  Space-time modulated metamaterials support extraordinary rich applications,
such as parametric amplification, frequency conversion and non-reciprocal
transmission. However, experimental realization of space-time modulation is
highly non-trivial, hindering many interesting physics that are theoretically
predicted to be experimentally demonstrated. Here, based on the proposed
virtualized metamaterials with software-defined impulse response, we
experimentally realize non-Hermitian space-time varying metamaterials for
efficient and asymmetric frequency conversion by allowing material gain and
loss to be tailor-made and balanced in the time domain. In the application of
frequency conversion, the combination of space-time varying capability and
non-Hermiticity allows us to diminish the main band through gain-loss balance
and to increase the efficiency of side band conversion at the same time. In
addition, our approach of software-defined metamaterials is flexible to realize
the analogy of quantum interference in an acoustic system with design
capability. Applying an additional modulation phase delay between different
atoms allows to control such interference to get asymmetric amplification in
frequency conversion.
