code: def evolve(self, rho: Density) -> Density:
          
          N = rho.qubit_nb
          qubits = rho.qubits
  
          indices = list([qubits.index(q) for q in self.qubits]) + \
              list([qubits.index(q) + N for q in self.qubits])
  
          tensor = bk.tensormul(self.tensor, rho.tensor, indices)
          return Density(tensor, qubits, rho.memory)
docs: Apply the action of this channel upon a density