Quantum Feedback Channels

In Shannon information theory the capacity of a memoryless communication channel cannot be increased by the use of feedback. In quantum information theory the no-cloning theorem means that noiseless copying and feedback of quantum information cannot be achieved. In this paper, quantum feedback is defined as the unlimited use of a noiseless quantum channel from receiver to sender. Given such quantum feedback, it is shown to provide no increase in the entanglement--assisted capacities of a memoryless quantum channel, in direct analogy to the classical case. It is also shown that in various cases of non-assisted capacities, feedback may increase the capacity of memoryless quantum channels.Comment: 5 pages, requires IEEEtran.cls, expanded, proofs added, references adde

Classical information capacity of superdense coding

Classical communication through quantum channels may be enhanced by sharing entanglement. Superdense coding allows the encoding, and transmission, of up to two classical bits of information in a single qubit. In this paper, the maximum classical channel capacity for states that are not maximally entangled is derived. Particular schemes are then shown to attain this capacity, first for pairs of qubits, and second for pairs of qutrits

Noise enhancing the classical information capacity of a quantum channel

We present a simple model of quantum communication where a noisy quantum channel may benefit from the addition of further noise at the decoding stage. We demonstrate enhancement of the classical information capacity of an amplitude damping channel, with a predetermined detection threshold, by the addition of noise in the decoding measurement.Comment: 4 pages, 2 figure

Bounds on classical information capacities for a class of quantum memory channels

The maximum rates for information transmission through noisy quantum channels has primarily been developed for memoryless channels, where the noise on each transmitted state is treated as independent. Many real world communication channels experience noise which is modelled better by errors that are correlated between separate channel uses. In this paper, upper bounds on the classical information capacities of a class of quantum memory channels are derived. The class of channels consists of indecomposable quantum memory channels, a generalization of classical indecomposable finite-state channels.Comment: 4 pages, 1 figure, RevTeX, coding theorem remove

Stochastic resonance effects in quantum channels

We provide some examples of quantum channels where the addition of noise is able to enhance the information transmission rate. This may happen for both quantum and classical uses and realizes stochastic resonance effects.Comment: 4 pages, 3 figure