Screening nuclear field fluctuations to generate highly indistinguishable photons from negatively charged self-assembled InGaAs quantum dots

Quantum dots (QDs) can generate highly coherent and indistinguishable single photons. However, a ground-state electron spin interacts with a QD's nuclear spins to create an effective Overhauser field ($\delta B_n$) of $\sim$30mT. We probe this interaction using resonance fluorescence. We observe the effect of $\delta B_n$ in high resolution (27 MHz) spectroscopy of the elastic and inelastic scattered photons, and characterize the effect of $\delta B_n$ on photon indistinguishability by monitoring the visibility of two-photon interference. With no external magnetic field ($B_z=0$), $\delta B_n$ effectively splits the ground state, and at low Rabi frequencies we observe two broad ($\Gamma=200$MHz) peaks equally spaced by $\sim$100MHz from the central elastic peak. The ratio of elastic to inelastic photons in the spectra gives a dephasing time $T_2=0.52~T_1=406$ps, far from the transform limit. With an external field $B_z>\delta B_n$, we can successfully screen the fluctuating nuclear field. For $B_z=300$mT, nearly all photons in the spectrum are elastically scattered and we extract $T_2=1.94~T_1=1512$ps. This transform limited linewidth enables us to demonstrate very high visibility two-photon interference. These results point towards robust generation of indistinguishable photons.