Quantum Key Distribution and Qubits ECES Practice Questions (Page 6)

Questions 26–30

1. 26expert · hard

   A financial institution wants to deploy QKD between two data centers 300 km apart. They are considering using fiber-based QKD with trusted repeaters. What is a critical security consideration they must address?

   • ATrusted repeaters are not needed for 300 km distances, as fiber QKD can span that distance without them
   • BTrusted repeaters increase the key rate, making the system more efficient and secure
   • CTrusted repeaters amplify the quantum signal without disturbing it, ensuring end-to-end security
   • DTrusted repeaters must be physically secured because they have access to the key material
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2. 27application · medium

   A team is implementing BB84 and needs to encode bits into qubits. They decide to use the polarization of photons. Which mapping is correct for BB84?

   • AUse only one basis with two non-orthogonal states to represent 0 and 1.
   • BUse three bases to increase the key rate.
   • CUse horizontal/vertical polarization for basis Z, and diagonal/anti-diagonal for basis X, with each basis mapping 0 and 1 to orthogonal states.
   • DUse the same basis for all qubits to simplify the protocol.
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4. 28application · medium

   A quantum cryptography researcher is explaining to a client why QKD can detect an eavesdropper who tries to copy the transmitted qubits. Which principle best supports this claim?

   • AThe Heisenberg uncertainty principle limits the speed at which qubits can be transmitted.
   • BThe observer effect guarantees that the eavesdropper will be detected by the sender's measurement.
   • CQuantum entanglement ensures that any measurement collapses the qubit to a known state.
   • DThe no-cloning theorem prevents an eavesdropper from making an exact copy of an unknown quantum state.
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5. 29application · medium

   A developer is implementing BB84 and needs to ensure that the key is not compromised. After sifting, they have a raw key of 1000 bits. They want to reduce the key size to account for potential eavesdropping. Which process should they use?

   • APrivacy amplification to reduce the key size and eliminate any information an eavesdropper might have.
   • BSifting again to remove more bits.
   • CKey expansion to increase the key size for better security.
   • DError correction to fix any bit errors in the key.
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6. 30foundation · easy

   What happens to a qubit when it is measured in a basis that is not aligned with its state?

   • AThe qubit collapses to one of the basis states with a probability determined by the original state.
   • BThe qubit is destroyed and cannot be used further.
   • CThe qubit automatically aligns to the measurement basis without any change in its information.
   • DThe qubit's state is unchanged, and the measurement result is deterministic.
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