Experimental Demonstration of a Quantum Receiver Beating the Standard Quantum Limit at Telecom Wavelength
Shuro Izumi, Jonas S. Neergaard-Nielsen, Shigehito Miki, Hirotaka Terai, Ulrik L. Andersen · Physical Review Applied · 2020
Discrimination of coherent states beyond the standard quantum limit (SQL) is an important task not only for quantum information processing but also for optical coherent communication. To optimize long-distance optical fiber networks, it is of practical importance to develop a quantum receiver beating the SQL and approaching the quantum bound at telecom wavelength. In this paper, we experimentally demonstrate a receiver beating the conventional SQL at telecom wavelength. Our receiver is composed of a displacement operation, a single-photon counter, and a real-time adaptive-feedback operation. By using a high-performance single-photon detector operating at the telecom wavelength, we achieve a discrimination error beyond the SQL. The demonstration in the telecom band is an enabler for quantum and classical communication beyond the SQL using a coherent state alphabet, and we envision that the technology can be used for long-distance quantum-key distribution, effective quantum-state preparation, and quantum estimation.