Optical Codeword Demodulation with Error Rates Below Standard Quantum Limit Using a Conditional Nulling Receiver
The quantum states of two laser pulses-coherent states-are never mutually orthogonal, making perfect discrimination impossible. Even so, coherent states can achieve the ultimate quantum limit for capacity of a classical channel, the Holevo capacity. Attaining this requires the receiver to make joint-detection measurements on long codeword blocks, optical implementations of which remain unknown. The authors report the first experimental demonstration of a joint-detection receiver, demodulating quaternary Pulse-Position-Modulation (PPM) codewords at a word error rate of up to 40% (2:2 dB) below that attained with direct-detection, the largest error-rate improvement over the standard quantum limit reported to date.