class_docstr: Tests for integration with the PassthruQNode
class_docstr_tok: ["Tests","for","integration","with","the","PassthruQNode"]
docstr: Test that the density matrix can be differentiated
docstr_tok: ["Test","that","the","density","matrix","can","be","differentiated"]
code: def test_density_matrix_differentiability(self, wires, tol):
      """Test that the density matrix can be differentiated"""
      dev = qml.device("default.mixed", wires=wires)
  
      @qml.qnode(dev, diff_method="backprop", interface="torch")
      def circuit(a):
          qml.RY(a, wires=wires[0])
          qml.CNOT(wires=[wires[0], wires[1]])
          return qml.density_matrix(wires=wires[1])
  
      a = torch.tensor(0.54, dtype=torch.float64, requires_grad=True)
  
      state = circuit(a)
      res = torch.abs(state) ** 2
      res = res[1][1] - res[0][0]
      res.backward()
  
      expected = torch.sin(a)
      assert torch.allclose(a.grad, expected, atol=tol, rtol=0)
  
imports: import pytest
  import numpy
  import pennylane
  from pennylane import numpy
  from pennylane.devices.default_mixed import DefaultMixed
label_desc: Use image node_id 6 for calling the TestPassthruIntegration obj's underlying member method code with example usage: obj.test_density_matrix_differentiability(wires, tol) and returns: qml