Quantum fluctuations of k-strings: A case study

K strings in Yang-Mills theory can be considered as bound states of k elementary confining strings carrying one unit of colour flux. Current estimates of k-string tension sigma_k are very sensitive to the leading corrections due to quantum fluctuations of the string. In this study we address this problem by comparing Polyakov-Polyakov correlators in the fundamental representation (k=1) with the corresponding ones with k=2 in the confining phase of a Z_4 gauge theory in three dimensions. Highly efficient simulation techniques are available in this case. Although the k=1 Polyakov-Polyakov correlator matches nicely with the expected bosonic string effects up to the Next-to-Leading-Order, the k=2 Polyakov-Polyakov correlators show large deviations. This is an important source of potential systematic errors in the current estimates of sigma_k.