# Tag Info

Accepted

### Density matrices for pure states and mixed states

Motivation The motivation behind density matrices is to represent a lack of knowledge about the state of a given quantum system, encapsulating within a single description of this system all the ...
• 47.1k
Accepted

Accepted

• 3,902
Accepted

### What is the superop simulator in Qiskit for?

If you have an ideal quantum circuit, you can easily get its superoperator representation using qiskit.quantum_info.SuperOp as follows, ...
• 4,648
### How to find the distance between a given $\rho$ and the nearest pure state(s)?
Recall that for any Hermitian operator $A$ and any unit vector $|\psi\rangle$ the real number $\langle \psi|A|\psi\rangle$, known as the Rayleigh quotient, is bounded by the largest eigenvalue \$\...