The power of entangled quantum channels

All communication channels are at bottom quantum mechanical. Quantum mechanics contributes both obstacles to communication in the form of noise, and opportunities in the use of intrinsically quantum representations for information. This paper investigates the trade-off between power and communication rate for coupled quantum channels. By exploiting quantum correlations such as entanglement, coupled quantum channels can communicate at a potentially higher rate than unentangled quantum channels given the same power. In particular, given the same overall power, M coupled, entangled quantum channels can send M bits in the same time it takes a single channel to send a single bit, and in the same time it takes M unentangled channels to send √ M bits. As communication technologies push down to the quantum level, a considerable effort has been made to uncover the physical limits to the communication process. Much progress has been made in investigating the physical limits to the bosonic communications channel (1-22) (e.g., quantum optical communication (1)) and to quantum communications in general (23-26). Quantum systems can be correlated with eachother in ways that classical systems cannot, a feature known as entanglement. It has been speculated that entangle-ment might be used enhance the capacity of quantum channels as compared to classical channels (24-28). Perhaps the best known example of the use of entanglement to enhance communication capacity is that of super-dense coding (25), in which two parties who initially possess an entangled state can send two bits of classical information by sending a single quantum bit. In addition, shared prior entanglement may enhance the transmission capacity of quantum channels in the presence of noise (28). In general, the enhancement in channel capacity that can be attained using entanglement is not known. This letter investigates the situation in which channels are coupled via a nonlinear dynamics to induce an entangled state in the process of transmission. For fixed power, I show that M coupled,