  We find two systems via holography that exhibit quantum
Berezinskii-Kosterlitz-Thouless (BKT) phase transitions. The first is the ABJM
theory with flavor and the second is a flavored (1,1) little string theory. In
each case the transition occurs at nonzero density and magnetic field. The BKT
transition in the little string theory is the first example of a quantum BKT
transition in (3+1) dimensions. As in the "original" holographic BKT transition
in the D3/D5 system, the exponential scaling is destroyed at any nonzero
temperature and the transition becomes second order. Along the way we construct
holographic renormalization for probe branes in the ABJM theory and propose a
scheme for the little string theory. Finally, we obtain the embeddings and
(half of) the meson spectrum in the ABJM theory with massive flavor.
