  Synthetic nonconservative systems with parity-time (PT) symmetric gain-loss
structures can exhibit unusual spontaneous symmetry breaking that accompanies
spectral singularity. Recent studies on PT symmetry in optics and weakly
interacting open quantum systems have revealed intriguing physical properties,
yet many-body correlations still play no role. Here by extending the idea of PT
symmetry to strongly correlated many-body systems, we report that a combination
of spectral singularity and quantum criticality yields an exotic universality
class which has no counterpart in known critical phenomena. Moreover, we find
unconventional low-dimensional quantum criticality, where superfluid
correlation is anomalously enhanced owing to non-monotonic renormalization
group flows in a PT-symmetry-broken quantum critical phase, in stark contrast
to the Berezinskii-Kosterlitz-Thouless paradigm. Our findings can be
experimentally tested in ultracold atoms and predict critical phenomena beyond
the Hermitian paradigm of quantum many-body physics.
