Allow caching subcircuits

src/tequila/circuit/_gates_impl.py

@@ -72,7 +72,8 @@ def __init__(self, name, target: UnionList, control: UnionList = None, generator
         self._target = tuple(list_assignment(target))
         self._control = tuple(list_assignment(control))
         self.finalize()
-        self.generator = generator
+        if generator:
+            self.generator = generator
 
     def copy(self):
         return copy.deepcopy(self)

src/tequila/circuit/circuit.py

@@ -718,6 +718,9 @@ def map_variables(self, variables: dict, *args, **kwargs):
 
         return QCircuit(gates=new_gates)
 
+    def as_subcircuit_gate(self) -> QCircuit:
+        return QCircuit.wrap_gate(SubcircuitGate(self))
+
 
 class Moment(QCircuit):
     """
@@ -1060,3 +1063,20 @@ def find_unused_qubit(U0: QCircuit = None, U1: QCircuit = None) -> int:
     assert free_qubit not in active_qubits
 
     return free_qubit
+
+
+class SubcircuitGate(QGateImpl):
+    """
+    Wraps a QCircuit object. When this is added to another circuit,
+    the structure of this subcircuit is kept (instead of flattening
+    it to a list of basic gates). This allows reusing this subcircuit
+    and e.g. compilation results while only storing it once. For
+    example, this is useful when building QSVT circuits.
+    """
+
+    def __init__(self, circuit: QCircuit):
+        super().__init__(name="CachedSubcircuit", target=tuple(range(circuit.n_qubits)))
+        self.circuit = circuit
+
+    def dagger(self):
+        return SubcircuitGate(self.circuit.dagger())

src/tequila/circuit/compiler.py

@@ -2,7 +2,7 @@
 import warnings
 
 from tequila import TequilaException
-from tequila.circuit.circuit import QCircuit
+from tequila.circuit.circuit import QCircuit, SubcircuitGate
 from tequila.circuit.gates import Rx, Ry, H, X, S, Rz, ExpPauli, CNOT, Phase, T, Z, GlobalPhase
 from tequila.circuit._gates_impl import (
     RotationGateImpl,
@@ -318,7 +318,16 @@ def compile_circuit(self, abstract_circuit: QCircuit, variables=None, *args, **k
         # Keep a dicitionary of compiled pauli_rotations
         pauli_dict = {}
 
+        compiled_subcircuits = {}
+
         for idx, gate in gatelist:
+            if isinstance(gate, SubcircuitGate):
+                if id(gate.circuit) not in compiled_subcircuits:
+                    compiled_subcircuits[id(gate.circuit)] = self.compile_circuit(gate.circuit)
+                cg = compiled_subcircuits[id(gate.circuit)]
+                compiled_gates.append((idx, cg))
+                continue
+
             cg = gate
             controlled = gate.is_controlled()
 

src/tequila/circuit/gates.py

@@ -1,4 +1,4 @@
-from tequila.circuit.circuit import QCircuit
+from tequila.circuit.circuit import QCircuit, SubcircuitGate
 from tequila.objective.objective import Variable, assign_variable
 from tequila.circuit import _gates_impl as impl
 import typing

src/tequila/simulators/simulator_base.py

@@ -1,6 +1,6 @@
 from tequila.circuit._gates_impl import GlobalPhaseGateImpl
 from tequila.utils import TequilaException, to_float, TequilaWarning
-from tequila.circuit.circuit import QCircuit
+from tequila.circuit.circuit import QCircuit, SubcircuitGate
 from tequila.utils.keymap import KeyMapSubregisterToRegister
 from tequila.utils.misc import to_float
 from tequila.wavefunction.qubit_wavefunction import QubitWaveFunction
@@ -314,7 +314,9 @@ def create_circuit(self, abstract_circuit: QCircuit, circuit=None, *args, **kwar
             result = self.initialize_circuit(*args, **kwargs)
 
         for g in abstract_circuit.gates:
-            if g.is_parameterized():
+            if isinstance(g, SubcircuitGate):
+                self.create_circuit(g.circuit, result)
+            elif g.is_parameterized():
                 self.add_parametrized_gate(g, result, *args, **kwargs)
             else:
                 self.add_basic_gate(g, result, *args, **kwargs)