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Alice and Bob in the Quantum Wonderland

Alice and Bob in the Quantum Wonderland. Superposition. +. The mystery of . How can a particle be a wave?. Polarisation. Three obstacles are easier than two. =. +. =. +. Addition of polarised light. . The individual photon. PREPARATION. MEASUREMENT. Yes. No.

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Alice and Bob in the Quantum Wonderland

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  1. Alice and Bob in theQuantum Wonderland

  2. Superposition + The mystery of

  3. How can a particle be a wave?

  4. Polarisation

  5. Three obstacles are easier than two

  6. = + = + Addition of polarised light 

  7. The individual photon PREPARATION MEASUREMENT Yes No

  8. How it looks to the photon in the stream (2) PREPARATION MEASUREMENT MAYBE!

  9. = + = + States of being |W |NE  |N  |NW |N  |NE 

  10. Quantum addition + = + = Alive Dead = ? +

  11. Schrödinger’s Cat |CAT = |ALIVE + |DEAD

  12. Entanglement + Observing either side breaks the entanglement

  13. + Entanglement killed the cat According to quantum theory, if a cat can be in a state |ALIVE  and a state |DEAD, it can also be in a state|ALIVE + |DEAD. Why don’t we see cats in such superposition states?

  14. ? ? [ ] ? + [ ] [ ] + Entanglement killed the cat ANSWER: because the theory actually predicts…..

  15. Entangled every which way = + +

  16. Einstein-Podolsky-Rosen argument If one photon passes through the polaroid, so does the other one. Therefore each photon must already have instructions on what to do at the polaroid.

  17. The no-signalling theorem I know what message Bob is getting right now Quantum entanglement can never be used to send information that could not be sent by conventional means. But I can’t make it be my message!

  18. Quantum cryptography 0 0 1 1 0 0 0 0 1 1 Alice and Bob now share a secret key which didn’t exist until they were ready to use it.

  19. Quantum information Yes θ No 1 qubit Θ=0.0110110001… 1 bit 0 or 1 To calculate the behaviour of a photon, infinitely many bits of information are required – but only one bit can be extracted. Yet a photon does this calculation!

  20. Available information: one qubit 0 1 qubit 1 bit 1 or x 1 qubit 1 bit y

  21. + W X - + Y - Z or 2 qubits 2 bits Available information: two qubits 0 0 0 1 1 0 1 1 2 qubits 2 bits

  22. Teleportation Transmission Reception Reconstruction Measurement ?

  23. Quantum Teleportation Measure W,X,Y,Z?

  24. Dan Dare, Pilot of the Future. Frank Hampson, Eagle (1950)

  25. Dan Dare, Pilot of the Future. Frank Hampson, Eagle (1950)

  26. Computing INPUT N digits COMPUTATION Running time T OUTPUT How fast does T grow as you increase N?

  27. + + 100 In 1 unit of time, many calculations can be done but only one answer can be seen Quantum Computing 6+4 20/3 But you can choose your question E.g. Are all the answers the same?

  28. Two Easy Sums 7873 x 6761 = ? ? x ? = 26 292 671 53 229 353

  29. Not so easy . But on a quantum computer, factorisation can be done in roughly the same time as multiplication T ≈ N 2 (Peter Shor, 1994)

  30. No cats were harmed in the preparation of this lecture

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