Fix num_qubits property on AsymetricDepolarizing Channel (quantumlib#…

…4652)

This is a breaking change!

It removes the `num_qubits` property and instead one should use the `num_qubits` method.

cirq-core/cirq/ops/common_channels.py

@@ -22,6 +22,7 @@
 from cirq import protocols, value
 from cirq.ops import raw_types, common_gates, pauli_gates, gate_features, identity
 
+
 if TYPE_CHECKING:
     import cirq
 
@@ -53,6 +54,10 @@ def __init__(
         (including identity). The input $\rho$ is the density matrix before the
         depolarization.
 
+        Note: prior to Cirq v0.14, this class contained `num_qubits` as a property.
+        This violates the contract of `cirq.Gate` so it was removed.  One can
+        instead get the number of qubits by calling the method `num_qubits`.
+
         Args:
             p_x: The probability that a Pauli X and no other gate occurs.
             p_y: The probability that a Pauli Y and no other gate occurs.
@@ -178,11 +183,6 @@ def p_z(self) -> float:
             raise ValueError('num_qubits should be 1')
         return self._error_probabilities.get('Z', 0.0)
 
-    @property
-    def num_qubits(self) -> int:
-        """The number of qubits"""
-        return self._num_qubits
-
     @property
     def error_probabilities(self) -> Dict[str, float]:
         """A dictionary from Pauli gates to probability"""
@@ -193,7 +193,7 @@ def _json_dict_(self) -> Dict[str, Any]:
 
     def _approx_eq_(self, other: Any, atol: float) -> bool:
         return (
-            self.num_qubits == other.num_qubits
+            self._num_qubits == other._num_qubits
             and np.isclose(self.p_i, other.p_i, atol=atol).item()
             and np.isclose(self.p_x, other.p_x, atol=atol).item()
             and np.isclose(self.p_y, other.p_y, atol=atol).item()

cirq-core/cirq/ops/common_channels_test.py

@@ -64,6 +64,8 @@ def test_asymmetric_depolarizing_channel():
     )
     assert cirq.has_kraus(d)
 
+    assert cirq.AsymmetricDepolarizingChannel(p_x=0, p_y=0.1, p_z=0).num_qubits() == 1
+
 
 def test_asymmetric_depolarizing_mixture():
     d = cirq.asymmetric_depolarize(0.1, 0.2, 0.3)
@@ -729,7 +731,7 @@ def test_default_asymmetric_depolarizing_channel():
     assert d.p_x == 0.0
     assert d.p_y == 0.0
     assert d.p_z == 0.0
-    assert d.num_qubits == 1
+    assert d.num_qubits() == 1
 
 
 def test_bad_error_probabilities_gate():