Mark M. Wilde, Saikat Guha
The pure-interference bosonic multiple access channel has two senders and one receiver, such that the senders each communicate with a single mode of light and the effect of the channel is to mix their transmissions on a beamsplitter. The receiver then obtains a signal from one of the output ports. In prior work, Yen and Shapiro found the capacity region of this channel if the senders are restricted to coherent-state encodings. Here, we demonstrate how to achieve the Yen-Shapiro region (in some cases) with a sequential decoding strategy, such that the receiver performs binary-outcome quantum measurements for every codeword pair in the senders' codebooks. A crucial component of this scheme is a non-destructive "vacuum-or-not" measurement that projects an n-symbol modulated codeword onto the n-fold vacuum state or its orthogonal complement, such that the post-measurement state is either the n-fold vacuum or has the vacuum removed from the support of the n symbols' joint quantum state. This receiver requires the additional ability to perform multimode optical phase-space displacements which are realizable using a beamsplitter and a laser.
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http://arxiv.org/abs/1204.0521
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