Minor fixes for new adder and multiplier gates classes (#13530)

* Fixes to AdderGate classes

Added define method to HalfAdder, FullAdder, and ModularAdder
classes to allow constructing Operators from quantum circuits
containing such adder gates.

Fixing plugins related to adder_ripple_v95, the plugins can be used
only if n-1 clean ancillas are available.

Improved the default adder plugins to choose the best decomposition
based on the number of state qubits and the number of ancilla qubits.

* Adding tests for the case that adder plugins do not apply

* Constructing Operators from circuits with MultiplierGates

* release notes

* docstring fix

* apply suggestions from code review

* futher improving tests based on additional review comments

pyproject.toml

@@ -109,7 +109,7 @@ sk = "qiskit.transpiler.passes.synthesis.solovay_kitaev_synthesis:SolovayKitaevS
 "HalfAdder.ripple_c04" = "qiskit.transpiler.passes.synthesis.hls_plugins:HalfAdderSynthesisC04"
 "HalfAdder.ripple_v95" = "qiskit.transpiler.passes.synthesis.hls_plugins:HalfAdderSynthesisV95"
 "HalfAdder.qft_d00" = "qiskit.transpiler.passes.synthesis.hls_plugins:HalfAdderSynthesisD00"
-"FullAdder.default" = "qiskit.transpiler.passes.synthesis.hls_plugins:FullAdderSynthesisC04"
+"FullAdder.default" = "qiskit.transpiler.passes.synthesis.hls_plugins:FullAdderSynthesisDefault"
 "FullAdder.ripple_c04" = "qiskit.transpiler.passes.synthesis.hls_plugins:FullAdderSynthesisC04"
 "FullAdder.ripple_v95" = "qiskit.transpiler.passes.synthesis.hls_plugins:FullAdderSynthesisV95"
 "Multiplier.default" = "qiskit.transpiler.passes.synthesis.hls_plugins:MultiplierSynthesisR17"

qiskit/circuit/library/arithmetic/adders/adder.py

@@ -116,6 +116,15 @@ def num_state_qubits(self) -> int:
         """
         return self._num_state_qubits
 
+    def _define(self):
+        """Populates self.definition with some decomposition of this gate."""
+        from qiskit.synthesis.arithmetic import adder_qft_d00
+
+        # This particular decomposition does not use any ancilla qubits.
+        # Note that the transpiler may choose a different decomposition
+        # based on the number of ancilla qubits available.
+        self.definition = adder_qft_d00(self.num_state_qubits, kind="half")
+
 
 class ModularAdderGate(Gate):
     r"""Compute the sum modulo :math:`2^n` of two :math:`n`-sized qubit registers.
@@ -162,6 +171,15 @@ def num_state_qubits(self) -> int:
         """
         return self._num_state_qubits
 
+    def _define(self):
+        """Populates self.definition with some decomposition of this gate."""
+        from qiskit.synthesis.arithmetic import adder_qft_d00
+
+        # This particular decomposition does not use any ancilla qubits.
+        # Note that the transpiler may choose a different decomposition
+        # based on the number of ancilla qubits available.
+        self.definition = adder_qft_d00(self.num_state_qubits, kind="fixed")
+
 
 class FullAdderGate(Gate):
     r"""Compute the sum of two :math:`n`-sized qubit registers, including carry-in and -out bits.
@@ -208,3 +226,10 @@ def num_state_qubits(self) -> int:
             The number of state qubits.
         """
         return self._num_state_qubits
+
+    def _define(self):
+        """Populates self.definition with a decomposition of this gate."""
+        from qiskit.synthesis.arithmetic import adder_ripple_c04
+
+        # In the case of a full adder, this method does not use any ancilla qubits
+        self.definition = adder_ripple_c04(self.num_state_qubits, kind="full")

qiskit/circuit/library/arithmetic/multipliers/multiplier.py

@@ -190,3 +190,12 @@ def num_result_qubits(self) -> int:
             The number of result qubits.
         """
         return self._num_result_qubits
+
+    def _define(self):
+        """Populates self.definition with some decomposition of this gate."""
+        from qiskit.synthesis.arithmetic import multiplier_qft_r17
+
+        # This particular decomposition does not use any ancilla qubits.
+        # Note that the transpiler may choose a different decomposition
+        # based on the number of ancilla qubits available.
+        self.definition = multiplier_qft_r17(self.num_state_qubits)

qiskit/transpiler/passes/synthesis/hls_plugins.py

@@ -300,13 +300,18 @@
       - :class:`.ModularAdderSynthesisD00`
       - 0
       - a QFT-based adder
+    * - ``"default"``
+      - :class:`~.ModularAdderSynthesisDefault`
+      - any
+      - chooses the best algorithm based on the ancillas available
 
 .. autosummary::
    :toctree: ../stubs/
 
    ModularAdderSynthesisC04
    ModularAdderSynthesisD00
    ModularAdderSynthesisV95
+   ModularAdderSynthesisDefault
 
 Half Adder Synthesis
 ''''''''''''''''''''
@@ -330,13 +335,18 @@
       - :class:`.HalfAdderSynthesisD00`
       - 0
       - a QFT-based adder
+    * - ``"default"``
+      - :class:`~.HalfAdderSynthesisDefault`
+      - any
+      - chooses the best algorithm based on the ancillas available
 
 .. autosummary::
    :toctree: ../stubs/
 
    HalfAdderSynthesisC04
    HalfAdderSynthesisD00
    HalfAdderSynthesisV95
+   HalfAdderSynthesisDefault
 
 Full Adder Synthesis
 ''''''''''''''''''''
@@ -356,12 +366,17 @@
       - :class:`.FullAdderSynthesisV95`
       - :math:`n-1`, for :math:`n`-bit numbers
       - a ripple-carry adder
+    * - ``"default"``
+      - :class:`~.FullAdderSynthesisDefault`
+      - any
+      - chooses the best algorithm based on the ancillas available
 
 .. autosummary::
    :toctree: ../stubs/
 
    FullAdderSynthesisC04
    FullAdderSynthesisV95
+   FullAdderSynthesisDefault
 
 
 Multiplier Synthesis
@@ -1212,10 +1227,26 @@ def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **
         if not isinstance(high_level_object, ModularAdderGate):
             return None
 
-        if options.get("num_clean_ancillas", 0) >= 1:
-            return adder_ripple_c04(high_level_object.num_state_qubits, kind="fixed")
+        # For up to 5 qubits, the QFT-based adder is best
+        if high_level_object.num_state_qubits <= 5:
+            decomposition = ModularAdderSynthesisD00().run(
+                high_level_object, coupling_map, target, qubits, **options
+            )
+            if decomposition is not None:
+                return decomposition
 
-        return adder_qft_d00(high_level_object.num_state_qubits, kind="fixed")
+        # Otherwise, the following decomposition is best (if there are enough ancillas)
+        if (
+            decomposition := ModularAdderSynthesisC04().run(
+                high_level_object, coupling_map, target, qubits, **options
+            )
+        ) is not None:
+            return decomposition
+
+        # Otherwise, use the QFT-adder again
+        return ModularAdderSynthesisD00().run(
+            high_level_object, coupling_map, target, qubits, **options
+        )
 
 
 class ModularAdderSynthesisC04(HighLevelSynthesisPlugin):
@@ -1264,8 +1295,8 @@ def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **
 
         num_state_qubits = high_level_object.num_state_qubits
 
-        # for more than 1 state qubit, we need an ancilla
-        if num_state_qubits > 1 > options.get("num_clean_ancillas", 1):
+        # The synthesis method needs n-1 clean ancilla qubits
+        if num_state_qubits - 1 > options.get("num_clean_ancillas", 0):
             return None
 
         return adder_ripple_v95(num_state_qubits, kind="fixed")
@@ -1309,10 +1340,26 @@ def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **
         if not isinstance(high_level_object, HalfAdderGate):
             return None
 
-        if options.get("num_clean_ancillas", 0) >= 1:
-            return adder_ripple_c04(high_level_object.num_state_qubits, kind="half")
+        # For up to 3 qubits, ripple_v95 is better (if there are enough ancilla qubits)
+        if high_level_object.num_state_qubits <= 3:
+            decomposition = HalfAdderSynthesisV95().run(
+                high_level_object, coupling_map, target, qubits, **options
+            )
+            if decomposition is not None:
+                return decomposition
 
-        return adder_qft_d00(high_level_object.num_state_qubits, kind="half")
+        # The next best option is to use ripple_c04 (if there are enough ancilla qubits)
+        if (
+            decomposition := HalfAdderSynthesisC04().run(
+                high_level_object, coupling_map, target, qubits, **options
+            )
+        ) is not None:
+            return decomposition
+
+        # The QFT-based adder does not require ancilla qubits and should always succeed
+        return HalfAdderSynthesisD00().run(
+            high_level_object, coupling_map, target, qubits, **options
+        )
 
 
 class HalfAdderSynthesisC04(HighLevelSynthesisPlugin):
@@ -1360,8 +1407,8 @@ def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **
 
         num_state_qubits = high_level_object.num_state_qubits
 
-        # for more than 1 state qubit, we need an ancilla
-        if num_state_qubits > 1 > options.get("num_clean_ancillas", 1):
+        # The synthesis method needs n-1 clean ancilla qubits
+        if num_state_qubits - 1 > options.get("num_clean_ancillas", 0):
             return None
 
         return adder_ripple_v95(num_state_qubits, kind="half")
@@ -1381,18 +1428,38 @@ def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **
         return adder_qft_d00(high_level_object.num_state_qubits, kind="half")
 
 
-class FullAdderSynthesisC04(HighLevelSynthesisPlugin):
+class FullAdderSynthesisDefault(HighLevelSynthesisPlugin):
     """A ripple-carry adder with a carry-in and a carry-out bit.
 
-    This plugin name is:``FullAdder.ripple_c04`` which can be used as the key on
+    This plugin name is:``FullAdder.default`` which can be used as the key on
     an :class:`~.HLSConfig` object to use this method with :class:`~.HighLevelSynthesis`.
+    """
 
-    This plugin requires at least one clean auxiliary qubit.
+    def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **options):
+        if not isinstance(high_level_object, FullAdderGate):
+            return None
 
-    The plugin supports the following plugin-specific options:
+        # FullAdderSynthesisC04 requires no ancilla qubits and returns better results
+        # than FullAdderSynthesisV95 in all cases except for n=1.
+        if high_level_object.num_state_qubits == 1:
+            decomposition = FullAdderSynthesisV95().run(
+                high_level_object, coupling_map, target, qubits, **options
+            )
+            if decomposition is not None:
+                return decomposition
+
+        return FullAdderSynthesisC04().run(
+            high_level_object, coupling_map, target, qubits, **options
+        )
 
-    * ``num_clean_ancillas``: The number of clean auxiliary qubits available.
 
+class FullAdderSynthesisC04(HighLevelSynthesisPlugin):
+    """A ripple-carry adder with a carry-in and a carry-out bit.
+
+    This plugin name is:``FullAdder.ripple_c04`` which can be used as the key on
+    an :class:`~.HLSConfig` object to use this method with :class:`~.HighLevelSynthesis`.
+
+    This plugin requires no auxiliary qubits.
     """
 
     def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **options):
@@ -1409,7 +1476,7 @@ class FullAdderSynthesisV95(HighLevelSynthesisPlugin):
     an :class:`~.HLSConfig` object to use this method with :class:`~.HighLevelSynthesis`.
 
     For an adder on 2 registers with :math:`n` qubits each, this plugin requires at
-    least :math:`n-1` clean auxiliary qubit.
+    least :math:`n-1` clean auxiliary qubits.
 
     The plugin supports the following plugin-specific options:
 
@@ -1422,8 +1489,8 @@ def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **
 
         num_state_qubits = high_level_object.num_state_qubits
 
-        # for more than 1 state qubit, we need an ancilla
-        if num_state_qubits > 1 > options.get("num_clean_ancillas", 1):
+        # The synthesis method needs n-1 clean ancilla qubits
+        if num_state_qubits - 1 > options.get("num_clean_ancillas", 0):
             return None
 
         return adder_ripple_v95(num_state_qubits, kind="full")

releasenotes/notes/fix-adder-gates-39cf3d5f683e8880.yaml

@@ -0,0 +1,18 @@
+---
+fixes:
+  - |
+    Added default definitions for :class:`.FullAdderGate`, :class:`.HalfAdderGate`,
+    :class:`.ModularAdderGate` and :class:`.MultiplierGate` gates, allowing to 
+    contruct :class:`.Operator`\s from quantum circuits containing these gates.
+  - |
+    Fixed the number of clean ancilla qubits required by
+    :class:`.FullAdderSynthesisV95`, :class:`.HalfAdderSynthesisV95`, and
+    :class:`.ModularAdderSynthesisV95` plugins.
+  - |
+    Added missing :class:`.FullAdderSynthesisDefault` plugin that chooses the best
+    decomposition for :class:`.FullAdderGate` based on the number of clean ancilla qubits
+    available.
+  - |
+    Fixed :class:`.HalfAdderSynthesisDefault` and :class:`.ModularAdderSynthesisDefault`
+    plugins, for :class:`.HalfAdderGate` and :class:`.ModularAdderGate` respectively,
+    to choose the best decomposition based on the number of clean ancilla qubits available.