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Quantum Information and Computation

Qubit channels which require four inputs to achieve capacity

HayashiMasahito, ImaiHiroshi, MatsumotoKeiji, RuskaiMary Beth, ShimonoToshiyuki

Published Jan 1, 2005• 1 citation
Publisher

Abstract

An example is given of a qubit quantum channel which requires four inputs to maximize the Holevo capacity. The example is one of a family of channels which are related to 3-state channels. The capa...
DOI
10.5555/2011608.2011610

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An example is given of a qubit quantum channel which requires four inputs to maximize the Holevo capacity. The example is one of a family of channels which are related to 3-state channels. The capacity of the product channel is studied and numerical evidence presented which strongly suggests additivity. The numerical evidence also supports a conjecture about the concavity of output entropy as a function of entanglement parameters. However, an example is presented which shows that for some channels this conjecture does not hold for all input states. A numerical algorithm for finding the capacity and optimal inputs is presented and its relation to a relative entropy optimization discussed.
Four signalling states are sufficient to achieve the Holevo capacity for qubit channels, but in many cases are not necessary. There are examples known where the capacity is achieved for two orthogonal input states, two nonorthogonal states, and examples where three states are necessary and sufficient. Many previous results were obtained for a class of channel for which three states are sufficient. In this study, a systematic theory for this class of channel was developed. Simple criteria are presented that, when satisfied, mean that two states are sufficient for the ensemble. When these criteria are satisfied, there is a simple method to determine whether the states in the ensemble should be orthogonal or nonorthogonal. When the criteria are not satisfied, it is still possible that two states are sufficient, though it is possible that three states are necessary. In the case where three states are necessary, the form of the optimal ensemble is predicted. These results provide an efficient method for calculating the Holevo capacity for all channels in this class.
This paper considers a class of qubit channels for which three states are always sufficient to achieve the Holevo capacity. For these channels, it is known that there are cases where two orthogonal states are sufficient, two nonorthogonal states are required, or three states are necessary. Here a systematic theory is given which provides criteria to distinguish cases where two states are sufficient, and determine whether these two states should be orthogonal or nonorthogonal. In addition, we prove a theorem on the form of the optimal ensemble when three states are required, and present efficient methods of calculating the Holevo capacity.
The qubit depolarizing channel with noise parameter η transmits an input qubit perfectly with probability 1−η , and outputs the completely mixed state with probability η . We show that its complementary channel has positive quantum capacity for all η>0 . Thus, we find that there exists a single parameter family of channels having the peculiar property of having positive quantum capacity even when the outputs of these channels approach a fixed state independent of the input. Comparisons with other related channels, and implications on the difficulty of studying the quantum capacity of the depolarizing channel are discussed.