Institute of Technical Physics Entanglement – Beamen – Quantum cryptography The weird quantum world Bernd Hüttner CPhys FInstP DLR Stuttgart

Institute of Technical Physics Quantum mechanics … is strange!  Particles behave like waves  Particles can be simultaneously in two states  Quanten cannot be cloned  Entangled particles are correlated to all times at arbitrarily distances

Institute of Technical Physics Double-slit experiment Richard Feynman: „Wave particle dual behavior contains the basic mystery of quantum mechanics. In reality it contains the only mystery. Complementarity: Which way versus interference

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Double-slit experiment with Fullerenes Diameter: C 60  1 nm dB  3 pm Laser

Institute of Technical Physics Feynman: Regardless of the quantum system, any information – recorded or not – about the alternative taken by a quantum process capable of following more than one alternative destroys the interference between alternatives.

Institute of Technical Physics Niels Bohr: “Anyone who is not shocked by quantum theory has not understood it.” Richard Feynman: “… I think I can safely say that nobody understands quantum mechanics.” Quotes to ponder …

Institute of Technical Physics Entanglement is the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought Independent on the basis Independent on the distance Leads to strong correlations – stronger than in classical physics Demonstrates the differences between quantum and classical physics in the most drastical way Entanglement and Bell‘s inequality test hidden variables theories Reveals nonlocality – a classical theory of the world is impossible Entanglement - Verschränkung

Institute of Technical Physics What is entanglement really? If a quantum system consists of more than one particle (photon, atom…) then superposition leads to an entangled, non separable state. Einstein: „Spooky action on a distance“ Example: Polarisation |H> und |V> → Superposition:  = 1/√2 {|H> + |V>} 2 Photons: possible polarizations: |H> 1, |H> 2 und |V> 1, |V> 2 Superposition: Measurement of only 1 photon gives with P=0.5 |H> or |V> (complete uncertainty) measurement of both photons results in perfect correlations.

Institute of Technical Physics Production of entangled photons More exactly: polarizations are each other perpendicular

Institute of Technical Physics Bell‘s inequality Henry R. Stapp (Berkeley) called Bell‘s theorem „the most profound discovery of science“! BI provides value for the correlation S of 2 simultaneously produced particles S HVT £ 2 ( local realism) S QM > 2 (nonlocal) That means, measurement on particle 1 forces immediately the property of particle 2 in any distance! No contradiction to theory of relativity. Quantum theory is not understandable - but right!

Institute of Technical Physics Violation of Bell‘s inequality experimental results GHZ-states: S HVT = 1 S QM = S exp = -1 QM LR |S| Voltage (~Winkel)

Institute of Technical Physics Beamen Quantum teleportation

Institute of Technical Physics  Bob and Alice create photons  Can’t just measure one and copy it  So Alice makes EPR pair, send one photon to Bob  Lets own photon interact with EPR photon (Bell measurement) - records outcome - destroys own photon  Instantaneously changes state of Bob’s EPR Photon  Send Bob outcome of Alice’s observations of interaction  Bob uses observations to adjust EPR photon to interact with own photon to make own photon copy of Alice’s Note, not the object (photon) is teleported but just its state.

Institute of Technical Physics Bell states TELEPORTATION

Institute of Technical Physics Cryptography - One-Time-Pad plain text: (22 Bit)binär: (22 Bit)key: (22 Bit)Cipher: XOR operation plain text: (22 Bit) XOR operation

Institute of Technical Physics Guessing the Secret Key

Institute of Technical Physics Quantum computer versus digital computer Net work of 100 computers Number of N 1024bit 4096bit Calculation time years over 30 billion years Quantum computer with 100MHz Number of N 1024bit 4096bit Calculation time 4.5 minutes 4.8 hours

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Quantum cryptography or better quantum key distribution

Institute of Technical Physics Detect if someone is listening

Institute of Technical Physics Example

Institute of Technical Physics DARPA Quantum Network

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D Ifm Bomb D Interaction-free measurement Elitzur, Vaidman (1993), Kwiat et al. (1995) D D Ifm

Institute of Technical Physics Summary and outlook Why is this so very important? We are in the age of quantum information processing

Institute of Technical Physics Gordon Moore, 1965: “Every 18 months microprocessors double in speed.” Quantum teleportation Quantum communication Quantum computing

Institute of Technical Physics