We propose a simple design of a rotary nanomotor comprised of three quantum dots attached to the rotating ring (rotor)
in the presence of an in-plane dc electric field. The quantum dots (sites) can be coupled to or decoupled from source and
drain carrier reservoirs, depending on the relative positions of the leads and the dots. We derive equations for the site
populations and solve these equations numerically jointly with the Langevin-type equation for the rotational angle. It is
shown that the synchronous loading and unloading of the sites results in unidirectional rotation of the nanomotor. The
corresponding particle current, torque, and energy conversion efficiency are determined. Our studies are applicable both
to biologically-inspired rotary nanomotors, the F0 motor of ATP synthase and the bacterial flagellar motor, which use
protons as carriers, and to novel artificial semiconductor systems using electrons. The efficiency of this semiconductor
analog of the rotary biomotors is up to 85% at room temperature.
We have analyzed and measured the quantum coherent dynamics of a circuit containing two coupled superconducting charge qubits. Each qubit is based on a Cooper pair box connected to a reservoir electrode through a Josephson junction. Two qubits are coupled electrostatically by a small island overlapping both Cooper pair boxes. Quantum state manipulation ofthe qubit circuit is done by applying non-adiabatic voltage pulses to the common gate. We read out each qubit by means of probe electrodes connected to Cooper pair boxes through high-Ohmic tunnel junctions. With such a setup the measured pulse-induced probe currents are proportional to the probability for each qubit to have an extra Cooper pai1r after the manipulation. As expected from theory and observed experimentally the measured pulse-induced current in each probe has two frequency components whose position on the frequency axis can be externally controlled. This is a result ofthe inter-qubit coupling which is also responsible for the avoided level crossing that we observed in the qubits' spectra. Our simulations show that in the absence of decoherence and with a rectangular pulse shape the system remains entangled most ofthe time reaching maximally entangled states at certain instances.