class_docstr: Tests for integration with the PassthruQNode
docstr: Test that the density matrix can be differentiated
code: def test_density_matrix_differentiability(
      self, decorator, wires, tol
  ):
      """Test that the density matrix can be differentiated"""
      dev = qml.device("default.mixed", wires=wires)
  
      @qml.qnode(dev, diff_method="backprop", interface="jax")
      def circuit(a):
          qml.RY(a, wires=wires[0])
          qml.CNOT(wires=[wires[0], wires[1]])
          return qml.density_matrix(wires=wires[1])
  
      a = jnp.array(0.54)
  
      def cost(a):
          res = jnp.abs(circuit(a)) ** 2
          return res[1][1] - res[0][0]
  
      grad = decorator(jax.grad(cost))(a)
      expected = np.sin(a)
  
      assert np.allclose(grad, expected, atol=tol, rtol=0)
  
imports: from functools import partial
  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(decorator, wires, tol) and returns: qml, unknown