BB84 Polarization Phase-Plane & QBER Monitor: Quantum Key Distribution in Action

What the BB84 Protocol Is

The BB84 protocol is a quantum key distribution (QKD) method that allows two parties to establish a shared secret key using entangled photons. It leverages the principles of quantum mechanics, particularly superposition and measurement-induced disturbance, to ensure secure communication.

In this protocol, Alice sends qubits to Bob in one of four polarization states: horizontal (H), vertical (V), diagonal at 45 degrees (D), or anti-diagonal at -45 degrees (A). By measuring these qubits with different bases, both parties can agree on a shared secret key without Eve, the eavesdropper, being able to intercept and copy the information.

The Polarization Phase-Plane

The polarization phase-plane is a graphical representation of the possible states of a photon's polarization. In this context, it helps visualize the different bases (H, V, D, A) and how they relate to each other. The plane can be divided into two orthogonal axes representing the horizontal and vertical components of the polarization.

By plotting the qubits on this phase-plane, we can see how the states change as Alice and Bob choose their measurement bases. This visualization aids in understanding the probabilistic nature of quantum measurements and the inherent randomness that ensures security.

Quantum Bit Error Rate (QBER)

The Quantum Bit Error Rate (QBER) is a critical metric in QKD. It quantifies the rate at which bits are incorrectly transmitted between Alice and Bob during their key distribution process. A high QBER indicates that Eve, the eavesdropper, has likely intercepted and modified some of the qubits.

The BB84 protocol includes a threshold for QBER: if it exceeds 11%, the communication is aborted to prevent any potential leakage of the secret key.

Eve's Intercept-Resend Attack

In the BB84 protocol, Eve can attempt an intercept-resend attack where she measures Alice’s qubits with her own bases and then resends new qubits to Bob. This action introduces errors in the sifted key, which are detected through QBER monitoring.

By observing how the QBER changes when Eve is active, we can determine if the communication channel has been compromised.

Frequently asked questions

Try it live

Everything above runs in your browser — open BB84 Polarization Phase-Plane & QBER Monitor and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.