Single-Photon Superradiance from a Quantum Dot

We report on the observation of single-photon superradiance from an exciton in a semiconductor quantum dot. The confinement by the quantum dot is strong enough for it to mimic a two-level atom, yet sufficiently weak to ensure superradiance. The electrostatic interaction between the electron and the hole comprising the exciton gives rise to an anharmonic spectrum, which we exploit to prepare the superradiant quantum state deterministically with a laser pulse. We observe a fivefold enhancement of the oscillator strength compared to conventional quantum dots. The enhancement is limited by the base temperature of our cryostat and may lead to oscillator strengths above 1000 from a single quantum emitter at optical frequencies.

Figure 1

Superradiant excitons in quantum dots. (a) A quantum dot defined by intentional monolayer fluctuations weakly confines electrons (e) and holes (h), which are mutually bound by electrostatic attraction. Notably, the spectrum is anharmonic due to interactions; i.e., the energy $\hslash {\omega }_{\mathrm{X}\mathrm{X}}$ of a biexciton is less than the energy $\hslash {\omega }_{\mathrm{X}}$ of a single exciton. The swirling arrow indicates superradiantly enhanced light-matter coupling. (b) SPS is defined in an ensemble of noninteracting emitters as a symmetric superposition of different excitations. (c) The excitonic enhancement of light-matter interaction may be regarded as a generalization of SPS: the exciton is a symmetric superposition of excitations. (d) Measured photoluminescence spectrum at 10% of the exciton saturation power $P_{\mathrm{sat}}=20\mathrm{nW}$. Only the exciton is observed. (e) At the saturation power, the biexciton becomes discernible. (f) The excitons and biexcitons are distinguished by their power-law dependence on excitation power $P$: the fits yield $P^{0.86}$ and $P^{2.01}$, respectively.

Figure 2

Deterministic preparation of superradiant excitons. (a) Photoluminescence-excitation spectrum obtained by integrating the emission of the $1s$ transition while scanning the excitation wavelength. It features a quasicontinuum and resolves the lowest-energy states of the exciton manifold, labeled $1s$, $2s$, and $3s$. (b) Two excitation schemes are used in our study. With $C$-type excitation, an equal bright- and dark-exciton population is prepared, which is important for extracting the impact of nonradiative processes. Deterministic preparation of the bright exciton is achieved by pumping into the $2s$ state.

Figure 3

Experimental demonstration of single-photon superradiance from a quantum dot. (a) Time-resolved decay (black points) of the bright $1s$ exciton obtained under $2s$ excitation. We obtain an excellent fit (yellow line) to the theoretical model when convoluting with the instrument-response function of the detector. After separating nonradiative from radiative effects we extract a radiative decay rate of $(8.3\pm 0.1){\mathrm{ns}}^{-1}$ (red line), which is deeply in the superradiant regime (green area). (b) HBT measurement of the emitted photons showing $g^{(2)}(0)=0.13$, which proves single-photon emission. (c) Long-time-scale HBT measurement where each coincidence peak has been numerically integrated. No blinking is observed.