The E91 Protocol is a Quantum Key Distribution (QKD) method proposed by physicist Artur Ekert in 1991. It was a significant development in quantum cryptography because it uses quantum entanglement rather than single-photon polarization like in BB84.
The E91 protocol stands out because:
- It uses pairs of entangled particles to create a secure key.
- It relies on Bell’s Theorem to detect eavesdropping.
- It provides a stronger security guarantee by showing that any interception breaks the fundamental laws of quantum physics.
Core Concepts Behind E91
Before jumping into the protocol steps, here are the basic quantum principles it uses:
1. Entanglement
Two quantum particles can be “entangled” such that their states are perfectly correlated—no matter how far apart they are. Measuring one instantly tells you the state of the other.
2. Bell’s Theorem
Bell’s Theorem proves that if two particles violate certain mathematical inequalities (Bell inequalities), they must be entangled in a way that cannot be explained by classical physics. This makes it possible to test whether the system is truly quantum or has been tampered with.
3. No Hidden Variables
E91 assumes the universe doesn’t hide information in advance (no “hidden variables”), so any measurements must be fundamentally uncertain until they’re made.
Objective of E91
To allow two users—Alice and Bob—to:
- Share a secure encryption key over a quantum channel.
- Use entanglement and Bell’s inequality testing to detect if someone (like Eve) tries to intercept their communication.
Step-by-Step Breakdown of the E91 Protocol
Step 1: Entangled Photon Source
Instead of Alice preparing photons, the E91 protocol begins with a source (can be a trusted third party or even an untrusted one) that generates entangled photon pairs.
- Each pair of photons is entangled such that their quantum states are perfectly correlated.
- One photon from each pair is sent to Alice, and the other to Bob, through separate quantum channels.
This happens repeatedly, creating a stream of entangled pairs.
Step 2: Alice and Bob Make Random Measurements
Both Alice and Bob have polarizers (or detectors) that can be set to different angles or bases.
- Each time they receive a photon, they randomly choose a basis (or angle) to measure it in.
- For each measurement, they record:
- The basis they used
- The result of the measurement (0 or 1)
📌 Note: The measurement results will be correlated only if the photons were truly entangled and no one tampered with them.
Step 3: Public Basis Announcement
After enough photon pairs have been measured:
- Alice and Bob communicate over a public classical channel.
- They announce which basis they used for each photon.
- They do not reveal the measurement outcomes.
Step 4: Keep Matching Bases and Discard the Rest
- Alice and Bob keep only the measurements where their bases matched (or are compatible for correlation).
- These remaining results form the raw key.
The idea is: If the entangled particles were untouched, their results will be perfectly correlated when measured in the same basis.
Step 5: Bell Inequality Test
This is the key innovation in E91.
- Alice and Bob use a subset of their measurements (with different basis combinations) to calculate whether their results violate Bell’s Inequality.
Why this matters:
- If Bell’s inequality is violated, it means their photons were truly entangled and the communication was not intercepted.
- If not, it’s likely that Eve has tampered with the photons, and they discard the key.
This test is done after the quantum exchange, during the public discussion phase.
Step 6: Key Extraction and Error Checking
If Bell’s inequality is successfully violated (i.e., the entanglement is preserved):
- Alice and Bob proceed to refine the key:
- Use error correction to align their keys (due to imperfections or channel noise)
- Use privacy amplification to reduce any possible knowledge Eve might have
The final result is a shared, secret encryption key that can now be used for secure communication.
Why Is the E91 Protocol Secure?
- No Trust Needed in the Source
Even if the entangled photon source is controlled by Eve, she can’t know the results unless she breaks the entanglement—which is detectable through Bell’s inequality. - No Cloning or Copying
Entangled particles cannot be copied or cloned. Any interception disturbs the entanglement and changes the measurement statistics. - Measurement Choices Are Private
Since Alice and Bob randomly and independently choose their measurement settings, Eve can’t predict or match them. - Testable Security via Bell’s Theorem
The protocol offers a mathematical test (Bell test) to prove that quantum entanglement exists and is undisturbed.
Comparison with BB84
| Feature | BB84 | E91 |
|---|---|---|
| Uses single photons | Yes | No, uses entangled pairs |
| Uses entanglement | No | Yes |
| Needs trust in source | Yes | No (E91 is device-independent) |
| Bell inequality test | No | Yes |
| Security proof based on | Measurement disturbance | Non-local correlations (Bell violation) |
Practical Considerations
Advantages
- Higher theoretical security (device-independent security)
- Can be more robust against certain attacks (like detector-based hacking)
- Can be adapted for quantum networks and satellites
Challenges
- Harder to implement due to need for high-quality entanglement
- Requires precise synchronization
- More complex equipment needed compared to BB84
Real-World Applications
E91 is gaining attention in:
- Quantum satellite networks
- Quantum internet prototypes
- Secure government and military communications
- Long-distance quantum communication, where entanglement plays a key role
Several experimental QKD systems, including some by European Space Agency, China, and IBM, have explored E91-style entanglement-based protocols.