Single Spin Dynamics and Decoherence in a Quantum Dot via Charge Transport

We investigate the spin dynamics of a quantum dot with a spin-1/2 ground state in the Coulomb blockade regime and in the presence of a magnetic rf field leading to electron spin resonance (ESR). We show that by coupling the dot to leads, spin properties on the dot can be accessed via the charge current in the stationary and nonstationary limits. We present a microscopic derivation of the current and the master equation of the dot using superoperators, including contributions to decoherence and energy shifts due to the tunnel coupling. We give a detailed analysis of sequential and cotunneling currents, for linearly and circularly oscillating ESR fields, applied in cw and pulsed modes. We show that the sequential tunneling current exhibits a spin satellite peak whose linewidth gives a lower bound on the decoherence time T-2 of the spin-1/2 state on the dot. Similarly, the spin decoherence can be accessed also in the cotunneling regime via ESR-induced spin flips. We show that the conductance ratio of the spin satellite peak and the conventional peak due to sequential tunneling saturates at the universal conductance ratio of 0.71 for strong ESR fields. We describe a double-dot setup which generates spin-dependent tunneling and acts as a current pump (at zero bias) and as a spin inverter which inverts the spin polarization of the current, even in a homogeneous magnetic field. We show that Rabi oscillations of the dot spin induce coherent oscillations in the time-dependent current. These oscillations are observable in the time-averaged current as function of ESR pulse duration, and they allow one to access the spin coherence directly in the time domain. We analyze the measurement and readout process of the dot spin via currents in spin-polarized leads and identify measurement time and efficiency by calculating the counting statistics, noise, and the Fano factor. We point out that single spin dynamics can also be accessed with STM techniques.