Can a quantum detector surpass the non-linear Shannon limit?

Catherine Laflamme, Aashish A. Clerk

Matteo Rosati

Non-linear effects are believed to reduce the communication rate in fiber networks, leading to the introduction of the non-linear Shannon (NLS) limit. Here, we analyze, for the first time to our knowledge, the effect of quantum detection techniques in amplified optical fibers affected by non-linearity, by computing their Holevo channel capacity. First, we report that the Holevo capacity under lumped Raman amplification (RA) can surpass the NLS capacity of a coherent-detection system with distributed Raman amplification (DRA) for a moderate number of spans. Then, we observe that the Holevo RA system offers a reduction of amplification power costs at a fixed communication rate, with respect to the DRA coherent-detection system. Our results rely on the ability of a quantum joint-detection receiver (JDR) to better extract information from low-power, low-noise signals, determining the type of amplifier that is best suited to work in conjunction with the JDR.

Alfredo Luis

H. A. Haus

Ya-Long Ren, Rong-Teng Cao, Sheng-Li Ma +4 more

Linear quantum amplifiers are indispensable tools for quantum technologies, yet their performance is fundamentally limited by quantum noise, precluding any signal-to-noise ratio (SNR) enhancement unless supplemented by post-selection or non-classical resources. To surpass this limitation, we propose a nonlinear quantum amplification strategy that exploits the interplay between a gain-stabilized bright eigenmode of a coupled two-mode bosonic system and Kerr nonlinearity. We demonstrate that this interplay enables the signal gain to surpass the noise gain in a selected quadrature, leading to a net increase in the SNR beyond the quantum limits of conventional linear amplifiers. Our work thus establishes a novel nonlinear amplification paradigm capable of enhancing the SNR, with promising applications across quantum information processing, quantum communications, and quantum metrology.

Jasminder S. Sidhu, Shuro Izumi, Jonas Schou Neergaard-Nielsen +2 more

Quantum enhanced receivers are endowed with resources to achieve higher sensitivities than conventional technologies. For application in optical communications, they provide improved discriminatory capabilities for multiple non-orthogonal quantum states. In this work, we propose and experimentally demonstrate a new decoding scheme for quadrature phase-shift encoded signals. Our receiver surpasses the standard quantum limit and outperforms all previously known non-adaptive detectors at low input powers. Unlike existing approaches, the receiver only exploits linear optical elements and on-off photo-detection. This circumvents the requirement for challenging feed-forward operations that limit communication transmission rates and can be readily implemented with current technology.