Replace existing calls of SupportsChannel to SupportsKraus (#4438)

cirq-core/cirq/qis/channels.py

@@ -45,7 +45,7 @@ def kraus_to_superoperator(kraus_operators: Sequence[np.ndarray]) -> np.ndarray:
     return m
 
 
-def operation_to_choi(operation: 'protocols.SupportsChannel') -> np.ndarray:
+def operation_to_choi(operation: 'protocols.SupportsKraus') -> np.ndarray:
     r"""Returns the unique Choi matrix associated with an operation .
 
     Choi matrix J(E) of a linear map E: L(H1) -> L(H2) which takes linear operators
@@ -68,7 +68,7 @@ def operation_to_choi(operation: 'protocols.SupportsChannel') -> np.ndarray:
 
 
 @deprecated(deadline='v0.14', fix='use cirq.operation_to_superoperator instead')
-def operation_to_channel_matrix(operation: 'protocols.SupportsChannel') -> np.ndarray:
+def operation_to_channel_matrix(operation: 'protocols.SupportsKraus') -> np.ndarray:
     """Returns the matrix representation of an operation in standard basis.
 
     Let E: L(H1) -> L(H2) denote a linear map which takes linear operators on Hilbert space H1
@@ -85,7 +85,7 @@ def operation_to_channel_matrix(operation: 'protocols.SupportsChannel') -> np.nd
     return operation_to_superoperator(operation)
 
 
-def operation_to_superoperator(operation: 'protocols.SupportsChannel') -> np.ndarray:
+def operation_to_superoperator(operation: 'protocols.SupportsKraus') -> np.ndarray:
     """Returns the matrix representation of an operation in standard basis.
 
     Let E: L(H1) -> L(H2) denote a linear map which takes linear operators on Hilbert space H1

cirq-core/cirq/qis/channels_test.py

@@ -20,7 +20,7 @@
 import cirq
 
 
-def apply_channel(channel: cirq.SupportsChannel, rho: np.ndarray) -> np.ndarray:
+def apply_channel(channel: cirq.SupportsKraus, rho: np.ndarray) -> np.ndarray:
     ks = cirq.kraus(channel)
     d_out, d_in = ks[0].shape
     assert rho.shape == (d_in, d_in)
@@ -38,7 +38,7 @@ def generate_standard_operator_basis(d_out: int, d_in: int) -> Iterable[np.ndarr
             yield e_ij
 
 
-def compute_choi(channel: cirq.SupportsChannel) -> np.ndarray:
+def compute_choi(channel: cirq.SupportsKraus) -> np.ndarray:
     ks = cirq.kraus(channel)
     d_out, d_in = ks[0].shape
     d = d_in * d_out
@@ -48,7 +48,7 @@ def compute_choi(channel: cirq.SupportsChannel) -> np.ndarray:
     return c
 
 
-def compute_superoperator(channel: cirq.SupportsChannel) -> np.ndarray:
+def compute_superoperator(channel: cirq.SupportsKraus) -> np.ndarray:
     ks = cirq.kraus(channel)
     d_out, d_in = ks[0].shape
     m = np.zeros((d_out * d_out, d_in * d_in), dtype=np.complex128)

cirq-core/cirq/qis/measures.py

@@ -287,7 +287,7 @@ def von_neumann_entropy(
     return 0.0
 
 
-def entanglement_fidelity(operation: 'cirq.SupportsChannel') -> float:
+def entanglement_fidelity(operation: 'cirq.SupportsKraus') -> float:
     r"""Returns entanglement fidelity of a given quantum channel.
 
     Entanglement fidelity $F_e$ of a quantum channel $E: L(H) \to L(H)$ is the overlap between

cirq-core/cirq/sim/act_on_density_matrix_args.py

@@ -115,7 +115,9 @@ def _act_on_fallback_(
         raise TypeError(
             "Can't simulate operations that don't implement "
             "SupportsUnitary, SupportsConsistentApplyUnitary, "
-            "SupportsMixture, SupportsChannel or is a measurement: {!r}".format(action)
+            "SupportsMixture, SupportsChannel or SupportsKraus or is a measurement: {!r}".format(
+                action
+            )
         )
 
     def _perform_measurement(self, qubits: Sequence['cirq.Qid']) -> List[int]:

cirq-core/cirq/sim/density_matrix_simulator.py

@@ -48,7 +48,7 @@ class DensityMatrixSimulator(
         * measurements
         * a `_decompose_` that eventually yields one of the above
     That is, the circuit must have elements that follow on of the protocols:
-        * `cirq.SupportsChannel`
+        * `cirq.SupportsKraus`
         * `cirq.SupportsMixture`
         * `cirq.SupportsConsistentApplyUnitary`
         * `cirq.SupportsUnitary`

cirq-core/cirq/sim/sparse_simulator.py

@@ -56,7 +56,7 @@ class Simulator(
     protocol, the `cirq.SupportsMixture` protocol, or the
     `cirq.CompositeOperation` protocol. It is also permitted for the circuit
     to contain measurements which are operations that support
-    `cirq.SupportsChannel` and `cirq.SupportsMeasurementKey`
+    `cirq.SupportsKraus` and `cirq.SupportsMeasurementKey`
 
     This simulator supports four types of simulation.