Bandwidth and energy are two fundamental resources needed to exchange information. In digital communication, information is encoded in a finite set of physical states known as symbols. In optics, modulated laser pulses are often used as information carriers.
Indeed, coherent laser light is easy to generate, modulate, and detect even in lossy communication channels. Modulation schemes differ in their use of parameters such as frequency, phase and/or amplitude of coherent states. The number of states used for data encoding or the alphabet length may also vary. The modulation scheme and the alphabet length are selected to optimize data transfer given the properties of a practical communication channel.
However, there is a fundamental limitation on such optimization. Even when the classical receiver is ideal and the communication channel is noiseless, communication is limited by shot noise of the measurement. The Shot Noise Limit (SNL) is defined as the minimal possible probability of error also known as Symbol Error Rate (SER) that can be attained by an ideal classical measurement with zero loss and 100% efficient detector, further we will refer to this approximation as to unit system efficiency. Therefore, SNL sets the minimum energy required at the receiver to classically discriminate states with the desired SER. A lower error bound is set by quantum theory, known as the Helstrom bound.
Researchers have presented a systematic study of quantum receivers and modulation methods enabling resource efficient quantum-enhanced optical communication. They have introduced quantum-inspired modulation schemes that theoretically yield a better resource efficiency than legacy protocols.
Experimentally, they demonstrated below the SNL symbol error rates for M ≤ 16 legacy and quantum-inspired communication alphabets using software-configurable optical communication time-resolving quantum receiver testbed.
Further, they experimentally verified that this quantum-inspired modulation schemes boost the accuracy of practical quantum measurements and significantly optimize the combined use of energy and bandwidth for communication alphabets that are longer than M = 4 symbols.
The paper has been published in npj Quantum Information.
