Speaker
Description
Applications in quantum computing and communication require entangled photon pairs featuring simultaneously a high degree of entanglement as well as a high single photon indistinguishability. Epitaxially grown quantum dots (QDs), especially gated GaAs QDs have shown to both be able to emit highly indistinguishable single photons as well as highly polarization entangled photon pairs from the biexciton (XX) - exciton (X) decay cascade. However, the state purity of the individual photons from a cascade is limited since the states are not entirely separable due to an entanglement in time between them. The reduction of the state purity $\mathbb{P}$ and thereby the indistinguishability of the photons is given by the lifetime ratio of the cascade $r = \tau_{XX}/\tau_X$ due to an unwanted temporal coupling with $ \mathbb{P} =1/(r+1)$. In GaAs QDs, this puts an upper bound on the reachable indistinguishability with typical lifetime ratios of $r=0.5$ at about $0.66$ which is too low for many applications. The ability to independently tune the lifetimes of the two decays therefore is required to increase this theoretical limit. Here we present a novel approach, achieving this by manipulating the vertical electric field.
Instead of working in forward bias, here we apply negative a gate voltage, which will increase the total field and thereby increase the band bending. The resulting decrease in overlap between the electron and hole wave function facilitates a change in lifetimes. While the exciton lifetime increases towards higher fields, the lifetime of the biexciton seems largely unaffected. This leads to an improvement of the raw measured indistinguishability from $0.60(1)$ to $0.74(1)$.
