Add support for target.num_qubits=None in transpiler pipeline. Set nu…

…m_qubits in BasicSimulator to None to avoid circuits being transpiled to the simulator's max number of qubits, thus blowing up the size of the computed statevector.

Apply suggestions from Julien's code review

Co-authored-by: Julien Gacon <gaconju@gmail.com>

qiskit/providers/backend_compat.py

@@ -84,7 +84,7 @@ def convert_to_target(
         "switch_case": SwitchCaseOp,
     }
 
-    in_data = {"num_qubits": configuration.n_qubits}
+    in_data = {"num_qubits": getattr(configuration, "n_qubits", None)}
 
     # Parse global configuration properties
     if hasattr(configuration, "dt"):

qiskit/providers/basic_provider/basic_provider.py

@@ -11,7 +11,7 @@
 # that they have been altered from the originals.
 
 
-"""Provider for test simulator backends, formerly known as `BasicAer`."""
+"""Provider for basic simulator backends, formerly known as `BasicAer`."""
 
 from __future__ import annotations
 

qiskit/providers/basic_provider/basic_simulator.py

@@ -21,7 +21,7 @@
 
    BasicSimulator().run(run_input)
 
-Where the input is a ``QuantumCircuit`` object and the output is a ``BasicProviderJob object``,
+Where the input is a ``QuantumCircuit`` object and the output is a ``BasicProviderJob`` object,
 which can later be queried for the Result object. The result will contain a 'memory' data
 field, which is a result of measurements for each shot.
 """
@@ -62,10 +62,11 @@
 
 
 class BasicSimulator(BackendV2):
-    """Python implementation of a basic (slow, non-efficient) quantum simulator. This implementation
-    was originally based on the :class:`.BackendV1` interface, and later migrated to :class:`.BackendV2`.
-    However, certain legacy methods and properties from :class:`.BackendV1` have been kept for
-    backwards compatibility purposes, these are:
+    """Python implementation of a basic (slow, non-efficient) quantum simulator.
+
+    This implementation was originally based on the :class:`.BackendV1` interface, and later
+    migrated to :class:`.BackendV2`. However, certain legacy methods and properties from
+    :class:`.BackendV1` have been kept for backwards compatibility purposes, these are:
 
         #. :meth:`.BasicSimulator.configuration`
         #. :meth:`.BasicSimulator.properties`
@@ -148,9 +149,13 @@ def _build_basic_target(self) -> Target:
         Returns:
             Target: the configured target.
         """
+        # Set num_qubits to None so that the transpiler doesn't unnecessarily
+        # resize the circuit to fit a specific (potentially too large)
+        # number of qubits. The number of qubits in the circuits given to the
+        # `run` method will determine the size of the simulated statevector.
         target = Target(
             description="Basic Target",
-            num_qubits=min(24, self.MAX_QUBITS_MEMORY),
+            num_qubits=None,
         )
         basis_gates = ["h", "u", "p", "u1", "u2", "u3", "rz", "sx", "x", "cx", "id", "unitary"]
         required = ["measure", "delay", "reset"]
@@ -172,7 +177,7 @@ def configuration(self) -> BackendConfiguration:
         """Return the simulator backend configuration.
 
         Returns:
-            BackendConfiguration: the configuration for the backend.
+            The configuration for the backend.
         """
         # The ``target`` input argument overrides the ``configuration`` if provided:
         if self._configuration and not self._target:
@@ -737,7 +742,7 @@ def run_experiment(self, experiment: QasmQobjExperiment) -> dict[str, ...]:
     def _validate(self, qobj: QasmQobj) -> None:
         """Semantic validations of the qobj which cannot be done via schemas."""
         n_qubits = qobj.config.n_qubits
-        max_qubits = self.configuration().n_qubits
+        max_qubits = self.MAX_QUBITS_MEMORY
         if n_qubits > max_qubits:
             raise BasicProviderError(
                 f"Number of qubits {n_qubits} is greater than maximum ({max_qubits}) "

qiskit/transpiler/passes/layout/full_ancilla_allocation.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2017, 2019.
+# (C) Copyright IBM 2017, 2023.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -81,7 +81,7 @@ def run(self, dag):
 
         idle_physical_qubits = [q for q in layout_physical_qubits if q not in physical_bits]
 
-        if self.target:
+        if self.target and self.target.num_qubits is not None:
             idle_physical_qubits = [
                 q for q in range(self.target.num_qubits) if q not in physical_bits
             ]

qiskit/transpiler/passes/optimization/optimize_1q_decomposition.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2017, 2018.
+# (C) Copyright IBM 2017, 2023.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -94,7 +94,8 @@ def __init__(self, basis=None, target=None):
         self.error_map = self._build_error_map()
 
     def _build_error_map(self):
-        if self._target is not None:
+        # include path for when target exists but target.num_qubits is None (BasicSimulator)
+        if self._target is not None and self._target.num_qubits is not None:
             error_map = euler_one_qubit_decomposer.OneQubitGateErrorMap(self._target.num_qubits)
             for qubit in range(self._target.num_qubits):
                 gate_error = {}
@@ -118,7 +119,8 @@ def _resynthesize_run(self, matrix, qubit=None):
         When multiple synthesis options are available, it prefers the one with the lowest
         error when the circuit is applied to `qubit`.
         """
-        if self._target:
+        # include path for when target exists but target.num_qubits is None (BasicSimulator)
+        if self._target and self._target.num_qubits is not None:
             if qubit is not None:
                 qubits_tuple = (qubit,)
             else:
@@ -128,9 +130,9 @@ def _resynthesize_run(self, matrix, qubit=None):
             else:
                 available_1q_basis = set(self._target.operation_names_for_qargs(qubits_tuple))
                 decomposers = _possible_decomposers(available_1q_basis)
-                self._local_decomposers_cache[qubits_tuple] = decomposers
         else:
             decomposers = self._global_decomposers
+
         best_synth_circuit = euler_one_qubit_decomposer.unitary_to_gate_sequence(
             matrix,
             decomposers,

qiskit/transpiler/passes/synthesis/high_level_synthesis.py

@@ -216,7 +216,8 @@ def __init__(
 
         self._top_level_only = self._basis_gates is None and self._target is None
 
-        if not self._top_level_only and self._target is None:
+        # include path for when target exists but target.num_qubits is None (BasicSimulator)
+        if not self._top_level_only and (self._target is None or self._target.num_qubits is None):
             basic_insts = {"measure", "reset", "barrier", "snapshot", "delay"}
             self._device_insts = basic_insts | set(self._basis_gates)
 
@@ -314,12 +315,13 @@ def _recursively_handle_op(
         controlled_gate_open_ctrl = isinstance(op, ControlledGate) and op._open_ctrl
         if not controlled_gate_open_ctrl:
             qargs = tuple(qubits) if qubits is not None else None
+            # include path for when target exists but target.num_qubits is None (BasicSimulator)
             inst_supported = (
                 self._target.instruction_supported(
                     operation_name=op.name,
                     qargs=qargs,
                 )
-                if self._target is not None
+                if self._target is not None and self._target.num_qubits is not None
                 else op.name in self._device_insts
             )
             if inst_supported or (self._equiv_lib is not None and self._equiv_lib.has_entry(op)):

qiskit/transpiler/target.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2021
+# (C) Copyright IBM 2021, 2023.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -702,6 +702,9 @@ def operation_names_for_qargs(self, qargs):
         Raises:
             KeyError: If ``qargs`` is not in target
         """
+        # if num_qubits == 0, we will return globally defined operations
+        if self.num_qubits == 0 or self.num_qubits is None:
+            qargs = None
         if qargs is not None and any(x not in range(0, self.num_qubits) for x in qargs):
             raise KeyError(f"{qargs} not in target.")
         res = self._qarg_gate_map.get(qargs, set())
@@ -784,6 +787,9 @@ def check_obj_params(parameters, obj):
                     return False
             return True
 
+        # Handle case where num_qubits is None by always checking globally supported operations
+        if self.num_qubits is None:
+            qargs = None
         # Case a list if passed in by mistake
         if qargs is not None:
             qargs = tuple(qargs)
@@ -1502,7 +1508,9 @@ def target_to_backend_properties(target: Target):
                         }
                     )
         else:
-            qubit_props: dict[int, Any] = {x: None for x in range(target.num_qubits)}
+            qubit_props: dict[int, Any] = {}
+            if target.num_qubits:
+                qubit_props = {x: None for x in range(target.num_qubits)}
             for qargs, props in qargs_list.items():
                 if qargs is None:
                     continue