Sensitivity of entanglement decay of quantum-dot spin qubits to the external magnetic field

We study the decay of entanglement of quantum-dot electron-spin qubits under hyperfine-interaction-mediated decoherence. We show that two-qubit entanglement of a single entangled initial state may exhibit decay characteristic of two disentanglement regimes in a single sample, when the external magnetic field is changed. The transition is manifested by the suppression of time-dependent entanglement oscillations which are superimposed on the slowly varying entanglement decay related to phase decoherence which results in oscillatory behavior of entanglement sudden death time as a function of the magnetic field. This unique behavior allows us to propose the double-quantum-dot two-electron spin Bell state as a promising candidate for precise measurements of the magnetic field.