Quantum Key Distribution and Qubits ECES Practice Questions (Page 1)

Questions 1–5

1. 1application · medium

   A startup is building a secure communication system and wants to use QKD to distribute symmetric keys. They plan to use the keys for AES-256 encryption. Which statement correctly describes the role of QKD in this system?

   • AQKD provides a method to securely exchange symmetric keys, which are then used with AES-256 for data encryption.
   • BQKD is only useful for key distribution over short distances and cannot be combined with AES.
   • CQKD uses quantum entanglement to encrypt data directly, so no separate encryption algorithm is needed.
   • DQKD replaces AES-256 encryption entirely, providing confidentiality without the need for symmetric encryption.
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2. 2foundation · easy

   In BB84, how are bits encoded onto photons?

   • ABy varying the intensity of the photon to represent 0 or 1.
   • BBy using the photon's wavelength to represent 0 or 1.
   • CBy using two different polarizations, each representing a 0 or 1, and randomly choosing between two bases.
   • DBy using the photon's phase, but only in one basis.
   Reveal Solution Discussion Select an answer first
4. 3application · medium

   During a BB84 QKD session, Alice sends a photon encoded in the rectilinear basis with a bit value of 1. Bob, however, chooses to measure in the diagonal basis. What is the outcome of Bob's measurement?

   • ABob will measure either 0 or 1 with equal probability, and the bit will be discarded during sifting
   • BBob will detect an error and abort the QKD session immediately
   • CBob will always measure a 1 because the photon's state is definite
   • DBob will measure a 0 because the diagonal basis flips the bit value
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5. 4foundation · easy

   Why does the no-cloning theorem make eavesdropping in QKD detectable?

   • AAn eavesdropper cannot measure the photon without knowing the basis, but the basis is secret.
   • BAn eavesdropper can clone the state, but the clone is always imperfect and can be detected.
   • CAn eavesdropper can only intercept a limited number of photons, making detection possible.
   • DAn eavesdropper cannot copy the quantum state without disturbing it, so any interception introduces errors.
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6. 5application · medium

   A security engineer is setting up a QKD link between two data centers using the BB84 protocol. During the sifting phase, Alice and Bob publicly compare a subset of their measurement bases. What is the primary purpose of this comparison?

   • ATo establish a shared secret key by exchanging the actual bit values over the public channel
   • BTo synchronize the clocks of Alice and Bob so that photon arrival times align correctly
   • CTo verify that the photons were not absorbed by the optical fiber before reaching Bob
   • DTo determine which photons were transmitted with matching bases and can be used to form the raw key
   Reveal Solution Discussion Select an answer first