We resume the investigation of the problem of independent local compression of correlated quantum sources, the classical case of which is covered by the celebrated Slepian-Wolf theorem. We focus specifically on classical-quantum (cq) sources, for which one edge of the rate region, corresponding to the compression of the classical part, using the quantum part as side information at the decoder, was previously determined by Devetak and Winter [Phys. Rev. A 68, 042301 (2003)]. Whereas the Devetak-Winter protocol attains a rate-sum equal to the von Neumann entropy of the joint source, here we show that the full rate region is much more complex, due to the partially quantum nature of the source. In particular, in the opposite case of compressing the quantum part of the source, using the classical part as side information at the decoder, typically the rate sum is strictly larger than the von Neumann entropy of the total source.We determine the full rate region in the generic case, showing that, apart from the Devetak-Winter point, all other points in the achievable region have a rate sum strictly larger than the joint entropy. We can interpret the difference as the price paid for the quantum encoder being ignorant of the classical side information. In the general case, we give an achievable rate region, via protocols that are built on the decoupling principle, and the principles of quantum state merging and quantum state redistribution. Our achievable region is matched almost by a single-letter converse, which however still involves asymptotic errors and an unbounded auxiliary system.
I. SOURCE AND COMPRESSION MODELData compression can be regarded as the foundation of information theory in the treatment of Shannon [1], and it remains one of the most fruitful problems to be considered, especially when additional constraints on the source, the encoders or the decoder are imposed. In particular, the Slepian-Wolf problem of two sources correlated in a known way, but subject to separate, local compression [2] has proved to provide a unifying principle for much of Shannon theory, giving rise to natural information theoretic interpretations of entropy and conditional entropy, and exhibiting deep connections with error correction, channel capacities and mutual information (cf. [3]). The quantum case has been investigated for two decades, starting with the second author's PhD thesis [4] and subsequently in [5], up to the systematic study [6], and while we still do not have a complete understanding of the rate region, it has become clear that the problem is of much higher complexity than the classical case. The quantum Slepian-Wolf problem, and specifically quantum data compression with side information at the decoder, has resulted in many fundamental advances in quantum information theory, including the protocols of quantum state merging [7,8] and quantum state redistribution [9], which have given operational meaning to the conditional von Neumann entropy, the mutual information and the conditional quantum mutual information, r...