Saikat Guha, Si-Hui Tan, Mark M. Wilde
An important practical open question has been to design explicit, structured
optical receivers that achieve the Holevo limit in the contexts of optical
communication and "quantum reading." The Holevo limit is an achievable rate
that is higher than the Shannon limit of any known optical receiver. We
demonstrate how a sequential decoding approach can achieve the Holevo limit for
both of these settings. A crucial part of our scheme for both settings 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. The sequential
decoder for optical communication requires the additional ability to perform
multimode optical phase-space displacements---realizable using a beamsplitter
and a laser, while the sequential decoder for quantum reading also requires the
ability to perform phase-shifting (realizable using a phase plate) and online
squeezing (a phase-sensitive amplifier).
View original:
http://arxiv.org/abs/1202.0518
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