  Spontaneous emission of a two--level atom in free space is modified by other
atoms in its vicinity leading to super- and sub-radiance. In particular, for
atomic distances closer than the transition wavelength the maximally entangled
antisymmetric superposition state of two individually excited atomic dipole
moments possesses no total dipole moment and will not decay spontaneously at
all. Such a two-atom dark state does not exist, if the atoms possess
alternative decay channels towards other atomic lower energy states. However,
we show here that for any atomic state with $N-1$ independent spontaneous decay
channels one can always find a $N$-particle highly entangled state, which
completely decouples from the free radiation field and does not decay.
Moreover, we show that this state is the unique state orthogonal to the
subspace spanned by the lower energy states with this property. Its sub-radiant
property largely survives at finite atomic distances.

Quantum Physics