A microscopic theory of a non-dissipative drag in a two-component superfluid Bose gas is developed. The expression for the drag current in the system with the components of different atomic masses, densities and scattering lengths is derived. It is shown that the drag current is proportional to the square root of the gas parameter. The temperature dependence of the drag current is studied and it is shown that at temperature of order or smaller than the interaction energy the temperature reduction of the drag current is rather small. A possible way of measuring the drag factor is proposed. A toroidal system with the drag component confined in two half-ring wells separated by two Josephson barriers is considered. Under certain condition such a system can be treated as a Bose-Einstein counterpart of the Josephson charge qubit in an external magnetic field. It is shown that the measurement of the difference of number of atoms in two wells under a controlled evolution of the state of the qubit allows to determine the drag factor.

This review is devoted to the theory of electron-hole superfluidity in bilayer systems. This is a discussion of the state in which electrons and holes from adjacent layers form bound pairs, and a nondissipative transfer of charge is caused by the motion of pairs as wholes. The occurrence of the flow of bound pairs is equivalent to the flow in conducting layers of equal in modulus but oppositely directed electric currents. Therefore, such superfluidity can be examined as a unique superconductivity that is often called counterflow superconductivity. Although this phenomenon was predicted about 40 years ago, the intensive study of the problem of electron-hole superconductivity in bilayer systems continues to date. An extensive description of theoretical representations and approaches to the problem of electron-hole superconductivity is presented in this review.

It is shown that in a bilayer excitonic superconductor dissipative losses emerge under transmission of the current from the source to the load. These losses are proportional to the square of the interlayer tunneling amplitude and are independent of the value of the input current. The case of a quantum Hall bilayer is considered. The bilayer may work as a transmission line if the input current exceeds a certain critical value. An input current higher than the critical one induces Josephson vortices in the bilayer. The difference in electrochemical potentials is required to feed the load and it forces Josephson vortices to move. The state becomes non-stationary which leads to dissipation.