Quantum Definition and 998 Threads

In quantum mechanics if I repeat a measurement of the same observable in succession I get the same quantum state if it is not a degenerate state. If I make the system under consideration interact with another quantum system and meanwhile keep measuring it what happens? Does the system not...

In https://phys.org/news/2016-09-cold-black-holes.html it is stated that a supermassive black hole interior could be 10^-14 degrees Kelvin. Is there a limit, perhaps due to quantum effects, below which a temperature (in a black hole or elsewhere) can go? Or do the possibilities approach 0...

I am following [this YouTube lecture by Schuller][1] where he finds the appropriate formalism for the quantum mechanics in the physical curved space. Everything makes sense to me but at the very end I see that we find the pull backed connection one-form on the base manifold. He says to the end...

Starting from the Heisenberg equation of motion, we have $$ih \frac{\partial p}{\partial t} = [p, H]$$ which simplifies to $$ih \frac{\partial p}{\partial t} = -ih\frac{\partial V}{\partial x}$$ but this just results in ## \frac{\partial p}{\partial t} = -ih\frac{\partial V}{\partial x}## and...

At the heart of the theory of open quantum systems is the idea that the measurement statistics of many-body systems can be expressed in terms of a reduced density matrix, obtained by tracing over degrees of freedom that are irrelevant to the system of interest. In general, given a pure state...

What is the Schrodinger equation in QFT? is it the nonrelativistic approximation of a Klein-Gordon scalar field? or Is there more? I have read that the Schrodinger equation describes a QFT in 0 dimensions. I accept every answer

I thought this was a fun and quick video of an engineer giving a tour of their quantum computer. Only about 30 seconds, but it looks well, quite beautiful! The engineer mentioned they achieved milestone 2 which has something to do with size. I also haven't heard of the field "Quantum Metrology"...

Hi everybody. Some years ago I came across a video on youtube where they talked about an experiment with small and large envelops, when the small ones were placed into the large ones and then it resulted in something interesting. It might have been an instance of delayed choice, but I am not...

What is meant when a phase is said to have "symmetry protected"?

The many online articles about the future security issues posed by quantum supremacy — at least those pitched at the interested layperson (like myself) — tend for quite understandable reasons to focus on the outcomes that QS will have on today’s digital-based security systems. . . not as they...

It this correct explanation why quantum is not valid for large object? Why would small object want to hide their information?

It is often argued that Dirac Equation is not valid as relativistic quantum mechanics requires the creation of antiparticles. But, there are also some arguments that suggest otherwise. For example, I saw Arnold Neumaier's website on this that there are multiparticle relativistic quantum...

In this 2011 paper, Lundeen & colleagues used weak measurement to map both imaginary and real components of a wavefunction directly, without destroying the state. It says: “with weak measurements, it’s possible to learn something about the wavefunction without completely destroying it”. And...

I have the equations to calculate transmission probability, my problem is that the barrier is given in Volts not electron volts. $$200V = e \cdot 200 eV = 3.2 \cdot 10^{-17} eV$$ I am not even sure if that's a correct conversion. But if it is then this "barrier" is extremly small and 99.999%...

The classical "power method" for solving one special eigenvalue of an operator works, in a finite-dimensional vector space, as follows: suppose an operator ##\hat{A}## can be written as an ##n\times n## matrix, and its unknown eigenvectors are (in Dirac bra-ket notation) ##\left|\psi_1...

quantum connection vs. quantum entanglement Do they mean the same? Are there any differences? Is the second one more common/newer/more standard than the first one? Could someone please answer these questions or recommend something to read?

Hello, I hope you are doing well. I had a question about the eigenvalue problem of quantum mechanics. In a past class, I remember it was strongly emphasized that the eigenvalues of an eigenvalue problem is what we measure in the laboratory. ##A\psi = a\psi## where A would be the operator...

This thread picks up a discussion between myself and @A. Neumaier in another thread. In particular, this comment: Reference: https://www.physicsforums.com/threads/nobody-understands-quantum-physics.1049370/page-8 First some basic background. If we consider the usual presentation of a...

I am using BURAI for quantum espresso calculations and am getting the error ATOMIC_POSITION nonexistent. I have attached the scf .in file and the crash file. I am using the CIF file of LaFeO3 I downloaded from materials project. Any help would be appreciated as I am not very familiar with these...

Listen just 1 minute, what does it mean when he said nobody understand quantum mechanics? This sound like comedy

I think Atomic physics gets into specific examples and experiments but I think I’m probably wrong. So, I have to ask, what is atomic physics and how is it different than quantum physics?

The proton and electron are described by separate wavefunctions. When they come together in the hydrogen atom are they quantum entangled and have a joint wavefunction.

Einstein showed (via general relativity) that spacetime is curved by mass, mass moves in relation to this curvature, and that gravitation arises as secondary effect. Why then are we looking for quantum gravity as some sort of mass<->mass interaction? Aren't the fundamental interactions better...

Ok so just tell me this, if you flip a coin with your eyes closed and it lands on your hand and then you look at the coin and it is heads… there was a time in the duration of the coin being in the air at which the coin was in a state of neither heads or tales. But only once it hits your hand it...

I'm watching a nice video that tries to explain how linear algebra enters the picture in quantum physics. A quick summary: Classical physics requires that physical quantities are single-valued and vary smoothly as they evolve in time. So a natural way to model classical physical quantities is...

Hey everyone, A quick question that I hope all can participate: In your educated opinion, what is the biggest question or the biggest problem to be solved in quantum mechanics for 2023? Warm regards, Thiago Munhoz da Nóbrega

Hi. I’ve seen a number of schematic diagrams for the Delayed Choice Quantum Eraser experiment. Some include a prism to make the two idler photon paths diverge. However, the prism is shown refracting the ‘idler rays’ in a way which contravenes Snell’s law. E.g. see prism marked ‘PS’ (not the...

What would people here say are good modern Quantum Optics textbooks?

I recently watched this lecture "Quantum Fields: The Real Building Blocks of the Universe" by David Tong where the professor provides a succinct explanation of QFT in about 6 minutes around the midway mark. The main point being that there are fields for particles and fields for forces and the...

inspiration: How does one predict the effects of small modifications (ordering of optical devices etc)? https://www.physicsforums.com/threads/the-experiment-of-kim-et-al-1999-with-small-modification.1047803/ In chapter 5 and 6 of A Guide to Experiments in Quantum Optics, the authors deal with...

I am entering into the 2nd year of my masters and I plan to do my phd in quantum chromodynamics. So for my specialization, would it be helpful if I choose nuclear and particle physics or quantum field theory?

Dennis Dieks is a respected Phycisist who works in Utrecht University.He has been developing his unique understanding of quantum mechanics namely Quantum Perspectivalism,close to Rovelli's RQM.But his works seem highly unpopular . https://link.springer.com/article/10.1007/s10701-019-00264-0 This...

The quantum number n determines the energy, and for each n the allowed values for the angular momentum quantum number are -(n-1),...,(n-1). This doesn't seem resonable to me. Classically increasing the orbital angular momentum will result in an increase in the energy of the system. But why is it...

The titular paper can be found here, https://doi.org/10.1088/1751-8121/ac6f2f, and on arXiv as https://arxiv.org/abs/2101.10931 (which is paginated differently, but the text and equation and section numbers are the same). Please see the abstract, but in part this 24 page paper argues that we...

The system at my college died and I did a fresh install of ubuntu and tried to install quantum espresso after installing the required dependancies according to the instructions in the link "https://osuginoissp.wixsite.com/mysite/a-install-quantum-espresso" . But the make all command keeps...

Hi Pfs, I wonder if in quantum mechanics the question "Did this event occur?" has a yes or no answer like in classical phyics or in our daily life. We are accustomed to consider linear superpositions of states like "dead" and "alive" but id "dead" is a state the death of the cat is an event. i...

I've been reading about how language around virtual particle fluctuations is metaphorical. This is helpful: https://www.physicsforums.com/insights/vacuum-fluctuation-myth/ I'm just trying to understand a bit more from a layman's point of view. I found Matt Strassler's article 'Virtual...

it is known that azimutal quantum number takes the values from 0 to n-1. how did they find it ?

Then please explain how the transition in conceptual language from a single quantum field (extending all over spacetime, or at least over the lab during a day) to an ensemble of particles can be justified from the QFT formalism.

Normalize function f(r) = Nexp{-alpha*r} Where alpha is positive const and r is a vector I was just wondering if the fact that we have a vector value in our equation changes anything about the solution

So, in a rare instance I actually read APS News, I came across “New Experiment Suggests Imaginary Numbers Must be Part of Real Quantum Physics.” In November 2022, Volume 31, Number 10. Since complex numbers are isomorphic to a real 2x2 matrix algebra, I was confused how such a claim can be...

Hi, I'm working on a problem where I need to find the different energies allowed for a potential, and I found this link https://quantummechanics.ucsd.edu/ph130a/130_notes/node151.html, which is similar of what I'm doing. I'm using mathematica to find the values of E. However, I'm not sure how...

Hello to everyone, I would like to ask you to brief questions. The first one is whether you could recommend any pedagogical books on Quantum Information and Computation. I tried Nielsen and Chuang but I found it too dense for a beginner in the field. The second question is the following: to...

I got a one liner for QP, Electron behave like wave when no observer is there but when we put observer ELECTRON saw this and they told themself hey they are observing us lets change our behaviour, WE are particle now. That type of strange QP is. Am I right?

What i mean if we change state/spin at one end it will immediately effect the other. Can we see that live using two camera which may be 10 meter apart so that minium time delay. Is there any video proof exist such kind?

Is the answer 0.748?

As QP says it is uncertain that where you found electron. But its not the same phenomena that one fan blade running so fast that you cant see where the blade is. You can only see the blade on a particular position if you use high resulation camera and its position depend on when you take the...

Hi I’m looking for a textbook that shows the derivations of equations such as the different forms of the schrodinger equation fully and step by step.

I have no idea where to start with this problem. I am interested in any hints, or ways to proof this. But i would especially like to know how the commutator is connected to the identity.

Suppose some quantum system has a Hamiltonian with explicit time dependence ##\hat{H} := \hat{H}(t)## that comes from a changing potential energy ##V(\mathbf{x},t)##. If the potential energy is changing slowly, i.e. ##\frac{\partial V}{\partial t}## is small for all ##\mathbf{x}## and ##t##...