Overcoming power broadening of the quantum dot emission in a pure wurtzite nanowire

One of the key challenges in developing quantum networks is to generate single photons with high brightness, purity, and long temporal coherence. Semiconductor quantum dots potentially satisfy these requirements; however, due to imperfections in the surrounding material, the coherence generally degrades with increasing excitation power yielding a broader emission spectrum. Here we overcome this power-broadening regime and demonstrate an enhanced coherence at exciton saturation where the detected count rates are highest. We detect single-photon count rates of 460 000 counts per second under pulsed laser excitation while maintaining a single-photon purity greater than 99%. Importantly, the enhanced coherence is attained with quantum dots in ultraclean wurtzite InP nanowires, where the surrounding charge traps are filled by exciting above the wurtzite InP nanowire band gap. By raising the excitation intensity, the number of possible charge configurations in the quantum dot environment is reduced, resulting in a narrower emission spectrum. Via Monte Carlo simulations we explain the observed narrowing of the emission spectrum with increasing power. Cooling down the sample to 300 mK, we further enhance the single-photon coherence twofold as compared to operation at 4.5 K, resulting in a homogeneous coherence time, $T_2$, of 1.2 ns, and two-photon interference visibility as high as 83% under strong temporal postselection ($\sim 5\%$ without temporal postselection).

Figure 1

Single-photon interference measurements. (a) SEM image of tapered nanowire waveguide containing a single quantum dot. Scale bar: 200 nm. (b) Typical PL spectrum of a quantum dot that is predominantly charged. ${\mathrm{X}}^{-}$: charged exciton. (c) Autocorrelation measurement of ${\mathrm{X}}^{-}$ demonstrating a single-photon purity of greater than 99%. A fit to the data (blue line) yields $g^2\left(0\right)<0.008$. (d) Single-photon interference measurements of a single quantum dot at 300 mK (blue triangles), 1.7 K (red squares), and 12 K (black circles). The solid lines are a fit to the data using a Voigt profile (described in the text). (e) Raw single-photon interference fringes with a step size of 20 nm at maximum fringe visibility for a temperature of 300 mK.

Figure 2

Single quantum dot coherence. (a) Single quantum dot emission linewidth is represented by a Voigt profile (blue) with a full width at half maximum of 880(130) MHz, which is a convolution of a Lorentzian (black) and Gaussian (red) line shape. The parameters for the homogeneous broadening [Lorentzian, $T_2=1.2\left(2\right)\mathrm{ns}$, corresponding to 260(40) MHz] and inhomogeneous broadening [Gaussian, $T_c=0.9\left(1\right)\mathrm{ns}$, corresponding to 730(50) MHz] are extracted from the fit of the Michelson data at 300 mK in Fig. 1. For comparison, the dotted black line shows the lifetime Fourier-transform limit of 100 MHz. (b) Coherence time, $T_2$, and corresponding pure dephasing time, $T_2^{*}$, extracted from the fits of Fig. 1. At 300 mK the pure dephasing time is longer than the exciton lifetime, $T_1$, of 1.6(1) ns.

Figure 3

Power narrowing of the single quantum dot emission linewidth. (a) Single-photon interference measurements of a single quantum dot at 300 mK as a function of excitation power, $P$. In contrast to conventional self-assembled quantum dots, importantly, the best coherence is observed at exciton saturation of the quantum dot emission where the detected count rate is highest. The laser power, $P_{\mathrm{sat}}$, used to saturate the exciton transition is $\sim 3 \mu \mathrm{W}$ (spot size of $\sim 1$ $\mu \mathrm{m}$). (b) Power dependence of charged exciton emission line. The exciton emission saturates at 460 000 counts per second as measured on a single-photon avalanche photodiode. (c) Emission linewidth as a function of excitation power, $P$, as extracted from a fit to the data by a Voigt profile as presented in (a). $P_{\mathrm{sat}}$ corresponds to the excitation power used to saturate the charged exciton transition.

Figure 4

Photon indistinguishability. (a) Schematic of HOM setup for two subsequently created photons from a quantum dot in a nanowire. The charged exciton line of the quantum dot emission is filtered in a spectrometer. A nonpolarizing beam splitter (NPBS) splits the photons in a short and long path. The delay is precisely matched to the repetition rate of the laser (20 MHz) so that the two photons interfere at the fiber-coupled NPBS. That is, a single photon traveling the long path interferes with the next subsequent photon traveling the short path all in polarization-maintaining (PM) fibers. SSPD: Superconducting nanowire single photon detector. (b) HOM interference measured at saturation for parallel (indistinguishable case) and perpendicular polarizations (distinguishable case). (c) HOM interference as in (b), but for one-fifth of the saturation power. The coincidence data in (b) and (c) consist of 64 ps time bins.

Figure 5

Monte Carlo simulations of Gaussian line broadening mechanism for single quantum dot emission line. (a) A projection of randomly positioned trap sites for both electrons (blue) and holes (red dots) over a 400 nm nanowire segment. The nanowire diameter, $D$, is 200 nm. The trap density used for the simulations to reproduce the measured exciton linewidth as a function of power is $\sim 1\times {10}^{16}{\mathrm{cm}}^{-3}$. The quantum dot, QD, in the center of the nanowire is represented as a dotted circle with diameter of 20 nm. (b) Resulting histogram of Stark shift induced on the exciton transition for 10 000 possible charge configurations distributed across trap sites surrounding the quantum dot for an average trap occupancy probability of 0.5. Due to the random nature of the possible charge configurations, the broadening mechanism of the emission line is a Gaussian (fit curve in red). (c) Monte Carlo simulations showing the quantum dot emission linewidth as a function of excitation power. The simulations are in excellent agreement with the experimental power dependent emission linewidth of dot B presented in (d). $P_{\text{trap}}\text{saturation}$ corresponds to the laser power used to fill trap sites, whereas $P_{\mathrm{sat}}$ corresponds to the excitation power used to saturate the charged exciton transition.