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 09.07.2010   Карта сайта     Language По-русски По-английски
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09.07.2010

Ultrabright source of entangled photon pairs





Journal name:

Nature

Volume:

466,

Pages:

217–220

Date published:

(08 July 2010)

DOI:

doi:10.1038/nature09148


Received


Accepted







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


    left


    1. 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 = 3µm and various centre to centre distances. The molecule modes are labelled M1–M5. Dashed lines are guides to the eye.




    2. Figure 2: Polarization properties of modes of the photonic molecule.


      a, Energy of the optical modes of the molecule for D = 3.5µm 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 60µeV. The polarization splitting of mode M3 is also smaller than 70µeV (not shown). The sources are fabricated with a moderate-quality-factor sample corresponding to a linewidth of 300–400µeV (indicated as the grey-shaded region). b, Measurement of the radiation pattern for the modes M1, M2 and M3 of photonic molecule A (D = 2.4µm, CC′ = 1.8µm) 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.




    3. 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 = 2.4μm, CC′ = 1.9μm.




    4. 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 130nW. 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.






    right







    Дизайн и программирование N-Studio 
    А Б В Г Д Е Ё Ж З И Й К Л М Н О П Р С Т У Ф Х Ц Ч Ш Щ Ъ Ы Ь Э Ю Я
  • Chen Wev .  honorary member of ISSC science council

  • Harton Vladislav Vadim  honorary member of ISSC science council

  • Lichtenstain Alexandr Iosif  honorary member of ISSC science council

  • Novikov Dimirtii Leonid  honorary member of ISSC science council

  • Yakushev Mikhail Vasilii  honorary member of ISSC science council

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