ensure round still works

qiskit/quantum_info/states/densitymatrix.py

@@ -480,11 +480,13 @@ def probabilities(self, qargs=None, decimals=None):
  probs = self._subsystem_probabilities(
  np.abs(self.data.diagonal()), self._op_shape.dims_l(), qargs=qargs
  )
- if decimals is not None:
- probs = probs.round(decimals=decimals)
 
  # to account for roundoff errors, we renormalize and clip
  probs = np.clip(probs / np.sum(probs), a_min=0, a_max=1)
+
+ if decimals is not None:
+ probs = probs.round(decimals=decimals)
+
  return probs
 
  def reset(self, qargs=None):

qiskit/quantum_info/states/statevector.py

@@ -575,11 +575,13 @@ def probabilities(self, qargs=None, decimals=None):
  probs = self._subsystem_probabilities(
  np.abs(self.data) ** 2, self._op_shape.dims_l(), qargs=qargs
  )
- if decimals is not None:
- probs = probs.round(decimals=decimals)
 
  # to account for roundoff errors, we renormalize and clip
  probs = np.clip(probs / np.sum(probs), a_min=0, a_max=1)
+
+ if decimals is not None:
+ probs = probs.round(decimals=decimals)
+
  return probs
 
  def reset(self, qargs=None):

test/python/quantum_info/states/test_densitymatrix.py

@@ -1209,6 +1209,18 @@ def test_clip_probabilities(self):
  self.assertTrue(np.allclose(dm.probabilities(), [1, 0], atol=0))
  self.assertDictAlmostEqual(dm.probabilities_dict(), {"0": 1, "1": 0}, delta=0)
 
+ def test_round_probabilities(self):
+ """Test probabilities are correctly rounded.
+
+ This is good to test to ensure clipping, renormalizing and rounding work together.
+ """
+ p = np.sqrt(1 / 3)
+ amplitudes = [p, p, p, 0]
+ dm = DensityMatrix(np.outer(amplitudes, amplitudes))
+ expected = [0.33, 0.33, 0.33, 0]
+
+ self.assertTrue(np.allclose(dm.probabilities(decimals=2), expected))
+
 
 if __name__ == "__main__":
  unittest.main()

test/python/quantum_info/states/test_statevector.py

@@ -1315,6 +1315,17 @@ def test_clip_probabilities(self):
  self.assertTrue(np.allclose(sv.probabilities(), [1, 0], atol=0))
  self.assertDictAlmostEqual(sv.probabilities_dict(), {"0": 1, "1": 0}, delta=0)
 
+ def test_round_probabilities(self):
+ """Test probabilities are correctly rounded.
+
+ This is good to test to ensure clipping, renormalizing and rounding work together.
+ """
+ p = np.sqrt(1 / 3)
+ sv = Statevector([p, p, p, 0])
+ expected = [0.33, 0.33, 0.33, 0]
+
+ self.assertTrue(np.allclose(sv.probabilities(decimals=2), expected))
+
 
 if __name__ == "__main__":
  unittest.main()