We consider Grover's algorithm of quantum search for one or several integers out of N = 2n, where n is a number of quantum bits in the memory register. There is a black-box or subroutine containing information about hidden integers and it can easily recognize these integers but we do not know which ones out of N they are. To find the hidden items we can do no better with a classical computer than to apply the subroutine repeatedly to all possible integers until we hit on the special one and in the worst case we have to repeat this procedure N times. We have analyzed the Grover algorithm carefully and showed that it enables to speed up this search quadratically, although its realization requires to know a number of hidden items. The lower bound for the probability of successful solving the search problem has been obtained. The validity of the results was demonstrated by simulation of the Grover search algorithm using the package QuantumCircuit written in the Wolfram Mathematica language.
We consider Grover's algorithm of quantum search for one or several integers out of N = 2n, where n is a number of quantum bits in the memory register. There is a black-box or subroutine containing information about hidden integers and it can easily recognize these integers but we do not know which ones out of N they are. To find the hidden items we can do no better with a classical computer than to apply the subroutine repeatedly to all possible integers until we hit on the special one and in the worst case we have to repeat this procedure N times. We have analyzed the Grover algorithm carefully and showed that it enables to speed up this search quadratically, although its realization requires to know a number of hidden items. The lower bound for the probability of successful solving the search problem has been obtained. The validity of the results was demonstrated by simulation of the Grover search algorithm using the package QuantumCircuit written in the Wolfram Mathematica language.
The nature of the search Suppose you know that exactly one n -bit integer satisfies a certain condition, and suppose you have a black-boxed subroutine that acts on the N = 2 n different n -bit integers, outputting 1 if the integer satisfies the condition and 0 otherwise. In the absence of any other information, to find the special integer you can do no better with a classical computer than to apply the subroutine repeatedly to different random numbers until you hit on the special one. If you apply it to M different integers the probability of your finding the special number is M/N . You must test ½ N different integers to have a 50% chance of success. If, however, you have a quantum computer with a subroutine that performs such a test, then you can find the special integer with a probability that is very close to 1 when N is large, using a method that calls the subroutine a number of times no greater than. This very general capability of quantum computers was discovered by Lov Grover, and goes under the name of Grover's search algorithm . Shor's period-finding algorithm and Grover's search algorithm, together with their various modifications and extensions, constitute the two masterpieces of quantum-computational software. One can think of Grover's black-boxed subroutine in various ways. The subroutine might perform a mathematical calculation to determine whether the input integer is the special one. Here is a simple example.
A quantum algorithm for the computation the order of an integer, which uses the quantum Fourier transform, is discussed. The cases of the exact and approximate Fourier transform are considered, and estimates of the probability of the successful solution of the problem that significantly improve the available results are obtained. The quantum algorithm for order finding is simulated using the QuantumCircuit package written in the computer algebra system Wolfram Mathematica , and its efficiency in combination with the approximate Fourier transform is demonstrated.
Since Grover’s seminal work, quantum search has been studied in great detail. In the usual search problem, we have a collection of n items x 1,…,x n and we would like to find i:x i =1. We consider a new variant of this problem in which evaluating x i for different i may take a different number of time steps. Let t i be the number of time steps required to evaluate x i . If the numbers t i are known in advance, we give an algorithm that solves the problem in O(t12+t22+…+tn2) steps. This is optimal, as we also show a matching lower bound. The case, when t i are not known in advance, can be solved with a polylogarithmic overhead. We also give an application of our new search algorithm to computing read-once functions.
Since Grover's seminal work, quantum search has been studied in great detail. In the usual search problem, we have a collection of n items and we would like to find a marked item. We consider a new variant of this problem in which evaluating the i-th item may take a different number of time steps for different i. Let t_i be the number of time steps required to evaluate the i-th item. If the numbers t_i are known in advance, we give an algorithm that solves the problem in O(\sqrt{t_1^2+t_2^2+...+t_n^2}) steps. This is optimal, as we also show a matching lower bound. The case, when t_i are not known in advance, can be solved with a polylogarithmic overhead. We also give an application of our new search algorithm to computing read-once functions.
In this paper, Grover's quantum search algorithm is analyzed using a classical computer by calculating the amplitudes and the probabilities of finding a single marked state for n=5, 10, 15, 20, 25, and 27 qubit states.The calculations show that the marked state can be found in 𝒪(√𝑁) iterations, where 𝑁 = 2 ( is the number of items.The possibility of improving Grover's search algorithm to find a single item in N search elements is discussed by calculating the amplitudes and hence the probabilities of finding a single marked state for n=5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 qubit states.The calculations showed that the marked state could be found with sufficiently high probability in 𝒪(ln (𝑁)) iterations.This is quite a remarkable speed-up that can be achieved to find a single marked element in an unsorted N search element.
In the classical model of a computer, the most fundamental building block, the bit, can only exist in one of two distinct states, a 0 or a 1. Computations are carried out by logic gates that act on these bits to produce other bits. Unless there is duplicate (parallel) hardware, only one problem instance (i.e. input data set) can be handled at a time. In this classical computing, increasing the number of bits increases the complexity of the problem and the time necessary to arrive at a solution. A quantum algorithm consists of a sequence of operations on a register, to transform it into a state which, when measured, yields the desired result with high probability. An n-bit quantum register can store an exponential amount of information. This paper aims at taking advantage of the superiority of quantum computing over classical computing to solve the problem of searching unstructured databases for a particular item or more than one item in good time. The general aim of this work is to establish the correctness and optimality of Grover’s quantum database search algorithm compared against classical database search methods in order to investigate the superiority or otherwise of quantum computing over classical computing. This is followed by a simulation of the algorithm using a classical computer, namely through functions that are present in MATLAB, referred to as “Quantum Computing Functions”. Povzetek: Clanek predstavlja implementacijo kvantnega iskanja v nestrukturiranih bazah.