  Quantum limits of power dissipation in spintronic computing are estimated. A
computing element composed of a single electron in a quantum dot is considered.
Dynamics of its spin due to external magnetic field and interaction with
adjacent dots is described via the Bloch equations. Spin relaxation due to
magnetic noise from various sources is described as coupling to a reservoir.
Resulting dissipation of energy is calculated and is shown to be much less than
the thermal limit, ~kT per bit, if the rate of spin relaxation is much slower
than the switching rate. Clues on how to engineer an energy efficient
spintronic device are provided.
