Q: What does it mean for light to be stopped or stored?

Physicist: We’ve gotten a handful of questions since this was published and led to articles like this, this, and, this.  In a nutshell, some dudes in Germany (Georg Heinze, Christian Hubrich, and Thomas Halfmann) have found a method to shoot a pulse of light, “stop” that light for about a minute and then get the light going again, using “Electromagnetically Induced Transparency” (EIT).  It’s not important to know what EIT involves.

So, what does it mean to “stop light”?  Light by it’s very nature always travels at the same speed, 2.99 x 108 meters per second: the aptly named “speed of light”.  So, for example, when it travels through water or glass and it “slows down” it’s actually just getting absorbed and re-emitted over and over by atoms in whatever it’s traveling through.  When it does travel, it travels at full speed.  And if “stopping light” means holding the energy for a while and then re-releasing it later, then what’s the difference between what Heinze, Hubrich, and Halfmann did and what a rock does when it heats up in sunlight and then radiates that heat later (as infrared light)?

The answer to that question, and what makes this experiment important, is that the process preserves the photons’ information.  The rock that absorbs sunlight and radiates that energy as heat later is scrambling the sunlight’s information completely, but what H and H and H did preserves the light’s information almost perfectly.  It’s a little profound how perfectly it preserves the light’s information.

An image, which is "classical information", is shown after several different storage times.

An image, which is “classical information”, is shown after several different storage times.  This shows that not only is the energy of the light pulse being stored, but the information in the light pulse is being stored as well.

Not only is it possible to store information like this image, but quantum information can be stored as well.  The difference between regular information and quantum information is a little hard to communicate (the exact definition of “information” is already plenty technical before lumping on quantum information).  Quantum information is the backbone behind things like entanglement, “action at a distance”, and quantum computation.  That last use is the one that physicists are excited about.  “Stopping light” is nothing new, but finding a way to store quantum information is a big deal.

Quantum information is the “delicate” part of a thing’s quantum state.  When the outside world interacts with a quantum system, it tends to screw it up.  We say “the wave function collapsed” or “the system decohered“.  Quantum information is only useful and different from classical information when the system (in this case a bunch of light) is allowed to be in superpositions or is entangled with something else.  We can tell that the EIT technique preserves quantum information, because we can do experiments on the entanglement between a photon that’s stored, and another that’s not and we find that the entanglement between the two is preserved.  Basically, this kind of storage is so “gentle” that it isn’t even a measurement, and all of the quantum state is preserved (quantum information and all).


The UNIVAC I could store as much as 1000 words with 12 letters each in this tank of mercury. This was built right around when “bits” were first becoming the universal standard for information.

Storing light in the way the three H’s have done, is akin to building an early memory device called a “delay line memory“.  For comparison, way back in the day (1950 or so) computers were built that used tanks of liquid mercury to store data.  Acoustic pulses travel through the mercury tank much slower than electrical signals travel through wires, so you could store tiny chunks of data by reading the pulses from one end of the tank, then re-sending those pulses at the other.  The “light storage” technique would be a similar (although much longer time span) memory system for quantum computers.

So, light isn’t being “stopped” it’s “imprinting” on some of the electrons in the crystal that are in very, very carefully prepared states.  This imprint isn’t light (so it doesn’t have to move), it’s just excited electrons.  That imprint lasts for as much as a minute; slowly accruing errors and fading.  After some amount of time that imprint is turned back into light, and it exits the crystal at exactly the speed you’d expect.  What makes the experiment most exciting is that this experiment has proven to be an extremely long term method for storing quantum information, which has traditionally been a major hurdle.  Normally a quantum computer (such as they are) has to get all of its work done in a fraction of a second.

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14 Responses to Q: What does it mean for light to be stopped or stored?

  1. Locutus says:


  2. jaseg says:

    For comparison, way back in the day (1950 or so) computers were built that used tanks of liquid mercury to store data. Electrical pulses travel slower through mercury than regular wires, so you could store tiny chunks of data by reading the pulses from one end of the tank, then re-sending those pulses at the other.

    These delay lines operate acoustically by sending the data into the mercury with a piezo crystal. The choice of mercury was due to its acoustic impedance matching that of the used piezo transducers.

  3. River says:

    Very interesting. In another post, could you give an introduction to EIT?

  4. The Physicist The Physicist says:

    That’s amazingly interesting! I was massively misinformed!
    That “electrical pulse” thing never did quite sit right.

  5. James Williams says:

    Thank you for that explanation, I understand what the researchers did much better now. I believe, however, that you are incorrect about mercury delay lines using electrical pulses. They were, in fact, acoustic. See this article on Wikipedia:

  6. K2 says:

    Hi, all. I had assumed that various experiments that allegedly slowed the speed of light were actually bouncing it back and forth in a medium.
    I would like to see someone try to speed up light.

    What would be the effect – Split a light beam into two parts. Send one down a path that delayed it for some part of a second. Combine this output with the other split beam that had not been delayed. Carefully tune both so that their characteristics match or mesh when combined.
    You might insert a delay pulse in the original to keep tract of time.
    When combined –
    Would the delayed beam ‘catch up’ in time with the unaltered beam?
    Or, would the unaltered be slowed to the other beam’s delayed speed (arrival time)?

  7. Lex Corvus says:

    Excellent article. N.B. Quick “it’s” correction: “Light by its very nature”

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  9. Mehrdad says:

    Lene Hau measured a light speed of only 17m/s in sodium Bose-Einstein Condensate in 1998. How’s that for stopping light?

  10. kopernik says:

    Re: Stopping light in its tracks
    To my analogue wetstate brain it appears – the photon(s) that went into the system are not the same that came out.
    Still would like to know – After splitting a beam and sending one down a path that delays it, when recombined (or not) are the two still sharing the same coherence?
    Have they have maintained their quantum connection?
    Might give the original beam some rotation to make it more definitive. K

  11. Xerenarcy says:

    you’ve just described an interferometer – a device that measures sensitive changes in distance by examining light interfering with itself.

    also, the photons going in are not the same as the photons going out, logically. also because we can control the delay and the speed of light is set, we must be creating photons at some point. however in the quantum scale this does not matter – you can’t tell apart quantum particles since aside their quantum states they are identical in every respect we have tested. to that end we can say the photons going in are identical to the photons going out to the best of reality/physics allowing us to tell them apart.

  12. James Fitzsimmons says:

    There is a terrific science fiction story based on the idea of ‘slow glass’ that absorbs an image by detouring photons through minute channels so that the image is delayed and released months or years later. So if you bought this slow glass and hung it in your downtown apartment that has no view, the glass would appear to be a window, and the image would not be from a dvd or flash drive or any kind of recording device, but the actual image itself (e.g. lake, mountain range, girls at the beach, etc.).

    The story is ‘Light of Other Days’ by Bob Shaw from 1966. Just a wonderful piece.

  13. Greg Robert says:


    Ah, I remember that story well.
    It was sad though. The man was using it to watch his deceased wife in what had been their back yard. He would wait hours on end for the rare and unpredictable chance of glimpsing her, knowing all the time that one day the show would end.

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