In quantum mechanics, the propagatorK(xf,tf;xi,ti)
gives the probability amplitude that a particle
starting at xi at ti ends up at position xf at tf.
It is defined as follows:
K(xf,tf;xi,ti)≡⟨xf∣U^(tf,ti)∣xi⟩
Where U^≡exp(−itH^/ℏ) is the time-evolution operator.
The probability that a particle travels
from (xi,ti) to (xf,tf) is then given by:
P=K(xf,tf;xi,ti)2
Given a general (i.e. non-collapsed) initial state ψi(x)≡ψ(x,ti),
we must integrate over xi:
P=∫−∞∞K(xf,tf;xi,ti)ψi(xi)dxi2
And if the final state ψf(x)≡ψ(x,tf)
is not a basis vector either, then we integrate twice:
Given a ψi(x), the propagator can also be used
to find the full final wave function:
ψ(xf,tf)=∫−∞∞ψi(xi)K(xf,tf;xi,ti)dxi
Sometimes the name “propagator” is also used to refer to
the fundamental solutionG
of the time-dependent Schrödinger equation,
which is related to K by: