On-demand semiconductor source of 780-nm single photons with controlled temporal wave packets

We report on a fast, bandwidth-tunable single-photon source based on an epitaxial GaAs quantum dot. Exploiting spontaneous spin-flip Raman transitions, single photons at 780 nm are generated on demand with tailored temporal profiles of durations exceeding the intrinsic quantum dot lifetime by up to three orders of magnitude. Second-order correlation measurements show a low multiphoton emission probability $[g^2\left(0\right)\sim 0.10\text{–}0.15]$ at a generation rate up to 10 MHz. We observe Raman photons with linewidths as low as 200 MHz, which is narrow compared to the 1.1-GHz linewidth measured in resonance fluorescence. The generation of such narrow-band single photons with controlled temporal shapes at the rubidium wavelength is a crucial step towards the development of an optimized hybrid semiconductor-atom interface.

Figure 1

(a) Reduced energy-level diagram of a quantum dot charged with a single hole subject to a magnetic field in the Faraday geometry. The inset illustrates the two-step sequence used for generating a “red” or “blue” single Raman photon with controlled temporal wave form. (b) Polarization-based dark-field microscope with tailored excitation pulses.

Figure 2

Observation of optical spin pumping. (a) $X^{1+}$ resonance fluorescence (RF) spectrum in the Faraday configuration at $B_z=2.8$ T. Light scattered on the red transitions {$➀$, $➁$} (red trace) and on the blue transitions {$➂$, $➃$} (blue trace) is detected on two neighboring pixels of the CCD spectrometer (9-GHz resolution), with residual leakage of the red RF signals from the transitions {$➀$, $➁$} also detected on the “blue” pixel. The dip in the red (blue) trace compared to the fitted Lorentzian profile (dashed line) shows the enhancement of the optical spin pumping that depopulates the ground state ${|\Uparrow \rangle }_z$ (${|\Downarrow \rangle }_z$) when the driving laser is resonant with the spin-flipping transition $➁$ ($➂$). (b) Time-resolved fluorescence observed under pulsed resonant excitation, alternately pumping the transitions $➃$ and $➀$ at saturation with a delay $\tau$. The exponential decays result from optical spin pumping that sequentially prepares ${|\Uparrow \rangle }_z$ and ${|\Downarrow \rangle }_z$ with time constant ${\tau }_{\mathrm{opt}}=50$ ns. (c) Spin-relaxation dynamics. The exponential fit (dashed line) gives an effective $1/e$ spin thermalization time of $0.95\mu \mathrm{s}$. The dotted line shows Boltzmann equilibrium. (d) Same as (a), but with additional cw laser 1 driving the spin-preserving transition $➀$ at saturation. The spin remains optically pumped in ${|\Downarrow \rangle }_z$ except when the scanning laser is resonant with the spin-preserving transition $➃$ (photon scattering at transitions $➀$ and $➃$) or the spin-flipping transition $➂$ (photon scattering mostly at transition $➀$, with a residual leak of red fluorescence counted on the blue pixel).

Figure 3

Single-photon pulse shaping. (a) Exponential photon wave forms obtained with a square control pulse. The single-photon wave forms are shown (with an offset for visibility) as the intensity of the control pulse decreases. Inset: Tuning of the Raman photon duration ${\tau }_R$ with control-pulse intensity. (b) Gaussian photon wave forms (Gaussian control pulses) with FWHM durations of 5, 15, 23, and 64 ns. (c) Double-Gaussian photon wave form. Each curve from (a) to (c) corresponds to 10-min integration and 2-ns time resolution. (d) Intensity autocorrelation of Raman photons with Gaussian wave form ($\mathrm{FWHM}=5$ ns, 10-MHz repetition rate, 11-h acquisition).

Figure 4

Spectral properties of the Raman photons. (a) Spectra of 50-ns Gaussian single photons measured for different control-laser detunings ${\mathrm{\Delta }}_L$ (2-MHz repetition rate, 100-s integration). Voigt fits (solid lines) are used to extract values and error bars for (b) the center frequency, (c) peak, and linewidth of the Raman light stream deconvoluted from the (Lorentzian) etalon transmission profile. (d) Increase of the spectral linewidth with control-pulse intensity for a fixed temporal profile with the minimum nonresonant intensity (the dashed line indicates a linear fit). Inset: Raw spectrum. A Gaussian emission linewidth (FWHM) of 200 MHz is obtained by a Voigt fit with the Lorentzian instrument response (FWHM of 250 MHz). (e) Increase of the spectral linewidth with nonresonant intensity. The saturation fit (dashed line) is a guide to the eye.