Quantum Key Distribution and Qubits ECES Practice Questions (Page 4)

Questions 16–20

1. 16application · medium

   A company is considering using QKD to secure communication between two branches. They want to ensure that the key exchange is secure against future quantum computers. What is the primary reason QKD is considered quantum-safe?

   • AQKD's security relies on the laws of quantum mechanics, not on computational hardness, so it is immune to quantum computer attacks
   • BQKD is a post-quantum cryptographic algorithm that is resistant to quantum attacks
   • CQKD uses quantum computers to generate the keys, making them resistant to classical attacks
   • DQKD uses extremely long keys that are impossible to brute-force even with quantum computers
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2. 17expert · hard

   A QKD system is under attack. The eavesdropper is using a photon number splitting (PNS) attack, which exploits multi-photon pulses. What is the best defense against this attack?

   • AIncrease the intensity of the laser to ensure single-photon pulses.
   • BUse a longer wavelength to reduce photon loss.
   • CUse a trusted relay to prevent the attack.
   • DUse decoy states to detect the presence of multi-photon pulses.
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4. 18expert · hard

   A quantum physicist is designing a BB84 implementation and needs to choose a physical system for qubits. They require high transmission rates and low error rates over a fiber link. Which qubit implementation is most suitable?

   • ATrapped ions, because they have long coherence times.
   • BNuclear spins, because they are stable.
   • CPolarization-encoded photons, because they are easy to generate and detect, and are widely used in QKD.
   • DSuperconducting circuits, because they operate at high speeds.
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5. 19expert · hard

   A QKD system using BB84 is deployed on a noisy optical fiber link. The system's error rate is 8%, which is above the typical security threshold. The operator must decide whether to use the key or abort. What is the most appropriate action?

   • AUse the key after applying error correction and privacy amplification to reduce the error rate
   • BUse the key for non-critical data, as the error rate is not high enough to be a security concern
   • CIncrease the laser intensity to reduce the error rate and then use the key
   • DAbort the key and restart the QKD process, as the high error rate indicates possible eavesdropping or excessive noise
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6. 20foundation · easy

   Which property of quantum mechanics is exploited by QKD to ensure secure key exchange?

   • AMeasuring a quantum state inevitably disturbs it, revealing the presence of an eavesdropper.
   • BQuantum states can be cloned perfectly, allowing error correction.
   • CQuantum entanglement allows instantaneous communication over any distance.
   • DQuantum bits can be stored indefinitely without loss, ensuring reliable key exchange.
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