This tag is for questions about the quantum mechanical measurement problem, such as e.g. the wavefunction collapse/decoherence. DO NOT USE THIS TAG for non-quantum measurements.

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100 views

How does one compute the state of a quantum system following imperfect measurement?

Suppose I have a quantum system $S$ ("system") with Hamiltonian $H_S$ and initial density matrix $\rho_S(0)$. I allow $S$ to interact with another system $P$ ("probe"), which has Hamiltonian $H_P$ and ...
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1answer
113 views

Wave Function State Reduction As a Result of Quantum Measurement

This is a question about indirect quantum measurement, involving an observable of an object of interest and a probe that is used to measure that observable. In this experiment, an observable of the ...
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2answers
93 views

Theoretically, how does quantum decoherence induce noise?

The decoherence process has allowed us to explain various (classical and decoherence) sources of measurement noise in quantum systems. I intuitively understand this physical concept of ...
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2answers
47 views

Collapsing a wave function without hitting the detector

If a free particle is placed at the origin (in 1D) with a wave function that consists of a superposition of the particle moving in both the +/- X direction, and a single detector is placed on the ...
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5answers
183 views

Looking for clarification on superposition [closed]

I have always had a hard time accepting the concept of superposition from quantum mechanics. I know that the leading physicists say that the cat is both alive and dead until it is observed and that an ...
0
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1answer
55 views

Generalized Born Rule for partial measurements

I'm looking for the mathematical formulation of this trait (from an introductory Quantum Computing course): How can I generalize this into higher dimensions? Or other observables? It does not seem ...
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1answer
105 views

If this were possible, would it count as a modification of quantum mechanics? [closed]

This question will be re elaborated to meet the standards of this site in shot
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2answers
119 views

Physical quantities have definite values?

I don't really know if this question has an anwser but I thought it was worth to try asking. My point here is the following: in Quantum Mechanics, to describe the states of a system we use a Hilbert ...
2
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2answers
97 views

How to we measure quantum purity experimentally?

Quantum purity $\gamma$ of a general state $\rho$, where $\rho$ is a density matrix, is defined as $\gamma = \text{tr} (\rho^2)$. I'd like to understand how $\gamma$ can be measured experimentally. ...
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3answers
95 views

Calibration of a clock [duplicate]

I was inspired by this interesting question on this forum: How do I measure an earth year without a clock? Say you're stranded on an alien planet without any significant tools. How would you ...
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1answer
118 views

Time evolution operator acting on a non-eigenket

I'm taking a course in QM at my university, and I'm trying to work out an assignment given to the class by our professor. The setup is as follows: The problem is about a simplified description of ...
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0answers
70 views

Wave function: what does “1% chance of finding the particle in this area” mean

Say I have 1 electron in some quantum state Defined by some wave function, and it's doing its thing fluctuating the probabilities of where it might be. What if I put a measuring device in an area ...
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2answers
77 views

In quantum mechanics does the truth/accuracy of a measurement really matter?

Once John Wheeler said "the past has no meaning or existence unless it exist as a record in the present". So, in a experiment of delayed choice entanglement swapping, if we used a faulty detector to ...
2
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2answers
157 views

Do our eyes act as observers at the quantum level? [duplicate]

This is a very high level question. I was just thinking about the idea than in quantum physics, the act of observing has a "strange" effect on some properties (e.g. double slit). If I'm staring at a ...
2
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3answers
201 views

Does the Copenhagen interpretation require Nature to solve NP problems instantaneously? [closed]

Let's say we have a quantum computer with two registers taking in $m$ and $n$ qubits respectively, with $m$, $n$ suitably large. Let $f:\{0,1\}^m \rightarrow \{0,1\}^n$ be a one-way function. Set up ...
6
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4answers
822 views

Is uncertainity a postulate? [duplicate]

I heard the standard interpretation of Heisenberg Uncertainty Principle: Just the measurement affects the position of the body because always you want to see a body (=to measure the position), you ...
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1answer
49 views

Observers in a Double Slit Experiment

Does it matter what the observer is in the double slit experiment? Could it be anything from an ant to a dog? Has it ever been tried with a non-human observer?
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1answer
225 views

Meaning of expectation value of product of non-commuting operators

Let $\hat{A}$ and $\hat{B}$ be Hermitian observables with spectra labeled by $a$ and $b$. Then we can write \begin{equation} \hat{A} = \sum_a a\, \hat{P}_a \end{equation} \begin{equation} \hat{B} = ...
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1answer
68 views

Problem with tracing over the environment and obtaining reduced density operator [closed]

I'm reading an article by Bas Henson given here. On page 25 he deals with the double slit experiment Given the wavefunction which is his equation 1.75, he claims he has traced over the environment ...
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0answers
30 views

Regarding IMU derivation of position/orientation

If you are familiar with IMUs (Inertial Measurement Unit) , you know that before the double integration to arrive at estimating the position, one has to subtract the gravity from the measured ...
6
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3answers
311 views

How to understand the motion of a particle in Quantum Mechanics?

In Classical Mechanics when we talk about the motion of a particle it is the same as talking about the idea of trajectory. The fact is that in Classical Mechanics, a particle has a definite position ...
0
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1answer
79 views

Is it possible to plot a quantum particle's position with respect to time?

If I shoot an electron through a substance, is it theoretically possible to plot its path through said substance? Has this been experimentally done?
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4answers
356 views

Is the uncertainty principle a statement about limits on our predictive rather than our measurement abilities?

Here's what I know. The Uncertainty Principle states that $$\sigma_x \cdot \sigma_p \geq {{\hbar} \over 2}$$ However, I also know that this principle refers to measurements performed over many ...
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1answer
100 views

How to construct the operator and the physical experiment needed to perform an arbitrary 'measurement in a basis'?

I have taken an introductory level course in QM and have covered some advanced topics by myself and don't really understand what it means to 'measure in a particular basis'. A projective measurement ...
4
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2answers
101 views

Do quantum wave functions curve spacetime before they are measured

Do wave functions cause spacetime curvature before they are measured, or would curvature only happen upon measurement? I guess the question becomes, do quantum wavefunctions carry energy while they ...
5
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3answers
406 views

Reason for Uncertainty principle

$$\Delta x \Delta p_x \geq \frac{\hbar}{2} $$ I understand what does Heisenberg's uncertainty principle states i.e. it's definition and it has been proven experimentally. But, can anyone please ...
6
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2answers
139 views

What does it mean to observe? [duplicate]

This is a layman's question. The only thing I know about quantum physics is from casual reading and documentaries. I can imagine electrons being probabilistic waves. Their position is an infinite ...
3
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1answer
370 views

A particle in a 1D box: what is the meaning of velocity?

In the box $x = 0$ to $x = L$, $V = 0$, and for $x < 0$ and $x > L$, $V = \infty$ (infinite potential well). The eigenvalues of the Hamiltonian are: $$E_n = \frac{n^2 h^2}{8L^2} \, .$$ Since ...
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1answer
63 views

Do the eigenstates of the Pauli operators correspond to the six directions of the 3D world?

I understand that the six eigenstates of the three Pauli operators $X, Y, Z$ correspond to the six poles of the Bloch sphere. By fixing an orthonormal basis of our physical word, does "measuring Pauli ...
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2answers
152 views

Quantum Entanglement - How To Interpret [duplicate]

I have thought about quantum entanglement for some time, and I still don't quite understand the reasoning behind the conclusion that entangled particles somehow can communicate their state to each ...
2
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0answers
106 views

Can the Born rule be derived? [duplicate]

$\renewcommand{ket}[1]{|#1\rangle}$ If we have a particle and we know the initial state $|\psi\rangle$ of everything that is relevant, and we know the full Hamiltonian $H$, then we should be able to ...
4
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4answers
207 views

How does one describe a state with a density matrix after measuring position?

My question is about position measurement in non relativistic quantum mechanics. I've been taught that when you measure the value of an observable for some state of a system described by ...
2
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0answers
141 views

Weak measurement and weak value

The concept of weak measurements (and weak values) have become popular in Quantum information community, as I can see quite a few papers in arXiv. Since I am from Mathematical background (and the ...
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0answers
81 views

Can I measure distance with sound through soil?

Can you measure distance (cca. with precision 0.1m-1m) with sound through soil. I know this is really crazy think to do, but I have this idea quite for a while. Basically I will set speakers on fix ...
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1answer
43 views

ideally accurate measurement

In the address below http://en.wikipedia.org/wiki/Measurement_in_quantum_mechanics it's written: For pedagogic reasons, the measurement [in quantum mechanics] is usually assumed to be ideally ...
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1answer
147 views

Momentum uncertainty of free particle

I've read several Q&A's regarding free particles and the associated wave packet in this website, but found the answer to my question nowhere. It's OK to attribute a Gaussian wave packet to the ...
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5answers
185 views

Does measurement change the evolution of wave function?

Basically any measurement is on wave function $|\psi\rangle$ is done by operator $X$ such that $X|\psi\rangle$ results observable $x$ with some probability. But what happens to $|\psi\rangle$? Does ...
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1answer
85 views

Quantum computing can be done via measurement alone, why is this significant?

I read in the Afterword section of Nielsen and Chuang's book Quantum Computation and Quantum Information that A second area of progress has been in understanding of what physical resources are ...
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1answer
44 views

How is measurement on system in a Hilbert space seen?

I am a bit confused about different kinds of measurements on a system in state $W$ where $W$ is the density operator in Hilbert space $H$. A general measurement can be given by POVM's, let ...
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4answers
676 views

Heisenberg uncertainty principle clarification

I found these two examples in a books which demonstrate Heiseberg's uncertainty relation: 1) It shows that when we try to locate a moving electron,we transferred momentum via the photon that we ...
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0answers
36 views

Quantum superposition of macroscopic objects [duplicate]

An extract from this article: Extrapolated to the scales of our everyday life quantum theory leads to situations such as the famous example of Schroedinger's cat: the cat is neither dead nor ...
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2answers
175 views

The relation between classical from quantum vs measurement problem

What is the relation between classical from quantum vs measurement problem. On the one hand they seem to be related on the other they seem to be of different nature. We always see our screens in ...
4
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1answer
274 views

Transition from one state to another in Quantum Mechanics

When we measure an electron's position we know that the wave function $\psi$ peaks at the measured position and the wave function as a function of momentum is a harmonic function. When it makes the ...
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2answers
124 views

Post mortem measurement of particle-wave duality

I was wondering what the outcome of this experiment would be: You shoot single photons at a double slit. On their way there you preform a measurement in $x$ so you get the time ($t_0$) the particle ...
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2answers
171 views

Doesn't Schrodinger's Cat depend on whether there's a quantum/classical boundary?

Everyone knows how Schrodinger's Cat is set up, so the question becomes whether there's a quantum/classical boundary and what that boundary is. Some people say everything is quantum while some may ...
5
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1answer
784 views

What is the difference between general measurement and projective measurement?

Nielsen and Chuang mention in Quantum Computation and Information that there are two kinds of measurement : general and projective ( and also POVM but that's not what I'm worried about ). General ...
0
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1answer
87 views

The double slit experiment - methods used to observe single photons prior to striking the target

I can accept that when single photons are used in the double slit experiment that a diffraction pattern results at the target due to their wave property. What I am puzzled about is exactly what ...
3
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2answers
135 views

How to account for the movement of stars during measurement of parallax

In my GCSE physics class today I was doing revision for my upcoming GCSEs, and we came to parallax measurements, as before my teacher explained that two measurements are taken six months apart as the ...
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1answer
248 views

How to physically prepare a qubit in a certain state?

I earlier asked the question about definition of a qubit. From it I understood that its the experimental setup that actually defines the qubit. But I don't get it's physical realization. How a qubit ...
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0answers
71 views

Why isn't everthing in a superposition state? [duplicate]

I know that a superposition is a state in which there are 50% and 50% (other % also) chances of two things to happen so why am I not in a superposition state? Example, I am at rest and i now choose to ...