One-shot for noise in Python

pennylane_qrack/qrack_device.py

@@ -189,6 +189,8 @@ def __init__(self, wires=0, shots=None, **kwargs):
             self.isHostPointer = options['isHostPointer']
         if 'noise' in options:
             self.noise = options['noise']
+            if (self.noise != 0) and (shots is None):
+                raise ValueError("Shots must be finite for noisy simulation (not analytical mode).")
         super().__init__(wires=wires, shots=shots)
         self._state = QrackSimulator(
             self.num_wires,
@@ -200,6 +202,7 @@ def __init__(self, wires=0, shots=None, **kwargs):
             isHostPointer=self.isHostPointer,
             noise = self.noise
         )
+        self._circuit = []
 
     def _reverse_state(self):
         end = self.num_wires - 1
@@ -216,7 +219,14 @@ def apply(self, operations, **kwargs):
             operations (List[pennylane.Operation]): operations to be applied
         """
 
-        for i, op in enumerate(operations):
+        self._circuit = self._circuit + operations
+        if self.noise == 0:
+            self._apply()
+            self._circuit = []
+        # else: Defer application until shots or expectation values are requested
+
+    def _apply(self):
+        for op in self._circuit:
             if isinstance(op, QubitStateVector):
                 self._apply_qubit_state_vector(op)
             elif isinstance(op, BasisState):
@@ -647,20 +657,36 @@ def expval(self, observable, **kwargs):
         # estimate the ev
         return np.mean(self.sample(observable))
 
+
+    def _generate_sample(self):
+        rev_sample = self._state.m_all()
+        sample = 0
+        for i in range(self.num_wires):
+            if (rev_sample & (1 << i)) > 0:
+                sample |= 1 << (self.num_wires - (i + 1))
+        return sample
+
+
     def generate_samples(self):
         if self.shots is None:
             raise qml.QuantumFunctionError(
                 "The number of shots has to be explicitly set on the device "
                 "when using sample-based measurements."
             )
 
+        if self.noise != 0:
+            self._samples = []
+            for _ in range(self.shots):
+                self._state.reset_all()
+                self._apply()
+                self._samples.append(self._generate_sample())
+            self._samples = QubitDevice.states_to_binary(np.array([self._generate_sample()]), self.num_wires)
+            self._circuit = []
+
+            return self._samples
+
         if self.shots == 1:
-            rev_sample = self._state.m_all()
-            sample = 0
-            for i in range(self.num_wires):
-                if (rev_sample & (1 << i)) > 0:
-                    sample |= 1 << (self.num_wires - (i + 1))
-            self._samples = QubitDevice.states_to_binary(np.array([sample]), self.num_wires)
+            self._samples = QubitDevice.states_to_binary(np.array([self._generate_sample()]), self.num_wires)
 
             return self._samples