Time dependant collapse operator in mesolve

[Pascal Obled]

Hi

I would like to solve for a time dependant collapse operator in mesolve with a time dependence that cannot be separated from the operator. The form is a complex exponential depending on the time and the number operator like this: e^( -i t N ). It seems mesolve will only accept operators where the time dependence is a scalar and not a matrix. Is it possible to do this?

[Simon Cross]

Hi Pascal,

This is indeed somewhat difficult to achieve in QuTiP 4. In the
upcoming QuTiP 5, QobjEvo supports taking a function f(t) --> Qobj
directly, but that is perhaps not immediately helpful to you.

In your case though, there is a semi straight forward approach. The
number operator N is diagonal, so you can easily calculate e^(-itN)
yourself and then write the resulting time-dependent matrix as a sum
over the projectors.

For example, if your Hilbert space is dimension 4, you could write:

e_itN = [
[ qutip.basis(4, n).proj(), f"exp(-1j * {n} * t)" ]
for n in range(4)
]
q = qutip.QobjEvo(e_itN)

Hopefully that helps!

Just because I'm curious, where does this collapse operator arise?

Regards,
Simon