Monolithic Source of Photon Pairs

The creation of monolithically integratable sources of single and entangled photons is a top research priority with formidable challenges: The production, manipulation, and measurement of the photons should all occur in the same material platform, thereby fostering stability and scalability. Here we demonstrate efficient photon pair production in a semiconductor platform, gallium arsenide. Our results show type-I spontaneous parametric down-conversion of laser light from a 2.2 mm long Bragg-reflection waveguide, and we estimate its internal pair production efficiency to be $2.0\times {10}^{-8}$ (pairs/pump photon). This is the first time that significant pair production has been demonstrated in a structure that can be electrically self-pumped and which can form the basis for passive optical circuitry, bringing us markedly closer to complete integration of quantum optical technologies.

Figure 1

Device geometry: A schematic of a ridge Bragg-reflection waveguide as an integrated source of quantum light. The waveguide core layer (yellow) supports index-guided light as well as light confined by the transverse Bragg reflectors (alternating light and dark blue layers). The Bragg layer thicknesses and material composition along with the width ($W$) and the ridge height ($D$) are design parameters that determine the wavelengths at which photon pairs can be created. Pump photons (represented by the blue arrow) are injected into the structure and photon pairs (represented by the red and green arrow, respectively) emerge at the output facet. The structure is grown on a [001]-GaAs substrate (dark gray).

Figure 2

The SPDC experiment: TM polarized picosecond pulses of light near a wavelength of 775 nm were injected into the waveguide. Light emitting from the waveguide was color filtered to remove the pulsed input. What remained was split probabilistically before being sent to two single photon detectors. The path to one of the detectors was optically delayed by 60 ns to allow it to be conditionally armed upon the detection of the earlier photon at the other detector. Time differences between (detection events at) the two detectors were recorded, and a histogram of all recurring time differences was created.

Figure 3

Coincidence measurements: A peak at $\tau =0$ indicates that both detectors responded in coincidence. The top figure (a) shows the effect of rotating a polarizer from TE (black) to TM (red) in the output while injecting TM polarized light on resonance with the phase-matching wavelength into the BRW. The bottom figure (b) shows the typical effect found when injecting light at (black) and away from (red) the resonant phase-matching wavelength. Peak widths are essentially determined by the finite detector time resolution of approximately 1 ns. The periodic signal on either side of the zero delay time is predominantly a result of pairs being produced in pulses preceding or succeeding the true coincidence pulse. The signal between the peaks is due to background light, detector dark counts, or (slowly decaying) fluorescence, thought to arise from deep level impurities in the sample.

Figure 4

Cross pulse to coincidence ratio: A demonstration of the dependence of the cross pulse coincidences to true coincidence ratio on the input power. Ignoring random events, the cross pulse signal is an indication of the pair per pulse probability ($p_{\mathrm{pulse}}$) which, as shown here, increases linearly with laser power. This linear scaling of $p_{\mathrm{pulse}}$ builds the case that the signals we see originate from a pair production process like SPDC. By taking the ratio, system losses are largely eliminated.