We want to describe a system with a fixed number of particles. We model our system using the quspin package (https://quspin.github.io/QuSpin/index.html). The problem is, we havent found a way to fix the total number
of particles in our combined basis defined as “basis”:

Lferm=8
Nbosons=20

basis_fermion=spinless_fermion_basis_1d(L=Lferm, Nf=range(0,Lferm+1))

basis_boson = boson_basis_1d(L=1, sps=Nbosons+1, Nb=range(0,Nbosons+1))

basis = tensor_basis(basis_fermion, basis_boson)

While it is easy to find all combinations of fermionic & bosonic basis vector that have a total
particle number of f.e. 10, and extract all the parameters that identify these basis vectors (f.e. index,
integer representation etc.), we have not been able to redefine our basis using this information, i.e.
we cannot generate a new basis that is still of the “quspin basis type” containing only basis vectors with a fixed number of particles.

Does anybody know a solution/workaround to this problem WITHOUT using the user_basis function?

We have tried various things to convert the gathered information back to a “quspin.basis” object but none have worked.