33 research outputs found
Trade-off capacities of the quantum Hadamard channels
Coding theorems in quantum Shannon theory express the ultimate rates at which
a sender can transmit information over a noisy quantum channel. More often than
not, the known formulas expressing these transmission rates are intractable,
requiring an optimization over an infinite number of uses of the channel.
Researchers have rarely found quantum channels with a tractable classical or
quantum capacity, but when such a finding occurs, it demonstrates a complete
understanding of that channel's capabilities for transmitting classical or
quantum information. Here, we show that the three-dimensional capacity region
for entanglement-assisted transmission of classical and quantum information is
tractable for the Hadamard class of channels. Examples of Hadamard channels
include generalized dephasing channels, cloning channels, and the Unruh
channel. The generalized dephasing channels and the cloning channels are
natural processes that occur in quantum systems through the loss of quantum
coherence or stimulated emission, respectively. The Unruh channel is a noisy
process that occurs in relativistic quantum information theory as a result of
the Unruh effect and bears a strong relationship to the cloning channels. We
give exact formulas for the entanglement-assisted classical and quantum
communication capacity regions of these channels. The coding strategy for each
of these examples is superior to a naive time-sharing strategy, and we
introduce a measure to determine this improvement.Comment: 27 pages, 6 figures, some slight refinements and submitted to
Physical Review
Quantum Information Transmission over a Partially Degradable Channel
We investigate a quantum coding for quantum communication over a PD
(partially degradable) degradable quantum channel. For a PD channel, the
degraded environment state can be expressed from the channel output state up to
a degrading map. PD channels can be restricted to the set of optical channels
which allows for the parties to exploit the benefits in experimental quantum
communications. We show that for a PD channel, the partial degradability
property leads to higher quantum data rates in comparison to those of a
degradable channel. The PD property is particular convenient for quantum
communications and allows one to implement the experimental quantum protocols
with higher performance. We define a coding scheme for PD-channels and give the
achievable rates of quantum communication.Comment: 7 pages, 2 figures, Journal-ref: IEEE Acces
Superadditivity of the Classical Capacity with Limited Entanglement Assistance
Finding the optimal encoding strategies can be challenging for communication
using quantum channels, as classical and quantum capacities may be
superadditive. Entanglement assistance can often simplify this task, as the
entanglement-assisted classical capacity for any channel is additive, making
entanglement across channel uses unnecessary. If the entanglement assistance is
limited, the picture is much more unclear. Suppose the classical capacity is
superadditive, then the classical capacity with limited entanglement assistance
could retain superadditivity by continuity arguments. If the classical capacity
is additive, it is unknown if superadditivity can still be developed with
limited entanglement assistance. We show this is possible, by providing an
example. We construct a channel for which, the classical capacity is additive,
but that with limited entanglement assistance can be superadditive. This shows
entanglement plays a weird role in communication and we still understand very
little about it.Comment: 13 page
Private Quantum Coding for Quantum Relay Networks
The relay encoder is an unreliable probabilistic device which is aimed at
helping the communication between the sender and the receiver. In this work we
show that in the quantum setting the probabilistic behavior can be completely
eliminated. We also show how to combine quantum polar encoding with
superactivation-assistance in order to achieve reliable and capacity-achieving
private communication over noisy quantum relay channels.Comment: 15 pages, 3 figures, Journal-ref: Lecture Notes in Computer Science,
Vol. 7479, pp. 239-250. Springer-Verlag, 2012, presented in part at the 11th
Intl. Conference on Quantum Communication, Measurement and Computing
(QCMC2012), v2: minor formatting change