Entanglement dynamics of molecular exciton states in coupled quantum dots

We theoretically model the electronic dynamics of a coupled quantum dot pair in a static electric field. We then investigate the possibility of polarization-entangled photon emission from the radiative cascade of the molecular biexciton state. Through numerical simulations, we analyze the dependence of entanglement fidelity on temperature and electric field, as well as tunnel coupling. We establish a regime of direct-indirect exciton detunings for which coupled quantum dots are superior to single dots for entangled photon generation, yielding near-unit fidelity over a larger range of exchange splittings.

Figure 1

(a) Energy level diagram of the molecular biexciton radiative cascade, depicting polarizations of all allowed recombination pathways, as well as spin-flip and phonon-assisted hole tunneling channels. Thick vertical arrows indicate direct exciton recombination, while thin vertical arrows indicate a slower interdot recombination. Excitation and detection pathways are indicated by green shading. (b) Simulated electric-field-dispersed photoluminescence spectrum, mapping out each of the relevant transition energies in the vicinity of the exciton and biexciton hole tunneling resonances. Single-exciton hole tunneling resonance occurs at $\mathrm{\Delta }=0\mathrm{meV}$, while biexciton hole tunneling resonances occur at $\mathrm{\Delta }=-1.9$ and $27.2\mathrm{meV}$.

Figure 2

(a) Dependence of entanglement fidelity on direct exciton bright-state splitting at $T=20\mathrm{K},t_h=0.2\mu \mathrm{eV}$, and $\mathrm{\Delta }=7\mathrm{meV}$ (green, top curve) and $\mathrm{\Delta }=3\mathrm{meV}$ (red, bottom curve), compared to the analogous SQD case. (b) Maximum fidelity and (c) width of $F^{+}\left(S_D\right)$ curve as a function of exciton detuning and temperature.

Figure 3

(a), (b) Dependence of fidelity parameters $F_{\text{max}}^{+}$ (black, left $y$ axis) and $\mathrm{\Delta }{F}^{+}$ (red, right $y$ axis) on (a) detuning and (b) temperature, with $t_h=0.2\mu \mathrm{eV}$. (c), (d) Total dephasing rate ${\gamma }_d^{iX}$ of the indirect exciton bright-state coherence, including contributions from incoherent transitions, as a function of detuning and temperature, respectively. Also included is the effective $\left|iX\right.〉$ population decay rate ${\mathrm{\Gamma }}_{\text{eff}}$.

Figure 4

(a) Maximum fidelity and (b) width of $F^{+}\left(S_D\right)$ curve as a function of hole tunnel coupling and temperature, with $\mathrm{\Delta }=7\mathrm{meV}$.