# Questions tagged [density-operator]

The density operator describes a quantum system in an (in general mixed) state.

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### Pure state vs mixed state in this example

Consider, I have a quantum state $|\Psi\rangle$, such that : $$|\Psi\rangle=c_1|\psi_1\rangle+c_2|\psi_2\rangle$$ This is defined as a pure state, since I have complete information about the system. ...
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### Master equation with a coherent bath

When we consider an oscillator $a$ acting with a bath of oscillators $b_i$ with the interaction Hamiltonian reads $$H_{int}=\sum_{i}g_ia b_i^{\dagger}+g_i^*a^{\dagger}b_i,$$ with the free Hamiltonian: ...
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### Variational principle of statistical mechanics

I was just reading an article called "Extractable work from ensembles of quantum batteries". in the third section of the article, it refers to 'variational principle of statistical ...
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### Compressing the Hilbert Space in Traditional DMRG

The traditional, non Matrix Product State, formulation of the Density Matrix Re-normalization Group (DMRG) algorithm can be coded in python. Such a code can be found in the following link: https://...
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### Quantum Mechanical Pictures and Operator Expansion

In Quantum Mechanics, there are 3 pictures: Schrodinger, Heisenberg and Dirac (Interaction) picture. In Schrodinger picture, the states evolve in time and operators are constant in time, and vice ...
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### Does a subsystem being mixed imply the state is entangled?

If a pure state, $\rho_{AB}$, has subsystems described by mixed density matrices, the overall state is entangled (as far as I understand). Can you conclude the same with an initially mixed bipartite ...
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### Concurrence between 2 qubits of tripartite system

If we consider a tripartite system (say of 3 qubits) described by density matrix $\rho_{ABC}$, does the concurrence $C(\rho_{BC})$ still accurately measure the entanglement between subsystems B and C? ...
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### Interpretation of the Bose-Einstein Condensate Wavefunction squared and Density Matrix

I'm currently trying to get some background information about the theoretical treatment of Bose-Einstein Condensate, and I'm reading through this paper on the topic by J. Rogel Salazar. I have also ...
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### Definition of density operator/ Matrix

In Sakurai's book, the density operator is defined as $$\rho\equiv \sum_i w_i|i\rangle \langle i|$$ where $w_i$ is statistical weight. Now, I'm reading a book by Parisi, In which it says in Sec 5.1, ...
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### The Density Matrix of a Pure State

It is my understanding that any pure quantum state $|\psi\rangle$ can be represented by the density matrix $|\psi\rangle\langle\psi|$. It is also my understanding that $|\psi\rangle\langle\psi|$ ...
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