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Question about the Quantum Zeno Effect

  1. Aug 14, 2014 #1
    If I get it right, the Quantum Zeno effect shown in the experiment of W. Itano et al in 1990 is one of the most amusing and easy to understand for non-experts about quantum superposition and the "role of observation" in quantum systems, which I would choose to explain these subjects to non-experts (that is, people even less expert than me who am myself a layman). But I want to make sure that I get it right.

    So here is what I get of the experiment as I would explain it in easy to understand layman terms (I first read about it being called "the quantum pot that never boils"):

    As we know, atoms can be “excited” into a higher energy state by providing them electromagnetic energy. If you throw photons to an atom, the photons may be absorbed by the atom’s electrons which will “jump” into a higher energy orbit. This is the “excited” state of the atom (if you leave it alone it will eventually radiate the photon away and return to its “ground state”).

    Now, the experiment consists of a “pot” containing around 5000 beryllium atoms. By applying a shower of microwave radiation (photons of a certain energy) we can excite them and set them in a higher energy state, which metaphorically we may call as “the beryllium atom being boiling”. The time it takes for the microwave shower to bring a beryllium atom into the excited “boiling” state is 256 milliseconds.

    However this boiling -which is the expected thing to happen for atoms under the radiation shower- occurs only if the atoms are not observed during the 256 milliseconds period. If we do not observe the pot, after 256 milliseconds all of its atoms will be “boiling”.
    But if we attempt to observe the atoms “en route to becoming boiling”, we will not see them getting to the boiling state. The more the atoms are observed, the longer they take to boil. If the atoms are observed frequently enough -every 4 milliseconds- the pot will never boil at all, no atoms will get to the boiling state.

    This shows that quantum systems evolve differently when observed than when unobserved, and this is what the metaphor that "the moon is not there when nobody is looking at it" stands for.

    The reason why the pot never boils if observed is that an atom, while not observed, can be in a superposition of different states, partly not-boiling and partly boiling (the percentage of each state shifting gradually from “not boiling” to “definitely boiling” along the 256 millisecond period). It is important to realise that there is no intermediate state between not-boiling and boiling, the electrons can only be either in the ground orbit or in the higher one, but they can never be in between. The “superposition state” is not an in-between state between not-boiling and boiling, but really a superposition of “not boiling” and “boiling” mutually exclusive states. But if the atom is observed, it needs to “choose” a definite state, either “boiling” or “not boiling”. If we observe the atom at 128 milliseconds, the chances that it may “choose” to be boiling or not boiling are exactly 50%.
    When observed, each atom can only be in any of either states, boiling or not-boiling, but nothing in between. They can only be in between (in a quantum superposition of both states) while not observed. This is the key.

    Now, those atoms which at the moment of being observed (let's say after 128 milliseconds) "choose" to take still the unexcited (non-boiling) state, need to start again from scratch, and need again 256 milliseconds to boil, not just 128 milliseconds more.

    The microwave radiation shower is continuous, even while the atoms are being observed.

    The observation is done by shining a laser beam through the "fog" of beryllium, and the scattering of the laser tells how many atoms were boiling and how many were not (because non-boiling atoms absorb some energy from the laser and boiling ones don't).

    During an unobserved 256 milliseconds radiation shower, every atom will evolve from a quantum superposition of states 100% not-boiling + 0% boiling, to a superposition of 0% not-boiling + 100% boiling.
    At 128 milliseconds, the superposition is 50%-50%, at 64 milliseconds it's of 75%-25%, and so on.

    When observed, each atom must abandon the superposition and "choose" between any of both states.

    If you only observe after 256 milliseconds, all the atoms could get to the 0% not-boiling + 100% boiling superposition, so you find all of them boiling.

    If you observe after 128 milliseconds, they are in a superposition of 50%-50%, therefore half of them will choose the non-boiling state and the other half the boiling state.

    But for the 50% who take the non-boiling state, the superposition returns to 100% non-boiling + 0% boiling. Therefore they need again 256 milliseconds unobserved to evolve to 0%+100%, they have to start from scratch again.

    Therefore if you observe them very repeatedly -every 4 milliseconds was the test so the probability of an atom “chosing” to be boiling was neglectable because of the short time elapsed-, causing them to return to the 100% non-boiling + 0% boiling, they can never make it to boil even if the radiation shower is never stopped.

    So far so good, this is my understanding of the experiment.
    Now an important question, I would be tempted to say to my interlocutors something like:

    "An interesting fact is that both the radiation shower (it is radio waves radiation) and the laser beam, are BOTH electromagnetic radiation being showered to the atoms. However the radio waves shower does not cause the collapse of the superposition, and the laser does. Why?
    The only difference is that we use the laser to observe, while we don't with the radio waves. It is our attempt to use the radiation to acquire knowledge of the system which determines if the radiation will change the atoms state or not. As long as we do not attempt to gain information from it, radiation will not cause the atoms to choose a definite state and they can remain in a superposition of multiple states, but if we try to use that radiation to get information on the state of the atoms, they will collapse into a single state."

    Would this last statement be correct? or is there some characteristic of the laser radiation crucially different from the microwave radiation making the laser to cause the collapse and not the microwaves, regardless if we attempt to obtain information or not?
  2. jcsd
  3. Aug 14, 2014 #2


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    They need 256 milliseconds to "boil" with certainty. Some of those atoms will be in an excited states even if you make another measurement within a few milliseconds.

    I would expect 85.4% / 14.6%, following a squared sine. This nonlinearity is also the reason why very frequent observations reduce excitations so much.

    And the others go from excited to ground state in that time.

    The radio wave interaction is coherent, the laser interaction (with much more energy per photon) is incoherent. It's a bit like an object floating in the water (radio waves - you don't notice individual "interactions with water") and then getting shot by a bullet (laser photon).
  4. Aug 14, 2014 #3
    Thanks, so if I understand your point well, the fact that the laser causes collapse and the microwaves do not is simply due to the energy of their photons, not with the fact that we try to observe the state of the atoms. So according to you shining the laser but not attempting to observe or record the results would result in exactly the same, i.e. the atoms would return to their ground state due to the action of the laser even if we will never know it because we did not observe it.
  5. Aug 14, 2014 #4
    Or, even more clearly, you are saying that if we still shine the laser every 4 ms but we do not attempt to observe or record the results, and only at 256 ms we do observe, we will find all the atoms in the ground state because even if we did not observe it, each laser shot collapsed them back to their ground state each 4 ms.
  6. Aug 16, 2014 #5


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    Well, they are related. We use lasers with different photon energy because they have different tasks.

    Any human intention is not relevant for the experiment itself. The experiment works even if you do not analyze it afterwards. "Shining the laser on the atoms" is the observation.
  7. Aug 17, 2014 #6
    ok thanks!
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