Nondestructive measurement of electron spins in a quantum dot

We propose and implement a nondestructive measurement that distinguishes between two-electron spin states in a quantum dot. In contrast to earlier experiments with quantum dots, the spins are left behind in the state corresponding to the measurement outcome. By measuring the spin states twice within a time shorter than the relaxation time T₁, correlations between the outcomes of consecutive measurements are observed. They disappear as the wait time between measurements becomes comparable to T₁. The correlation between the postmeasurement state and the measurement outcome is measured to be ~90% on average.