⚡ Speed up HRS and graph-state construction (#1119)

* perf: reuse HRS gates and build graph states from edges

* docs: link construction improvements to PR 1119

---------

Signed-off-by: simon1hofmann <119581649+simon1hofmann@users.noreply.github.com>

Author: simon1hofmann

CHANGELOG.md

@@ -36,6 +36,8 @@ that minor releases may include breaking changes.
 
 ### Changed
 
+- ⚡ Reuse the controlled adder in HRS multipliers and build graph-state gates
+  directly from sparse edges ([#1119]) ([**@simon1hofmann**])
 - ⚡ Test all Python versions on Linux and Python 3.15 on other platforms for
   ready pull requests. Keep the full matrix on main, merge queues, and manual
   runs ([#1112]) ([**@simon1hofmann**])
@@ -282,6 +284,7 @@ _📚 Refer to the [GitHub Release Notes] for previous changelogs._
 
 <!-- PR links -->
 
+[#1119]: https://github.com/munich-quantum-toolkit/bench/pull/1119
 [#1118]: https://github.com/munich-quantum-toolkit/bench/pull/1118
 [#1117]: https://github.com/munich-quantum-toolkit/bench/pull/1117
 [#1116]: https://github.com/munich-quantum-toolkit/bench/pull/1116

src/mqt/bench/benchmarks/graphstate.py

@@ -12,7 +12,6 @@
 
 import networkx as nx
 from qiskit.circuit import QuantumCircuit, QuantumRegister
-from qiskit.circuit.library import GraphStateGate
 
 from ._registry import register_benchmark
 
@@ -35,7 +34,11 @@ def create_circuit(num_qubits: int, degree: int = 2, seed: int | None = None) ->
     qc = QuantumCircuit(q, name="graphstate")
 
     g = nx.random_regular_graph(degree, num_qubits, seed=seed)
-    a = nx.convert_matrix.to_numpy_array(g)
-    qc.compose(GraphStateGate(a), inplace=True)
+    # Keep NetworkX's node order to preserve seeded wire assignments.
+    indices = {node: index for index, node in enumerate(g)}
+    edges = sorted(sorted((indices[a], indices[b])) for a, b in g.edges)
+    qc.h(q)
+    for a, b in edges:
+        qc.cz(a, b)
     qc.measure_all()
-    return qc.decompose(gates_to_decompose="graph_state")
+    return qc

src/mqt/bench/benchmarks/hrs_cumulative_multiplier.py

@@ -60,6 +60,7 @@ def create_circuit(num_qubits: int) -> QuantumCircuit:
 
     # prepare adder as controlled gate
     adder = adder_ripple_c04(num_state_qubits, kind="half")
+    controlled_adder = adder.to_gate().control(1)
 
     # get the number of helper qubits needed
     num_helper_qubits = adder.num_ancillas
@@ -73,10 +74,7 @@ def create_circuit(num_qubits: int) -> QuantumCircuit:
     qc = QuantumCircuit(*qregs)
 
     for i in range(num_state_qubits):
-        num_adder_qubits = num_state_qubits
-        adder_for_current_step = adder
-        controlled_adder = adder_for_current_step.to_gate().control(1)
-        qr_list = [qr_a[i], *qr_b[:num_adder_qubits], *qr_out[i : num_state_qubits + i + 1]]
+        qr_list = [qr_a[i], *qr_b[:], *qr_out[i : num_state_qubits + i + 1]]
         if num_helper_qubits > 0 and qr_helper:
             qr_list.extend(qr_helper[:])
         qc.append(controlled_adder, qr_list)

tests/test_bench.py

@@ -19,11 +19,12 @@
 from importlib import metadata
 from typing import TYPE_CHECKING, NoReturn, cast
 
+import networkx as nx
 import numpy as np
 import pytest
 from qiskit import QuantumCircuit, qasm2, qasm3, qpy
 from qiskit.circuit import ClassicalRegister, ForLoopOp, IfElseOp, Parameter
-from qiskit.circuit.library import CXGate, HGate, RXGate, RZGate, TGate, XGate
+from qiskit.circuit.library import CXGate, GraphStateGate, HGate, RXGate, RZGate, TGate, XGate
 from qiskit.compiler import transpile
 from qiskit.converters import circuit_to_dag
 from qiskit.primitives import StatevectorSampler
@@ -212,6 +213,7 @@ def test_arithmetic_circuits(benchmark_name: str, input_value: int) -> None:
         ("multiplier", 8, None, (4, 5, 6, 7), [1, 3], "0100"),  # 2 * 2 = 4
         ("rg_qft_multiplier", 8, None, (4, 5, 6, 7), [1, 3], "0100"),  # 2 * 2 = 4
         ("hrs_cumulative_multiplier", 9, None, (4, 5, 6, 7), [1, 3], "0100"),  # 2 * 2 = 4
+        ("hrs_cumulative_multiplier", 9, None, (4, 5, 6, 7), [0, 1, 2, 3], "1001"),  # 3 * 3 = 9
     ],
 )
 @pytest.mark.parametrize("level", list(BenchmarkLevel))
@@ -678,6 +680,19 @@ def test_dj_constant_oracle() -> None:
     assert Statevector(qc).probabilities([0, 1, 2, 3])[0] == pytest.approx(1)
 
 
+@pytest.mark.parametrize(("num_qubits", "degree", "seed"), [(1, 0, 42), (6, 2, 0), (6, 3, 42)])
+def test_graphstate_seeded_operator(num_qubits: int, degree: int, seed: int) -> None:
+    """Direct graph-state construction preserves the seeded adjacency-matrix operator."""
+    graph = nx.random_regular_graph(degree, num_qubits, seed=seed)
+    expected = GraphStateGate(nx.to_numpy_array(graph))
+    circuit = get_benchmark_alg("graphstate", num_qubits, degree=degree, seed=seed)
+
+    assert circuit.count_ops().get("cz", 0) == num_qubits * degree // 2
+    assert circuit.count_ops()["measure"] == num_qubits
+    circuit.remove_final_measurements()
+    assert Operator(circuit).equiv(Operator(expected))
+
+
 @pytest.mark.parametrize("benchmark", ["graphstate", "randomcircuit", "vqe_real_amp"])
 def test_benchmark_seed(benchmark: str) -> None:
     """The seed controls both random construction and parameter binding."""