03_superdense_coding.py

import sys
from pathlib import Path

PROJECT_FOLDER = Path(__file__).resolve().parents[1]
sys.path.insert(0, str(PROJECT_FOLDER))

from quantum_core.circuit import apply_gate
from quantum_core.gates import CNOT, H, X, Z
from quantum_core.measurement import get_probabilities
from quantum_core.state import make_zero_state


def alice_encodes_message(state, message):
    if type(message) != str:
        raise TypeError("message must be a string")

    if len(message) != 2:
        raise ValueError("message must have exactly 2 bits")

    for bit in message:
        if bit != "0" and bit != "1":
            raise ValueError("message can only contain 0 and 1")

    if message[1] == "1":
        state = apply_gate(state, X, [0])

    if message[0] == "1":
        state = apply_gate(state, Z, [0])

    return state


def send_and_decode(message):
    state = make_zero_state(2)

    state = apply_gate(state, H, [0])
    state = apply_gate(state, CNOT, [0, 1])

    state = alice_encodes_message(state, message)

    state = apply_gate(state, CNOT, [0, 1])
    state = apply_gate(state, H, [0])

    return get_probabilities(state)


print("Superdense coding")
print("-----------------")

messages = ["00", "01", "10", "11"]

for message in messages:
    answer = send_and_decode(message)
    print("Alice message:", message, "Bob receives:", answer)