Floquet mesolve not working as expected

Hi,

It seems there are some errors in the Floquet mesolve-equivalent function, which are not seen when considering a simple qubit.

Consider for instance an harmonic oscillator with a drive, which should converge towards a coherent state:

%matplotlib notebook

import numpy as np
from qutip import (basis, destroy, expect, fsesolve, sigmax, sigmaz, rand_ket, num,
                   mesolve)
from qutip.floquet import (floquet_modes, floquet_modes_table,
                     floquet_modes_t_lookup, fmmesolve)

# Parameters
wc = 1.0 * 2 * np.pi
wp = 1.5 * 2 * np.pi
T = 2 * np.pi / wp
N = 20
epsilon_p = np.sqrt(2) * np.sqrt(8) * (wp - wc)

psi0 = basis(N,0)
a = destroy(N)
H = [
    wc * a.dag() * a,
    [1.0j * epsilon_p * (a.dag() - a), 'cos(wp * t)']
]

args = {
    'wp': wp
}

# noise power spectrum
gamma1 = 0.05
def noise_spectrum(omega):
    return 0.5 * gamma1 * omega / (2 * np.pi)

tlist = np.linspace(0, 5 / gamma1, 100)

# find the floquet modes for the time-dependent hamiltonian
f_modes_0, f_energies = floquet_modes(H, T, args)

# precalculate mode table
f_modes_table_t = floquet_modes_table(f_modes_0, f_energies,
                                      np.linspace(0, T, 500 + 1),
                                      H, T, args)

# solve the floquet-markov master equation
output = fmmesolve(H, psi0, tlist,
                   [a], [], [noise_spectrum], T, args)

# For reference: calculate the same thing with mesolve
output_ref = mesolve(H, psi0, tlist, [np.sqrt(gamma1) * a],
                     [], args)

output_ref.states[-1] is a coherent state as expected.

But output.states[-1] is far from being a coherent state.

I tried to debug it, without real success for now 😕

EDIT: Might be partially related to #624.

[Phyks]

Sure, I’ll try to keep my PR up to date with my latest checks.

For anyone who might be using qutip for Floquet simulations, would be cool to have some reviews on my PR 😃

[jakelishman]

See #1248 that implements the correct A. I’m not certain about the comment about c_ops - I’ll need to look into it more before we merge #1248.