  Indirect excitons (IX) in semiconductor heterostructures are bosons, which
can cool below the temperature of quantum degeneracy and can be effectively
controlled by voltage and light. IX quantum Bose gases and IX devices were
explored in GaAs heterostructures where an IX range of existence is limited to
low temperatures due to low IX binding energies. IXs in van der Waals
transition-metal dichalcogenide (TMD) heterostructures are characterized by
large binding energies giving the opportunity for exploring excitonic quantum
gases and for creating excitonic devices at high temperatures. TMD
heterostructures also offer a new platform for studying single-exciton
phenomena and few-particle complexes. In this work, we present studies of IXs
in MoSe$_2$/WSe$_2$ heterostructures and report on two IX luminescence lines
whose energy splitting and temperature dependence identify them as neutral and
charged IXs. The experimentally found binding energy of the indirect charged
excitons, i.e. indirect trions, is close to the calculated binding energy of 28
meV for negative indirect trions in TMD heterostructures [Deilmann, Thygesen,
Nano Lett. 18, 1460 (2018)]. We also report on the realization of IXs with a
luminescence linewidth reaching 4~meV at low temperatures. An enhancement of IX
luminescence intensity and the narrow linewidth are observed in localized
spots.
