Quantum Communication Nicolas Gisin Group of Applied Physics Geneva University, Switzerland detectors Quantum nonlocality splitter (entanglement) photon The photon Quantum explores both paths randomness Geneva University GAP Optique 1 Quantum Randomness Quantum Randomness is the only intrinsic source of pure randomness in today’s physics. Nature offers us pure randomness. This gift of Nature is extremely useful in our information society: -PIN numbers, - Internet lotteries, - Numerical simulations. Geneva University It would be absurd not to exploit this gift of Nature. GAP Optique 2 Quantum Entanglement Quantum Theory: AliceIf Alice and Bob make Bob the same measurement - V + (both H/V or both +/-), - V + then the result is H random, private and secret. H This is Nonlocal Secret Randomness! Geneva University Let’sH or V exploit ? The result this is random, gift of but Natureit is the same at Alice and at Bob + or - ? The result is random, but it is the same at Alice and at Bob GAP Optique Secret & Private Randomness is nothing but a cryptographic key: QKD. This can be usedQuantum in many ways: Magic, Entanglement 3 QKDQKD forfor encryptionencryption One-time pad or AES Cipher text Plain text ⊕ ⊕ Plain text Secret key Secret key Use the key to encode and decode the plain text. Geneva University Photons Quantum key distribution (QKD) Slide taken from the Japanese UQCC: GAP Optique Updating Quantum Cryptography and Communication http://www.uqcc.org 4 QKDQKD forfor authentificationauthentification W.-C. Message authentication Scheme to ensure that data are genuine and have not been altered. Text Text Tag Verification Tag Use the key to choose a hash function to generate the tag. Geneva University Photons GAP Optique Quantum key distribution (QKD) QKD can be used for any application that requires secret keys5 RMP 74, 145-195, 2002 QKD over 67 km 4 cm Geneva University GAP Optique D. Stucki et al., New Journal of Physics 4, 41.1-41.8, 2002. Quant-ph/0203118 + aerial cable (in Ste Croix, Jura) !6 Quantum Cryptography: Challenges Faster Engineering Cheaper Longer distances Geneva University More protocols Science GAP Optique Easier to test (Device Independent) 7 Long distance QKD: World records 150 km of installed fibers, Optics Express 17, 13326 (2009) Lausanne 250 km in the lab. NJP 11, 075003 (2009) Geneva University GAP Optique 8 Distance limitation Secret Key Rate P2P + WDM 10MHz +4 Years 1MHz Today Lab Today Commercial 100kHz 10kHz 1kHz 1Hz Geneva University 100 200 300 400 Distance [km] There is a hard wall around 400 km ! With the best optical fibers, perfect noise-free detectors and ideal 10 GHz GAP Optique single-photon sources, it would take centuries to send 1 qubit over 1000 km ! 9 Beating the hard wall: Teleportation of entanglement Q teleportation Entanglement Entanglement ⇒ Entanglement over twice the distance Geneva University The synchronisation requires Quantum memories GAP Optique 10 Quantum Memory crystal doped with billions of ions photon out at desired time photon in in same Q state Quantum Memories are still in the labs The quantum state of the photon is now coded in a huge entangled states Geneva University of billions of « atoms » Today’sGAP Optique efficiencies ≈ 20 % 11 Nature 456, 773, 2008 Single-photon detectors Today, the best detectors are all produced in the United-Sates. This is the case both for semiconductor detectors and for superconducting detectors. Geneva University Europe must react ! GAP Optique 12 Quantum Networks as Quantum Many Body Systems Simulators “spin”“spin” Geneva University “interaction”“interaction” GAP Optique Borrowed from Prof. H.J. Kimple 13 CalTec, USA Conclusions Quantum “Magic” (Entanglement) is hard to understand. However, it would be absurd not to exploit this incredible gifts of Nature: Nonlocal Secret Private Randomness, to increase the security of today’s and tomorrow’s communications. Big challenges: - quantum memories, Geneva University - more protocols exploiting entanglement, - Self-testing: Device Independent. And - improved detectors and sources. GAP Optique 14.