I have a question which may be very naive yet I have no answer. I studied undergraduate quantum mechanics 4 years ago now and even if I studied more advanced stuff like QFT I feel like I don't understand the basics yet, so feel free to answer me with &quot;take a QM book and study it&quot; and close the question. My problem is about quantum measurements and superposition of states. I really don't understand why measurement is not happening all the time. I feel like I may be confusing measurement with interaction, but, for example if we have a magnetic field and a an electron passing through it, depending on how it is deflected it means the interaction with the magnetic field fixed a component of the spin: the electron now has spin up, it's not anymore in a superposition of spin up and down. Now, these kind of interactions happen all the time, every particle interacts with its environment constantly so I feel like the successions of all these interactions constantly fixes the quantum states of all the particles in the universe. Why is this wrong? What am I missing and what are the flaws of this view? I'm not sure this is the final version of my question and edits may arrive as soon as some comments/answers clarifies for me what I am actually not understanding.
79807
Let's say we are doing the double slit experiment with electrons. We get an interference pattern, and if we put detectors at slits, then we get two piles pattern because we measure electrons' positions when going through slits. But an electron interacts with other particles in a lot of different ways, e.g. electric field, gravity. Seems like the whole universe is receiving information about the electron's position. Why is it not the case and the electron goes through slits "unmeasured"?  Bonus question: in real experiments do we face the problem of not "shielding" particles from "measurement" good enough and thus getting a mix of both patterns on the screen?
77459
When an excited state couples to the vacuum, it has an infinite number of directions of the quantized electromagnetic field to couple to. Does it evolve into a superposition of all those directions at the same time and only collapses once the photon is measured, or does it couple to only one? (Or, of course, is there no experimental way to tell?)
81438
Text books say that when you measure a particle's position, its wave function collapses to one eigenstate, which is a delta function at that location. I'm confused here.   A measurement always have limited accuracy. Does the wave function collapse to exactly a eigenstate no matter what accuracy I have? When a particle is in an eigenstate of position, I can represent the state in momentum basis, and calculate it's expected value (average) of kinetic energy. This gives me infinity. Can a particle ever be in such a state?
34787
As I understand it (from a lay physics perspective), quantum entanglement has been experimentally demonstrated - it is a reality. As I understand it, you can measure something like the spin of an electron and know that its entangled pair will, in that same instant, no matter where in the universe it is, have the opposite spin.   This would not seem to have any utility as the foundation of a superluminal communications device. Is this true, or has it been established that is there some aspect of quantum entanglement that can ultimately lead to the development of such a device.  In other words: is superluminal communication via quantum entanglement an open scientific question, has it been settled as an impossibility, or is it currently more of an engineering problem than a scientific one?
18988
I apologize if this question is posted on the wrong portion of the website, but I knew no better place to ask.  I've come to see that some effects, such as super-conduction may happen at either extremely low temperatures, or at extreme temperatures (the recent discovery of superconducting iron in the bowels of the earth, for one), and this is fascinating.  My question is: can (does?) the same principle be applied to quantum effects? Would it be applicable to, for example, say that although quantum effects happen at nano scales and below they may happen on a grand scale of say, the entire Universe (as a solitary unit)?  Can said quantum effects happen at moments of say, picoseconds after the Big Bang due to the turbulence of forces involves in such an event? Or even within the core of a dying star?  Thank you for any answers or clarification of my misinformation.
74882
I think I understood very well how operations on one qubit are done - if qubit is electron, we just apply magnetic field in direction we want to make spin precess (unitary operations on single qubit).  I sort of understand how entanglement happens - you let 2 electrons interact and the lowest possible energy state is entanglement, so nature just puts them there. (?) But why all of a sudden we can make unitary operations on 2 qubit hilbert space? What physically lets us do so?   Rephrasing questions - How is CNOT operation realized physically?
56827
I followed a basic course on QFT and know some basic concepts but I focussed mainly on calculations and understanding the Feynman rules e.g.. However, sometimes I have the idea that there a quite some fundamental concepts I might be missing, especially regarding the interpretation of QFT and &quot;what's happening&quot;. One of the things I wonder is how does wave collapsing of QM fit into QFT? In QM there was a lot of focus on measurements, probabilities, and (the collapsing) of the wave function. Of course, particles in QM and QFT are fundamentally different. In QM they are probability waves and in QFT they are (excitations of) fields. And if I'm not mistaken I would say that for the field of a particle, the excitations of the field are governed by the rules of Quantum Mechanics. However, is the wave function really completely gone, or is there some way to link fields to wave functions again and effects of measurements etc.
23997
My layman understanding of the Uncertainty Principle is that you can't determine the both the position and momentum of a particle at the same point in time, because measuring one variable changes the other, and both cannot be measured at once.  But what happens if I measure a charged particle with any number of detectors over a period of time? Can I use a multitude of measurements to infer these properties for some point in the past? If not, how close can we get? That is, how precise can our estimate be?
122924
Classical computers store information in bits, which can either be $0$ or $1$, but, in a quantum computer, the qubit can store $0$, $1$ or a state that is the superposition of these two states. Now, when we make a "measurement" (to determine the state of qubit), it changes the state of the qubit, collapsing it from its superposition of $0$ and $1$ to the specific state consistent with the measurement result.  If from a measurement of a qubit we obtain only a single bit of information ($0$ or $1$, because the qubit collapses to either of the states), then, how is a qubit better than a classical bit? Since ultimately we are only able to get either $0$ or $1$ from either classical or quantum bit.  If you can explain with an example, it may be helpful.
107146
I hear the word "measurement" thrown around a lot in quantum mechanics, and I have yet to hear a scientific definition that makes sense. How do we define it?
83063
So I know that two particles can be entangled in a quantum way, but is it possible that more than two particles be entangled in a quantum way? Most descriptions provide with two-particles cases, so I wonder. (It's hard to think of three particles entangled in spin, or so.)
55273
I was wondering if quantum particles do actually  exists in two different states simultaneously and if it has been proven they do indeed exists in a superposition of states.  How has it been figured out since observing it would collapse the wave-function into one single state (two superposition of states into one ) as it has been mentioned in the Schrodinger's cat experiment?
104380
What's wrong in taking the cat as an observer in Schrodinger's experiment? Plz kindly elaborate! And if possible also describe about possible logics if the question bears the answer No.
81219
Consider a particle in a potential well. Let’s assume it’s a simple harmonic oscillator potential and the particle is in its ground state with energy E0 = (1/2) &#8463;&omega;0. We measure its position (measurement-1) with a high degree of accuracy which localises the particle, corresponding to a superposition of momentum (and therefore energy) states.  Now we measure the particle’s energy (measurement-2) and happen to find that it’s E10 = (21/2) &#8463;&omega;0. Where did the extra energy come from?  In the textbooks it’s claimed that the extra energy comes from the act of observation but I wonder how that could work. Measurement-1 which probed the position of the particle can’t have delivered to it a precise amount of energy, while measurement-2 might just have been passive. No doubt there is entanglement here between the particle state and the measuring device but where, and which measurement?
62401
In Lawrence Krauss's book "A Universe From Nothing"; page 62 mentions that for a very short period of time, so small it cannot be measured, an electron due to the uncertainty principle can appear to be moving faster than the speed of light; another way to interpret this is that it's moving back in time.  It further proceeds to say that Feynman used this to suggest that what happens was that an electron and a positron were created, the positron will annihilate with the original electron while the new electron will continue on its merry way.  It finally says that this behavior is confirmed by the spectrum of the Hydrogen atom, among other things.   My question is, how does this not violate special relativity? How is it possible to get an electron moving faster than light? And how did we say it's okay to assume that and simply consider it moving back in time?   (Note, I do not have a degree in Physics so please take that into account while explaining the basic idea).
73681
new finding by Minev et al. seems to suggest that transitions between atomic states are not instantaneous, but continuous processes wherein a superposition smoothly adjusts from favoring one state to another (if I understand it correctly). The authors also claim to be able to catch a system "mid-jump" and reverse it. Popular articles are  and .  I am curious if this finding rules out any interpretations of QM. It seems to generally go against the Copenhagen attitude, which describes measurements as collapsing physical systems into a definite classical state. The popular articles indeed claim that the founders of QM would have been surprised by the new finding.  The link with the asterisk mentions that something called "quantum trajectories theory" predicts what was observed. Is this an interpretation, or a theory? And are they implying that other interpretations/theories don't work?
114896
I was thinking about the many interpretations of quantum physics, and one thing that never made sense to me was the many world's interpretation. Basically at any given moment for which something exists in a superposition, isn't this interpretation basically saying that there would have to exist a new universe for every possible discrete outcome that could result once the measurement is made?   So let's say an electron's location is a superposition of two distinct locations. Doesn't that mean there would at that moment have to be two distinct universes for each distinct location that is superposed? If so, then clearly the amount of universes being spawned every moment must be astronomical.   There could be trillions upon trillions of electrons(for instance) having superposed states, and then every moment in time (I suppose every planck time?), it seems there would have to be yet another set of universes spawning for every superposition.  How could this be? Surely there must be some kind of thermodynamic cost to replicate an entire universe. What would fuel or supply the additional mass and energy for every universe at every moment for every superposed state?  To my lay person's mind, this seems to be a very big weakness of this interpretation, but I needed to ask people with the physics background required to answer this question. Could there be some kind of loophole that could allow alternate universes to spawn so rapidly? Otherwise, doesn't this present a bit of a problem for taking this interpretation seriously?