# Also in the Article

Channel capacity
This protocol is extracted from research article:
Beating the channel capacity limit for superdense coding with entangled ququarts

Procedure

The channel capacity (10, 11) is the maximal mutual information H (x; y). In our experiment, the mutual information can be calculated as$H(x;y)=∑y=04∑x=04p(x)p(y|x)log(p(y|x)∑x=04p(x)p(y|x))$(7)where p(x) = {p11), p12), p13), p14), p23)}. The channel capacity in our experiment is 2.09 ± 0.01, which is slightly smaller than the limit of 2.32 owing to the imperfect four-dimensional Bell-state measurement.

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