What is E91 protocol?

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1. Definition

The E91 protocol is a quantum key distribution (QKD) protocol proposed in 1991 by Artur Ekert.
Unlike BB84 (which uses photon polarization states), E91 relies on quantum entanglement and the violation of Bell’s inequalities to ensure secure key exchange.

2. Core Idea

• In E91, Alice and Bob share pairs of entangled particles (usually photons).

• Due to entanglement, the measurement outcome of one particle is strongly correlated with the other, no matter how far apart they are.

• The correlations can be tested using Bell’s inequality to detect eavesdropping.

• If an eavesdropper (Eve) tries to interfere, the entanglement is disturbed, and the correlations break down, exposing her presence.

3. How E91 Works (Step by Step)

1. Entangled Pair Generation

   • A source (trusted or untrusted) creates entangled photon pairs.

   • One photon goes to Alice, the other to Bob.

2. Random Measurement Choices

   • Alice and Bob each randomly choose one of several measurement bases (different polarization angles).

   • They record their results.

3. Public Comparison

   • Alice and Bob publicly announce which bases they used, but not the actual results.

   • For the subset of measurements where their chosen bases match, their results are perfectly correlated → used to form the raw key.

4. Bell’s Inequality Test

   • From another subset of measurements, they check whether Bell’s inequality is violated.

   • Violation confirms true quantum entanglement and rules out classical correlations or eavesdropping.

5. Error Correction & Privacy Amplification

   • Just like in BB84, they apply classical post-processing to correct errors and remove leaked information.

   • The final result is a shared secret key.

4. Why It’s Secure

• Security comes from the nonlocal correlations of entangled particles.

• Any eavesdropping attempt introduces noise and reduces the violation of Bell’s inequality, which Alice and Bob can detect.

• Even if the entanglement source is controlled by an adversary, the test ensures security.

5. Advantages of E91

• Uses fundamental quantum entanglement, offering stronger security guarantees.

• Eavesdropping can be detected more robustly.

• Inspired later device-independent QKD protocols, where security does not depend on trusting the source.

6. Limitations

• Requires reliable generation and transmission of entangled particles (still challenging in practice).

• More complex and expensive to implement than BB84.

• Sensitive to photon losses and noise.

✅ Key Tip:
Think of E91 as Alice and Bob sharing magically linked dice: whenever they roll them, the results are mysteriously correlated. If Eve tampers, the magical link breaks, and Alice and Bob immediately notice.

Read More  :

What are the limitations of quantum computing today?

What is BB84 protocol?

What is quantum cryptography?

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