Wigner Calculation unstable for the Fock backend

Photonic Software Strawberry Fields
3

Hi @Sebastian_Duque_Mesa! Eve is peeping :eyes: .

I am new and not familiar with this forum, and I hope I asked my question in the right chat.
Like @alicewithoutbob, I am experimenting with Strawberry Fields and have also noticed the same instability in the Wigner function calculation, even with a ‘reasonable’ amount of squeezing (around 6 dB).

I am using this definition for the squeezed state:

def Squeeze(r, cutoff):
    
    """This function aims to build a vacuum squeezed state in the Fock basis
    r      : Squeezing parameter
    """ 
    prog = sf.Program(1)
    eng = sf.Engine("fock", backend_options={"cutoff_dim": cutoff})
    with prog.context as q:
        S = Sgate(r)
        S | q[0]
    state = eng.run(prog).state
    print("State trace : {} ".format(state.trace()))
    return [state, format(state.trace())]

Compute a vacuum squeezed state with different cutoff, and calculate the Wigner function

ds = [150, 200, 250]

Ws = []
states = []
X = np.linspace(-15, 15, 401)
P = np.linspace(-15, 15, 401)

for i in ds:
    state = Squeeze(-0.7, i)
    states.append(state[0])
    Ws.append(state[0].wigner(mode=0, xvec=X, pvec=P))

The following plot is the wigner function I obtained :

6dB_subplot
6dB_subplot1000×300 15.2 KB

The issue is that at around a cutoff of 200, the calculation of the Wigner function for squeezed vacuum states computed with the Fock backend seems unstable, even with a reasonable amount of squeezing.

I reviewed the code used to calculate the Wigner function and did not find what causes these instabilities.

I need to increase the cutoff with the Fock backend for further studies, and I wonder if there is a way to correct this.
I also thought that the more the cutoff is increased, the more accurate on the calculation you are but it is not the case here and I was wondering why.

Thank you for your attention and your time and have a good day.