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Comment by bilbo0s | Hacker News Reader

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?

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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.

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