Global Quantum Communication

We discuss the power-minimum and maximum capacity communications predicted by quantum communication theory. Quantum communication theory has clarified the ultimate capacity limit, given a power constraint. The optimal solution is known such that the transmitter sends densely modulated coherent states (conventional laser light), while the receiver detects coherent state pulses in a unit of block sequence, by projecting it onto appropriate quantum superposition state bases to decode the signal. This optimal decoding essentially requires quantum computing with coherent states to transform the received codeword state into an appropriate quantum state, and the final measurement on it afterward. This new decoding system is called quantum decoder. In this presentation, we show predicted performances of the transmission rate versus the launch power for several kinds of technologies, for both fiber communication and deep space communication to make global network. In the case of fiber communication, we compare near future optical communication with a fiber, the ideal coherent optical communication, quantum communication with a fiber and quantum decoder, and the very final bound, which limits any communications using electromagnetic fields. The results show that, to attain a given certain transmission rate, we could eventually reduce the power budget by more than 40dB from the current level of technology, if we could realize the large scale quantum decoder. In the case of deep space optical communications, quantum decoder enable to extend a Gbps link to Mars from Earth. We have assumed that 1 THz bandwidth, 8-channel WDM with the repetition rate of 125 G pulse/s/channel, the launch power of 1W, and the optical antenna of 0.3m for the transmitter and 10m for the receiver. When one could realize quantum decoder for 1PHz bandwidth, it is possible to realize Tbps link to Mars.

    Experimental demonstration of quantum decoder is a grand challenge in communication technology. Currently we could have demonstrated a quantum receiver for binary coherent states using superconducting transition edge sensor, which could beat the shot noise limit in bit error rate (the limit of coherent communication). There is nothing to do with the entanglement in decoding process yet, but it is a first step toward the paradigm explained above.