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Generation of quantum states of light by a semiconductor quantum dot. Thermal light : bunched photons (superpoissonian). Coherent light : independent photons (poissonian). Regulated single photon source. Single photon sources. Applications: quantum cryptography

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Generation of quantum states of light by a semiconductor quantum dot


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    1. Generation of quantum states of light by a semiconductor quantum dot

    2. Thermal light : bunched photons (superpoissonian) Coherent light : independent photons (poissonian) Regulated single photon source Single photon sources Applications: quantum cryptography single photon optics (interferences…) linear optics quantum computing

    3. What is a single photon source ? Source able to emit single photons pulses on demand clic clic N.B. : Non-classical state of light => Impossible to generate using a thermal source or a laser

    4. Applications of single-photon sources Deterministic emission of light pulses containing one and only one photon Quantum Key Distribution Polarisation-encoded single photons 2005-2010 Metrology flux+energy standard Quantum computing single photons as Q-bits 2030?

    5. l/2 + l/4 waveplates PBS Bob Telecom fiber ‘1’  1 0 1 ….  Single Photon Detectors ‘0’ Spy?? Quantum cryptography Alice SPS Alice and Bob share a binary key The protocol is intrinsically secure (Heisenberg !!) Þ whatever the technological abilities of the spy Bennett & Brassard 84 : spying Þ error > 25%

    6. Constraints for secure QKD : 1) dark counts Alice <n> photon per pulse transmission t <1 Bob d dark counts per pulse • Error correction possible  <n> t > 10 d • limited secure transmission length (~100km at most at telecom wavelength)

    7. Constraints for secure QKD : 2) multi-photon pulses Lossy channel => « Photon-Number Splitting » attack Eve Alice P(n>1) >0 Bob Analyser Source . resends correct photons to Bob . adjusts bit rate to expected value t <n> analyses only multi-photon pulses PNS works  p(n>1) > t <n>

    8. Single photon sources for quantum cryptography • Attenuated laser : • - poissonian distribution of nphotons per pulse • for <n>=0.1 P(0)=90.5%, P(1)=9.05%, P(2)=0.45% • Vulnerable to photon number splitting attack • for <n>=4.5 10-3 P(0)=99.5%, P(1)=0.45%, P(2)=10-3% • Parametric downconversion: • - poissonian process • - detection of a photon allows to trigger the gate of the • receiver’s detector • Individual emitters

    9. Strategies for single photon generation Injection controlled by Coulomb blockade (Yamamoto et al, Nature 99) Implementation of a discrete emitter atom molecule quantum dot... 1e- 1h+ hn T<0.1K, no microcavity yet

    10. Molecule F center Lounis et Moerner, Nature 407, 491 (2000) Nanocristal Self-assembled quantum dots Core (CdSe) Shell (ZnS, CdS) Solid-state single photon emitters at 300 K ! Kurtsiefer et al, PRL 85, 290, 00 Michler et al, Nature 406, 968, 00 T<200K until now!

    11. E CB X XX VB QD assets for single-photon generation stable (no blinking, no bleaching) efficient (h ~1) fast (t~1ns) spectrally narrow at low T QDs are + adjustable bandgap/non-resonant pumping OK A QD is not a 2-level system!

    12. X 3X @w3 XX @w2 X @w1 XX Single photon generation from a quantum dot Proposal: J.M. Gérard et B. Gayral, J. Lightwave Technol. 17, 2089 (1999) CB VB Pulsed non resonant excitation + spectral filtering • Conversion of a poissonian excitation into a non-classical regulated stream of single photons

    13. Photon correlation setup Spectrometer l ~ 920 nm He-flow Cryostat (4K) Sample Start Analyser Laser Ti:Sa P=1W l=790 nm X Stop X 50/50 beamsplitter Optical densities

    14. Photon Antibunching– Proof of a Two-Level Emitter Probability to detect a stop photon at t=t1 given that a start photon was detected at t=0 t1

    15. Antibunching from a Single Quantum Dot 105 W/cm2 g2(0)=0.1 t = 750 ps 55 W/cm2 g2(0)=0.0 t = 1.4 ns 15 W/cm2 g2(0)=0.0 t = 3.6 ns PL spectrum P=55W/cm2 T=4K

    16. Autocorrelation experiment Start=X1 Stop=X1 X1 Average g1

    17. XX-X1 cross-correlation Start=X1 Stop=XX XX Detecting X1 after XX Detecting XX after X1 X1 average g1 Direct evidence of cascaded emission

    18. X line correlation histogram attenuated laser light -20 0 20 40 60 First reports: P. Michler et al, Science 290, 2282 (2000) C. Santori et al, PRL 86, 1502 (2000) 80 60 Number of coincidences 40 20 0 -20 0 20 40 60 Delay t (ns) « X » photons are emitted one by one Protocol works also for electrical pumping (Yuan et al, Science 195, 102, 2002)

    19. Fast emission t0/Fp directional « Slow » emission t0 PURCELL EFFECT Collecting the single photon with the Purcell effect PL Intensity • - Spontaneous emission • modification • Selective enhancement • in a cavity mode 0 500 1000 1500 2000 Time (ps)

    20. Pump laser injection of several e-h pairs Collecting the single photon 1 QD on resonance with the fundamental mode

    21. Recombination last e-h pair Pump laser injection of several e-h pairs Collecting the single photon Experimental single mode emission: 40% Recombination of excess pairs => Demonstration by Gerard (CEA) and Yamamoto (Stanford)

    22. Few QDs in a micropillar cavity mode

    23. Elliptical cross section 1.4 µm 0.7 µm Polarization control in single-mode micropillars x-polarized y-polarized PL Intensity (lin.) Non degenerate fundamental mode Gayral et al, APL 72, 1421, 1998 ~ 90% linear polarisation degree of single QD emission Moreau et al, APL 79, 2865, 2001 1.265 1.270 1.275 1.280 Energy (eV)

    24. b photons emitted into the mode scattering by sidewall roughness Q spoiling e < b SPS efficiency for GaAs/AlAs micropillars Where do they escape???

    25. SPS efficiency for GaAs/AlAs micropillars Efficiencies around 70% can be reached for state of the art pillars J.M. Gérard et al, quant-ph/0207115

    26. PCs for improved QD-SPS ? Monomode PC waveguide QD in high Q PC cavity Moderate Fp (>20) => b>0.95 Q ~ 1000 => X/XX filtering OK Quncoupled > 10 => e > 0.9 b > 0.85 Q e > 0.7 ? YES!

    27. Current issues for single photon sources Room temperature operation =>QDs with large X-XX splitting (CdSe, GaN...…) Generation of indistinguishable single photons for optical quantum logical gates Improvement of the collection efficiency (optimized pillars, PBG cavities…) Developpement of a « plug and play » source for quantum key distribution (1.3 µm + electrical pumping + microcavity) …and finding a real market for single photon sources !

    28. Two photon interferences + = 0 The two photons must be indistinguishable : i) arrive at the same time ii) have the same polarization iii) have the same spectrum Possibility to make two photons emitted by the same quantum dot interfere?

    29. Experimental demonstration (Stanford) C. Santori, D. Fattal, J. Vuckovic, G.S. Solomon, Y. Yamamoto, Nature (Oct 2002) • Understand and master dephasing processes!!

    30. Specifications : QKD vs quantum computing Specifications much tougher for quantum computing!!!!