Quantum Networking: Entanglement-Based Communication 2026

class QuantumProperties:
    """
    Key quantum mechanical properties for networking.
    """
    
    def superposition(self):
        """
        Qubit exists in combination of |0> and |1>.
        """
        return {
            'notation': 'α|0⟩ + β|1⟩',
            'probability': '|α|² for |0⟩, |β|² for |1⟩',
            'visualization': 'Point on Bloch sphere',
            'application': 'Quantum parallelism in computation'
        }
    
    def entanglement(self):
        """
        Correlated quantum states.
        """
        return {
            'bell_state': '|Φ⁺⟩ = (|00⟩ + |11⟩)/√2',
            'correlation': 'Measuring one affects other instantly',
            'distance': 'Works over any distance',
            'application': 'Quantum teleportation, QKD'
        }
    
    def no_cloning(self):
        """
        Cannot copy unknown quantum state.
        """
        return {
            'theorem': 'No arbitrary quantum state can be copied',
            'implication': 'Quantum messages cannot be duplicated',
            'benefit': 'Inherently secure against interception'
        }

class BB84QKD:
    """
    BB84: First quantum key distribution protocol.
    """
    
    def __init__(self):
        self.bases = ['Z', 'X']  # Rectilinear, Diagonal
        self.bases_states = {
            'Z': {'0': [1, 0], '1': [0, 1]},  # Computational
            'X': {'0': [1/√2, 1/√2], '1': [1/√2, -1/√2]}  # Hadamard
        }
    
    def alice_prepare_qubits(self, key_bits, bases):
        """
        Alice prepares qubits.
        """
        qubits = []
        
        for bit, base in zip(key_bits, bases):
            state = self.bases_states[base][bit]
            qubits.append(state)
        
        return qubits
    
    def bob_measure(self, qubits, bases):
        """
        Bob measures in random bases.
        """
        results = []
        
        for qubit, base in zip(qubits, bases):
            # Measure in chosen basis
            if base == 'Z':
                # Z basis: measure |0⟩ or |1⟩
                prob = qubit[0]**2
                result = '0' if random.random() < prob else '1'
            else:
                # X basis: measure |+⟩ or |-⟩
                prob = (qubit[0] + qubit[1])**2 / 2
                result = '+' if random.random() < prob else '-'
            
            results.append(result)
        
        return results
    
    def sifting(self, alice_bases, bob_bases, bob_results):
        """
        Keep only qubits measured in same basis.
        """
        sifted_key = []
        
        for i, (a_base, b_base) in enumerate(zip(alice_bases, bob_bases)):
            if a_base == b_base:
                sifted_key.append(bob_results[i])
        
        return sifted_key
    
    def error_check(self, sifted_key, sample_size):
        """
        Check for eavesdropping via error rate.
        """
        sample = random.sample(sifted_key, sample_size)
        
        # In perfect QKD, error rate should be 0
        # Any error indicates eavesdropping
        error_rate = count_errors(sample) / sample_size
        
        return error_rate

class QuantumNetwork:
    """
    Building quantum networks.
    """
    
    def network_layers(self):
        """
        Quantum network architecture.
        """
        return {
            'quantum_links': 'Fiber or free-space connections',
            'trusted_nodes': 'Repeat and measure',
            'quantum_repeaters': 'Extend range without measurement',
            'quantum_memories': 'Store qubits for routing',
            'quantum_processors': 'Local quantum computation'
        }
    
    def satellite_network(self):
        """
        China's quantum satellite network.
        """
        return {
            'satellite': 'Micius (2016)',
            'range': 'Beijing to Vienna',
            'technology': 'Entangled photon distribution',
            'application': 'Intercontinental QKD'
        }

class QuantumRepeater:
    """
    Extend quantum communication range.
    """
    
    def entanglement_swapping(self):
        """
        Connect two entangled pairs.
        """
        # Alice has A1 entangled with B1
        # Bob has B2 entangled with C1
        # Measure B1, B2 together → A1 becomes entangled with C1
        
        return {
            'technique': 'Bell state measurement',
            'result': 'Extends entanglement distance',
            'challenge': 'Requires quantum memory'
        }
    
    def entanglement_purification(self):
        """
        Improve entanglement quality.
        """
        return {
            'create_pairs': 'Generate multiple entangled pairs',
            'measure': 'Compare some pairs',
            'select': 'Keep high-fidelity pairs',
            'result': 'Higher quality entanglement'
        }
    
    def memory_based(self):
        """
        Quantum repeater with memory.
        """
        return {
            'components': [
                'Quantum memory (store qubits)',
                'Entanglement sources',
                'Bell state measurements',
                'Classical communication'
            ],
            'performance': 'Exponential improvement with segments',
            'status': 'Research, not yet commercial'
        }

class QuantumInternetStack:
    """
    Analogous to classical internet stack.
    """
    
    def layer_architecture(self):
        """
        Proposed quantum network layers.
        """
        return {
            'physical': 'Photon sources, detectors, quantum memories',
            'link': 'Entanglement generation between neighbors',
            'network': 'Routing entangled pairs',
            'transport': 'Quantum state transmission',
            'application': 'QKD, distributed quantum computing'
        }
    
    def protocols(self):
        """
        Quantum network protocols.
        """
        return {
            'QKD': 'Quantum Key Distribution',
            'QRSA': 'Quantum Router Space Allocation',
            'QUIC': 'Quantum UDP Internet Connections',
            'Teleportation': 'Transfer quantum states'
        }

class QuantumTeleportation:
    """
    Transfer quantum state without physical transfer.
    """
    
    def protocol(self, alice_qubit, alice_entangled, bob_entangled):
        """
        Teleport quantum state.
        """
        # Alice has:
        # - Qubit to teleport: |ψ⟩
        # - One half of entangled pair: A
        
        # Bob has:
        # - Other half of entangled pair: B
        
        # Alice performs Bell measurement on |ψ⟩ and A
        # Sends 2 classical bits to Bob
        
        # Bob performs operation on B based on measurement
        # Now B = |ψ⟩
        
        return {
            'requirement': 'Entangled pair shared in advance',
            'classical_bits': '2 bits for correction',
            'quantum_bits': 'No quantum info transmitted',
            'speed': 'Speed of light (classical channel)'
        }

class DistributedQuantum:
    """
    Connect quantum computers.
    """
    
    def network_computing(self):
        """
        Quantum distributed computing.
        """
        return {
            'concept': 'Multiple quantum computers work together',
            'entanglement': 'Links quantum processors',
            'applications': [
                'Large-scale quantum simulation',
                'Distributed quantum algorithms',
                'Blind quantum computing'
            ],
            'challenge': 'Requires high-quality entanglement'
        }
    
    def blind_computing(self):
        """
        Delegate computation to quantum server.
        """
        return {
            'client': 'Has quantum state, encrypts with quantum technique',
            'server': 'Performs computation without knowing data',
            'privacy': 'Server learns nothing',
            'implementation': 'Measurement-based quantum computing'
        }

class QuantumSensing:
    """
    Distributed quantum sensors.
    """
    
    def quantum_magnetometer(self):
        """
        Ultra-sensitive magnetic field sensing.
        """
        return {
            'application': 'Medical imaging, geophysics',
            'network': 'Distributed NV centers',
            'benefit': 'Quantum advantage in precision'
        }

class PhotonicQubits:
    """
    Use photons for quantum communication.
    """
    
    def encoding(self):
        """
        Encode qubits in photons.
        """
        return {
            'polarization': 'Horizontal/Vertical = |0⟩/|1⟩',
            'time_bin': 'Early/Late = |0⟩/|1⟩',
            'frequency': 'Different frequencies',
            'choice': 'Fiber compatibility favors time bin'
        }
    
    def sources(self):
        """
        Photon sources.
        """
        return {
            'spontaneous_parametric_down_conversion': 'Entangled photon pairs',
            'quantum_dots': 'Single photon on demand',
            'atoms': 'Single photon from atoms/ions'
        }
    
    def detectors(self):
        """
        Single photon detectors.
        """
        return {
            'spad': 'Single photon avalanche diode',
            'superconducting': 'SNSPD, high efficiency',
            'detection_efficiency': 'Up to 99% (superconducting)'
        }

class QuantumMemories:
    """
    Store quantum states.
    """
    
    def technologies(self):
        """
        Quantum memory approaches.
        """
        return {
            'cold_atoms': 'Optical lattice',
            'diamond_nv': 'Nitrogen-vacancy centers',
            'rare_earth_ions': 'Doped crystals',
            'optical_fibers': 'Storage in fiber loops'
        }
    
    def requirements(self):
        """
        Memory specifications.
        """
        return {
            'fidelity': '>99%',
            'storage_time': 'Seconds to minutes',
            'efficiency': '>90%',
            'readout': 'On-demand, not destructive'
        }

class ExistingNetworks:
    """
    Operating quantum networks.
    """
    
    def china_network(self):
        """
        World's largest quantum network.
        """
        return {
            'ground': '2,000 km Beijing-Shanghai',
            'satellite': 'Micius, intercontinental',
            'users': 'Banks, government'
        }
    
    def europe_network(self):
        """
        EuroQCI initiative.
        """
        return {
            'goal': 'EU-wide quantum network by 2027',
            'partners': '27 EU countries',
            'use_cases': 'QKD, distributed computing'
        }
    
    def us_network(self):
        """
        DoE quantum testbeds.
        """
        return {
            'testbeds': '3 networks operational',
            'length': '100-300 km each',
            'research': 'Advanced repeaters'
        }

class QKDImplementation:
    """
    Deploy QKD today.
    """
    
    def requirements(self):
        """
        What's needed.
        """
        return {
            'hardware': 'QKD endpoints ($50K-500K)',
            'fiber': 'Dedicated fiber (or trusted node)',
            'integration': 'VPN or encryption appliance',
            'maintenance': 'Calibration, monitoring'
        }
    
    def use_cases(self):
        """
        Best applications.
        """
        return {
            'banking': 'High-value transactions',
            'government': 'Classified communications',
            'datacenters': 'Cloud-to-cloud encryption',
            'telecom': 'Long-haul quantum-safe links'
        }