66 points by rbanffy
2 months ago
After many years of reading about quantum effects it's just became clear that most of the unintuitive nature of it is due to confusing language.
"Measuring" is usually hitting the system with a photon. At those scales it is similar to measuring the state of a person by shooting him with a canon, you would not be surprised if the "Measurement" changed the state of the person.
Why would you say the opposite of the what wiki says?
Shooting a person with a canon over and over does not prevent time evolution of the person.
Alright I'll give you another analogy that is more relevant to this specific experiment.
You have a balloon in the air that is slowly descending from state A (in the air) to state B (on the ground). You measure if the balloon is in the air by throwing a ball at it, each hit of the ball makes the balloon temporally bounce a bit higher. You then realize that if you "measure" fast enough the balloon will never transition from state A (in the air) to state B (on the ground) AKA the xeno effect.
This is incorrect. You are not actually analogizing.
You are literally saying the opposite of the article. Tell me what eigenstate the balloon is in?
No, we don't know what measurement is, and it's nothing to do with the fact we're hitting it with something, that's the observer effect. Heisenberg Uncertainty Principle is a fundamental property of reality, separate from "if we hit things they move"
I should repeat again: we still have no idea yet what exactly is a quantum measurement and what isn't, it's one of the great unsolved questions of physics
> In 1989, David J. Wineland and his group at NIST observed the quantum Zeno effect for a two-level atomic system that was interrogated during its evolution. Approximately 5,000 9Be+ ions were stored in a cylindrical Penning trap and laser-cooled to below 250 mK. A resonant RF pulse was applied, which, if applied alone, would cause the entire ground-state population to migrate into an excited state. After the pulse was applied, the ions were monitored for photons emitted due to relaxation. The ion trap was then regularly "measured" by applying a sequence of ultraviolet pulses during the RF pulse. As expected, the ultraviolet pulses suppressed the evolution of the system into the excited state. The results were in good agreement with theoretical models.
> Sometimes this effect is interpreted as "a system cannot change while you are watching it"
So it _is_ true that the water won't boil if you look at it!
I realize it's a joke but I'll explain why it doesn't follow. The amount of measuring required to prevent the water from boiling would look more like some impossibly intimate process that was actively and constantly interrogating almost every single detail of every water molecule in the pot. It's very different from the passive collection that your eyes do when you look at the pot. You'd need to really get in there and strongly interact with the water to get out the necessary information. A closer analogy than looking at it would be sticking your hand in it, but that's still not nearly harsh enough.
Measurement in quantum mechanics is still so weird and foreign to me, even as a postdoc in quantum optics. It will never amaze me how many weird and amazing things we know about measurement, while still not having defined or understood what a measurement really is! Quantum mechanics is terrifying in its opaqueness, it is almost less like a physical theory and more like a mathematical tool
The Zeno effect can still occur if there's no measurements.
Let there be a qubit B and a register of N qubits C, C, ..., C[N-1]. Now repeat the following N times: rotate B by an N'th of a full turn and then perform C[k] ^= B (a CNOT controlled by B targeting C[k] where k is the index of the iteration). These are all unitary operations; no measurement. If you consider doing this for larger and larger N, you find that if B started in the |0> state then B ends up nearer and nearer the |0> state as N increases. B's final density matrix limits to |0><0| as N goes to infinity.
Interesting! This seems related to the idea that quantum measurement is nothing but unitary evolution with a large number of degrees of freedom
I wonder if this could be an artifact created by the sensitivity, or rather the lack thereof, of the measuring instrument. Or to say another way, the instrument is limited by the same physical bounds that the system being measured is. Or maybe it’s a stroboscopic effect like car wheels spinning so fast they look like they’re not moving.
Here is an example of the effect straight from the article:
> The ion trap was then regularly "measured" by applying a sequence of ultraviolet pulses during the RF pulse. As expected, the ultraviolet pulses suppressed the evolution of the system into the excited state.
Not sure why this parent comment is bothering others so much.
If applying pulses of UV light is considered as an example of taking a measurement then I don’t see how I could be surprised when someone wonders whether or not the measurement itself is more than just an observation.
Atoms are sensitive to changes in electric and magnetic fields. Wine glasses are sensitive to being struck with a hammer. I can detect whether I just smashed glass with a hammer by listening for the sound the glass made when it was smashed to a billion pieces.
All measurement, but especially so at the quantum scale, involves interacting with the observed subject. One of the observations made by quantum mechanics is that you can't observe a system without interacting / altering it in some way.
To give you a macroscopic example, the way you detect the color of an object is to apply electromagnetic radiation in the visible light spectrum to the object and see which wavelengths are absorbed and reflected. The process of absorption and reflection is a physical interaction with the object you're looking at.
Or maybe it's what the wikipedia article says?
It's always a bit terrifying to me how often I see people on Hacker News confidently asserting some phenomenon can be explained by something that any rational person would recognize as non-nonsensical.
One person’s terror is another person’s Appeal to the Stone.
Really? An entire Wikipedia article filled with detailed explanation of the phenomenon and its cause is an appeal to the stone? Really? Sorry, but that is absurd.
I’m simply offering a possible underlying cause. I’m not denying it’s a real phenomenon.
You're suggesting there's a misleading flaw in the measurement device itself which is a different phenomenon than the article describes (in which a particle's normal evolution through time _actually_ decays when measured).
This effect is derived from the mathematical axioms that define quantum mechanics. It has nothing to do with instrument error or strobing. In fact it works best if there's no instrument error. Instrument error creates chances for the system you're trying to pin down to escape.
I think the poster you are replying to is suggesting a physical interpretation of QM
It's important to note that the anti-quantum zeno effect is the more common of the two.
Why do you find this important to note?
The effect was of interest for insulating quantum systems but it ends up that most times you touch a system, the system will decay.
Quantum mechanics looks like the borrow checker from Rust. You're observing a particular state, so that state is "borrowed" into your observation and cannot be mutated by anything else. Once you stop observing, you've freed the universe's memory so to speak and it can now change the state.
Maybe this is an artifact of memory management in the simulation's runtime?
How does watching paint dry work then? Does it only dry when I'm not looking?
Measurements slow down time, that’s why gravity exists
reminds me of this https://www.youtube.com/watch?v=UKxQTvqcpSg
Does this mean that state transition depends on output of previous state ONLY, and that output is produced only once (unicast, rather than broadcast)?
Thus, when you measure previous state, you effectively deprive the "next state computation" on those inputs, and hence, block state transition?
It reminds me @DDOS-Attacks and some at the same time a type of Kybernetic to solve a work-around, um maybe ?!
So to bind at real problems, my sanity and reasoning say, i do and by doing i am treatin (therapy'ing) myself. P-:
This would seem to impose an upper boundary on the speed of quantum computational systems related to the “distance” between states in their qbits. It would be roughly similar to parasitic capacitance in purely electronic computers.
The "you need to travel the distance between states" speed limit on quantum computation is called the Margolus-Levitin Theorem .
This seems like gibberish to me, can you explain further? How did you derive this.. hypothesis
TFA explains it as:
> As an outgrowth of study of the quantum Zeno effect, it has become clear that applying a series of sufficiently strong and fast pulses with appropriate symmetry can also decouple a system from its decohering environment.