Estimating divergently routed nonlinearly interfering channel powers using cross phase modulation.

The deployment of coherent transceivers in legacy networks requires significant investment in installation. We propose a method enabling autonomous (re-)configuration of an optical channel, which would be advantageous in legacy networks and necessary in proposed future networks utilizing a flexible frequency grid and software defined components such as reconfigurable optical add drop multiplexers (ROADM). We consider potential interfering optical channels propagating with the prospective channel along part of the fiber link which are dropped before arrival at the receiver. The method uses a commercially available line card transmitting a 40Gbit/s polarization multiplexed quadrature phase shift keying (PM-QPSK) probe channel to characterize an optical channel. The power of the nearest neighboring channels is then inferred by examining its bit error rate (BER) which is impacted by cross phase modulation (XPM) from the aggressor channels. In a 4 node network with 2 ROADMs using up to 6 aggressor OOK channels, we successfully deduce the neighboring channel power down to -2 dBm, and an extra improvement down to -3.5 dBm is gained, by measuring at an offset to the international telecommunication union (ITU) grid which also reveals inequality in neighbor channel powers.

Long-distance continuous-variable quantum key distribution over 100-km fiber with local local oscillator.

Quantum key distribution (QKD) enables two remote parties to share encryption keys with security based on the laws of physics. Continuous-variable (CV) QKD with coherent states and coherent detection integrates well with existing telecommunication networks. Thus far, long-distance CV-QKD has only been demonstrated using a highly complex scheme where the local oscillator is transmitted, opening security loopholes for eavesdroppers and limiting potential applications. Here, we report a long-distance CV-QKD experiment with a locally generated local oscillator over a 100-kilometer fiber channel with a total loss of 15.4 decibels. This record-breaking distance is achieved by controlling the phase noise-induced excess noise through a machine learning framework for carrier recovery and optimizing the modulation variance. We implement the full CV-QKD protocol and demonstrate the generation of keys secure against collective attacks in the finite-size regime. Our results mark a substantial milestone for realizing CV quantum access networks with a high loss budget and pave the way for large-scale deployment of secure QKD.

Practical continuous-variable quantum key distribution with composable security.

A quantum key distribution (QKD) system must fulfill the requirement of universal composability to ensure that any cryptographic application (using the QKD system) is also secure. Furthermore, the theoretical proof responsible for security analysis and key generation should cater to the number N of the distributed quantum states being finite in practice. Continuous-variable (CV) QKD based on coherent states, despite being a suitable candidate for integration in the telecom infrastructure, has so far been unable to demonstrate composability as existing proofs require a rather large N for successful key generation. Here we report a Gaussian-modulated coherent state CVQKD system that is able to overcome these challenges and can generate composable keys secure against collective attacks with N ≈ 2 × 108 coherent states. With this advance, possible due to improvements to the security proof and a fast, yet low-noise and highly stable system operation, CVQKD implementations take a significant step towards their discrete-variable counterparts in practicality, performance, and security.