BB84 QKD - Securing the Quantum Future

Project Overview

Demonstrating unconditional cryptographic security through quantum mechanics

💡 The Problem: Post-Quantum Cryptography Crisis

Classical cryptography faces an existential threat from quantum computers. Shor's algorithm can efficiently break RSA and ECC encryption schemes that protect today's internet. BB84 offers a solution: a key distribution method whose security is physically impossible to compromise, regardless of adversarial computational power.

🔐

Unconditional Security

Security guaranteed by the laws of quantum mechanics, not computational hardness. No future technology can break information-theoretic security.

🔍

Eavesdropping Detection

Any measurement attempt disturbs quantum states due to the no-cloning theorem, making eavesdropping statistically detectable through elevated QBER.

📡

Production-Ready

Demonstrated over 100+ km fiber optic cables. Commercial QKD systems already secure government and financial communications worldwide.

Protocol Implementation

Six critical phases of the BB84 quantum key distribution protocol

1

Preparation

Alice encodes random bits into quantum states using random bases (Z or X)

2

Transmission

Qubits transmitted through quantum channel with optional noise/eavesdropper

3

Measurement

Bob measures qubits using independently chosen random bases

4

Sifting

Bases compared publicly; only matching bases retained (~50% efficiency)

5

Error Check

QBER calculated; if >11%, protocol aborts (eavesdropper detected)

6

Final Key

Privacy amplification produces secure shared key

Quantum State Encoding

            
Z-basis (Computational): bit=0 → |0⟩, bit=1 → |1⟩

X-basis (Hadamard):      bit=0 → |+⟩ = (|0⟩+|1⟩)/√2, bit=1 → |−⟩ = (|0⟩−|1⟩)/√2

Core Implementation

            
class BB84Protocol:

    def encode_qubit(self, bit: int, basis: int) -> QuantumCircuit:

        """Encodes classical bit into quantum state"""

        qc = QuantumCircuit(1, 1)

        if bit == 1:

            qc.x(0)  # Apply X gate to get |1⟩

        if basis == 1:  # X-basis

            qc.h(0)  # Hadamard transforms Z-basis to X-basis

        return qc

    def measure_qubit(self, qc: QuantumCircuit, basis: int) -> QuantumCircuit:

        """Measures qubit in specified basis"""

        if basis == 1:

            qc.h(0)  # Transform back to Z-basis

        qc.measure(0, 0)

        return qc

⚠️ Security Assumptions

Critical Requirements:

Authenticated classical channel (prevents man-in-the-middle attacks)
Single-photon sources or decoy states (prevents photon-number-splitting)
Reliable single-photon detectors (minimizes detector vulnerabilities)
Privacy amplification with secure hash functions

Experimental Results

Comprehensive analysis across multiple scenarios and noise conditions

Protocol Workflow & Information Flow

Visualization of the complete BB84 protocol stages showing efficiency degradation from preparation (100%) through final key generation (22%). The sifting phase reduces bits by 50% due to basis matching, while error checking and privacy amplification further compress the key.

Comprehensive BB84 protocol dashboard showing six-panel visualization with protocol flow, basis encoding scheme, QBER distribution over 50 runs, security analysis under different conditions including eavesdropper attacks, theoretical key rate versus distance analysis, and comparative performance metrics between BB84 and E91 quantum key distribution protocols

Noise Tolerance & Security Threshold

Analysis of BB84 performance under varying depolarizing noise levels from 0% to 15%. The protocol maintains security below the 11% QBER threshold (green region), with key generation rate degrading gracefully as noise increases. Above the threshold (red region), the protocol correctly aborts to prevent compromised key usage.

Dual-panel noise analysis showing QBER versus depolarizing noise probability with color-coded secure and insecure regions divided by 11 percent security threshold, and corresponding key generation efficiency degradation as noise increases from 0 to 15 percent with measurements at 16 discrete noise levels

Information-Theoretic Security Analysis

Shannon mutual information curves demonstrating the security foundation of BB84. The green shaded region represents extractable secret information where I(Alice:Bob) exceeds I(Alice:Eve). At QBER = 11%, mutual information converges to zero, defining the theoretical security threshold. Secret key rate calculation accounts for sifting efficiency and privacy amplification overhead.

Information-theoretic security visualization with two panels: left panel showing mutual information curves for Alice-Bob correlation versus Alice-Eve information as functions of QBER with green shaded region indicating secret information extraction zone, right panel displaying extractable secret key rate with security threshold marked at 11 percent QBER where key rate drops to zero

Quantum Circuit Examples

Representative quantum circuits for BB84 encoding schemes and E91 entanglement generation. Left: Z-basis encoding with X gate for bit=1. Center: X-basis encoding with double Hadamard transformation. Right: Maximally entangled Bell state |Φ+⟩ = (|00⟩ + |11⟩)/√2 for E91 protocol with barrier separating preparation from measurement.

Three quantum circuit diagrams showing BB84 Z-basis encoding with X gate and measurement for bit equals 1, BB84 X-basis encoding with Hadamard gates for bit equals 0, and E91 entangled Bell pair creation using Hadamard gate and CNOT gate followed by barrier and two-qubit measurement in computational basis

Performance Metrics Summary

Scenario	QBER	Key Rate (bits/sent)	Final Key Length	Secure?
Clean Channel	0.000	0.251	53 bits (from 200 sent)	✓ Yes
With Eavesdropper (I-R)	0.230	0.000	0 bits (protocol aborted)	✗ No
Low Noise (2%)	0.035	0.254	~50 bits (from 200 sent)	✓ Yes
High Noise (8%)	0.116	0.114	~23 bits (from 200 sent)	✓ Yes

✓ Key Findings

Eavesdropping Detection:

BB84 Quantum Key Distribution