Fundamental Impossibility of a Superradiant Neutrino Laser.

Here, we address the fundamental question of whether an idealized system of N atoms will show collective behavior and superradiance when it emits fermions instead of photons. We show that for single-fermion emission processes, the maximum emission is ∝N and not ∝N^{2}, which proves the absence of superradiance and shows that the recent proposal to realize a superradiant neutrino laser is impossible. This can be understood as either destructive interference of fermionic transition amplitudes, or Pauli blockade by collective excitations with fermionic nature. We derive the exact solution of the fermionic Dicke problem and analyze the decay dynamics in various regimes. We extend the proof to arbitrary Hamiltonians and show that the jump rate operator for neutrino emission has a maximum eigenvalue of N times the single-particle rate Γ_{0}. States with low excitation can show collective behavior and emit at a rate of NΓ_{0}.