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Results tagged with density-operator
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user 28512
The density operator describes a quantum system in a (in general mixed) state.
2
votes
What is the actual meaning of the density operator?
The density operator of a quantum system summarises the expectation values of the observables of that system alone. If you have two systems $S_1$ and $S_2$ that are entangled with one another, then th …
- 10.2k
0
votes
In Everettian quantum mechanics, what does branching look like if the universe is in a non-u...
The debate about the interpretation of quantum theory is about what is happening in reality to produce the results of quantum mechanical experiments. Non-MWI interpretations either deny that there is …
- 10.2k
4
votes
Meaning of the Reduced Density Operator
The reduced density operator of A contains all of the information about that system you can get by measuring that system alone. It does not contain information about what systems A is entangled with. …
- 10.2k
15
votes
What is the Reduced Density Matrix?
The reduced density matrix can be found by taking the trace over the subspaces of the Hilbert space that represent systems you're not interested in. For the Bell state the density matrix of the whole …
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0
votes
How do we prove that POVMs are the most general measurements?
A measurement is an interaction that produces a record that can be copied as many times as you like. When you measure a system you often do this by letting it interact with a measuring instrument or t …
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0
votes
Understanding the density matrix of a qubit
The way to work out the decomposition is to set your density matrix $\rho$ to be equal to a linear combination of the Pauli matrices:
$$
\rho = \sum_{i=0}^3\alpha_i\sigma_i.
$$
where $\sigma_0=I$ and …
- 10.2k
2
votes
Accepted
What is the state of a single electron in an entanglement?
The reduced density matrix of one electron in an entangled pair includes all of the information you can get by measuring that electron without measuring the other electron and comparing the results. T …
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1
vote
The eigenstates of a single EPR particle
There is no non-trivial observable for which the reduced density matrix of one half of an EPR system is an eigenstate. Your candidate observable $|A\rangle\langle A| + |B\rangle\langle B|$ is just the …
- 10.2k
0
votes
What is Decoherence?
When information is copied out of a quantum system interference between different values of the monitored observable is suppressed. A macroscopic object, that is anything you can see with the naked ey …
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1
vote
States of entangled particles after no/partial/full measurement
The answer to this question involves what interpretation of quantum theory you're using so I'll describe the options.
The Copenhagen and statistical interpretations say that quantum theory doesn't des …
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