Spin-to-charge conversion

A measurement method in which spin-dependent tunnelling or allowed charge transfer maps a spin outcome onto a distinguishable dot charge state. Quantum-dot charge sensing then records that charge. The measurement axis or singlet/triplet observable must be specified.

Map states with unequal escape rates ${\Gamma }_f\gg {\Gamma }_s$ onto distinct charge outcomes after an interval between their characteristic escape times. Both removal energies can lie above the reservoir chemical potential, so their splitting need not be thermally resolved. A useful rate difference must have a physical origin: a demonstrated single-dot example uses a two-electron singlet state and triplet state with different occupied orbitals. A bare single-electron Zeeman pair in the same orbital does not automatically provide the required rate contrast.^[ref]

For independent exponential escape, equal prior probabilities and no state relaxation, the mean classification error is $P_e(\tau )=[e^{-{\Gamma }_f\tau }+1-e^{-{\Gamma }_s\tau }]/2$. Setting its derivative to zero gives the displayed interval. The error increases both when the fast state is not given enough time to escape and when the slow state is given too much. Detector bandwidth, relaxation and reloading require an expanded measurement model.

Suppression of charge transfer by spin symmetry and the Pauli exclusion principle. For a double quantum dot, a $(1,1)$ singlet state can enter the lowest $(0,2)$ singlet, whereas a $(1,1)$ triplet state is blocked if the required $(0,2)$ triplet lies above the accessible energy window. Spin mixing or relaxation can lift blockade.

A resonantly driven local electron-spin rotation can give a blocked two-dot triplet a singlet component, allowing spin-conserving charge transfer to a lowest-orbital doubly occupied dot. A common rotation of two perfectly identical spins does not mix singlet and triplet sectors, so relative spin rotation or an additional mixing mechanism is essential. Leakage-current spectroscopy tests resonant lifting; pulse-duration oscillations add evidence for coherent control.

Place the reservoir chemical potential between the lower and higher Zeeman-split one-electron dot levels. The excited state can leave while the ground state remains trapped; a time-resolved charge sensor detects the temporary empty state. Thermal errors, missed fast tunnelling and relaxation before escape limit fidelity.