K. Tyagi

Optical networking for quantum key distribution and quantum communications

Modern optical networking techniques have the potential to greatly extend the applicability of quantum communications by moving beyond simple point-to-point optical links and by leveraging existing fibre infrastructures. We experimentally demonstrate many of the fundamental capabilities that are required. These include optical-layer multiplexing, switching and routing of quantum signals; quantum key distribution (QKD) in a dynamically reconfigured optical network; and coexistence of quantum signals with strong conventional telecom traffic on the same fibre. We successfully operate QKD at 1310 nm over a fibre shared with four optically amplified data channels near 1550 nm. We identify the dominant impairment as spontaneous anti-Stokes Raman scattering of the strong signals, quantify its impact, and measure and model its propagation through fibre. We describe a quantum networking architecture which can provide the flexibility and scalability likely to be critical for supporting widespread deployment of quantum applications.

Dense wavelength multiplexing of 1550 nm QKD with strong classical channels in reconfigurable networking environments

To move beyond dedicated links and networks, quantum communications signals must be integrated into networks carrying classical optical channels at power levels many orders of magnitude higher than the quantum signals themselves. We demonstrate the transmission of a 1550 nm quantum channel with up to two simultaneous 200 GHz spaced classical telecom channels, using reconfigurable optical add drop multiplexer (ROADM) technology for multiplexing and routing quantum and classical signals. The quantum channel is used to perform quantum key distribution (QKD) in the presence of noise generated as a by-product of the co-propagation of classical channels. We demonstrate that the dominant noise mechanism can arise from either four-wave mixing or spontaneous Raman scattering, depending on the optical path characteristics as well as the classical channel parameters. We quantify these impairments and discuss mitigation strategies.

Practical long-distance quantum key distribution system using decoy levels

Quantum key distribution (QKD) has the potential for widespread real-world applications, but no secure long-distance experiment has demonstrated the truly practical operation needed to move QKD from the laboratory to the real world due largely to limitations in synchronization and poor detector performance. Here, we report results obtained using a fully automated, robust QKD system based on the Bennett Brassard 1984 (BB84) protocol with low-noise superconducting nanowire single-photon detectors (SNSPDs) and decoy levels to produce a secret key with unconditional security over a record 140.6 km of optical fibre, an increase of more than a factor of five compared with the previous record for unconditionally secure key generation in a practical QKD system.

Demonstration of 1550 nm QKD with ROADM-based DWDM Networking and the Impact of Fiber FWM

We demonstrate compatibility of 1550 nm QKD with a MEMS-based ROADM and also show that four-wave mixing resulting from copropagating DWDM signals can become the dominant source of background noise within the QKD channel passband.

Experimental characterization of the separation between wavelength-multiplexed quantum and classical communication channels

Quantum key distribution (QKD) is a new technique for secure key distribution based on the laws of physics rather than mathematical or algorithmic computational complexity used by current systems. Understanding the compatibility of QKD at 1310 nm with the existing commercial optical networks bearing classical wavelength-division-multiplexed (WDM) channels at 1550 nm is important to advance the deployment of QKD systems in such networks. The minimum wavelength separation for multiplexing QKD and WDM channels on a shared fiber is experimentally determined for impairment-free QKD+WDM transmission.

Impact of spontaneous anti-Stokes Raman scattering on QKD+DWDM networking

This study presents an experimental demonstration of 1310 nm QKD multiplexing and transmission with amplified DWDM signals over a shared 10 km fiber span. This work identifies anti-Stokes Raman scattering generated during fiber propagation as the primary contributor of crosstalk noise at the QKD receiver. New results are presented on the characterization of spontaneous anti-Stokes Raman noise (SASRN), generated within the fiber by the high-power DWDM signals, and implications for QKD+DWDM networking architectures are also discussed.

Progress toward quantum communications networks: opportunities and challenges

Quantum communications is fast becoming an important component of many applications in quantum information science. Sharing quantum information over a distance among geographically separated nodes using photonic qubits requires a reconfigurable transparent networking infrastructure that can support quantum information services. Using quantum key distribution (QKD) as an example of a quantum communications service, we investigate the ability of fiber networks to support both conventional optical traffic and single-photon quantum communications signals on a shared infrastructure. The effect of Raman scattering from conventional channels on the quantum bit error rate (QBER) of a QKD system is analyzed. Additionally, the potential impact and mitigation strategies of other transmission impairments such as four-wave mixing, cross-phase modulation, and noise from mid-span optical amplifiers are discussed. We also review recent trends toward the development of automated and integrated QKD systems which are important steps toward reliable and manufacturable quantum communications systems.

Quantum cryptography for optical networks: a systems perspective

In this paper, we demonstrate for the first time, the coexistence architectures which enable quantum cryptographic systems to use the emerging optical network infrastructure. A vision of a future network that combines classical optical communications with ultra-low power quantum signals over a shared, reconfigurable fiber infrastructure is illustrated. We have used a fiber quantum key distribution (QKD) system to experimentally measure the effective QKD throughput for various types of optical switch elements, including opto-mechanical, lithium niobate, and 2D MEMS. We have investigated QKD coexistence with a commercial EDFA-amplified DWDM system using the experimental setup. These results are an important first step for scaling QKD beyond dedicated point-to-point fiber links and provide a context for understanding the applicability of QKD to the broader optical networking environment.

Long Distance Quantum Key Distribution in Optical Fiber

Results are presented from an experiment using quantum key distribution with decoy states and low- noise superconducting nanowire single photon detectors to distribute secure key across 145 km of optical fiber.

Compatibility of quantum key distribution with optical networking

Quantum key distribution (QKD) is an emerging technology for secure distribution of keys between users linked by free-space or fiber optic transmission facilities. QKD has usually been designed for and operated over dedicated point-to-point links. However, the commercial world has been developing increasingly sophisticated fiber networks, with basic networking functions such as routing and multiplexing performed in the optical domain. One of the most important practical questions for the future of QKD is to what extent it can benefit from these trends, either to expand the capabilities of dedicated quantum networks, or to avoid the need for dedicated networks by combining quantum and conventional optical signals onto a single infrastructure. In this paper, we report on systematic investigations of these issues using a 1310-nm weak-coherent, phase-encoded B92 prototype QKD system developed by Los Alamos that includes the implementation of error correction, privacy amplification, and authentication. We have demonstrated reconfigurability of QKD networks via optical switching and successful QKD operation in the presence of amplified DWDM signals over 10 km of fiber. We have identified anti-Stokes Raman scattering of the DWDM signals in the fiber as a dominant transmission impairment for QKD, and developed filtering architectures to extend transmission distances to at least 25 km. We have also measured noise backgrounds and polarization variations in network fibers to understand applicability to real-world networks. We will discuss the implications of our results for the choice of QKD wavelengths, wavelength-spacing between QKD and conventional channels, and QKD network architectures.