Fig. 1: Operation and fundamentals of the switching device.

a An incoming continuous wave (cw) laser (Input) excites a single quantum dot. A second pulsed laser controls the switching gate. When the pulse arrives at the sample, all quantum dot simple excitonic transitions are blocked. The switcher output consists of an antibunched train of single photons, modulated by the control pulse sequence. b 3 × 3 matrix representing all quantum dot microstates considered in our model: vacuum state \( \left|0\right\rangle \), single electron (\( \left|e\right\rangle \)) and single hole \( \left|h\right\rangle \) states, two electron \(\left|2e\right\rangle \) and two hole \( \left|2h\right\rangle \) states, neutral exciton \( \left|{x}^{0}\right\rangle \) and both negative \( \left|{X}^{-1}\right\rangle \) and positive trions \( \left|{X}^{+1}\right\rangle \) and neutral biexciton \( \left|X{X}^{0}\right\rangle \). Vertical lines represent single electron capture (downwards) and escape (upwards), while horizontal lines represent single hole capture (rightwards) and escape (leftwards). Downward diagonal lines represent the capture of a single exciton, and upwards curly diagonal arrows represent single-photon emission. c Model simulation example of the neutral exciton (X⁰) and biexciton population (XX⁰) probability evolution in time as the system is pumped with single-pulsed excitation (orange lines) and with dual excitation (bluish shadows). d Non-resonant pumping energy schemes for single-colour dual excitation (both cw and pulsed lasers are tuned to 790 nm). e Non-resonant pumping energy schemes for two-colour dual excitation (cw laser is tuned to 830 nm, resonant to the residual carbon impurities (C⁰–e) transition, and the pulsed laser is tuned to 790 nm). The energy diagram corresponds to the GaAs conduction and valence band energies, the bidimensional carrier reservoir (WL) and the quantum dot S-level electron and hole energies. White arrows indicate the carrier relaxation path. f Set-up arrangement. One cw diode laser and a second pulsed laser are coupled to a single-mode fibre. A second fibre coupler is used to induce a relative separation between pulses. The signal from both lasers is directed to the excitation arm of a confocal microscope. Photon emission from a single InAs quantum dot is collected and coupled to a single-mode fibre, and directed to the spectrum recorder or to the time resolved single-photon counter analyser.