Source code for qolumbina.programs.state_preparation.w_state

# The original code is sourced from:
# https://github.com/munich-quantum-toolkit/bench/blob/main/src/mqt/bench/benchmarks/wstate.py
#
# This program is adapted for use as a benchmark in controlled software testing experiments.
# Modifications made to the original code include (for Apache License 2.0):
# - Modify the code in the class
# - Remove the measurements from the circuit
#
# Copyright (c) 2023 - 2026 Chair for Design Automation, TUM
# Copyright (c) 2025 - 2026 Munich Quantum Software Company GmbH
# All rights reserved.
#
# SPDX-License-Identifier: MIT
#
# Licensed under the MIT License

"""W state benchmark definition."""

from __future__ import annotations

import numpy as np
from qiskit.circuit import QuantumCircuit, QuantumRegister
 
# ---------- benchmark registration ----------
from ..benchmark_registry import register_benchmark
from pathlib import Path
@register_benchmark(
    Path(__file__).stem,
    family=Path(__file__).resolve().parent.name,
    description="W state preparation circuit",
    class_name="WState",
    source={
        "repo": "https://github.com/munich-quantum-toolkit/bench/tree/main",
        "file": "src/mqt/bench/benchmarks/wstate.py",
        "language": "Qiskit",
        "available_doc": False
    },
    testability_refactoring=[
        "Structure reorganization"
    ] 
)
def create_w_state(num_qubits: int):
    return WState(num_qubits=num_qubits)



class WState(QuantumCircuit): 
    """
    Create a W State:
    A W state is an entangled quantum state of multiple qubits where a single excitation
    is delocalized over all qubits.  
    """
    def __init__(self, num_qubits: int, name: str | None = None) -> None:
        """
        Args:
            num_qubits: Number of qubits of the W state
            name: Name of the quantum circuit
        """
        self._num_qubits = num_qubits

        super().__init__(num_qubits, name="wstate" or name)

        # Build the W state circuit
        self._build()



    def f_gate(self, qc: QuantumCircuit, i: int, j: int, n: int, k: int) -> None:
        theta = np.arccos(np.sqrt(1 / (n - k + 1)))
        qc.ry(-theta, j)
        qc.cz(i, j)
        qc.ry(theta, j)


    def _build(self) -> None:
        """Build the W state circuit."""
        num_qubits = self._num_qubits

        q = QuantumRegister(num_qubits, "q")
        qc = QuantumCircuit(q, name="wstate")

        qc.x(q[-1])

        for m in range(1, num_qubits):
            self.f_gate(qc, num_qubits - m, num_qubits - m - 1, num_qubits, m)

        for k in reversed(range(1, num_qubits)):
            qc.cx(k - 1, k)
 
        self.compose(qc, qubits=self.qubits, inplace=True)