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Quantum Information Processing with a single photon source

Quantum Information Processing with a single photon source. David Fattal. Dept of Physics, Stanford University. Les Houches seminar, July 17 2003. GOOD. essentially no decoherence. single qubit unitaries are very easy and accurate. BAD. controlled interactions between qubits are hard.

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Quantum Information Processing with a single photon source

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  1. Quantum Information Processing with a single photon source David Fattal Dept of Physics, Stanford University Les Houches seminar, July 17 2003

  2. GOOD • essentially nodecoherence • single qubit unitaries are very easy and accurate BAD • controlled interactions between qubits are hard Use tricks to induce interactions : Quantum interference + photon counting Motivation Photons have few interactions with their environment Idea : encode quantum information in optical modes…

  3. Outline • Encoding quantum information in single optical modes • What single mode really means… • Entangling two independent photons with a beam-splitter… • Single-mode teleportation with linear-optics • Generation of single photons by optical excitation of a quantum dot

  4. Encoding QI in optical modes • Polarization • Dual-rail logic • Single-rail logic • gaussian states (squeezed displaced vacuum…) Discrete logic Continuous logic

  5. Dual rail logic 1 qubit = 1 quantum in 2 modes C-z C-NOT = sx sz p C-z does not utilize two of four input modes (representing two qubits)

  6. “wavefunction” for single photon Single modes… Single photon in time domain : t

  7. Single modes… t “Identical” photons if : t “overlap” between photons

  8. What can go wrong… Constant but fluctuates. Dephasing : and fluctuates. Time jitter : Deviation from single-mode

  9. Take home message ! To do (interesting) Quantum Information Processing with optical modes, photons conveyed in different modes must be (very close to) identical. (except for BB84 QKD) It is important to be able to measure the degree of indistinguishability … Mandel-type experiment : two photon “collision”

  10. Mandel-type experiment C.K. Hong, Z.Y. Ou and L. Mandel, PRL 59, 2044 (1987) b Photon counter a NPBS Overlap between photon a and b coincidence counts • Identical photons bunch at a beam splitter, due to quantum interference.

  11. 0.6m Test on QD single photon source t2 t1 Dt + 2 ns 2 ns b a Classical light pulses

  12. Results… Dip Coincidence counts Detection time difference “t2 – t1”(ns) Overlap between consecutive photons is 0.81 C. Santori et al., Nature 419, 594 (2002)

  13. Entangling single photons Input : Detector 1 1 NPBS Output : 2 Detector 2 if both detectors click (postselection)

  14. Violation of Bell’s inequality HWP • Input : • Output : from QD a V H H A 1 2 b Analyser angles used in experiment: a = 0/90oa’ = 45/135o b = 22.5/112.5ob’ = 67.5/157.5o SPCM NPBS B = 2.377± 0.025 > 2 Coincidence circuit = post-selection State tomography • Scheme relies on quantum interference between two independent single photons from a QD. • Entanglement is induced by the measurement: NO optical non-linearity required. • Ideal efficiency is ½. • Only single pairs are created. • Application to BBM92 QKD. • Opens the way to efficient generation of multi-particle • entanglement and linear-optics quantum computing…

  15. Single mode “teleportation” Coherence preserved |u |u target |d C output |u D ancilla p |d |d Condition : ‘C’ clicks and ‘D’ does not, or ‘D’ clicks and ‘C’ does not. • No entangled state needed for input • Proba of success = 1/2

  16. Single mode “teleportation”  Can be viewed as teleportation of half a qubit two half teleportations = teleportation • two ancillas already entangled • proba of success = ½ • two independent ancillas • proba of success = ¼ using linear-optics…

  17. simplified version Mach-Zehnder config to check coherence Piezo |u |u target |d C A |u D B ancilla p |d |d  Record coincidences between A/C and B/C We use single photons : if A and C click, D cannot click…

  18. Experimental setup Delay line (for ancilla) from QD ancilla target Piezo 1 2 SM-fiber BS 1 Pol. control C 3 BS a D BS 2 A 4 B ¼ of the times, ancilla takes long path and target takes short path…

  19. Experimental results

  20. Conclusion • We need identical photons to manipulate QI in optical modes • The residual distinguishability limits the fidelities • To make more identical photons, need new cavities… J. Vuckovic and Y. Yamamoto, APL 82, 2374 (2003)

  21. Acknowledgements Charles Santori Kyo Inoue Eleni Diamanti Glenn Solomon (QD growth) Jelena Vuckovic (microcavities) Collaborators : Advisor : Yoshihisa Yamamoto

  22. Quantum Dots Electron discrete levels Conduction band n=2 n=1 GaAs InAs GaAs n=1 n=2 Valence band Hole discrete levels

  23. Single photon emission Conduction band Fast non-radiative decay Resonant excitation Radiative decay = 1 photon n=1 n=2 Valence band

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