Quantum Computer. Jaewan Kim School of Computational Sciences Korea Institute for Advanced Study

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1 Quantum Computer Jaewan Kim School of Computational Sciences Korea Institute for Advanced Study

2 KIAS (Korea Institute for Advanced Study) Established in 1996 Located in Seoul, Korea Pure Theoretical Basic Science Schools Mathematics Physics Computational Sciences By the computation, For the computation 20 Prof s, 2 Distinguished Prof s, 20 KIAS Scholars, 70 Research Fellows, 20 Staff Members

3 Quantum Physics Information Science Quantum Information Science Quantum Parallelism Quantum computing is exponentially larger and faster than digital computing. Quantum Fourier Transform, Quantum Database Search, Quantum Many-Body Simulation (Nanotechnology) No Clonability of Quantum Information, Irrevesability of Quantum Measurement Quantum Cryptography (Absolutely secure digital communication) Quantum Correlation by Quantum Entanglement Quantum Teleportation, Quantum Superdense Coding, Quantum Cryptography, Quantum Imaging

4 Twenty Questions Animal Four legs? Herbivorous?... Tiger? Yes(1) or No(0) Yes(1) or No(0) Yes(1) or No(0)..

5 It from Bit John A. Wheeler

6 Yoot =Ancient Korean Binary Dice x x x x x x x x x

8 Taegeukgi (Korean National Flag) 1 0 Qubit j = a 0 + b 1 Qubit = quantum bit (binary digit)

9 Businessweek 21 Ideas for the 21st Century

10 Transition to Quantum Mathematics: Real Complex Physics: Classical Quantum Digital Information Processing Hardwares by Quantum Mechanics Softwares and Operating Systems by Classical Ideas Quantum Information Processing All by Quantum Ideas

11 Quantum Information Processing Quantum Computer Quantum Algorithms: Softwares Simulation of quantum many-body systems Factoring large integers Database search Experiments: Hardwares Ion Traps NMR Cavity QED, etc. NT IT BT

12 Quantum Information Processing Quantum Communication Quantum Cryptography Absolutely secure digital communication Generation and Distribution of Quantum Key ~100 km through optical fiber (Toshiba) 23.4 km wireless secure satellite communication Quantum Teleportation Photons Atoms, Molecules Quantum Imaging and Quantum Metrology IT

13 Cryptography and QIP Public Key Cryptosystem (Asymmetric) Computationally Secure Based on unproven mathematical conjectures Cursed by Quantum Computation One-Time Pad (Symmetric) Unconditionally Secure Impractical Saved by Quantum Cryptography Giving disease, Giving medicine. Out with the old, In with the new.

14 KIAS-ETRI, December km Quantum Cryptography T.G. Noh and JK

15 Classical Computation Hilbert (1900): 23 most challenging math problems The Last One: Is there a mechanical procedure by which the truth or falsity of any mathematical conjecture could be decided? Turing Conjecture ~ Sequence of 0 s and 1 s Read/Write Head: Logic Gates Model of Modern Computers

16 Turing Machine Finite State Machine: Head q q q = f( qs, ) s = gqs (, ) d = dqs (, ) d Bit {0,1} s s' Infinitely long tape: Storage

17 Bit and Logic Gate NOT AND OR XOR NAND C-NOT CCN ( Toffoli ) C-Exch ( Fredkin ) Universal Reversible Universal Reversible

18 DNA Computing Bit { 0, 1 } Tetrit (?) { A, G, T, C} Gate Enzyme Parallel Ensemble Computation Hamiltonian Path Problem Adleman

19 Complexity N= (# bits to describe the problem, size of the problem) (#steps to solve the problem) = Pol(N) P(polynomial) : Tractable, easy (#steps to verify the solution) = Pol(N) NP (nondeterministic polynomial) : Intractable P NP NP P NP P or NP=P?

20 Quantum Information Bit {0,1} Qubit N bits 2 N states, One at a time Linearly parallel computing AT BEST N qubits Linear superposition of 2 N states at the same time Exponentially parallel computing Quantum Parallelism But when you extract result, a 0 + b1 with a + b = 1 Deutsch you cannot get all of them. 2 2

21 Quantum Algorithms 1. [Feynman] Simulation of Quantum Physical Systems with HUGE Hilber space ( 2 N -D ) e.g. Strongly Correlated Electron Systems 2. [Peter Shor] Factoring large integers, period finding t q Pol (N) t cl Exp (N 1/3 ) 3. [Grover] Searching t q N t cl ~ N/2

22 N bits N bits Digital Computer Digital Computers in parallel m{ 1 N m N N bits N bits Quantum Computer : Quantum Parallelism N bits 2 N N bits

23 Quantum Gates Time-dependent Schrödinger Eq. ih ψ = H ψ t iht / h ψ() t = e ψ(0) = Ut () ψ(0) P ( ) = i 0 I θ θ e 0 1 X = = Z=P( π )= Y = XZ= H= Unitary Transform Norm Preserving Reversible H H 0 = = = = = = ( ) ( ) Hadamard

24 Hadamard Gate H1 H2... HN N 1 ( ) 1 ( ) 1 = ( ) N N = ( ) 1 2 N N N N 2 N = k( ) N 2 k = 0

25 Universal Quantum Gates General Rotation of a Single Qubit q -if q cos -ie sin Ł2ł Ł2ł V( qf, ) = + if q q -ie sin cos Ł Ł2ł Ł2ł ł X c : CNOT (controlled - NOT) or XOR Ł0 0ł Ł0 1ł Ł0 1ł Ł1 0ł = 0 0 I X V X a b = a a b c

26 Quantum Circuit/Network Scalable Qubits Initial State DiVincenzo, Qu-Ph/ Unitary evolution : Deterministic : Reversible Universal Gates Ψæ 0æ 0æ 0æ H t X X 1 H 2C X 13 Yæ M M M Cohere, Not Decohere Quantum measurement : Probabilistic : Irreversible change

27 Quantum Key Distribution [BB84,B92] QKD[E91] Quantum Repeater Quantum Teleportation Single-Qubit Gates Single- & Two-Qubit Gates 3 3 Quantum Error Correction Quantum Computer 7-Qubit QC Single- & Two-Qubit Gates

28 Physical systems actively considered for quantum computer implementation Liquid-state NMR NMR spin lattices Linear ion-trap spectroscopy Neutral-atom optical lattices Cavity QED + atoms Linear optics with single photons Nitrogen vacancies in diamond Electrons on liquid He Small Josephson junctions charge qubits flux qubits Spin spectroscopies, impurities in semiconductors Coupled quantum dots Qubits: spin,charge, excitons Exchange coupled, cavity coupled

29 15= 3 5 Chuang Nature 414, (20/27 Dec 2001) OR QP/

30 Concept device: spin-resonance transistor R. Vrijen et al, Phys. Rev. A 62, (2000)

31 Quantum-dot array proposal

32 Ion Traps Couple lowest centre-of-mass modes to internal electronic states of N ions.

33 Quantum Error Correcting Code Three Bit Code Encode Recover Decode φ 0 0 channel noise 0 0 M M U m1m2 φ

34 Molding a Quantum State Yæ 0æ 0æ H X X 1 H 2C X 13 Yæ M M 0æ t M Molding

35 Sculpturing a Quantum State - Cluster state quantum computing - One-way quantum computing -- +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ 1. Initialize each qubit in +æ state. 2. Contolled-Phase between the neighboring qubits. 3. Single qubit manipulations and single qubit measurements only [Sculpturing]. No two qubit operations!

36 Controlled-NOT XOR X X = 0 0 I X AB AA B AA B = = = A B A B AB AB AB

37 Qubit Copying Circuit? x x x x 0 y x y x ψ = a 0 + b1 0 ψ = a 0 + b1 a 00 + b 11 ψ = a 0 + b1

38 Entanglement by Two-Qubit Gates a 0æ + b 1æ 0æ a 0æ + b 1æ 0æ 0æ a 0æ 0æ + b 1æ 1æ a 0æ 0æ 0æ + b 1æ 1æ 1æ

39 Single Particle Entanglement Single Photon ( ) Single Electron + + H = t c1 c2 + c2c1 Beam Splitter 1 ψ = ( ) 2 1 or ψ = fe + ef 2 ( ) φ = + 2 ( )

40 Quantum Teleportation using a single particle entanglement Lee, JK, Qu-ph/ ; Phys. Rev. A 63, (2001)

41 No Cloning Theorem An Unknown Quantum State Cannot Be Cloned. <Proof> U ( α 0 ) ( 0 ) = α α Zurek, Wootters U β = β β α β 1 Let γ = ( α + β ). 2 1 Then U ( γ 0 ) = ( α α + β β ) γ γ 2

42 If If an unknow quantum state can be cloned Quantum States can be measured as accurately as possible??? yæ yæ, yæ, yæ, yæ measure, measure, Commnunication Faster Than Light? yæ = 0æ A 1æ B - 1æ A 0æ B for 0 = +æ A -æ B - -æ A +æ B for 1

43 Communication Faster Than Light? If an unknown quantum state can be copied; yæ = 0æ A 1æ B - 1æ A 0æ B for 0 = +æ A -æ B - -æ A +æ B for 1 Alice wants to send Bob 1. yæ = +æ A -æ B - -æ A +æ B for 1 Alice mesures her qubit in { +>, ->}. Alice s state will become +> or ->. Bob s state will become -> or +>. Let s assume it is +>. +æ Bob makes many copies of this. +æ, +æ, +æ, +æ, He measures them in { +>, ->}, and gets 100% +>. He measures them in { 0>, 1>}, and gets 50% 0> and 50% 1>. Thus Bob can conclude that Alice measured her state in { +>, ->}.

44 Mysterious Connection Between QM & Relativity Weinberg: Can QM be nonlinear? Experiments: Not so positive result. Polchinski, Gisin: If QM is nonlinear, communication faster than light is possible.

45 Cluster State 1. Qubits on lattice sites are initialized to be = = 2 H 0 2. Operate Control-Z on neighboring qubits. Cont-Z = 0 0 I+ 1 1 X = =

46 Cont-Z 0i e i 0 e 0 0 = = 0i e i e π H= σ1 z + σ2z σ1z σ2z + I = 0 1 I+ I I = = Cont-Z = π i H e 4

47 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ 0æ

48 +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ

49 +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ

50 One-way quantum computing +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ +æ Optical Lattice, Quantum Dots, Superconductors, etc. Single qubit manipulations and single qubit measurements only [Sculpturing]. No two qubit operations!

51 Quantum computing with quantum-dot cellular automata Géza Tóth and Craig S. Lent PRA 63, D array

52 Some comments 1-D cluster state quantum computing can be efficiently simulated by digital computing. 2-D cluster state quantum computing is equivalent to quantum circuit quantum computing, but needs more(just polynomially more) qubits. Proposal : 2-D array of vertical QDs 2-D cluster state does not seem to be a ground state of some Hamiltonian. Once the cluster state is prepared, only single qubit measurements are needed.

53 QUIPU [kí:pu] 1. Quantum Information Processing Unit [JK] 2. A device consisting of a cord with knotted strings of various colors attached, used by the ancient Peruvians for recording events, keeping accounts, etc. Computation Communication * Peruvian Information Processing Unit for Computation and Communication *Rosary *Buddhist Beads

Quantum Information Science. Jaewan Kim School of Computational Sciences Korea Institute for Advanced Study Seoul, Korea

Quantum Information Science. Jaewan Kim School of Computational Sciences Korea Institute for Advanced Study Seoul, Korea Quantum Information Science Jaewan Kim (jaewan@kias.re.kr) School of Computational Sciences Korea Institute for Advanced Study Seoul, Korea Quantum Physics Information Science Quantum Information Science