Quantum Key Distribution (QKD) uses quantum mechanics to securely share cryptographic keys. The BB84 protocol, a popular QKD method, leverages quantum principles like superposition and entanglement to detect eavesdropping attempts during key exchange.
BB84 involves preparing qubits, transmitting them over a quantum channel, measuring them, and discussing results publicly. Through error estimation, correction, and privacy amplification, Alice and Bob can establish a secure key for encrypted communication.
Quantum Key Distribution (QKD) and the BB84 Protocol
Principles of quantum key distribution
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Utilizes quantum mechanics principles (superposition, entanglement) to securely distribute cryptographic keys between two parties (Alice and Bob)
Quantum states (qubits) are used to encode and transmit the key
Qubits can exist in multiple states simultaneously until measured
Measuring a qubit collapses its state to a single value (0 or 1)
Any attempt to intercept or measure the qubits during transmission alters their states irreversibly
This alteration introduces detectable errors in the key
Presence of errors alerts Alice and Bob to potential eavesdropping attempts
Advantages over classical key distribution methods:
Unconditional security guaranteed by laws of quantum mechanics
Eavesdropping attempts are detectable due to introduced errors
Security does not rely on assumed computational limitations of adversaries
Steps in BB84 protocol
State preparation:
Alice prepares a sequence of qubits, each randomly in one of four states:
∣0⟩, ∣1⟩ (computational basis)
∣+⟩, ∣−⟩ (Hadamard basis)
Basis choice for each qubit is random and independent
Quantum channel transmission:
Alice sends the prepared qubits to Bob over a quantum channel (optical fiber, free space)
Qubits maintain their quantum states during transmission
Measurement:
Bob measures each received qubit independently in a randomly chosen basis
Computational basis (∣0⟩, ∣1⟩) or Hadamard basis (∣+⟩, ∣−⟩)
Bob records the measurement result (0 or 1) and the corresponding basis for each qubit
Public discussion and key sifting:
Alice and Bob communicate over an authenticated classical channel (not secure)
They compare their chosen bases for each qubit
Qubits measured in mismatched bases are discarded
Remaining qubits form the raw key (sifted key)
Error estimation and correction:
Alice and Bob compare a random subset of their raw key bits
Differences indicate potential eavesdropping or channel noise
If error rate is below a predetermined threshold, they proceed with error correction
Corrects errors using classical error correction techniques (CASCADE, LDPC)
Privacy amplification:
Alice and Bob apply a hash function to the error-corrected key
Reduces any potential information leakage to an eavesdropper
Resulting final key is used for secure communication (one-time pad encryption)
Security of BB84 protocol
Eavesdropping attacks:
Intercept-resend attack:
Eve intercepts qubits, measures them, and sends new qubits to Bob
Introduces errors due to measuring in bases incompatible with Alice and Bob's
Detected during key comparison and error estimation
Beam-splitting attack:
Eve attempts to split the quantum signal and measure a portion
Introduces detectable errors and reduces signal strength received by Bob
Unconditional security:
BB84 protocol's security relies on fundamental principles of quantum mechanics
No-cloning theorem: Qubits cannot be perfectly copied without altering the original
Uncertainty principle: Measuring a qubit in one basis disturbs its state in other bases