shadow to use initial state for faster simulation

qadence/measurements/shadow.py

@@ -6,7 +6,7 @@
 
 from qadence.backend import Backend
 from qadence.backends.pyqtorch import Backend as PyQBackend
-from qadence.blocks import AbstractBlock, KronBlock, chain, kron
+from qadence.blocks import AbstractBlock, KronBlock, kron
 from qadence.blocks.block_to_tensor import HMAT, IMAT, SDAGMAT
 from qadence.blocks.composite import CompositeBlock
 from qadence.blocks.primitive import PrimitiveBlock
@@ -16,7 +16,7 @@
 from qadence.measurements.utils import get_qubit_indices_for_op
 from qadence.noise import NoiseHandler
 from qadence.operations import H, I, SDagger, X, Y, Z
-from qadence.types import Endianness
+from qadence.types import BackendName, Endianness
 
 pauli_gates = [X, Y, Z]
 pauli_rotations = [
@@ -134,20 +134,38 @@ def classical_shadow(
     shadow: list = list()
     all_rotations = extract_operators(unitary_ids, circuit.n_qubits)
 
+    initial_state = state
+    backend_name = backend.name if hasattr(backend, "name") else backend.backend.name
+    if backend_name == BackendName.PYQTORCH:
+        # run the initial circuit without rotations
+        # to save computation time
+        conv_circ = backend.circuit(circuit)
+        initial_state = backend.run(
+            circuit=conv_circ,
+            param_values=param_values,
+            state=state,
+            endianness=endianness,
+        )
+        all_rotations = [
+            QuantumCircuit(circuit.n_qubits, rots) if rots else QuantumCircuit(circuit.n_qubits)
+            for rots in all_rotations
+        ]
+    else:
+        all_rotations = [
+            QuantumCircuit(circuit.n_qubits, circuit.block, rots)
+            if rots
+            else QuantumCircuit(circuit.n_qubits, circuit.block)
+            for rots in all_rotations
+        ]
+
     for i in range(shadow_size):
-        if all_rotations[i]:
-            rotated_circuit = QuantumCircuit(
-                circuit.register, chain(circuit.block, all_rotations[i])
-            )
-        else:
-            rotated_circuit = circuit
         # Reverse endianness to get sample bitstrings in ILO.
-        conv_circ = backend.circuit(rotated_circuit)
+        conv_circ = backend.circuit(all_rotations[i])
         batch_samples = backend.sample(
             circuit=conv_circ,
             param_values=param_values,
             n_shots=1,
-            state=state,
+            state=initial_state,
             noise=noise,
             endianness=endianness,
         )