The BB84 Protocol, proposed by Charles Bennett and Gilles Brassard in 1984, is the first and most widely known Quantum Key Distribution (QKD) method. It allows two parties—commonly called Alice (the sender) and Bob (the receiver)—to securely generate and share a secret key over an insecure channel, with guaranteed detection if anyone tries to eavesdrop.
BB84 uses basic properties of quantum mechanics—like superposition, measurement disturbance, and the no-cloning theorem—to provide unconditional security that isn’t dependent on computational difficulty.
The Goal
To allow Alice and Bob to:
- Share a random, secret key securely
- Be able to detect any eavesdropper (usually called Eve)
- Use that key to encrypt messages using classical methods (e.g., One-Time Pad)
The Core Ideas Behind BB84
Let’s go over a few fundamental quantum principles that the protocol uses:
- Qubits can be in a superposition of states (not just 0 or 1).
- Measuring a qubit changes it, unless done in the correct basis.
- There are two types of polarization bases used:
- Rectilinear basis (+): Horizontal (|0⟩) and Vertical (|1⟩)
- Diagonal basis (×): 45° and 135° polarizations
- No-cloning theorem: A quantum state can’t be copied exactly—meaning Eve can’t clone qubits and measure them later.
Step-by-Step: How the BB84 Protocol Works
Step 1: Alice Generates Random Bits and Bases
- Alice prepares a random sequence of bits (0s and 1s).
- For each bit, she randomly selects a basis—either rectilinear (+) or diagonal (×).
- Then she encodes each bit as a photon in the chosen polarization.
For example:
- Bit = 0, Basis = + → Horizontal polarization
- Bit = 1, Basis = × → 135° polarization
She then sends this stream of encoded photons to Bob over a quantum channel (like fiber optics).
Step 2: Bob Measures the Incoming Qubits
- Bob doesn’t know which basis Alice used.
- So, for each incoming photon, he randomly picks a basis (either + or ×) and measures the qubit accordingly.
- If Bob’s basis matches Alice’s, he gets the correct bit.
- If not, his measurement is random.
At this point, Bob has a string of bits—but he doesn’t know which are correct yet.
Step 3: Public Basis Reconciliation
- After all qubits are sent and measured, Alice and Bob communicate over a classical public channel (which can be monitored by Eve but not altered).
- They announce the basis they used for each qubit—but not the actual bit values.
- They keep only the bits where their bases matched. This shared subset becomes the raw key.
Typically, about 50% of the bits are discarded at this stage, because the bases didn’t match.
Step 4: Eavesdropping Detection (Error Estimation)
- To detect Eve, Alice and Bob sacrifice a small part of their raw key and publicly compare a few bits.
- If there’s no or very low error, they assume Eve hasn’t interfered.
- If the error rate is high, they know the transmission was compromised, and they abort the protocol.
Why does this work? Because Eve doesn’t know the correct basis. If she measures a qubit with the wrong one, she disturbs the photon’s state, introducing errors in Bob’s results.
Step 5: Key Refinement
If no eavesdropping is detected:
- Alice and Bob apply error correction protocols to ensure their raw keys are identical.
- They also apply privacy amplification, which reduces Eve’s potential knowledge of the key to near zero, even if she got partial information.
What remains is a shared, secure encryption key that can be used with classical cryptography.
A Real-World Analogy: Colored Glasses
Imagine Alice is sending Bob colored balls (red and blue), but she uses one of two types of lenses—regular or tinted. Bob also uses one of the two types of lenses to look at the balls. If they both use the same type, Bob sees the true color. If not, he sees a distorted color.
Eve, trying to intercept the ball and look at it, doesn’t know which lens Alice used—so she might distort the color. Later, when Bob checks the colors and compares lens types with Alice, they’ll notice any suspicious alterations.
Why Is BB84 So Secure?
- Eve can’t copy qubits: She can’t intercept and duplicate without disturbing them.
- Interception introduces errors: If Eve measures using the wrong basis, she collapses the quantum state, leading to errors.
- Errors are detectable: Alice and Bob can detect Eve’s interference just by comparing a few bits.
This makes BB84 provably secure, with security based on the laws of nature, not just complex math.
BB84 in Practice
Advantages:
- Simple and elegant
- Strong theoretical security
- Works with current photon-based technology
Challenges:
- Photon loss in long-distance fiber optics
- Expensive and sensitive hardware needed (single-photon detectors)
- Limited by environmental noise
Solutions:
- Quantum repeaters (future tech) to extend distances
- Decoy states to prevent photon-number splitting attacks
- Satellite QKD for global-scale communication (already demonstrated by China’s Micius satellite)
BB84’s Real-World Use
The BB84 protocol has moved from theory to reality:
- Used in secure banking and government communications
- Incorporated in quantum networks across cities and countries
- Backbone of the developing Quantum Internet
Major telecom and cybersecurity firms are already working to integrate QKD—often using BB84 or its variants—into their security infrastructure.
Summary
- BB84 is the first quantum key distribution protocol, leveraging quantum mechanics to securely share encryption keys.
- It uses random bases, quantum states, and measurement disturbance to detect eavesdropping.
- It enables tamper-evident communication, ensuring that any interception attempt is revealed.
- Although practical implementation has some hurdles, BB84 remains the foundation of quantum cryptography.