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In quantum mechanics, Kossakowski–Lindblad equation (after Andrzej Kossakowski and Göran Lindblad) or master equation in Lindblad form is the most general type of Markovian and time-homogeneous master equation describing non-unitary evolution of the density matrix that is trace-preserving and completely positive for any initial condition.
Lindblad master equation for an -dimensional system's reduced density matrix can be written:
where is a (Hermitian) Hamiltonian part, the are an arbitrary orthonormal basis of the operators on the system's Hilbert space, and the are constants which determine the dynamics. The coefficient matrix must be positive to ensure that the equation is trace-preserving and completely positive. The summation only runs to because we have taken to be proportional to the identity operator, in which case the summand vanishes. Our convention implies that the are traceless for . The terms in the summation where can be described in terms of the Lindblad superoperator, .
If the terms are all zero, then this is quantum Liouville equation (for a closed system), which is the quantum analog of the classical Liouville equation. A related equation describes the time evolution of the expectation values of observables, it is given by the Ehrenfest theorem.
Note that is not necessarily equal to the self-Hamiltonian of the system. It may also incorporate effective unitary dynamics arising from the system-environment interaction.
where the eigenvalues are non-negative. If we define another orthonormal operator basis
we can rewrite Lindblad equation in diagonal form
This equation is invariant under a unitary transformation of Lindblad operators and constants,
and also under the inhomogeneous transformation
However, the first transformation destroys the orthonormality of the operators (unless all the are equal) and the second transformation destroys the tracelessness. Therefore, up to degeneracies among the , the of the diagonal form of the Lindblad equation are uniquely determined by the dynamics so long as we require them to be orthonormal and traceless.
Harmonic oscillator example
The most common Lindblad equation is that describing the damping of a quantum harmonic oscillator, it has , , , with all others . Here is the mean number of excitations in the reservoir damping the oscillator and is the decay rate. Additional Lindblad operators can be included to model various forms of dephasing and vibrational relaxation. These methods have been incorporated into grid-based density matrix propagation methods.
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