UPSC Prelims Practice Questions — India’s First 5.56 km Free‑Space Quantum Key Distribution Link Demonstrated

Q1. With reference to Quantum Key Distribution (QKD) and Post-Quantum Cryptography (PQC), consider the following statements: Which of the statements given above is/are correct?

  1. PQC algorithms can be deployed on existing classical computers and networks without any quantum transmission channel.
  2. The security of QKD rests on the assumed computational difficulty of a mathematical problem for which no efficient quantum algorithm is known.
  3. In a hybrid architecture, a PQC layer can keep communication protected during intervals when the quantum optical channel is unavailable.
  • A. 1, 2 and 3
  • B. 1 and 3 only
  • C. 1 and 2 only
  • D. 2 and 3 only

Q2. With reference to the Quantum Bit Error Rate (QBER) in a Quantum Key Distribution link, consider the following statements: Which of the statements given above is/are correct?

  1. Errors from atmospheric turbulence and detector noise also add to the QBER, so a non-zero QBER does not by itself prove that an eavesdropper is present.
  2. If the QBER of a run rises above the protocol's security threshold, the parties discard that key instead of using it for encryption.
  3. A low QBER over long distances is maintained by amplifying the photon signal at intermediate repeaters placed along the channel.
  • A. 1 only
  • B. 1, 2 and 3
  • C. 1 and 2 only
  • D. 2 and 3 only

Q3. Consider the following statements: Which of the following is correct in respect of the above statements?

  1. In Quantum Key Distribution, the ability to detect an eavesdropper does not depend on how much computing power the eavesdropper has.
  2. Measuring an unknown quantum state disturbs it, and such a state cannot be copied perfectly, so any interception leaves detectable traces in the transmitted key.
  • A. Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
  • B. Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
  • C. Statement-I is correct, but Statement-II is incorrect
  • D. Statement-I is incorrect, but Statement-II is correct

Q4. Consider the following statements: Which of the following is correct in respect of the above statements?

  1. Satellite-based links are being pursued for Quantum Key Distribution between ground stations that are thousands of kilometres apart.
  2. Quantum signals in optical fibre can be boosted by conventional optical amplifiers at regular intervals, so long-distance fibre QKD is limited mainly by the cost of laying fibre.
  • A. Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
  • B. Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
  • C. Statement-I is correct, but Statement-II is incorrect
  • D. Statement-I is incorrect, but Statement-II is correct

Q5. A 5.56 km free-space (through-air) Quantum Key Distribution link has been described as a stepping stone towards satellite-based QKD. In this context, consider the following statements: Which of the statements given above is/are correct?

  1. Over comparable distances, a satellite-to-ground quantum link loses far fewer photons than optical fibre, because most of its path lies above the dense lower atmosphere.
  2. A free-space quantum link needs continuous pointing, acquisition and tracking to keep transmitter and receiver aligned, and this becomes harder when one terminal is in orbit.
  3. Because of the Earth's curvature, direct ground-to-ground line-of-sight links cannot stretch beyond a few hundred kilometres, which is one reason a satellite is used as an intermediate node.
  • A. 2 only
  • B. 1 and 3 only
  • C. 1 and 2 only
  • D. 1, 2 and 3

Q6. With reference to Quantum Random Number Generators (QRNGs) and the pseudo-random number generators (PRNGs) used in conventional computers, consider the following statements: Which of the statements given above are correct?

  1. A PRNG started from the same seed produces the same sequence every time, because it is a deterministic algorithm.
  2. A QRNG can derive random bits from which output path a single photon takes at a balanced beam splitter.
  3. Because its randomness is quantum in origin, a QRNG's output need not be checked against statistical randomness test suites before deployment.
  4. A QRNG can strengthen key generation even in a system that uses only classical post-quantum algorithms and has no quantum transmission channel.
  • A. 1, 2 and 4 only
  • B. 1, 2, 3 and 4
  • C. 2, 3 and 4 only
  • D. 1 and 2 only

Q7. Consider the following assertion: Government, defence and critical-infrastructure communications need quantum-safe protection now, even though large-scale quantum computers do not yet exist. Which of the following statements support/supports the above assertion?

  1. Free-space Quantum Key Distribution links currently deliver secure key rates of only a few hundred bits per second.
  2. Encrypted traffic intercepted and stored today could be decrypted later, once sufficiently powerful quantum computers become available.
  3. Much of today's public-key cryptography rests on mathematical problems whose hardness may not survive advanced quantum computers.
  • A. 2 and 3 only
  • B. 1 and 2 only
  • C. 3 only
  • D. 1, 2 and 3