Bell-Inequality Violation with a Triggered Photon-Pair Source

Here we demonstrate, for the first time, violation of Bell’s inequality using a triggered quantum dot photon-pair source without post-selection. Furthermore, the fidelity to the expected Bell state is increased above 90% using temporal gating to reject photons emitted at times when collection of uncorrelated light is more probable. A direct measurement of a CHSH Bell inequality is made showing a clear violation, highlighting that a quantum dot entangled photon source is suitable for communication exploiting nonlocal quantum correlations.

Figure 1

The two-photon biexciton decay process of a quantum dot with no bright exciton splitting. Recombination of a spin-up (-down) electron and a spin-down (-up) hole generates a left (right) hand circularly polarized photon. Since it is impossible to predict which photon will be emitted first, the emission is expected in the entangled state $(|L_1{R}_2⟩+|R_1{L}_2⟩)/\surd 2$. Subscripts denote the order of emission of the left, $L$ and right, $R$ polarized photons.

Figure 2

A single quantum-dot photoluminescence spectrum. Emission lines corresponding to electron-hole recombination from the neutral exciton ($X$), biexciton ($XX$), positively-charged exciton ($X^{+}$), and negatively charged exciton ($X^{-}$) are labeled. The energetic difference between the $X$ and $XX$ emission lines ($E_{X\mathrm{\text{−}}XX}$) offset by its average is plotted as a function of half-wave plate angle to reveal a bright structure splitting ($\delta$) for this dot of $\sim 0.3\mu \mathrm{eV}$.

Figure 3

(a) Zero-delay co- and cross-polarized second-order correlation measurements between the pair of photons emitted by the biexciton state of a single dot. The red (blue) bars represent data collected with (without) a temporal gate. (b) Time-resolved emission illustrating the position of the temporal gate on the exciton ($X$) and biexciton ($XX$) photons.