Generalize probability calculation

src/qibo/backends/abstract.py

@@ -203,6 +203,17 @@ def circuit_result_tensor(self, result): # pragma: no cover
         """
         raise_error(NotImplementedError)
 
+    @abc.abstractmethod
+    def circuit_result_probabilities(self, result, qubits=None):  # pragma: no cover
+        """Calculates measurement probabilities by tracing out qubits.
+
+        Args:
+            result (:class:`qibo.states.CircuitResult`): Result object that contains
+                the data required to represent the state.
+            qubits (list, set): Set of qubits that are measured.
+        """
+        raise_error(NotImplementedError)
+
     @abc.abstractmethod
     def calculate_symbolic(self, state, nqubits, decimals=5, cutoff=1e-10, max_terms=20): # pragma: no cover
         """Dirac representation of a state vector."""

src/qibo/backends/numpy.py

@@ -395,6 +395,16 @@ def circuit_result_representation(self, result):
     def circuit_result_tensor(self, result):
         return result.execution_result
 
+    def circuit_result_probabilities(self, result, qubits=None):
+        if qubits is None:  # pragma: no cover
+            qubits = result.circuit.measurement_gate.qubits
+
+        state = self.circuit_result_tensor(result)
+        if result.density_matrix:
+            return self.calculate_probabilities_density_matrix(state, qubits, result.nqubits)
+        else:
+            return self.calculate_probabilities(state, qubits, result.nqubits)
+
     def calculate_symbolic(self, state, nqubits, decimals=5, cutoff=1e-10, max_terms=20):
         state = self.to_numpy(state)
         terms = []

src/qibo/states.py

@@ -80,19 +80,10 @@ def __array__(self):
     def probabilities(self, qubits=None):
         """Calculates measurement probabilities by tracing out qubits.
 
-        Exactly one of the following arguments should be given.
-
         Args:
             qubits (list, set): Set of qubits that are measured.
         """
-        if qubits is None:  # pragma: no cover
-            qubits = self.circuit.measurement_gate.qubits
-
-        state = self.backend.circuit_result_tensor(self)
-        if self.density_matrix:
-            return self.backend.calculate_probabilities_density_matrix(state, qubits, self.nqubits)
-        else:
-            return self.backend.calculate_probabilities(state, qubits, self.nqubits)
+        return self.backend.circuit_result_probabilities(self, qubits)
 
     def samples(self, binary=True, registers=False):
         """Returns raw measurement samples.