E91 Quantum Key Distribution

This project is a Demonstration of the Ekert91 (E91) QKD protocol implemented in Qiskit.

In this project just like in BB84, the random bitstream used here are not from built-in pseudorandom generators.
It is sourced from my project, Chaos2Crypto, where I developed an FFT-based random number generator with bias correction and cryptographic extraction.

What this project demonstrates

• Entangled Bell pairs are generated and distributed between Alice and Bob.
• Alice and Bob randomly choose one of three measurement bases.
• The randomness comes from the cryptographically secure bits in bits_for_E91.txt generated using Chaos2Cryto Code.
• A subset of results is used to compute the CHSH correlation parameter (S-value):
• When Alice and Bob’s bases match, the results are used to form the raw secret key.
• The Quantum Bit Error Rate (QBER) is estimated to detect noise or eavesdropping.

Simulator Limitations

• Implemented with Qiskit’s AerSimulator.
• Each Bell pair is simulated in its own circuit.
• Runtime grows with the number of entangled pairs.
• For sufficiently large (N), the CHSH test converges toward the expected quantum bound.

Example Results

• CHSH S-value: typically ~2.8 (well above the classical bound of 2).
• QBER: a few percent in ideal simulation, depending on random choices.

Feel free to clone, modify, and extend this project!
(Ideas: add explicit noise models, simulate an eavesdropper, or extend key distillation with error correction.)