The efficient use of communication channels motivates extensive research in novel communication protocols. Modern communication protocols use large alphabets contain up to a few thousand symbols, thus optimizing the use of power and available frequency space. To date, quantum receivers that discriminate up to approximately 20 symbols were theoretically investigated and receivers with as many as 4 nonorthogonal coherent states have been experimentally demonstrated. However, all heretofore explored quantum receivers suffer from the sensitivity degradation with the alphabet size. Particularly, their Helstrom Bound (HB) nearly reaches the classical standard quantum limit (SQL).
Here we introduce an M-ary quantum receiver based on a coherent frequency shift keying (CFSK) protocol with a record power sensitivity, free of the above deficiency. The CFSK not only provides better accuracy for longer alphabets but also allows discrimination with a practically attainable symbol error rate (SER) situated much below the HBs of other encodings for large alphabets. Our receiver operates with a classical transmitter, and with any communication channel, including the existing global fiber network. It can be used to increase the amplification-free range in a network and/or reduce power requirements on the transmitter by more than 1000 times. In addition, the quantum measurement advantage can significantly optimize the use of the frequency space in comparison to classical frequency keying protocols. This advantage can be used in deep-space telecom links to enhance the satellite power budget. In existing fiber network links, quantum CFSK receivers can improve the amplification-free range by approximately the factor of 2.
Ivan Burenkov and Sergey Polykov, "Quantum coherent frequency-shift keying beyond the standard quantum limit (Conference Presentation)," Proc. SPIE 10547, Advances in Photonics of Quantum Computing, Memory, and Communication XI, 105470G (Presented at SPIE OPTO: January 30, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2286401.5751548030001.
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