Quantum Computing 1. Multi-Qubit System. Goutam Biswas. Lect 2

Transcription

1 Quantum Computing 1 Multi-Qubit System

2 Quantum Computing State Space of Bits The state space of a single bit is {0,1}. n-bit state space is {0,1} n. These are the vertices of the n-dimensional hypercube.

3 Quantum Computing 3 State Space of Qubits The state space of a single qubit is a -dimensional vector space over C. It is written as a 0 +b 1 where a,b C, such that a + b = 1 and { 0, 1 } are orthonormal base vectors. The state space of n-qubit system is a n -dimensional vector space over C. It is the tensor product of n copies of the single qubit spaces.

4 Quantum Computing 4 State Space Postulate A closed quantum mechanical system can be modelled as a vector space with inner product - it is a Hilbert Space. Each state of the system is a unit vector of the space.

5 Quantum Computing 5 Tensor Product R R 3 We start with an example. Consider -dimensional and 3-dimensional vector spaces over reals - R and R 3. The tensor product of these two spaces is a vector space of dimension 3 = 6. It is denoted by R R 3.

6 Quantum Computing 6 Tensor Product R R 3 The standard basis for R and R 3 are B = {(1,0),(0,1)} and B 3 = {(1,0,0),(0,1,0),(0,0,1)} respectively. Six elements of B B 3 forms a basis of R R 3. They are {(1,0) (1,0,0),(1,0) (0,1,0),(1,0) (0,0,1),(0,1) (1,0,0),(0,1) (0,1,0),(0,1) (0,0,1)}.

7 Quantum Computing 7 Tensor Product R R 3 Let (a,b) = a(1,0)+b(0,1) R and (p,q,r) = p(1,0,0)+q(0,1,0)+r(0,0,1) R 3. The tensor product (a,b) (p,q,r) = ap(1,0) (1,0,0)+aq(1,0) (0,1,0)+ar(1,0) (0,0,1)+bp(0,1) (1,0,0)+bq(0,1) (0,1,0)+br(0,1) (0,0,1).

8 Quantum Computing 8 We may view Tensor Product of R R 3 B B 3 = {(1,0),(0,1)} {(1,0,0),(0,1,0),(0,0,1)} {[ ] [ ] } 1 0 = [1,0,0],, [0,0,1] = = {[ 0 1[1,0,0] 0[1,0,0] {[ ] [,, ],, 1 0[0,0,1] 1[0,0,1] ]} [ ]} It may also be viewed as column vectors..

9 Quantum Computing 9 Tensor Product of R R 3 B B 3 = {(1,0,0,0,0,0),(0,1,0,0,0,0),(0,0,1,0,0,0), (0,0,0,1,0,0),(0,0,0,0,1,0),(0,0,0,0,0,1)}. So, (a,b) (p,q,r) = ap(1,0,0,0,0,0)+ aq(0,1,0,0,0,0)+ar(0,0,1,0,0,0)+ bp(0,0,0,1,0,0)+bq(0,0,0,0,1,0)+ br(0,0,0,0,0,1) = (ap,aq,ar,bp,bq,br).

10 Quantum Computing 10 Note 1. Every element of R R 3 is not a tensor product of the vectors of R and R 3 i.e. not of the the form (a,b) (p,q,r).. An example: (,4,6,8,10,1) R R 3, but there is no (a,b) R and (p,q,r) R 3 such that (a,b) (p,q,r) = (ap,aq,ar,bp,bq,br) = (,4,6,8,10,1).

11 Quantum Computing 11 Note 3. If possible, then ap aq = 4 p q = 1. But bp bq = 8 10 p q = a contradiction. 4. Every element of R R 3 is a linear combination of vectors of the form (a,b) (p,q,r).

12 Quantum Computing 1 Definition: Tensor Product Let A and B be two inner product spaces over a field F with base E a = { a 1,, a m } and E b = { b 1,, b n } respectively. The tensor product of A and B is an mn-dimensional inner product space with the base E a b = { a i b j : 1 i m,1 j n}.

13 Quantum Computing 13 Notation If a A and b B, then a b is also written as a b, a,b, or ab. A shorter notation for = 0110.

14 Quantum Computing 14 Notation Let E a = { a 1, a } and E b = { b 1, b }. Consider two vectors a = α 1 a 1 +α a A and b = β 1 b 1 +β b. Their tensor product a b = ab = (α 1 a 1 +α a ) (β 1 b 1 +β b ) = α 1 β 1 a 1 b 1 + α 1 β a 1 b +α β 1 a b 1 +α β a b.

15 Quantum Computing 15 Notation Consider the qubit space V = C with standard pair of base vectors 0, 1. The n-times tensor product of V, n {}}{ V V = V n = (C ) n has n standard base vectors, b 1 b n, where b 1,,b n {0,1}. Using the decimal notation the set of basis can be written as { 0, 1,, n 1 }.

16 Quantum Computing 16 Properties Let α F, a, a A and b, b B 1. α( a b ) = (α a ) b = a (α b ).. ( a + a ) b = ( a b )+( a b ). 3. a ( b + b ) = ( a b )+( a b ).

17 Quantum Computing 17 Inner product The inner product on A B (if both A and B are inner product spaces) is defined as follows: Inner product of a b with a b is defined as ( a b ) ( a b ) = ( a a )( b b ).

18 Quantum Computing 18 Orthonormal Basis We know that the set of basis { a 1,. a m } of an inner product space A is orthonormal if a i a j = δ ij.

19 Quantum Computing 19 Standard Basis We consider standard basis of a -qubit system - { 00, 01, 10, 11 }. Inner product of two of them b i b j and b k b l is b k b l b i b j = b k b i b l b j = 1 if b k = b i and b l = b j, 0 otherwise. So this is an orthonormal basis.

20 Quantum Computing 0 Hadamard Standard Basis We choose the Hadamard basis for the first first qubit and standard basis for the second qubit. So the basis of the -qubit system is B HS = { +, } { 0, 1 } = { +0, +1, 0, 1 }.

21 Quantum Computing 1 Hadamard Standard Basis +0 = 1 ( ) 0 = 1 ( ) +1 = 1 ( ) 1 = 1 ( ) Similarly other two base vectors can be constructed.

22 Quantum Computing Hadamard Standard Basis is Orthonormal Inner product of +0 with itself is 1 ( ) 1 ( ) = 1 ( ) = 1 ( ) = 1

23 Quantum Computing 3 Hadamard Standard Basis is Orthonormal Inner product of +0 with +1 is 1 ( ) 1 ( ) = 1 ( ) = 1 ( ) = 0 So B HS is orthonormal.

24 Quantum Computing 4 Bell Basis Φ + = 1 ( ), Φ = 1 ( ), Ψ + = 1 ( ), Ψ = 1 ( ).

25 Quantum Computing 5 Bell Basis is Orthonormal Inner product of Φ + with itself. = ( 1 ( )) ( 1 ( )) = 1 ( ) = 1 ( ) = 1.

26 Quantum Computing 6 Bell Basis is Orthonormal Inner product of Φ + with Φ. = ( 1 ( )) ( 1 ( )) = 1 ( ) = 1 ( ) = 0.

27 Quantum Computing 7 Phase Factor The concept of global phase is similar to the single qubit system with the only difference that the phase factor may be attached to any of the component vectors of the tensor product. So we have (e iφ a ) b = e iφ ( a b ) = a ) (e iφ b ), representing the same state.

28 Quantum Computing 8 Phase Factor The concept of relative phase is also similar to the single qubit system. Note that though e iφ a and a are equivalent states, e iφ a + b and a + b are not be equivalent.

29 Quantum Computing 9 Separable and Entangled States We have already shown that every vector of R R 3 cannot be expressed as a tensor product of vectors of R and R 3. This is true for tensor product of qubit spaces. We consider two -qubit states, ψ = 1 ( ) and Φ + = 1 ( ).

30 Quantum Computing 30 Separable and Entangled States The state ψ can be decomposed into the tensor product of two 1-qubit state - 1 ( ) 0. These types of states are known as separable.

31 Quantum Computing 31 Separable and Entangled States But Φ + cannot be decomposed in that way. If possible, let Φ + = (a 0 +b 1 ) (c 0 +d 1 ) = ac 00 +ad 01 +bc 10 +bd 11. But ad = 0 implies that a = 0 or d = 0. That makes either ac = 0 or bd = 0 - a contradiction. These types of states are known as entangled stater.

32 Quantum Computing 3 Separable and Entangled States If we perform a measurement in standard computational basis, on the second qubit of ψ = 1 ( ), we get a 0 with probability 1 and the state remains unchanged.

33 Quantum Computing 33 Separable and Entangled States If we perform a similar measurement on the second qubit of Φ + = 1 ( ), we get 0 or 1 with equal probability 1. But the outcome of this experiment also determines the first qubit with certainty - so the -qubit state changes. If two qubits in entangled state are separated out, taken far apart and one of them is measured, the outcome of the measurement on the other qubit is known.

34 Quantum Computing 34 Entanglement and Decomposition Consider the decomposition of 4-qubit state u = 1 ( ) to 4 i=1 (a i 0 +b i 1 ). Such a decomposition must satisfy, a 1 a a 3 a 4 = a 1 a b 3 b 4 = b 1 b 1 a 3 a 4 = b 1 b b 3 b 4 = 1/ and a 1 a a 3 b 4 = 0 - which is impossible. So u is an entangled state with respect to 1-qubit decomposition.

35 Quantum Computing 35 Entanglement and Decomposition But u = 1 ( ) is equal to Φ + Φ +. So u is separable in -qubit decomposition.

36 Quantum Computing 36 Direct Sum of Vector Spaces Consider the m and n-dimensional vector spaces A and B over a field F, with the corresponding bases E a and E b respectively. The direct sum of A and B is an (m + n)-dimensional vector space with the base E a E b. The space is written as A B. Any vector x A B can be written as x = a b, where a A and b B.

37 Quantum Computing 37 Direct Sum R R 3 The basis of R R 3 is B B 3 = {(1,0),(0,1),(1,0,0),(0,1,0),(0,0,1)}. Each of these base vectors is viewed as 5-dimensional vector i.e {(1,0,0,0,0),(0,1,0,0,0), (0,0,1,0,0),(0,0,0,1,0),(0,0,0,0,1)} where the first two components are for R and the last three components are for R 3. Addition and scalar multiplications are defined in a natural way.

38 Quantum Computing 38 Measurement Postulate Let B = { b i } i I is orthonormal basis for the state space V A of a system A. A von Neumann measurement on a state x = i I a i b i will output a real value v i with the the probability a i. The system will be in the state b i after the measurement.

39 Quantum Computing 39 Measurement Postulate Let the state space V A V C be a tensor product of V A with basis B = { b i } i I (orthonormal) and V C with basis D = { d j } j J (with unit norm). A von Neumann measurement of a state x = i,j a ij b i d j on the space V A will give a value v ij with the probability a ij. The system will be in the state b i d j after the measurement.

40 Quantum Computing 40 Multi-Qubit Measurement We have already stated that for a single-qubit system the every measuring device has an orthonormal basis B = { b 1, b } for the associated vector space V of 1-qubit states. Each element of B generates an 1-dimensional subspace, V 1 = a 1 b 1 and V = a b, where a 1,a C, such that V = V 1 V.

41 Quantum Computing 41 Multi-Qubit Measurement The n -dimensional vector space V corresponding to an n-qubit system is decomposed by a measuring device into k n orthogonal subspaces V 1,,V k so that V = V 1 V k.

42 Quantum Computing 4 Multi-Qubit Measurement Any n-qubit state x V on measurement on this device chooses a subspaces V i with a probability p i = a i, where a i is the amplitude of x in V i. So we have x = a 1 x 1 + +a k x k, where x i is the unit vector in the subspace V i.

43 Quantum Computing 43 Example 1 The device measures the first qubit state of a -qubit system in standard basis. Let V the state space of the -qubit system. We decompose V = V 1 V where V 1 is the space spanned by { 00, 01 } and V is the space spanned by { 10, 11 }. In other words V 1 = 0 W and V = 1 W, where W is 1-qubit state space.

44 Quantum Computing 44 Example 1 The state of a -qubit system is ψ = a a a 10 +a 3 11 where a 0 + a 1 + a + a 3 = 1. We rewrite the state as ψ = b 1 ψ 1 +b ψ, where ψ 1 V 1 and ψ V, b 1 = a 0 + a 1 and b = a + a 3.

45 Quantum Computing 45 Example 1 So, ψ 1 = 1 b 1 (a a 1 01 ) and ψ = 1 b (a 10 +a 3 11 ). When the first qubit is measured, the probability that the resulting state is ψ 1 is a 0 + a 1 = b 1, and the probability that the resulting state is ψ is a + a 3 = b

46 Quantum Computing 46 Example The device measures the first qubit state of a -qubit system in Hadamard basis. The state space V of the -qubit system is expressed as V = V 1 V where V 1 is the space spanned by { +0, +1 } and V is the space spanned by { 0, 1 }. In other words V 1 = + W and V = W, where W is 1-qubit state space.

47 Quantum Computing 47 Example The state of a -qubit system is ψ = a a a 10 +a 3 11 where a 0 + a 1 + a + a 3 = 1. We rewrite ψ as a 0 ( + + ) 0 + a 1 ( + + ) 1 + a ( + ) 0 + a 3 ( + ) 1

48 Quantum Computing 48 Example So ψ = a 0 +a +0 + a 1 +a a 0 a 0 + a 1 a 3 1 = c 1 ψ 1 +c ψ,

49 Quantum Computing 49 Example where ψ 1 = 1 ( a0 +a +0 + a ) 1 +a 3 +1 c 1 ψ = 1 ( a0 a 0 + a ) 1 a 3 1 c c 1 = 1 a0 +a + a 1 +a 3 c = 1 a0 a + a 1 a 3

50 Quantum Computing 50 Example If we consider the Bell Basis state Φ + = 1 ( ), then a 0 = 1 = a 3 and a 1 = 0 = a. So we have c 1 = 1 = c and ψ 1 = 1 ( ) ψ = 1 ( 0 1 ) Φ + = 1 ψ ψ

51 Quantum Computing 51 Example The result of the measurement on first qubit of the Bell basis Φ + is = 1 ( ) = 1 [ + ( + + )+ + ( + )+ ( + + ) ( + )] = 1 ( ++ + ). The state is still entangled.

52 Quantum Computing 5 Eckert 91 A symmetric key distribution protocol using entangled pair of qubits was proposed by A K Eckert in Quantum cryptography based on Bell s theorem, Physical Review Letters, vol. 67, no. 6, 5 August 1991, pp

53 Quantum Computing 53 e classical A quantum channel channel B pair of entangled qbits

54 Quantum Computing 54 Eckert A sequence of qubit pairs are generated. Each pair is at the state Φ + = 1 ( ).. The first qubit is sent to Alice and the second qubit is sent to Bob on quantum channels. 3. For each qubit they choose randomly either the standard basis or the Hadamard basis for measurement.

55 Quantum Computing 55 Eckert After the measurement, they compare the bases. If the bases are not identical, the bit is discarded.

56 Quantum Computing 56 Eckert 91 If they use the standard base and Alice gets 0 then the state of the entangled pair is 00. So Bob also gets 0 and both of them interpret this as bit 0. If they use the Hadamard base and Alice gets +, the state of the pair is ++. So Bob also gets + and both interpret it as bit 0.

57 Quantum Computing 57 Eckert 91 Alice and Bob need to ensure that the qubit pairs are indeed in the state Φ +. That test uses Bell s inequalities.

58 Quantum Computing 58 References [ERWP] Quantum Computing: A Gentle Introduction by Eleanor Rieffel & Wolfgang Polak, Pub. MIT Press, 011, ISBN [MNIC] Quantum Computation and Quantum Information by Michael A Nielsen & Isaac L Chuang, Pub. Cambridge University Press, 00, ISBN [SA] Quantum Computing Since Democritus by Scott Aaronson, Pub. Cambridge University Press, 013, ISBN [AAM] Classical and Quantum Computation by A Yu Kitaev, A H Shen & M N Vyalyi, Pub. American Mathematical Society (GSM vol 47) 00, ISBN

59 Quantum Computing 59 References [PRM] An Introduction to Quantum Computing by Phillip Kaye, Raymond Laflamme & Michele Mosca, Pub. Oxford University Press, 007, ISBN