BB84 Quantum Key Distribution: A Step-by-Step Photon Basis Diagram

What is BB84 Quantum Key Distribution

BB84, named after its creators Charles Bennett and Gilles Brassard, is a quantum cryptographic protocol designed to establish a shared secret key between two parties—Alice and Bob—in such a way that any eavesdropping by an adversary can be detected. The protocol relies on the principles of quantum mechanics, particularly the no-cloning theorem and the Heisenberg uncertainty principle.

In BB84, Alice encodes her bits into qubits using one of two bases: rectilinear (0° or 90°) for a '0' bit and diagonal (45° or 135°) for a '1' bit. She then sends these qubits to Bob over an insecure channel. Bob measures the qubits with randomly chosen bases, resulting in either matching or mismatching bases.

How BB84 Works

After both parties have sent and received their qubits, they publicly compare their bases over an authenticated channel. If a base mismatch occurs, the corresponding bits are discarded as errors. The remaining bits form the shared secret key that Alice and Bob can use for secure communication.

The protocol is robust against eavesdropping because any attempt by Eve to intercept or measure the qubits will alter them due to the no-cloning theorem, leading to a higher quantum bit error rate (QBER) which can be detected.

Eve's Role and Detection

The inclusion of an eavesdropper, Eve, in the BB84 protocol demonstrates how any attempt at intercepting the qubits will introduce errors. By adjusting the intercept probability using a slider, one can observe how Eve’s measurements affect the QBER.

If the QBER exceeds a certain threshold, Alice and Bob know that their communication has been compromised and they discard the key. This ensures the security of the shared secret key.

Why BB84 Matters

BB84 is significant because it provides a practical implementation of quantum cryptography, offering provable security against eavesdropping. The protocol has been experimentally verified and forms the basis for many modern quantum key distribution systems.

Moreover, BB84 highlights the potential of quantum mechanics in enhancing cybersecurity by leveraging its unique properties to create unbreakable encryption.

Frequently asked questions

Try it live

Everything above runs in your browser — open BB84 Quantum Key Distribution — Photon Basis Diagram (2D) and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.