Quantum error correction for communication with linear optics

Improving the signal-to-noise ratio in optical communication systems is a fundamental requirement for cost-effective data transmission. This is particularly important for the transmission of noise-intolerant quantum states: excess noise at the quantum level destroys the coherence of the states, rendering classical error correction or amplifier-based schemes(1) useless for quantum communication. Only quantum error correction(2,3) can remove the effects of noise without corrupting the fragile superpositions of quantum states. But difficulties arise in the practical implementation of such a correction process because nonlinear operations(4) have been thought to be required, greatly reducing the efficiency of any optical scheme. Here I report an efficient, compact scheme involving only linear optical elements and feedback,which performs error correction for both quantum and classical noise. In the classical case, the noise penalty incurred is no worse than for ideal amplification. But for low-noise quantum optical communication, this penalty may be eliminated entirely. This quantum error-correction scheme may thus find application in quantum cryptographic networks(5-7) (where low noise is equivalent to high security), possibly extending their range far beyond limits imposed by system losses(7).