Quantum Key Distribution and Qubits ECES Practice Questions (Page 5)

Questions 21–25

1. 21expert · hard

   A security architect is comparing QKD with classical key exchange methods like Diffie-Hellman. They need to choose a method for a system that must remain secure against future quantum computers. Which consideration is most important?

   • ADiffie-Hellman can be made quantum-safe by increasing the key size.
   • BQKD provides information-theoretic security based on quantum mechanics, while Diffie-Hellman relies on computational assumptions that quantum computers could break.
   • CDiffie-Hellman is more secure because it has been used for decades.
   • DQKD is easier to deploy over long distances than Diffie-Hellman.
   Reveal Solution Discussion Select an answer first
2. 22application · medium

   A security engineer is setting up a QKD link between two offices. The sender uses the BB84 protocol and transmits a sequence of qubits. After the transmission, the receiver announces which measurement bases were used. The sender then reveals which bases were correct. During sifting, the engineer notices that the error rate is above the expected threshold. What should the engineer conclude?

   • AAn eavesdropper may have intercepted and measured the qubits, causing disturbances.
   • BThe qubits were transmitted too slowly, causing decoherence and bit flips.
   • CThe receiver used the wrong measurement bases, so the key must be resent with different bases.
   • DThe key is still valid; the error rate only affects the final key length.
   Reveal Solution Discussion Select an answer first
4. 23expert · hard

   A quantum computing team is designing a new QKD protocol. They want to use qubits that are entangled pairs. How does entanglement enhance QKD security compared to BB84?

   • AEntanglement allows the key to be generated without transmitting any qubits, making it immune to interception
   • BEntanglement eliminates the need for sifting, simplifying the protocol
   • CEntanglement ensures that any measurement by an eavesdropper breaks the entanglement, which can be detected by the legitimate parties
   • DEntanglement allows the key to be generated at a higher rate than BB84, improving efficiency
   Reveal Solution Discussion Select an answer first
5. 24application · medium

   A government agency wants to deploy QKD for secure communication between two buildings in the same city, about 5 km apart. They are considering using free-space optical QKD. What is a key practical advantage of free-space QKD over fiber-based QKD in this scenario?

   • AFree-space QKD does not require a clear line of sight between the transmitter and receiver
   • BFree-space QKD can achieve higher key rates over short distances because it avoids fiber attenuation
   • CFree-space QKD is immune to weather conditions such as fog and rain
   • DFree-space QKD does not require any specialized equipment beyond standard networking gear
   Reveal Solution Discussion Select an answer first
6. 25application · medium

   A quantum computing engineer is explaining to a colleague how a qubit differs from a classical bit. They want to illustrate the concept of superposition. Which example best demonstrates superposition?

   • AA qubit can store more information than a classical bit because it has two states.
   • BA qubit can be in a state of 0, 1, or 2, unlike a classical bit which is only 0 or 1.
   • CA qubit can represent both 0 and 1 simultaneously until it is measured.
   • DA qubit is always in a superposition of 0 and 1, and measurement always yields 0.
   Reveal Solution Discussion Select an answer first