Purification of Single Photons from Room-Temperature Quantum Dots

Single photon emitters are indispensable to photonic quantum technologies. Here, we demonstrate waveform-controlled high-purity single photons from room-temperature colloidal quantum dots. The purity of the single photons does not vary with the excitation power, thereby allowing the generation rate to be increased without compromising the single-photon quality.

Figure 1

Single photons from room-temperature quantum dots. (a) Single quantum dots (QDs) are spin coated on a cover glass (CS) and optically excited by focused laser pulses using a 100X microscopic objective (MO). The emitted single photons are collected by the same MO, separated from the laser pulses at the dichroic mirror (DM), and detected by the single photon counting modules (SPCMs). To generate waveform-controlled single photons, an acousto-optic modulator (AOM) driven by a function generator is used to modulate the waveform of the single photons. Other optical elements shown are mirror ($M$), filter ($F$), pinhole (PH), lens ($L$) and beam splitter (BS). (b) Photoluminescence image ($20\times 20\mu \mathrm{m}$) of single quantum dots. (c) Antibunching in the Hanbury Brown–Twiss experiment. (d) Brightness as a function of excitation power $P$ normalized to the saturation power $P_{\mathrm{sat}}$. (e) Spectra of a single quantum dot.

Figure 2

Acousto-optic modulation of single photons. (a) Waveform of the unmodulated single photons from a single room-temperature quantum dot. The inset shows the biexponential shape. (b) Waveforms of the single photons modulated by a biased sinusoidal function of frequency 10 MHz (red) and without modulation (black). (c) Waveforms of single photons modulated by a biased square function with a period of 200 ns (red) and without modulation (black). The excitation power is $5.8P_{\mathrm{sat}}$.

Figure 3

Purification of the single photons from room-temperature quantum dots. (a) The modulation function generated by the acousto-optic modulator (red solid curve) compared to the Heaviside step function (black dashed curve). (b) The waveform and Hanbury Brown–Twiss measurement (inset) of the unmodulated single photons. (c)–(d) The waveforms and Hanbury Brown–Twiss measurements (insets) of the modulated single photons with time offset of $t_0=30\mathrm{ns}$ (c) and $t_0=45\mathrm{ns}$ (d). (e) Decomposition of the photoluminescence (black curve) into single-exciton (SX, pink) and biexciton (BX, gray) emission. (f) The biexciton percentage and relative total counts (inset) with different time offsets. (g) The biexciton percentage and relative total counts (inset) with different time offsets if the modulation function is Heaviside step. (h) The normalized photon correlation functions $g^{(2)}(0)$ at zero time delay. The time offset of the unmodulated single photons, whose values are marked by open circles, corresponds to $t_0=-50\mathrm{ns}$ due to the finite rising time of the modulation waveform.

Figure 4

Normalized photon correlation function at zero time delay. (a) The time offset of the modulation function is varied from 16 to 45 ns for different quantum dots. (b) The excitation power is varied from 1.4 to 7 $P_{\mathrm{sat}}$ for different quantum dots. $g^{(2)}(0)$ of the unmodulated and modulated single photons are shown by the open and solid symbols, respectively.