A source of triggered entangled photon pairs is a key component in quantum information science1; it is needed to implement functions such as linear quantum computation2, entanglement swapping3 and quantum teleportation4. Generation of polarization entangled photon pairs can be obtained through parametric conversion in nonlinear optical media5, 6, 7 or by making use of the radiative decay of two electron–hole pairs trapped in a semiconductor quantum dot8, 9, 10, 11. Today, these sources operate at a very low rate, below 0.01 photon pairs per excitation pulse, which strongly limits their applications. For systems based on parametric conversion, this low rate is intrinsically due to the Poissonian statistics of the source12. Conversely, a quantum dot can emit a single pair of entangled photons with a probability near unity but suffers from a naturally very low extraction efficiency. Here we show that this drawback can be overcome by coupling an optical cavity in the form of a ‘photonic molecule’13 to a single quantum dot. Two coupled identical pillars—the photonic molecule—were etched in a semiconductor planar microcavity, using an optical lithography method14 that ensures a deterministic coupling to the biexciton and exciton energy states of a pre-selected quantum dot. The Purcell effect ensures that most entangled photon pairs are emitted into two cavity modes, while improving the indistinguishability of the two optical recombination paths15, 16. A polarization entangled photon pair rate of 0.12 per excitation pulse (with a concurrence of 0.34) is collected in the first lens. Our results open the way towards the fabrication of solid state triggered sources of entangled photon pairs, with an overall (creation and collection) efficiency of 80%.
Figures at a glance
Figure 1: Principle of photon extraction using a photonic molecule. 
a, Sketch of the radiative cascade in a single quantum dot. See main text for nomenclature. b, Diagram of the source: two identical pillar microcavities with diameter D are coupled. The centre to centre distance is labelled CC′. A single quantum dot (QD) is inserted in one of the pillars. k is the photon wave vector. c, Energy of the optical mode of the photonic molecule for D = 3 µm and various centre to centre distances. The molecule modes are labelled M1–M5. Dashed lines are guides to the eye.
Figure 2: Polarization properties of modes of the photonic molecule. 
a, Energy of the optical modes of the molecule for D = 3.5 µm and various distance CC′ measured on a calibration sample with Q = 60,000 to allow high spectral resolution. Both M1 and M2 show polarization splitting smaller than 60 µeV. The polarization splitting of mode M3 is also smaller than 70 µeV (not shown). The sources are fabricated with a moderate-quality-factor sample corresponding to a linewidth of 300–400 µeV (indicated as the grey-shaded region). b, Measurement of the radiation pattern for the modes M1, M2 and M3 of photonic molecule A (D = 2.4 µm, CC′ = 1.8 µm) for two linear polarizations: H and V, respectively parallel and perpendicular to the molecule axis, x. k is the amplitude of k as defined in Fig. 1.
Figure 3: Photon correlation measurements on molecule B. 
a, Emission intensity (linear colour scale, arbitrary units) as a function of energy and temperature. b, Measured second order correlation function, g2X,XX. The black curve has been horizontally shifted for clarity. c, Zoomed-in view of the zero delay correlation peak in b. d, Correlation polarization C deduced from the data presented in c. The polarization detection settings are indicated as the first letter for the X line and the second letter for the XX line. R and L refer to the right- and left-handed circular basis, respectively; H and V to the linear basis parallel and perpendicular to the molecule axis, x, respectively; and D and D′ to the two linear diagonal polarizations. Molecule B has D = 2.4 μm, CC′ = 1.9 μm.
Figure 4: Characterization of the source: entanglement and brightness. 
a, Density matrix of the two-photon state measured on molecule B for positive delays for an excitation power of 130 nW. For simplicity, the absolute value of the imaginary part is plotted, the sign of the imaginary part matrix elements is indicated in the table (inset, right). b, Left: number of X and XX collected photons for each excitation pulse as a function of the excitation power. Right: collected entangled photon pair rate (MHz) as a function of the excitation power.
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