Some optimizations for the clifford backend

src/qibo/backends/_clifford_operations.py

@@ -3,6 +3,8 @@
 import numpy as np
 from scipy import sparse
 
+from qibo.config import raise_error
+
 name = "numpy"
 
 
@@ -424,7 +426,7 @@ def _determined_outcome(state, q, nqubits):
     return state, state[-1, -1]
 
 
-def _random_outcome(state, p, q, nqubits):
+def _random_outcome(state, p, q, nqubits, outcome):
     """Extracts the outcome for a measurement in case it is random."""
     p = p[0] + nqubits
     state[p, q] = 0
@@ -439,7 +441,7 @@ def _random_outcome(state, p, q, nqubits):
             False,
         )
     state[p - nqubits, :] = state[p, :]
-    outcome = np.random.randint(2, size=1).item()
+    # outcome = np.random.randint(2, size=1).item()
     state[p, :] = 0
     state[p, -1] = outcome
     state[p, nqubits + q] = 1
@@ -494,25 +496,46 @@ def _init_state_for_measurements(state, nqubits, collapse):
     return state.copy()
 
 
-# valid for a standard basis measurement only
-def M(state, qubits, nqubits, collapse=False):
+def _sample_random_outcomes(n_samples):
+    return np.random.randint(2, size=n_samples).tolist()
+
+
+def _M(state, qubits, nqubits, collapse=False):
     sample = []
     state = _init_state_for_measurements(state, nqubits, collapse)
-    # TODO: parallelize this and get rid of the loop
+    possible_outcomes = []
     for q in qubits:
         p = state[nqubits:-1, q].nonzero()[0]
         # random outcome, affects the state
         if len(p) > 0:
-            state, outcome = _random_outcome(state, p, q, nqubits)
+            state, _ = _random_outcome(state, p, q, nqubits, 0)
+            outcome = None
         # determined outcome, state unchanged
         else:
             _, outcome = _determined_outcome(state, q, nqubits)
+            outcome = int(outcome)
         sample.append(outcome)
     if collapse:
         state = _packbits(state, axis=0)
     return sample
 
 
+# valid for a standard basis measurement only
+def M(state, qubits, nqubits, collapse=False, nshots=1):
+    if collapse and nshots != 1:
+        raise_error(
+            RuntimeError,
+            "Cannot generate multiple shots with a collapsing measurement.",
+        )
+    sample = _M(state, qubits, nqubits, collapse)
+    samples = [
+        (_sample_random_outcomes(nshots) if outcome is None else nshots * [outcome])
+        for outcome in sample
+    ]
+    samples = list(zip(*samples))
+    return samples
+
+
 def cast(x, dtype=None, copy: bool = False):
     if dtype is None:
         dtype = "complex128"

src/qibo/backends/clifford.py

@@ -410,14 +410,6 @@ def execute_circuit(  # pylint: disable=R1710
         if self.platform == "stim":
             return self._execute_circuit_stim(circuit, initial_state, nshots)
 
-        for gate in circuit.queue:
-            if (
-                not gate.clifford
-                and not gate.__class__.__name__ == "M"
-                and not isinstance(gate, gates.PauliNoiseChannel)
-            ):
-                raise_error(RuntimeError, "Circuit contains non-Clifford gates.")
-
         if circuit.repeated_execution and nshots != 1:
             return self.execute_circuit_repeated(circuit, nshots, initial_state)
 
@@ -531,10 +523,10 @@ def sample_shots(
             qubits = tuple(qubits)
 
         if collapse:
-            samples = [self.engine.M(state, qubits, nqubits) for _ in range(nshots - 1)]
-            samples.append(self.engine.M(state, qubits, nqubits, collapse))
+            samples = self.engine.M(state, qubits, nqubits, nshots=nshots - 1)
+            samples.append(self.engine.M(state, qubits, nqubits, collapse, nshots=1))
         else:
-            samples = [self.engine.M(state, qubits, nqubits) for _ in range(nshots)]
+            samples = self.engine.M(state, qubits, nqubits, nshots=nshots)
         return self.engine.cast(samples, dtype=int)
 
     def symplectic_matrix_to_generators(

tests/test_backends_clifford.py

@@ -161,7 +161,8 @@ def test_two_qubits_gates(backend, gate):
     [
         15,
         25,
-    ],
+    ]
+    + list(range(100)),
 )
 def test_random_clifford_circuit(backend, prob_qubits, binary, seed):
     np.random.seed(seed)
@@ -287,7 +288,7 @@ def test_non_clifford_error(backend):
     clifford_bkd = construct_clifford_backend(backend)
     c = Circuit(1)
     c.add(gates.T(0))
-    with pytest.raises(RuntimeError) as excinfo:
+    with pytest.raises(AttributeError) as excinfo:
         clifford_bkd.execute_circuit(c)
         assert str(excinfo.value) == "Circuit contains non-Clifford gates."