qrw.py

# Example of a (Hadamard) quantum random walk

import math
import random
import sys
import time

from pyqrack import QrackSimulator


def main():
    lattice_qb_count = 8
    if len(sys.argv) > 1:
        lattice_qb_count = int(sys.argv[1])
    lattice_qubits = list(range(lattice_qb_count))
    coin_qubit = lattice_qb_count
    sign_qubit = lattice_qb_count - 1
    sign_power = 1 << sign_qubit
    step_count = sign_power - 1
    if len(sys.argv) > 2:
        step_count = int(sys.argv[2])

    sim = QrackSimulator(lattice_qb_count + 1)
    sim.x(sign_qubit)

    for _ in range(step_count):
        sim.h(coin_qubit)
        sim.mcadd(1, [coin_qubit], lattice_qubits)
        sim.x(coin_qubit)
        sim.mcsub(1, [coin_qubit], lattice_qubits)
        sim.x(coin_qubit)

    exp_pos = sim.permutation_expectation(lattice_qubits) - sign_power
    print("Expected position:", exp_pos)
    obs_pos = (sim.m_all() & ~(1 << coin_qubit)) - sign_power
    print("Observed position:", obs_pos)


if __name__ == '__main__':
    sys.exit(main())