Introduction to Quantum Computation

1. Introduction to Quantum Computation Neil Shenvi Department of Chemistry Yale University

2. Quantum Random Walks Noise in Grover’s Algorithm O Decoherence in Spin Systems Talk Outline • Background • What is Quantum Computation? • Quantum Algorithms • Decoherence and Noise • Implementations • Applications

3. Background: Classical Computation Input Computation Output 2 + 2 4 Hello World! C:\Hello.exe What is the essence of computation?

4. Input What is a Turing machine? …0100101101010010110… Finite State Automaton (control module) Computation Infinite tape …0000001011111111100… Read/Write head …0100101101010010110… Output …1110010110100111101… Classical Computation Theory Church-Turing Thesis: Computation is anything that can be done by a Turing machine. This definition coincides with our intuitive ideas of computation: addition, multiplication, binary logic, etc…

5. Classical Computation Theory What kind of systems can perform universal computation? DNA Billiard balls Desktop computers These can all be shown to be equivalent to each other and to a Turing machine! Cellular automata The Big Question: What next?

6. Talk Outline • Background • What is Quantum Computation? • Quantum Algorithms • Decoherence and Noise • Implementations • Applications

7. What Is Quantum Computation? Conventional computers, no matter how exotic, all obey the laws of classical physics. On the other hand, a quantum computer obeys the laws of quantum physics.

8. 1 0 The Bit The basic component of a classical computer is the bit, a single binary variable of value 0 or 1. At any given time, the value of a bit is either ‘0’ or ‘1’. 0 1 The state of a classical computer is described by some long bit string of 0s and 1s. 0001010110110101000100110101110110...

9. Valid qubit states: | = |0 | = |1 | = (|0- ei/4 |1)/2 | = (2|0- 3ei5/6 |1)/13 The Qubit A quantum bit, or qubit, is a two-state system which obeys the laws of quantum mechanics. Spin-½ particle =|0 =|1 The state of a qubit | can be thought of as a vector in a two-dimensional Hilbert Space, H2, spanned by the Basis vectors |0 and |1.

10. Quantum Computation Data unit: qubit =|0 =|1 Valid states: | = c1|0 + c2|1 | = (|0 + |1)/√2 | = |1 | = |0 0 1 0 1 Computation with Qubits How does the use of qubits affect computation? Classical Computation Data unit: bit = ‘0’ = ‘1’ Valid states: x = ‘0’ or ‘1’ x = 1 x = 0

11. Quantum Computation Operations: unitary Valid operations: σz = σX = 1-qubit 1 Hd = σy = √2 2-qubit CNOT = Computation with Qubits How does the use of qubits affect computation? Classical Computation Operations: logical Valid operations: in 1-bit NOT = out in 2-bit AND = out in

12. Quantum Computation Measurement: stochastic State Result of measurement | = |0 ‘0’ | = |1 ‘1’ ‘0’ 50% | = |0- |1 ‘1’ 50% 2 Computation with Qubits How does the use of qubits affect computation? Classical Computation Measurement: deterministic Result of measurement State ‘0’ x = ‘0’ ‘1’ x = ‘1’

13. c1|00 + c2|01 + c3|10 + c4|11 Arbitrary state | = = | = c1|0 + c2|1 = U| = Operator U| = More than one qubit Two qubits Single qubit |00,|01,|10,|11 |0,|1 Hilbert space , , , H2 = H22 = H2H2 = ,

14. ‘1’ |1 ‘1’ |0 Quantum Circuit Model Example Circuit Two-qubit operation One-qubit operation Measurement |1 CNOT |0 σx |0 |1 CNOT = σx I =

15. 50% 50% |0 + |1 ______ ? ‘1’ ‘0’ √2 or ? ‘1’ ‘0’ |0 or Entangled state: cannot be written as tensor product Separable state: can be written as tensor product | ≠|  | | = |  | Quantum Circuit Model Example Circuit |0 + |1 ______ CNOT σx √2 |0

16. Reversibility Since quantum mechanics is reversible (dynamics are unitary), quantum computation is reversible. |00000000 |00000000 | Quantum Superordinacy All classical quantum computations can be performed by a quantum computer. U No cloning theorem It is impossible to exactly copy an unknown quantum state | | | |0 Some Interesting Consequences

17. Talk Outline • Background • What is Quantum Computation? • Quantum Algorithms • Decoherence and Noise • Implementations • Applications

18. Quantum Algorithms: What can quantum computers do? • Grover’s search algorithm • Quantum random walk search algorithm • Shor’s Factoring Algorithm

19. Grover’s Search Algorithm Imagine we are looking for the solution to a problem with N possible solutions. We have a black box (or ``oracle”) that can check whether a given answer is correct. Question: I’m thinking of a number between 1 and 100. What is it? Oracle 78 No Oracle 3 Yes

20. Quantum computer Oracle 1+2+3+... No+No+Yes+No+... Superposition over all N possible inputs. Using Grover’s algorithm, a quantum computer can find the answer in N queries! Grover’s Search Algorithm Classical computer Oracle 1 No Oracle 2 No Oracle 3 Yes ... The best a classical computer can do on average is N/2 queries.

21. Grover’s Search Algorithm Pros: Can be used on any unstructured search problem, even NP-complete problems. Cons: Only a quadratic speed-up over classical search. O(N) iterations … σz |0 σz Hd Hd Hd Hd Hd … |0 Hd Hd Hd Hd Hd O O … … … … … … |0 Hd Hd Hd Hd Hd The circuit is not complicated, but it doesn’t provide an immediately intuitive picture of how the algorithm works. Are there any more intuitive models for quantum search?

22. Moves walkers based on coin Flips coin Quantum random walk: Quantum Random Walk Search Algorithm Idea: extend classical random walk formalism to quantum mechanics Classical random walk:

23. Quantum Random Walk Search Algorithm To obtain a search algorithm, we use our “black box” to apply a different type of coin operator, C1, at the marked node C1 C0 1 C0= C1= 2

24. Quantum Random Walk Search Algorithm Pros: As general as Grover’s search algorithm. Cons: Same complexity as Grover’s search algorithm. Slightly more complicated in implementation Slightly more memory used Interesting Feature: Search algorithm flows naturally out of random walk formalism. Motivation for new QRW- based algorithms?

25. Shor’s Factoring Algorithm Makes use of quantum Fourier Transform, which is exponentially faster than classical FFT. Find the factors of: 57 Find the factors of: 1623847601650176238761076269172261217123987210397462187618712073623846129873982634897121861102379691863198276319276121 3 x 19 whimper All known algorithms for factoring an n-bit number on a classical computer take time proportional to O(n!). But Shor’s algorithm for factoring on a quantum computer takes time proportional to O(n2 log n).

26. Shor’s Factoring Algorithm The details of Shor’s factoring algorithm are more complicated than Grover’s search algorithm, but the results are clear: with a classical computer # bits 1024 2048 4096 factoring in 2006 105 years 5x1015 years 3x1029 years factoring in 2024 38 years 1012 years 7x1025 years factoring in 2042 3 days 3x108 years2x1022 years with potential quantum computer (e.g., clock speed 100 MHz) # bits 1024 2048 4096 # qubits 5124 10244 20484 # gates 3x109 2X1011 X1012 factoring time 4.5 min 36 min 4.8 hours R. J. Hughes, LA-UR-97-4986

27. Talk Outline • Background • What is Quantum Computation? • Quantum Algorithms • Decoherence and Noise • Implementations • Applications

28. V Decoherence and Noise What happens to a qubit when it interacts with an environment? Environment Quantum computer … σN σ2 σ3 σ1 Quantum information is lost through decoherence.

29. T2 processes: transverse relaxation, system becomes entangled with the environment + V + What are the effects of decoherence? Types of Decoherence T1 processes: longitudinal relaxation, energy is lost to the environment V

30. Effects of Environment on Quantum Memory T1 – timescale of longitudinal relaxation T2 – timescale of transverse relaxation Fidelity of stored information decays with time.

31. Effects of Environment on Quantum Algorithms Ideal oracle O Grover’s algorithm success rate Noisy oracle O n = # of qubits Errors accumulate, lowering success rate of algorithm

32. 1. Remove or reduce V, i.e. build a better computer System isolated from environment 2. Increase B, i.e. increase level splitting |1 E When E >> V, decoherence is small E |0 B Suppressing Decoherence • Use decoherence free subspace (DFS) 4. Use pulse sequence to remove decoherence

33. Talk Outline • Background • What is Quantum Computation? • Quantum Algorithms • Decoherence and Noise • Implementations • Applications

34. Some Proposed Implementations for QC NMR Ion trap B Kane Proposal Optical Lattice

35. The Loss-Divincenzo Proposal D. Loss and D.P. DiVincenzo, Phys. Rev. A 57, 120 (1998); G. Burkhard, H.A. Engel, and D. Loss, Fortschr. der Physik 48, 965 (2000).

36. Si28 (no spin) Dipolar coupling Hyperfine coupling Si29 (spin ½) Solid State Electron Spin Qubit Electron wavefunction Phosphorus impurity External Magnetic Field, B Silicon lattice

37. Hyperfine coupling Dipolar coupling System Hamiltonian Electron spin N nuclear spins ~1011 Hz / T ~107 Hz / T ~105 Hz ~102 Hz

38. Hyperfine-Induced Longitudinal Decay Critical field for electron spin relaxation: For B > Bc, T1 is infinite

39. Spin echo pulse sequence Spin echo pulse sequence removes nearly all dephasing! Hyperfine-Induced Transverse Decay Free evolution

40. Talk Outline • Background • What is Quantum Computation? • Quantum Algorithms • Decoherence and Noise • Implementations • Applications

41. Applications • Factoring – RSA encryption • Quantum simulation • Spin-off technology – spintronics, quantum cryptography • Spin-off theory – complexity theory, DMRG theory, N-representability theory

42. Acknowledgements • Dr. Julia Kempe, Dr. Ken Brown, Sabrina Leslie, Dr. Rogerio de Sousa • Dr. K. Birgitta Whaley • Dr. Christina Shenvi • Dr. John Tully and the Tully Group