Romanenko, A. Checchin, M. Khabiboulline, T.N. Martinello, M. JACoW Geneva, Switzerland Potential application of SRF cavities for quantum computing requires their operation in the limit of very low fields (down to single photon) and temperatures (<100 mK). If niobium cavities are advantageous for this field depends on the Q value, which can be achieved, and therefore requires detailed studies in the low field Q slope regime. Here we report the first attempt toward such a characterization by extending the low field Q slope measurements in 1.3 GHz single cell cavities down to the lowest possible fields.