Challenges in Long-Distance Quantum Communication: Future Prospects and Solutions

1. Future Challenges in Long-Distance Quantum Communication Jian-Wei Pan Hefei National Laboratory for Physical Sciences at Microscale, USTC and Physikalisches Institut der Universität Heidelberg December 15, 2005

2. Quantum Superposition or Classical Physics: “bit” + Quantum Physics: “qubit” Entanglement: + Quantum foundations: Bell’s inequality, quantum nonlocality… Quantum information processing: quantum communication, quantum computation, high precision measurement etc …

3. Why Quantum Communication? • When information is encoded in quantum states one • may outperform classical communication systems in • terms of • absolute security • efficiency • channel capacity • Because quantum information systems allow encoding • information by means of • coherent superposition of quantum states.

4. Qubits: Polarization of Single Photons One bit of information per photon (encoded in polarization) Qubit: Non-cloning theorem: An unknown quantum state can not be copied precisely!

5. Polarization Entangled Photon Pair 1-2 Bell states – maximally entangled states: Singlet: where 45-degree polarization

6. Quantum Cryptographic Key Distribution • Single-particle-based secret key distribution: [C. H. Bennett & G. Brassard, BB84 protocol (1984) ] • Entanglement-based secret key distribution: [A. Ekert, Phys. Rev. Lett. 67, 661 (1991). ]

7. Quantum Teleportation where Initial state The shared entangled pair [C.H. Bennett et al., Phys. Rev. Lett. 73, 3801 (1993)]

8. Entanglement Swapping [M. Zukowski et al., Phys. Rev. Lett. 71, 4287 (1993)]

9. Key Distribution with Single Photons [C. Kurtsiefer et al., Nature 419, 450 (2002)] achieved distance: 100km fiber-based (Toshiba Research Europe) 23km free-space (TU Munich)

10. Generation of Photonic Entanglement [P. G. Kwiat et al., Phys. Rev. Lett.75, 4337 (1995).]

11. Key Distribution with Entangled Photons Fibre:[T. Jennewein et al., Phys. Rev. Lett.84, 4729 (2000).] [D. S. Naik, et al., Phys. Rev. Lett. 84, 4733 (2000).] [W. Tittel et al., Phys. Rev. Lett. 84, 4737 (2000).] Free-space: [M. Aspelmeyer et al., Science 301, 621 (2003).] achieved distance: 1km for both fibre-based and free-space

12. Experimental Quantum Teleportation The setup The result Teleportation: [D. Bouwmeester & J.-W. Pan et al., Nature 390, 575 (1997)] Entanglement Swapping: [J.-W. Pan et al., Phys. Rev. Lett. 80, 3891 (1998)]

13. Our dream: achieving long-distance quantum communication!

14. Difficulties in Long-Distance Quantum Communication However,due to the noisy quantum channel (1) absorption photon loss (2) decoherence degrading entanglement quality Free-Space Distribution of Entangled Photons

15. Free-Space Distribution of Entangled Photons over 13km [C.-Z. Peng et al., Phys. Rev. Lett. 94, 150501 (2005)] Free-space entanglement distribution - we are working on 20km and 500km scale…

16. Another Solution to Photon Loss and Decoherence Entanglement swapping: solution to photon loss: [N. Gisin et al., Rev. Mod. Phys. 74, 145 (2002)] Entanglement purification: solution to decoherence [C. H. Bennett et al., Phys. Rev. Lett. 76, 722 (1996)] [D. Deutsch et al., Phys. Rev. Lett. 77, 2818 (1996)]

17. Generating Entangled States over Long-Distance Quantum repeaters: [H. Briegel et al., Phys. Rev. Lett. 81, 5932(1998)] • Require • entanglement swapping with high precision • entanglement purification with high precision • quantum memory

18. Experimental Entanglement Purification and Swapping Before purification, F=3/4 After purification, F=13/14 [J.-W. Pan et al., Nature 410, 1067 (2001)] [J.-W. Pan et al., Nature 421, 721 (2003)] [J.-W. Pan et al., Nature 423, 417 (2003)]

19. Drawback in Former Experiments • Probabilistic entangled photon source • Probabilistic entanglement purification • Bad weather Quantum memory • In N -stage realization, the cost of resource • is proportional to • With the help of quantum memory, the total cost • isthen

20. Solution with Atomic Ensembles Storage of light in atomic ensembles [C. Liu et al., Nature 409, 490 (2001)] [D. F. Phillips et al., Phys. Rev. Lett. 86, 783 (2001)] motivate Storage of single-photon states in atomic ensembles [L.-M. Duan et al., Nature 414, 413 (2001)]

21. Entanglement Generation Maximally entangled in the number basis!

22. Entanglement Connection • Steps： • Apply a reverse read laser pulse to transfer • atomic excitation to optical exc. • 2.Succeeds if D1 or D2 registers one photon • 3.Fails otherwise, and repeat every step from entanglement generation

23. The most recent experiment results • Observation of Stokes and anti-Stokes photon • Harvard: M. D. Lukin… • [C. H. Van der Wal et al., Science 301, 196 (2003)] • Caltech: H. J. Kimble… • [A. Kuzmich et al., Nature 423, 731 (2003)] • Gatech: A. Kuzmich… • [D. N. Matsukevich et al., Science 306, 663 (2004)] • Heidelberg: J.-W. Pan … • long-life time quantum memory • [S. Chen et al., in preparation for Phys. Rev. Lett.] • working on a phase insensitive scheme… • Synchronization of two independent lasers • USTC: J.-W. Pan, J. Zhang and Z.-Y. Wei … • [T. Yang et al., submitted to Phys. Rev. Lett. (2005)]

24. + |Photons> |Atoms> Powerful Quantum Superposition Promising Long-Distance Quantum Communication