= Quantum coherence in a specified basis
{title2=$\rho_{jk}\ne0$, $j\ne k$}
= Quantum coherence
{synonym}
For a chosen <orthonormal basis>, the off-diagonal entries of a <density operator> encode quantum coherence between distinct basis states. A diagonal density operator is an incoherent mixture in that basis. This property is basis-dependent: diagonalizing the operator does not imply that it lacked coherence in the measurement or energy basis of interest. Relative phases affect these entries, while a global wavefunction phase cancels from the density operator. Dephasing damps the entries and can spoil <stimulated Raman adiabatic passage> by removing the ground-state interference that maintains the dark state.
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