Highly indistinguishable and strongly entangled photons from symmetric GaAs quantum dots

Abstract

The development of scalable sources of non-classical light is fundamental to unlocking the technological potential of quantum photonics. Semiconductor quantum dots are emerging as near-optimal sources of indistinguishable single photons. However, their performance as sources of entangled-photon pairs are still modest compared to parametric down converters. Photons emitted from conventional Stranski-Krastanov InGaAs quantum dots have shown non-optimal levels of entanglement and indistinguishability. For quantum networks, both criteria must be met simultaneously. Here, we show that this is possible with a system that has received limited attention so far: GaAs quantum dots. They can emit triggered polarization-entangled photons with high purity (g⁽²⁾(0) = 0.002±0.002), high indistinguishability (0.93±0.07 for 2 ns pulse separation) and high entanglement fidelity (0.94±0.01). Our results show that GaAs might be the material of choice for quantum-dot entanglement sources in future quantum technologies.

Figure 1. Sample structure and spectral properties of the used quantum dots.

(a) Cross-sectional 3D view of an atomic force microscopy (AFM) image of a nanohole in an AlGaAs layer. The colour scale reflects the local surface inclination (white for flat areas and black for inclinations >16°). The height-to-width ratio is amplified 17 times to highlight the nanohole shape. A GaAs quantum dot (QD) with a height of about 7 nm is obtained after its filling with GaAs and overgrowth with AlGaAs. (b) Sketch of the sample structure. (c) Top view of the AFM measurement with colour scale reflecting the local height (0 corresponds to the average height of the flat areas surrounding the nanohole). The lateral scale of the AFM is as in a −250 × 250 nm². The highly symmetric shape is crucial to obtain the high degree of entanglement observed here. (d) Microphotoluminescence spectrum of a representative QD under non-resonant excitation (laser photon energy E_L=2.54 eV). The neutral exciton emission line is labelled as X. The biexciton line (XX) is not visible in these excitation conditions. (e) Spectrum of the same QD as in d under resonant two photon excitation (E_L=1.5775, eV). The X and XX are visible and have very similar intensity. Two weaker charged states appear (C₁ and C₂). Inset: For resonant two-photon excitation, the 9 ps laser pulses are tuned to half of the energy difference between |XX> and ground state |0>.

Figure 2. Single-photon purity and two-photon interference.

(a) Integrated intensities of the excition (X) (blue) and biexciton (XX) (red) versus the square root of the pump power under resonant excitation of a representative quantum dot (QD). The abscissa is normalized in π-units with respect to the π-pulse of the XX state. The X as well as the XX show well-defined Rabi oscillations. (b) Auto-correlation measurements of X and (c) XX photons for a representative QD. The measurements show strong antibunching for both lines. (d) Two-photon interference with co-polarized (red) and cross-polarized (grey) XX photons. (e) Same as in d for three different QDs around zero time delay (see dotted line box in d). The data for both the X and XX co-polarized photons are reported. The dashed lines are Lorentzian fits of the peaks. The solid lines show the total sum of the single fits. The values of the visibility are also given in each panel.

Figure 3. Entangled photons from GaAs quantum dots.

(a) Real and imaginary part of the two-photon density matrix reconstructed via quantum state tomography on quantum dot QD2. The matrix has been calculated out of biexciton–exciton (XX–X) cross-correlation measurements in 16 polarization settings and using a maximum likelihood method. (b) XX–X cross-correlation measurements under resonant two-photon excitation for QD2 and (c) QD3. V_XX,X (H_XX,X), D_XX,X (A_XX,X) and R_XX,X (L_XX,X) indicate vertical (horizontal), diagonal (antidiagonal) and circularly right (circularly left) polarized photons. The graphs for co-polarized (red) and cross-polarized (blue) photons are temporally shifted by 3 ns for clarity.