Linked Questions

88
votes
9answers
29k views

How is quantum superposition different from mixed state?

According to Wikipedia, if a system has $50\%$ chance to be in state $\left|\psi_1\right>$ and $50\%$ to be in state $\left|\psi_2\right>$, then this is a mixed state. Now, consider the state $...
37
votes
5answers
6k views

Density matrix formalism

The density matrix $\hat{\rho}$ is often introduced in textbooks as a mathematical convenience that allows us to describe quantum systems in which there is some level of missing information. $\hat{\...
12
votes
3answers
3k views

How is quantum mechanics consistent with statistical mechanics?

Let's say we have an harmonic oscillator (at Temperature $T$) in a superposition of state 1 and 2: $$\Psi = \frac{\phi_1+\phi_2}{\sqrt{2}}$$ where each $\phi_i$ has energy $E_i \, .$ The probability ...
4
votes
3answers
101 views

What is meant by “maximum amount of information is available” in quantum measurements?

I’m reading Isham’s Lectures on Quantum Theory, and in Chapter 5, General Formalism of Quantum Theory, Section 5.1.2, he states the following rule. Rule 1. The predictions of results of ...
4
votes
2answers
580 views

What is the qualitative difference between quantum superpostion and mixed states? [duplicate]

As I understand it, if one has a complete knowledge of the state of a quantum system (insofar as one knows the statistical distributions of all the observables associated with the state) then one can ...
2
votes
3answers
162 views

Does an asteroid superpose in space?

I'm learning the basics of quantum physics and I've read that all objects have a wave function, but the only reason nothing in the macro-world superposes is that they're constantly interacting with ...
5
votes
2answers
234 views

Interpretation of density matrix

In Landau’s Statistical Physics (part 1) , section 5, he writes:" In particular, it would be quite incorrect to suppose that the description by means of the density matrix signifies that the subsystem ...
4
votes
4answers
107 views

Is the collapse of a classical probability density a puzzle analogous to the wave function collapse puzzle in quantum mechanics?

Famously, the collapse of the wave function is considered one of the biggest puzzles of quantum mechanics and motivates people to take ideas like the many-worlds interpretation seriously. Something ...
2
votes
1answer
108 views

Alice sends random states in a channel, what Bob receives?

Suppose Alice prepares $\rho_x$ with probabilities $p_x$ and sends it to Bob. I would say this is the same thing as "Alice prepares $\rho = \sum_x p_x \rho_x$ and sends it to Bob", but Preskill's ...
1
vote
1answer
101 views

Is quantum theory useful to describe the whole cosmos? [closed]

We often say that QFT describes the nature on a fundamental level. However this is indeed a very complicated theory for which the calculations related to the interaction of just a few particles simply ...
0
votes
1answer
67 views

Do physicists accept true randomness in nature? [duplicate]

I am not a physicist but I've started studying the subject and noticed that terms like "random", "randomness", "randomly" are widely used when talking about nature. For example, random movement of ...