dp/0060956437 http://www.amazon.com/Writing-Works-Communicate-Effectively-...
bilbo0s 13y ago
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?
- alanctgardner2 13y ago
  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.
  - netshade 13y ago
    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.
e3pi 13y ago
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.

AlisdairSH 13y ago

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)?

amirmc 13y ago
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.
selimthegrim 13y ago
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 http://en.wikipedia.org/wiki/Spin_wave
fsniper 13y ago
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 :) )
- gcb0 13y ago
  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.
  - fsniper 13y ago
    It's never proposed as solving the repeater problem in full.

guizzy 13y ago

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?

kbenson 13y ago
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...
john_b 13y ago
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).
- TeMPOraL 13y ago
  > excluding quantum entanglement IANAP either, but AFAIK (ah, acronyms :)) quantum entanglement can not transfer information FTL.
  - alcari 13y ago
    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.
    - nbouscal 13y ago
      IIRC it's not due to random instabilities, but rather due to the no cloning theorem: http://en.wikipedia.org/wiki/No-cloning_theorem http://en.wikipedia.org/wiki/No-cloning_theorem