4 research outputs found
The free space optical interference channel
Semiclassical models for multiple-user optical communication cannot assess the ultimate limits on reliable communication as permitted by the laws of physics. In all optical communications settings that have been analyzed within a quantum framework so far, the gaps between the quantum limit to the capacity and the Shannon limit for structured receivers become most significant in the low photon-number regime. Here, we present a quantum treatment of a multiple-transmitter multiple-receiver multi-spatial-mode free-space interference channel with diffraction-limited loss and a thermal background. We consider the performance of a laser-light (coherent state) encoding in conjunction with various detection strategies such as homodyne, heterodyne, and joint detection. Joint detection outperforms both homodyne and heterodyne detection whenever the channel exhibits very strong interference. We determine the capacity region for homodyne or heterodyne detection when the channel has strong interference, and we conjecture the existence of a joint detection strategy that outperforms the former two strategies in this case. Finally, we determine the Han-Kobayashi achievable rate regions for both homodyne and heterodyne detection and compare them to a region achievable by a conjectured joint detection strategy. In these latter cases, we determine achievable rate regions if the receivers employ a recently discovered minentropy quantum simultaneous decoder. © 2011 IEEE
Explicit receivers for pure-interference bosonic multiple access channels
The pure-interference bosonic multiple access channel has two senders and one
receiver, such that the senders each communicate with multiple temporal modes
of a single spatial mode of light. The channel mixes the input modes from the
two users pairwise on a lossless beamsplitter, and the receiver has access to
one of the two output ports. In prior work, Yen and Shapiro found the capacity
region of this channel if encodings consist of coherent-state preparations.
Here, we demonstrate how to achieve the coherent-state Yen-Shapiro region (for
a range of parameters) using a sequential decoding strategy, and we show that
our strategy outperforms the rate regions achievable using conventional
receivers. Our 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.Comment: v1: 9 pages, 2 figures, submission to the 2012 International
Symposium on Information Theory and its Applications (ISITA 2012), Honolulu,
Hawaii, USA; v2: minor change
Network information theory for classical-quantum channels
Network information theory is the study of communication problems involving
multiple senders, multiple receivers and intermediate relay stations. The
purpose of this thesis is to extend the main ideas of classical network
information theory to the study of classical-quantum channels. We prove coding
theorems for quantum multiple access channels, quantum interference channels,
quantum broadcast channels and quantum relay channels.
A quantum model for a communication channel describes more accurately the
channel's ability to transmit information. By using physically faithful models
for the channel outputs and the detection procedure, we obtain better
communication rates than would be possible using a classical strategy. In this
thesis, we are interested in the transmission of classical information, so we
restrict our attention to the study of classical-quantum channels. These are
channels with classical inputs and quantum outputs, and so the coding theorems
we present will use classical encoding and quantum decoding. We study the
asymptotic regime where many copies of the channel are used in parallel, and
the uses are assumed to be independent. In this context, we can exploit
information-theoretic techniques to calculate the maximum rates for error-free
communication for any channel, given the statistics of the noise on that
channel. These theoretical bounds can be used as a benchmark to evaluate the
rates achieved by practical communication protocols.
Most of the results in this thesis consider classical-quantum channels with
finite dimensional output systems, which are analogous to classical discrete
memoryless channels. In the last chapter, we will show some applications of our
results to a practical optical communication scenario, in which the information
is encoded in continuous quantum degrees of freedom, which are analogous to
classical channels with Gaussian noise.Comment: Ph.D. Thesis, McGill University, School of Computer Science, July
2012, 223 pages, 18 figures, 36 TikZ diagram