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# Introduction to Quantum Computation

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1. Introduction to Quantum Computation Andris Ambainis University of Latvia

2. What is quantum computation? • New model of computing based on quantum mechanics. • Quantum circuits, quantum Turing machines • More powerful than conventional models.

3. Quantum algorithms • Factoring: given N=pq, find p and q. • Best algorithm 2O(n1/3), n -number of digits. • Many cryptosystems based on hardness of factoring. • O(n2) time quantum algorithm [Shor, 1994] • Similar quantum algorithm solves discrete log.

4. Quantum algorithms ... 0 1 0 0 • Find if there exists i for which xi=1. • Queries: input i, output xi. • Classically, n queries. • Quantum, O(n) queries [Grover, 1996]. • Speeds up exhaustive search. x1 x2 x3 xn

5. Quantum cryptography • Key distribution: two parties want to create a secret shared key by using a channel that can be eavesdropped. • Classically: secure if discrete log hard. • Quantum: secure if quantum mechanics valid [Bennett, Brassard, 1984]. • No extra assumptions needed.

6. Quantum communication • Dense coding: 1 quantum bit can encode 2 classical bits. • Teleportation: quantum states can be transmitted by sending classical information. • Quantum protocols that send exponentially less bits than classical.

7. Experiments • ~10 different ideas how to implement QC. • NMR, ion traps, optical, semiconductor, etc. • 7 quantum bit QC [Knill et.al., 2000]. • QKD has been implemented.

8. Outline • Today: basic notions, quantum key distribution. • Tomorrow: quantum algorithms, factoring. • Friday: current research in quantum cryptography, coin flipping.

9. Model • Quantum states • Unitary transformations • Measurements

10. Quantum bit |1> • 2-dimensional vector of length 1. • Basis states |0>, |1>. • Arbitrary state: |0>+|1>, ,  complex, ||2+ ||2=1. |0>

11. Physical quantum bits • Nuclear spin = orientation of atom’s nucleus in magnetic field. •  = |0>,  = |1>. • Photons in a cavity. • No photon = |0>, one photon = |1>

12. Physical quantum bits (2) • Energy states of an atom • Polarization of photon • Many others. |0> |1> ground state excited state

13. General quantum states • k-dimensional quantum system. • Basis |1>, |2>, …, |k>. • General state 1|1>+2|2>+…+k|k>, |1|^2+…+ |k|^2=1 • 2k dimensional system can be constructed as a tensor product of k quantum bits.

14. Unitary transformations • Linear transformations that preserve vector norm. • In 2 dimensions, linear transformations that preserve unit circle (rotations and reflections).

15. Examples • Bit flip • Hamamard transform

16. Linearity • Bit flip |0>|1> |1>|0> • By linearity, |0>+|1> |1>+|0> • Sufficient to specify U|0>, U|1>.

17. Examples |1> |0>

18. Measurements • Measuring |0>+|1> in basis |0>, |1> gives: • 0 with probability | |2, • 1 with probability | |2. • Measurement changes the state: it becomes |0> or |1>. • Repeating measurement gives the same outcome.

19. Probability 1/2 Probability 1/2 Measurements |0> |1>

20. General measurements • Let |0>, | 1> be two orthogonal one-qubit states. • Then, |> = 0|0> + 1|1>. • Measuring | > gives | i> with probability |i|2. • This is equivalent to mapping |0>, | 1> to |0>, |1> and then measuring.

21. Measurements Probability 1

22. Probability 1/2 Probability 1/2 Measurements |1>

23. Measurements • Measuring 1|1>+2|2>+…+k|k> in the basis |1>, |2>, …, |k> gives |i> with probability |i|2. • Any orthogonal basis can be used.

24. Result 1 Result 0 Partial measurements • Example: two quantum bits, measure first.

25. Classical bits: can be measured completely, are not changed by measurement, can be copied, can be erased. Quantum bits: can be measured partially, are changed by measurement, cannot be copied, cannot be erased. Classical vs. Quantum

26. Copying One nuclear spin  Two spins ? Impossible! Related to impossiblity of measuring a state perfectly.

27. No-cloning theorem • Imagine we could copy quantum states. • Then, by linearity • Not the same as two copies of |0>+|1>.

28. Key distribution • Alice and Bob want to create a shared secret key by communicating over an insecure channel. • Needed for symmetric encryption (one-time pad, DES etc.).

29. Key distribution • Can be done classically. • Needs hardness assumptions. • Impossible classically if adversary has unlimited computational power. • Quantum protocols can be secure against any adversary. • The only assumption: quantum mechanics.

30. BB84 states |> = |1> |  >= |  >= |> = |0>

31. ... ... No Yes Yes Yes ... 0 0 1 BB84 QKD Alice ... Bob

32. BB84 QKD • Alice sends n qubits. • Bob chooses the same basis n/2 times. • If there is no eavesdropping/transmission errors, they share the same n/2 bits.

33. Eavesdropping • Assume that Eve measures some qubits in , | basis and resends them. • If the qubit she measures is |> or |>, Eve resends a different state ( or | ). • If Bob chooses |>, |> basis, he gets each answer with probability 1/2. • With probability 1/2, Alice and Bob have different bits.

34. Eavesdropping • Theorem: Impossible to obtain information about non-orthogonal states without disturbing them. • In this protocol:

35. Check for eavesdropping • Alice randomly chooses a fraction of the final string and announces it. • Bob counts the number of different bits. • If too many different bits, reject (eavesdropper found). • If Eve measured many qubits, she gets caught.

36. Next step • Alice and Bob share a string most of which is unknown to Eve. • Eve might know a few bits. • There could be differences due to transmission errors.

37. Classical post-processing • Information reconciliation: Alice and Bob apply error correcting code to correct transmission errors. • They now have the same string but small number of bits might be known to Eve. • Privacy amplification: apply a hash function to the string.

38. QKD summary • Alice and Bob generate a shared bit string by sending qubits and measuring them. • Eavesdropping results in different bits. • That allows to detect Eve. • Error correction. • Privacy amplification (hashing).

39. Eavesdropping models • Simplest: Eve measures individual qubits. • Most general: coherent measurements. • Eve gathers all qubits, performs a joint measurement, resends.

40. Security proofs • Mayers, 1998. • Lo, Chau, 1999. • Preskill, Shor, 2000. • Boykin et.al., 2000. • Ben-Or, 2000.

41.    EPR state • First qubit to Alice, second to Bob. • If they measure, same answers. • Same for infinitely many bases.

42. Bell’s theorem • Alice’s basis: • Bob’s basis: y instead of x. |1> |0>

43. Bell’s theorem Pr[b=0] Pr[b=1] Pr[a=0] Pr[a=1]

44. Classical simulation • Alice and Bob share random variables. • Someone gives to them x and y. • Can they produce the right distribution without communication?

45. Bell’s theorem • Classical simulation impossible: • Bell’s inequality: constraint satisfied by any result produced by classical randomness.

46. Ekert’s QKD • Alice generates n states sends 2nd qubits to Bob. • They use half of states for Bell’s test. • If test passed, they error-correct/amplify the rest and measure.

47. Equivalence • In BB84 protocol, Alice could prepare the state keep the first register and send the second to Bob.    

48. UI Ekert and BB84 states    

49. QKD summary • Key distribution requires hardness assumptions classically. • QKD based on quantum mechanics. • Higher degree of security. • Showed two protocols for QKD.

50. QKD implementations • First: Bennett et.al., 1992. • Currently: 67km, 1000 bits/second. • Commercially available: Id Quantique, 2002.