Decoherence-enhanced quantum measurement of a quantum-dot spin qubit

We study the effect of decoherence on a quantum von Neumann measurement process. We discuss the effect of phonon noise on the direct measurement of two-electron spin states in a double quantum dot achieved by monitoring the noise of the current flowing through a quantum point contact coupled to one of the dots. We show that although the decoherence is damaging to the procedure at the extremely low temperatures characteristic of spin-in-quantum-dots experiments, and may even be fatal, increasing the temperature leads to a revival of the usefulness of the protocol: At higher temperatures, when the reservoir becomes an effective source of energy, it can enhance system fluctuations, and under such conditions the decoherence becomes advantageous to the measurement scheme and leads to the enhancement of the distinguishability between the measured states. Hence, the uncontrollable interaction of the measured system with the environment can be either an advantage or a disadvantage for a quantum measurement, depending on the characteristics of the decoherence process.

Figure 1

The Fano factor for a relatively strong electron phonon-interaction, $\alpha =3$, as a function of temperature (red, solid line); Fano factor without phonons is marked by the blue (grey) dotted line. Inset: The Fano factor as a function of the strength of the electron-phonon interaction relative to the DQD-QPC interaction, $\alpha$, for different temperatures. In both plots, the dashed grey line is set at Fano factor equal to one (Poissonian noise).