Quantum Key Distribution and Qubits ECES Practice Questions (Page 2)

Questions 6–10

1. 6application · medium

   A network administrator is implementing BB84 for a secure link. After the qubits are transmitted, the receiver measures them using randomly chosen bases. The sender and receiver then perform sifting. What is the purpose of sifting in BB84?

   • ATo discard all qubits that were measured in the wrong basis and keep only those where the bases match.
   • BTo amplify the key by adding additional random bits.
   • CTo correct all errors in the key using error correction codes.
   • DTo detect eavesdropping by comparing a subset of the key bits.
   Reveal Solution Discussion Select an answer first
2. 7expert · hard

   A QKD system is deployed over a 50 km fiber link. The system is experiencing a high error rate, and the operator suspects it is due to environmental noise. They also notice that the error rate fluctuates with the time of day. What is the most likely cause?

   • AThe detector efficiency is degrading over time.
   • BTemperature changes affecting the fiber's birefringence and causing polarization drift.
   • CThe eavesdropper is only active during certain hours.
   • DThe laser power is fluctuating due to power supply variations.
   Reveal Solution Discussion Select an answer first
4. 8application · medium

   A quantum physicist is explaining to a software engineer why a qubit cannot be perfectly copied. Which principle directly prevents the copying of an unknown quantum state?

   • AThe Heisenberg uncertainty principle, which limits the precision of simultaneous measurements
   • BThe no-cloning theorem, which states that an unknown quantum state cannot be duplicated exactly
   • CThe entanglement principle, which links the states of multiple qubits
   • DThe superposition principle, which allows qubits to exist in multiple states simultaneously
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5. 9expert · hard

   A QKD system is deployed and the error rate is 15%. The system is configured to abort if the error rate exceeds 11%. The operator suspects the high error rate is due to environmental noise rather than eavesdropping. What is the best course of action?

   • AProceed with the key exchange because the error rate is likely just noise.
   • BAbort the key exchange and investigate the source of the noise before retrying.
   • CIncrease the error rate threshold to 20% to allow the key to be used.
   • DUse error correction to fix the errors and proceed with the key.
   Reveal Solution Discussion Select an answer first
6. 10foundation · easy

   In the BB84 protocol, what is the purpose of the sifting step?

   • ATo encrypt the key with a one-time pad before transmission.
   • BTo correct all errors in the key without any communication.
   • CTo discard all bits where the sender and receiver used different bases.
   • DTo verify that the quantum channel is secure before sending any bits.
   Reveal Solution Discussion Select an answer first