I am running a circuit with two modes in which I prepare two logical 0 GKP states, using the GKP class with epsilon=0.125 on both modes, and have no gates acting on them.
I noticed that when I plot the Fock probability distribution of either mode using cutoffs larger than 40, I get a set of positive and negative probabilities, some of them with magnitudes larger than 1; which seem to be close to the cutoff independently of the cutoff I use. I attached figures to show this.
This behavior also shows up in circuits where I actually have gates acting on the modes. And I just noticed, that if I were to increase the epsilon value, this would start happening at greater cutoffs (observed with epsilon=0.15 at cutoffs larger than 45)
I am thinking that because of this, I can only trust simulations for GKP states up to a cutoff of 40 (when using an epsilon of 0.125); but nothing beyond that.
Would you think there is a way around this? Or is there some background knowledge to have in mind here?