A solid-state source of strongly entangled photon pairs with high brightness and indistinguishability

Abstract

The generation of high-quality entangled photon pairs has been a long-sought goal in modern quantum communication and computation. So far, the most widely used entangled photon pairs have been generated from spontaneous parametric down-conversion (SPDC), a process that is intrinsically probabilistic and thus relegated to a regime of low rates of pair generation. In contrast, semiconductor quantum dots can generate triggered entangled photon pairs through a cascaded radiative decay process and do not suffer from any fundamental trade-off between source brightness and multi-pair generation. However, a source featuring simultaneously high photon extraction efficiency, high degree of entanglement fidelity and photon indistinguishability has been lacking. Here, we present an entangled photon pair source with high brightness and indistinguishability by deterministically embedding GaAs quantum dots in broadband photonic nanostructures that enable Purcell-enhanced emission. Our source produces entangled photon pairs with a pair collection probability of up to 0.65(4) (single-photon extraction efficiency of 0.85(3)), entanglement fidelity of 0.88(2), and indistinguishabilities of 0.901(3) and 0.903(3) (brackets indicate uncertainty on last digit). This immediately creates opportunities for advancing quantum photonic technologies.

FIG. 1:

Circular Bragg resonator on highly-efficient broadband reflector (CBR-HBR) for entangled-photon pair generation. Realization and calculated performance of the CBR-HBR are presented. (a) An illustration of a CBR-HBR with a single QD emitting entangled-photon pairs. The inset shows the XX-X cascaded radiative process for generating polarization-entangled photon pairs, in which the value of the fine structure splitting (FSS) plays an important role in determining the achievable entanglement fidelity without time-filtering. (b) Simulated Purcell factor (red) and collection efficiency (blue) of the CBR-HBR as a function of wavelength. The collection efficiency is based on a 40° azimuth angle, corresponding to a numerical aperture (NA) = 0.65. (c) and (d) are fluorescence images of the same QD before and after the fabrication of the CBR-HBR. (c) and (d) share the same scale bar.

FIG. 2:. Basic characterization of the QD-CBR-HBR device.

(a) PL spectrum of a QD in the CBR-HBR under two-photon resonant excitation (right y axis, indicated in red) and the cavity mode measured from white light reflection (left y axis, indicated in blue). The excitation power is chosen to maximize the intensity of the XX emission (”$\text{π}$ pulse” conditions), X and XX are equally populated and resonant with the cavity mode of the CBR-HBR. (b) PL lifetime of X and XX in bulk and in the CBR-HBR, showing pronounced Purcell enhancement for both X and XX. (c) Photon auto-correlation measured under ”$\text{π}$ pulse” two-photon resonant excitation, using a Hanbury-Brown and Twiss interferometer. The second-order correlation $g^{(2)}(0)=\mathrm{0.001\pm 0.001}$ for X and $g^{(2)}(0)=\mathrm{0.007\pm 0.001}$ for XX are calculated from the integrated area in the zero delay peak divided by the mean of the peaks away from zero-delay, and the uncertainty is a one standard deviation value. (d) Detected count rates of the X photons as a function of square root of the excitation power. The blue curve is a guide to the eyes.

FIG. 3:. Entanglement characterization.

Fidelity of the polarization entanglement is investigated. (a) Polarization-dependent measurement to determine the FSS of X. The relative energy difference between X and XX is plotted in order to obtain a higher measurement precision. An FSS value of 4.8(2) μeV is extracted from the amplitude of the sine-function fitting. (b) Theoretically predicted entanglement fidelity as a function of FSS for GaAs QDs in the CBR-HBR ($F_p=3.5$, black line), in bulk (blue line), Purcell enhanced InAs QDs ($F_p=3.5$, ruby line) and InAs QDs in bulk (green line). The vertical dashed line denotes a FSS of 4.8 μeV and the horizontal dashed line ($f=0.5$) is the boundary above which quantum entanglement exists. (c), (d) and (e) are the X-XX polarization dependent cross-correlation histogram under ”$\text{π}$ pulse” conditions for linear, diagonal, and circular basis respectively. Data for cross-polarization configurations are shifted deliberately for clarity.

FIG. 4:. Photon indistinguishability.

HOM interference for X and XX photons are performed individually. Two-photon interference for cross-polarized (a), co-polarized (b) X photons and cross-polarized (c), co-polarized (d) XX photons. The data are fitted by exponential decays (measured emitter decay response) convolved with a Gaussian (measured photon detector time response). The area of the central peaks is extracted to calculate the raw visibilities, which are 0.901(3) and 0.903(3) for X and XX respectively.