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Towards practical quantum repeaters (and quantum experiments with human eye detectors)

Towards practical quantum repeaters (and quantum experiments with human eye detectors). Christoph Simon. Group of Applied Physics, University of Geneva. Institute for Quantum Information Science, University of Calgary. Overview. Quantum repeaters: motivation and principle

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Towards practical quantum repeaters (and quantum experiments with human eye detectors)

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  1. Towards practical quantum repeaters (and quantum experiments with human eye detectors) Christoph Simon Group of Applied Physics, University of Geneva Institute for Quantum Information Science, University of Calgary

  2. Overview Quantum repeaters: motivation and principle Repeaters with atomic ensembles Multimode quantum memories CRIB and AFC: principles and experiments Requirements for practical repeaters Quantum experiments with human eyes Micro-macro entanglement?

  3. Long-distance quantum communication Fundamental motivation (how far can one stretch entanglement?), but also important for quantum cryptography, quantum networks. Big problem: transmission losses 1000 km of fiber at optimal wavelength – transmission 10-17. 3 years to distribute one entangled pair with 1 GHz source! Straightforward amplification impossible (no-cloning theorem). Potential solution: quantum repeaters.

  4. Quantum Repeater - Principle Z A Create entanglement independently for each link. Extend by swapping. n links with transmission t each Direct transmission Repeater Requires heralded creation, storage and swapping of entanglement. H.-J. Briegel et al., PRL 81, 5932 (1998)

  5. The DLCZ scheme: atomic ensembles and linear optics DLCZ = Duan, Lukin, Cirac, Zoller Entanglement creation through single photon detection. Entanglement swapping through efficient reconversion of memory excitation into photon

  6. Improved repeaters with ensembles and linear optics Entanglement swapping via two-photon detections Jiang, Taylor, Lukin, PRA 76, 012301 (2007) Entanglement generation via two-photon detections Zhao et al, PRL 98, 240502 (2007); Chen et al, PRA 76, 022329 (2007) Spatial multiplexing Collins et al, PRL 98, 060502 (2007). Photon pair sources and temporal multimode memories Simon et al, PRL 98, 190503 (2007). Single-photon source based protocol Sangouard et al, PRA 76, 050301(R) (2007) Local generation of entangled pairs plus two-photon detections Sangouard et al, PRA 77, 062301 (2008). Review: N. Sangouard, C. Simon, H. de Riedmatten, N. Gisin, arXiv:0906.2699

  7. Comparison of repeaters with ensembles and linear optics N. Sangouard, C. Simon, H. de Riedmatten, N. Gisin, arXiv:0906.2699 Assuming 90% memory and detection efficiency. Multiplexing is very attractive.

  8. Repeaters with photon pair sources and multi-mode memories Memories that are able to store and recall trains of N pulses. N entanglement creation attempts per waiting time interval L0 /c. Speedup by factor of N C. Simon et al, PRL 98, 190503 (2007).

  9. Temporal multi-mode memories Standard Photon Echo: thousands of modes, but not a quantum memory! J. Ruggiero, J.L. Le Gouet, C. Simon, T. Chaneliere, PRA 79, 053851 (2009) Stopped light (EIT): M.D. Lukin, RMP 75, 457 (2003) Controlled Reversible Inhomogeneous Broadening (CRIB): • B. Kraus et al., PRA 73, 020302 (2006); C. Simon et al, PRL 98, 190503 (2007) • Atomic frequency comb protocol: N independent of d. • M. Afzelius, C. Simon, H. de Riedmatten, N. Gisin, PRA 79, 052329 (2009). CRIB and AFC exploitstatic inhomogeneous broadening (e.g. in rare-earth doped solids) AFC promises efficient storage ofhundreds of temporal modes on seconds timescale. AFC

  10. CRIB: Controlled Reversible Inhomogeneous Broadening Natural broadening STEP 2 « Burn a hole » Absorption w Absorption w Optical pumping STEP 3 STEP 4 Controlled broadening Trigger re-emission Absorption Absorption w w Linear Stark shifts by EXTERNAL ELECTRIC FIELD Mirror broadening by changing the POLARITY of the E-field STEP 1 Light Storage ! Moiseev and Kröll, PRL 87, 173601 (2001), B. Kraus et al., PRA 73, 020302 (2006), N. Sangouard, et al, PRA 75, 032327 (2007)

  11. State after absorption Control fields Output mode Input mode Dephasing Backward readout Storage state Avoid re-absorption by constructive interference Periodic structure => Rephasing after a time Output mode Input mode Input mode Output mode Intensity Control fields kin Intensity k k Time kout Time Collective emission in the forward mode. Photon echo like emission Atomic Frequency Comb (AFC) Quantum Memory Ensemble of inhomogeneously broadened atoms D Atomic density Atomic detuning  M.Afzelius, C.Simon, H. de Riedmatten and N.Gisin, PRA 79, 052329 (2009).

  12. APPLY BROADENING Absorption w CRIB vs AFC CRIB AFC Absorption Absorption w w More atoms for the same bandwidth using an AFC  Higher efficiency! ~N2 ~N Many atoms are lost in the preparation step  Low efficiency! AFC

  13. +V -V CRIB at telecom wavelength – weak pulse storage at the single photon level Transmitted pulse ~0.6 photons per pulse Efficiency  0.5 % CRIB echo B. Lauritzen et al, arXiv:0908.2348

  14. CRIB at telecom wavelength Why is the efficiency low ? • Low efficiency due to: • Low optical depth • Poor optical pumping Both points can be improved (other crystals, cavities; higher B field, hyperfine states).

  15. Nd:YVO in Geneva: =1% Tu:YAG in Orsay/Paris: =9% Input Transmitted input Nd:YSO in Geneva: =5% Pr:YSO in Lund: =19% Output 19% 2-level AFC in various places, ions and crystals

  16. 0.10 Transmitted light 0.08 0.06 Intensity (au) 0.04 Internal Efficiency ≈ 0.5 % 0.02 Echo 0.00 0 100 200 300 Time (ns) … and one more that I got this morning! (W. Tittel, Calgary)

  17. Complete 3-level AFC storage experiment in Pr3+:Y2SiO5 5/2 3/2 Ts=15.6 µs 1/2 Ts=10.6 µs Ts=7.6 µs Control field Input mode Ts=5.6 µs 1/2 Decay of coherence due to inhomogeneous spin dephasing. Fitted spin distribution Gaussian FWHM: 26 kHz 3/2 5/2 M. Afzelius et al., arXiv:0908.2309 See also poster by H. De Riedmatten! Solution: Spin echo  1 s spin coherence !

  18. How good do memories and detectors have to be for repeaters? 1 percent drop in efficiency costs 7-19 percent in repeater rate Status quo: Memory efficiencies up to 50% for DLCZ (q) and EIT (cl) in atomic gases, 60% for CRIB (cl) in RE doped solid. Overall detection efficiency 95% (transition edge sensors) A.E. Lita, A.J. Miller, S.W. Nam, Opt. Exp. 16, 3032 (2008) Reducing coupling losses is essential! N. Sangouard, C. Simon, H. de Riedmatten, N. Gisin, arXiv:0906.2699

  19. What about intercontinental entanglement? Geneva – Calgary still seems out of reach. Can we do better than multi-mode memories plus linear optics?

  20. Repeaters with deterministic entanglement swapping Repeaters with trapped ions. N. Sangouard, R. Dubessy, C. Simon, PRA 79, 042340 (2009) Great gain in performance! Quantum gates with over 99% fidelity (R. Blatt) Ion-ion entanglement (C. Monroe) Efficient coupling to cavity (R. Blatt)

  21. Quantum experiments with human eye detectors based on quantum cloning via stimulated emission Cloning by stimulated parametric down-conversion. Simon, Weihs, Zeilinger, PRL ‘00, De Martini, Mussi, Bovino, Opt. Comm. ‘00 P. Sekatski et al., PRL 103, 113601 (2009) Realistic model of the eye as a threshold detector with losses Hecht, Shlaer, Pirenne, J. Gen. Physiol. 25, 819 (1942), Rieke and Baylor, RMP ‘98

  22. Micro-macro experiment following De Martini et al. Can violate Bell inequality with human eye detectors! Would prove micro-micro entanglement (amplifier part of detector) Different from usual detection (amplification before choice of basis). Brings observer closer to quantum phenomenon... But wait. Is the micro-macro state entangled?

  23. Micro-macro entanglement? Human-eye experiment does not prove micro-macro entanglement! Separable model: This also applies to De Martini group experiments. P. Sekatski et al., PRL 103, 113601 (2009) Micro-macro entanglement criterion: for all separable states for micro-macro experiment etc., Stokes parameters Micro-macro entanglement is present, even with loss. Requires precise counting of large photon numbers.

  24. Conclusions Multimode memories are attractive for quantum repeaters. First quantum memory at telecom wavelength via CRIB. First complete implementation of AFC memory protocol. Quantified importance of memory efficiency. Thousands of km distances will require deterministic swapping. Quantum experiments with human eye detectors seem realistic. Clarified the role of micro-macro entanglement. Graduate student applications welcome!

  25. Thanks to Thierry Chaneliere Romain Dubessy Jean-Louis Le Gouet Nicolas Sangouard Hugues de Riedmatten Pavel Sekatski Matthias Staudt Imam Usmani Hugo Zbinden Mikael Afzelius Cyril Branciard Nicolas Brunner Nicolas Gisin Bjoern Lauritzen Jiri Minar

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