Quantum decoherence of neutrino states is an effect that is proposed in various theories of quantum
gravity. It is envisaged to emerge from interactions of the neutrino as a quantum system with the environment and may destroy the superposition of neutrino mass states, which leads to a modification of neutrino oscillation probabilities.
KM3NeT/ORCA, a neutrino telescope designed to detect various neutrino flavours across a broad spectrum of energies, can probe this phenomenon.

ORCA is a water Cherenkov detector that is
currently under construction in the Mediterranean Sea. It was designed to detect atmospheric
neutrinos in the GeV energy range. The main objective of ORCA is the precise measurement of
neutrino oscillations. Therefore, the detector provides the possibility to investigate various beyond
Standard Model scenarios that may alter the oscillation pattern.
This contribution reports on first constraints on the strength of decoherence effects with KM3NeT.
We provide upper limits on the decoherence parameters 𝛾21 and 𝛾31. The high-purity neutrino
sample used for this analysis was collected with a six Detection Units configuration of ORCA with
an exposure of 433 kton-years.