Fundamental rate-loss tradeoff for optical quantum key distribution

Area of Science:

• Quantum Information Theory
• Quantum Cryptography
• Optical Communication Systems

Background:

• Optical quantum key distribution (QKD) protocols have been studied since 1984.
• Existing QKD protocols face an exponential decay in secret key generation rate with increasing distance.
• A fundamental question is whether protocols exist that can overcome this rate-distance tradeoff without quantum repeaters.

Purpose of the Study:

• To investigate the theoretical limits of secret key generation over lossy and noisy optical channels.
• To determine if optical QKD protocols can circumvent the established rate-distance tradeoff without quantum repeaters.

Main Methods:

• Theoretical analysis of secret key agreement capacity.
• Consideration of a lossy and noisy optical channel assisted by unlimited two-way public classical communication.
• Derivation of an upper bound for the secret key agreement capacity.

Main Results:

• The secret key agreement capacity is limited by an upper bound solely dependent on channel loss.
• This bound is independent of the optical power utilized by the protocol.
• The established rate-distance tradeoff for optical QKD cannot be circumvented without quantum repeaters.

Conclusions:

• The study provides a significant advancement in understanding the limits of secret key agreement in optical channels.
• The findings strongly indicate the necessity of quantum repeaters for achieving high-rate QKD over long distances.
• This research addresses a long-standing open problem in optical quantum information theory.