Correlations are fundamental in describing many body systems. However, in experiments, correlations are notoriously difficult to assess on the microscopic scale, especially for electron spins. While it is firmly established theoretically that the electrons in a Cooper pair of a superconductor form maximally spin-entangled singlet states with opposite spin projections, no spin correlation experiments have been demonstrated so far. Here, we report the direct measurement of the spin cross-correlations [1] between the currents of a Cooper pair splitter, an electronic device that emits electrons originating from Cooper pairs. We find a negative spin correlation of -1/3, which deviates from the ideal value mostly due to the overlap of the Zeeman split quantum dot states. Our results demonstrate a new route to perform spin correlation experiments in nano-electronic devices, especially suitable for those relying on magnetic field sensitive superconducting elements, like triplet or topologically non-trivial superconductors.  [1] A. Bordoloi et al., Nature 612, 454-458 (2022)

(Pizza and refreshments will be served after the talk.)