Provably-Secure and High-Rate Quantum Key Distribution with Time-Bin Qudits (1709.06135v1)

Abstract: The security of conventional cryptography systems is threatened in the forthcoming era of quantum computers. Quantum key distribution (QKD) features fundamentally proven security and offers a promising option for quantum-proof cryptography solution. Although prototype QKD systems over optical fiber have been demonstrated over the years, the key generation rates remain several orders-of-magnitude lower than current classical communication systems. In an effort towards a commercially viable QKD system with improved key generation rates, we developed a discrete-variable QKD system based on time-bin quantum photonic states that is capable of generating provably-secure cryptographic keys at megabit-per-second (Mbps) rates over metropolitan distances. We use high-dimensional quantum states that transmit more than one secret bit per received photon, alleviating detector saturation effects in the superconducting nanowire single-photon detectors (SNSPDs) employed in our system that feature very high detection efficiency (of over 70%) and low timing jitter (of less than 40 ps). Our system is constructed using commercial off-the-shelf components, and the adopted protocol can readily be extended to free-space quantum channels. The security analysis adopted to distill the keys ensures that the demonstrated protocol is robust against coherent attacks, finite-size effects, and a broad class of experimental imperfections identified in our system.

• The paper introduces a QKD system that leverages high-dimensional time-bin qudits to encode more information per photon and achieve megabit-per-second secure key rates.
• It employs superconducting nanowire single photon detectors with over 70% efficiency and under 40 ps timing jitter, reducing detector saturation.
• Experimental results validate a secret key rate of 26.2 Mbps over 20 km of fiber, highlighting the system's potential for integration into urban networks.

Overview of "Provably-Secure and High-Rate Quantum Key Distribution with Time-Bin Qudits"

The paper "Provably-Secure and High-Rate Quantum Key Distribution with Time-Bin Qudits," authored by Nurul T. Islam et al., presents a significant advancement in the field of quantum key distribution (QKD). The paper emphasizes a discrete-variable QKD system leveraging high-dimensional quantum states referred to as time-bin qudits, critically improving the key generation rates while ensuring provable security against various quantum attacks.

Key Contributions

The authors introduce a QKD system that uses high-dimensional time-bin qudits instead of traditional two-dimensional qubits. This approach inherently increases the amount of information encoded per photon, enabling the generation of secure keys at megabit-per-second (Mbps) rates over metropolitan-scale optical fiber networks. This advancement addresses a crucial bottleneck in QKD—the discrepancy between the lower key generation rates in existing QKD systems and the higher rates needed for practical use, parallel to current classical communication channels.

Several notable technical implementations are outlined:

1. Time-Bin Qudits: The system employs temporal states in a four-dimensional space to overcome challenges such as detector saturation, enhancing the system's efficacy over short to moderate distances typical of metropolitan networks.
2. High-Efficiency Detectors: The application of superconducting nanowire single photon detectors (SNSPDs) with over 70% detection efficiency and less than 40 ps timing jitter significantly amplifies the system's detection capability.
3. Security: The security framework is rigorously analyzed through entropic uncertainty relations, addressing vulnerabilities such as coherent attacks and finite-size effects. The protocol is validated for both collective and general attacks, ensuring robust security under realistic conditions.
4. Experimentation and Realization: The system implementation utilizes commercially available components, indicating its practical viability and potential for deployment in existing network infrastructures. The authors provide detailed experimental results demonstrating a secret key rate of 26.2 Mbps over a channel loss equivalent to 20 km of telecommunication-grade optical fiber.

Implications and Future Directions

The research contributes importantly to the evolution of quantum-safe communication, presenting a pathway to integrate QKD systems into current fiber-optic networks established in urban areas. By achieving high key-generation rates, the system enhances the feasibility of QKD for protecting data transfer against future threats posed by quantum computers.

Several future lines of work could further optimize this technology:

• Increased Dimensionality: Exploring qudits beyond four dimensions could potentially deliver greater enhancements in key rates and noise tolerance, increasing system resilience against practical imperfections.
• Integration with Dense Wavelength Division Multiplexing (DWDM): Implementing DWDM could permit simultaneous use of multiple spectral channels, thus amplifying the overall throughput of QKD systems without proportional increases in infrastructure complexity or cost.
• Enhanced Detection Technology: Continued advancements in SNSPDs might reduce dead times and further expand bandwidth, resulting in higher effective key generation even under elevated photon detection rates.
• Chip-Based Implementations: Developing chip-scale solutions for interferometric setups could lower insertion losses and streamline integration with existing communication technologies, increasing both practical utility and deployment ease.

Conclusion

Islam et al.'s work presents a sophisticated approach to high-rate QKD, employing time-bin qudits to bridge significant gaps in speed and security. This research marks a step forward in transitioning QKD from theoretical development stages to practical, widespread adoption, positioning quantum cryptography to address looming cybersecurity challenges in the quantum-computing era.