Find the Fourier transform of f :G→Cdefined by f(g

Lehr- und Forschungsgebiet

Mathematische Grundlagen der Informatik RWTH Aachen

Prof. Dr. E. Grädel, D. Fischer, T. Ganzow, B. Puchala

WS 09/10

Complexity Theory and Quantum Computing — Assignment 12 Due: Monday, February 01, 12:00

Exercise 1

LetG={g1, . . . , gn} be an abelian group and leti∈ {1, . . . , n}. Find the Fourier transform of f :G→Cdefined by

f(g) =

(1, ifg=gi

0, otherwise.

Exercise 2

Let n = n1n2, where gcd(n1, n2) = 1. Let also f : Zn1 ×Zn2 → Zn be the function given by f(k1, k2) = a1n2k1 +a2n1k2, where a1 (respectively a2) given by the Chinese Remainder Theorem, is the multiplicative inverse ofn2(respectivelyn1) modulon1 (respectivelyn2). Show thatf is an isomorphism.

Exercise 3

(a) We define the operator S : C^Z2n → C^Z2n as follows. For f : Z2ⁿ → C, the function S(f) :Z2ⁿ →Cis given byS(f)(x) =f((x+1) mod 2ⁿ). Compute the Fourier coefficients of S(f) in terms of the Fourier coefficients off.

(b*) Consider a black-box U_f that computes a function f : {0,1}ⁿ → {0,1}ⁿ as usual: U_f :

|xi|yi 7→ |xi|y⊕f(x)i. Construct a quantum circuit, which implements the following operation {0,1}ⁿ → {0,1}ⁿ, using two applications of the black box, some other gates and, if needed, some extra qubits.

|xi 7→e^2πif(x)2n |xi

where forx∈ {0,1}ⁿ we definex=^Pn−1_i=0 x_i·2ⁱ. Hint: Use the gatesR_j as presented in the lecture.

(c) Implement the following transformation{0,1}ⁿ → {0,1}ⁿ using only the transformation from (b) and the quantum Fourier transformation QFT overZ2ⁿ.

|xi 7→ |bin((x+ 1) mod 2ⁿ)i

where for a natural number k, bin(k) denotes the binary representation ofk.

Hint: Use the transformation from (b) wheref is the identity.

http://www.logic.rwth-aachen.de/Teaching/KTQC-WS09/