Future Challenges in LongDistance Quantum Communication JianWei Pan

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

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 …

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.

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!

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

Quantum Cryptographic Key Distribution • Single-particle-based secret key distribution: [C. H. Bennett & G. Brassard, BB 84 protocol (1984) ] • Entanglement-based secret key distribution: [A. Ekert, Phys. Rev. Lett. 67, 661 (1991). ]

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

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

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

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

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: 1 km for both fibre-based and free-space

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)]

Our dream: achieving long-distance quantum communication!

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

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

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)]

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

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

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 is then

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)]

Entanglement Generation Maximally entangled in the number basis!

Entanglement Connection Steps： 1. Apply a reverse read laser pulse to transfer 2. atomic excitation to optical exc. 3. 2. Succeeds if D 1 or D 2 registers one photon 4. 3. Fails otherwise, and repeat every step from entanglement generation

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)]

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