KM3NeT Time calibration with Nanobeacons

The KM3NeT Collaboration is building a neutrino telescope in the Mediterranean Sea. The detector is expected to achieve an angular resolution better than 0.1 degrees for energies above 10 TeV. This is critical for attaining one of the key goals of the experiment, i.e. the identification of cosmic neutrino sources. In order to achieve a good angular resolution, the detector requires a relative time calibration of the order of 1 ns. The Nanobeacon is a cost-effective time calibration device developed by the KM3NeT Collaboration to synchronise in situ the detector photomultipliers with a nanosecond relative accuracy. In this contribution we will describe the design and operation of the Nanobeacon. Moreover, we will present the results of data taken in real sea-conditions and we will show how they are used to validate the time calibration parameters measured onshore.