Until the pure fusion ages come true, present nuclear power is a crucial option if humanity takes aim for a zero net carbon society by the 2050s. A thorium subcritical reactor activated and controlled by muon-catalyzed fusion (MuCF) is an alternative until the completion of the pure fusion reactors. This proposal consists of two main technologies: a steady-state MuCF and the thorium subcritical fission reactor with cascaded neutron multipliers. It will be an environmentally friendly quantum energy source built only by the present science and technologies in a few decades.