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"""
High-Fidelity Quantum Communication Network Simulation using Cirq with Detailed Math Logging.
Simulates entanglement swapping by creating two Bell pairs, performing a Bell measurement,
and logging the intermediate and final measurement outcomes. This demonstrates how
entanglement can be extended across a network without direct interaction between end nodes.
"""
import cirq
import numpy as np
from cirq.contrib.svg import circuit_to_svg
def add_noise(circuit, noise_prob):
"""Add depolarizing noise to all qubits after each operation."""
noisy_ops = []
for op in circuit.all_operations():
noisy_ops.append(op)
for q in op.qubits:
noisy_ops.append(cirq.DepolarizingChannel(noise_prob).on(q))
return cirq.Circuit(noisy_ops)
def entanglement_swapping_cirq(noise_prob=0.0):
"""
Simulates quantum entanglement swapping for a quantum repeater network.
Entanglement swapping allows two parties who have never interacted to become
entangled through an intermediate node:
1. Node A and B share entangled qubits q0-q1
2. Node B and C share entangled qubits q2-q3
3. Node B performs a Bell measurement on q1 and q2
4. Based on B's measurement, q0 and q3 become entangled
Args:
noise_prob: Probability of depolarizing noise
Returns:
Dictionary with measurement results and log
"""
log = []
log.append("=== Entanglement Swapping Simulation ===")
# Create 4 qubits representing:
# q0: Node A's qubit
# q1: Node B's first qubit (shared with A)
# q2: Node B's second qubit (shared with C)
# q3: Node C's qubit
q0, q1, q2, q3 = cirq.LineQubit.range(4)
circuit = cirq.Circuit()
# Step 1: Create Bell pair between Node A and B (q0-q1)
log.append("Creating Bell pair between Node A (q0) and Node B (q1).")
circuit.append([cirq.H(q0), cirq.CNOT(q0, q1)])
# Step 2: Create Bell pair between Node B and C (q2-q3)
log.append("Creating Bell pair between Node B (q2) and Node C (q3).")
circuit.append([cirq.H(q2), cirq.CNOT(q2, q3)])
# Get state after Bell pair creation
simulator = cirq.Simulator()
initial_state = simulator.simulate(circuit).final_state_vector
log.append(f"State after creating Bell pairs: {np.round(initial_state[:4], 3)}")
# Add noise if specified
if noise_prob > 0:
circuit = add_noise(circuit, noise_prob)
log.append(f"Noise added with probability {noise_prob}.")
# Step 3: Node B performs Bell measurement on q1 and q2
log.append("Node B performing Bell measurement on q1 and q2.")
circuit.append(cirq.CNOT(q1, q2))
circuit.append(cirq.H(q1))
# Measure the middle qubits
circuit.append([cirq.measure(q1, key='m1'), cirq.measure(q2, key='m2')])
# Run simulation to get intermediate measurement
intermediate_result = simulator.run(circuit, repetitions=1)
m1 = int(intermediate_result.measurements['m1'][0][0])
m2 = int(intermediate_result.measurements['m2'][0][0])
log.append(f"Node B's measurement results: m1={m1}, m2={m2}")
# Step 4: Conditional operations on q3 based on Bell measurement
correction_circuit = cirq.Circuit()
if m1 == 1:
correction_circuit.append(cirq.Z(q3))
log.append("Applying Z correction to Node C's qubit (q3).")
if m2 == 1:
correction_circuit.append(cirq.X(q3))
log.append("Applying X correction to Node C's qubit (q3).")
# Measure the end nodes to verify entanglement
correction_circuit.append([cirq.measure(q0, key='m0'), cirq.measure(q3, key='m3')])
# Combine circuits
full_circuit = circuit + correction_circuit
# Run final simulation
final_result = simulator.run(full_circuit, repetitions=1)
m0 = int(final_result.measurements['m0'][0][0])
m3 = int(final_result.measurements['m3'][0][0])
log.append(f"Node A's measurement (q0): {m0}")
log.append(f"Node C's measurement (q3): {m3}")
# In ideal entanglement swapping, these measurements should be correlated
entanglement_success = (m0 == m3) if m1 == 0 else (m0 != m3)
log.append(f"Entanglement swapping successful: {entanglement_success}")
# Generate SVG diagram for visualization
circuit_svg = circuit_to_svg(full_circuit)
return {
'intermediate_measurements': (m1, m2),
'node_A_measurement': m0,
'node_C_measurement': m3,
'entanglement_success': entanglement_success,
'circuit_svg': circuit_svg,
'log': "\n".join(log)
}
if __name__ == '__main__':
# Run simulation
result = entanglement_swapping_cirq(noise_prob=0.0)
print("Quantum Entanglement Swapping Simulation:")
print(f"Node B's Bell measurement: {result['intermediate_measurements']}")
print(f"Node A measured: {result['node_A_measurement']}")
print(f"Node C measured: {result['node_C_measurement']}")
print(f"Entanglement swapping successful: {result['entanglement_success']}")
# Run with noise
noisy_result = entanglement_swapping_cirq(noise_prob=0.1)
print("\nWith noise:")
print(f"Entanglement swapping successful: {noisy_result['entanglement_success']}")
print("\nDetailed Log:")
print(result['log']) |