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Introduction to Photonic Quantum Logic IRC Summer School Aug. 2006 J. G. Rarity University of Bristol john.rarity@brist

EU FET: QAP. EPSRC 1-phot. Introduction to Photonic Quantum Logic IRC Summer School Aug. 2006 J. G. Rarity University of Bristol john.rarity@bristol.ac.uk. FP6:IP SECOQC www.ramboq.org. Bristol: Daniel Ho, J. Fulconis, J. Duligall, C. Hu, R. Gibson, O Alibart

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Introduction to Photonic Quantum Logic IRC Summer School Aug. 2006 J. G. Rarity University of Bristol john.rarity@brist

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  1. EU FET: QAP EPSRC 1-phot Introduction to PhotonicQuantum LogicIRC Summer School Aug. 2006J. G. RarityUniversity of Bristoljohn.rarity@bristol.ac.uk FP6:IP SECOQC www.ramboq.org Bristol: Daniel Ho, J. Fulconis, J. Duligall, C. Hu, R. Gibson, O Alibart Bath: William Wadsworth, Philip Russell Sheffield: M. Skolnick, D. Whittaker, M. Fox, J. Timpson Toshiba: A. Shields, A. Bennett Cambridge: D. Richie www.bris.ac.uk

  2. Structure • Lecture 1 • What is light? • Decoherence of photons • Single photon detection • Encoding bits with single photons and single bit manipulation. • Single photon sources • Entangled state sources • Lecture 2 • Linear logic, efficiency and scalability • Towards single photon non-linearity. • Lecture 3 • Quantum Cryptography (mainly free space)

  3. λ=1.5um Eph=0.8eV λ=0.33um Eph=4eV The electro-magnetic spectrum Optical Photon energy Eph=hf>>KT Particle like Particle like Wave-like during propagation V+

  4. Decoherence of photons: associated with loss • Storage time in fibre 5μs/km, loss 0.17 dB/km (96%) • Polarised light from stars==Storage for 6500 years!

  5. Photon counting using avalanche photodiodes Photon absorbed Photon is absorbed in the avalanche region to create an electron hole pair Electron and hole are accelerated in the high electric field Collide with other electrons and holes to create more pairs With high enough field the device breaks down when one photon is absorbed

  6. Equivalent Circuit Rq Rq Vb Rd Vb+V Total charge released on breakdown (Cd+Cg)V Recharge time (dead time) τD= (Cd+Cg)Rq Silicon devices total capacitance ~10pF and Rq~250-400Kohm τD 3-5us Ge 5pF and 33Kohm, τD 150ns, InGaAs2pF, ~56Kohm

  7. Actively Quenching photon counting module Output pulse rate proportional to intensity Constant intensityPoisson distributed

  8. Commercial detector module using Silicon APDEfficiency ~70% (at 700nm)Timing jitter~400ps (latest <50ps)Dark counts <50/secwww.perkinelmer.com InGaAs avalanche detectors: Gated modules operation at 1550nm Lower efficiency ~20-30% Higher dark counts ~1E4/sec Afterpulsing (10 us dead time) www.idquantique.com

  9. Other detectors • The Geiger mode avalanche diodes count one photon then switch off for a dead time before they are ready to detect another-NOT PHOTON NUMBER RESOLVING • Photon number resolving detectors may become available in the near future: • Cryogenic superconducting to resistive transitions Jaspan et al APL 89, 031112, 2006 • Impurity transitions in heavily doped silicon (Takeuchi)

  10. Interference effectswith single photons U Single photon can only be detected in one detector However interference pattern built up from many individual counts P. Grangier et al, Europhysics Letters 1986  L In the interferometer we have superposition state

  11. After the interferometer:

  12. Encoding one bit per photon and single qubit rotations Encoding single photons using two polarisation modes Superposition states of ‘1’ and ‘0’ |Ψ>= α|0> +β|1> Probability amplitudes α , β Detection Probability: |α|2 Single photon encoding showing QBER<5.10-4 (99.95% visibility)

  13. Rot or λ/2 Broad band source Polarising Prism PolarisinPrism Detector λ/4 λ/4 H z - 45° y x R L + 45° V Multiple waveplates reduce errors in brouad band sources, mapping onto NMR techniques L H V R

  14. See lecture 3:Bennett and Brassard 1984Secure key exchange using quantum cryptography Sends no. bit pol. 1 1 45 2 0 45 3 0 0 4 1 45 5 1 0 6 0 45 7 1 45 … 1004 0 45 1005 1 0 …. 3245 1 45 … Receives no. Bit Pol. 246 1 45 1004 0 45 2134 0 0 3245 0 0 4765 1 0 5698 0 45

  15. Single photon sources www.bris.ac.uk

  16. Approximate single photon source Attenuated laser = Coherent state Laser Mean number of photon per pulse = 0,1 Attenuator

  17. True single photon sources Particle like Particle like Wave-like during propagation V+ Single atom or ion (in a trap) Single dye molecule Single colour centre (diamond NV) Single quantum dot (eg InAs in GaAs) www.bris.ac.uk

  18. Quantum cryptography and linear-optics quantum logic needs single-photon sources with • High generation rate • High efficiency emission into single mode • Small multi-photon probability ( g(2) (0) 0 ) • Quantum indistinguishability (time-bandwidth limited, single mode, one polarization, etc. ) see lecture 2 Possible solution self-assembled InAs/GaAs QDs • Good approximation to two level atom • No bleaching effect and long-term stability • High generation rate (exciton lifetime~1ns) • Embedded in microcavity by in-situ growth • Standard semiconductor processing • Solid-state source • low extraction efficiency (~2%) (GaAs n=3.5)

  19. Cavity effects Single photon generation of QDs in 3D microcavity can be improved by Cavity Quantum Electrodynamics (CQED) see lecture 2 • Enhance spontaneous emission (Purcell effect) • Improve both coupling and extraction efficiency • Couple to a single cavity mode • Toward time-bandwidth limited photon pulse lifetime T1<< dephasing time T2

  20.  cavity between two GaAs/Al0.9Ga0.1As DBRs, bottom 27 pairs, top 20 or 14 pairs • One layer self-assembled InGaAs QDs at cavity center Samples FDTD modelling elliptical pillar circular pillar FIB etching ICP etching

  21. FDTD simulations: 0.50μm radius micropillar microcavity:Plane wave resonance=1001 nm 15 mirror pairs on top and 30 bottom

  22. FDTD simulations of micropillar microcavities Spontaneous emission enhancement Purcell factor= Total emission in cavity Emission into infinite GaAs >80% efficiency Into cavity mode!

  23. Experimental Setup Time Interval Analyser Hanbury-Brown Twiss measurement

  24. Single QD emission and temperature tuning • Single QD emission can be observed in smaller pillar at low excitation power • QD emission line shifts faster than cavity mode

  25. Single photon generation in circular pillars With increasing excitation power • QD emission intensity turns saturated • Cavity mode intensity develops

  26. Single photon generation in circular pillars • g(2) (0)=0.05indicates multi-photon emission is 20 times suppressed. • g(2) (0) increases with pump power due to the cavity mode

  27. Beyond this: • Time bandwidth limited single photons on demand from pillar microcavities (Journal of Optics B 7, 129-136 (2005). • Entangled photon pairs from Biexciton-Exciton cascaded emission (see , Nature 439, 179-182 ). • Linear gates mixing single photons/ entangled states (lecture 2). • Entangled solid state/atomic qubits after emission (Knight, Cirac). • Strong coupling, evidence of spectral splitting seen lecture 2 Long term • Inter-conversion to/from solid state Veff~(λ/n)3 • non-linear gates.

  28. Pair photons and entangled photons www.bris.ac.uk

  29. Why photon pairs? JGR,JMO 1998, 45, 595-604 • Heralded single photon source Triggered APD Heralded photons Source of Photon pairs Triggerphotons APD Mandel (1986) Experimental realization of a localized one-photon state, C. K. Hong and L. Mandel, Phys. Rev. Lett. 56, 58 (1986) Observation of sub-poissonian light in parametric downconversion. J.G. Rarity, P.R.Tapster and E. Jakeman, Opt. Comm., 62(3):201, 1987.

  30. Creating Entangled Photon Pairs C Phase matching and energy conservation: D

  31. Portable Crystal based entangled pair source150mW diode70000 detected coincidences/sec

  32. Pair photon generation in microstructured fibres With University of Bath kp ks χ(3) kp ki lpump=709nm lidler=897nm lsignal=587nm • Non-linear process in χ(3) • 2 pump photons  2 emitted photons Phase matching and energy conservation: • Use of the normal dispersion region: • no pump power dependence • wavelengths created far from the pump • Raman and fluorescence amplification effects reduced W. J. Wadsworth et al. (Opt. Express 2004)

  33. Coincidences~3.105 s-1 Spectra Experiment

  34. Coincidence results Idler (895nm): single-modefiber ~ 14% of coincidences Signal (583nm): multimode fiber ~ 33% of coincidences : single-modefiber ~ 22% of coincidences 2 photon peak 4 photon peak • Ppump=540μW: • Coincidence rate: ~3.5.105 s-1 • Overall coupling efficiency in SMF 60% Confirmation of bright pair photon emission: -potential for 80% coupling to SMF

  35. Entangled state, preliminary measurement Latest result by narrow band filtering: 90% in HV and 80% in +/-45.

  36. Co-workers • Universiy of Bristol: Daniel Ho, J. Fulconis, J. Duligall, C. Hu, R. Gibson, O.Alibart, J. O’Brien Fibres • University of Bath: W. Wadsworth, P. Russell Quantum dots and microcavities • University of Sheffield: M. Skolnick, D. Whittaker, M Fox • Toshiba Europe: A. Shields, A. Bennett • Univ Cambridge: D. Ritchie Linear optics RAMBOQ IST38864: Vienna, LMU, HP Bristol, Geneva, Erlangen, Queensland, Toshiba, Cambridge, Thales, MPQ, IdQ, ….

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