Читать книгу Quantum Computing - Hafiz Md. Hasan Babu - Страница 23

A magnitude comparator is a logic circuit that first compares the sizes of A and B and then determines the result among A>B, A<B, and A = B. When the two numbers in the comparator circuit are two one-bit numbers, the result will be only one bit from 0 and 1. Thus the circuit is called a one-bit magnitude comparator, which is a basis of comparison of the two numbers of n bits. A quantum bit string comparator is designed for the quantum bit string comparator circuit. In this chapter two quantum states are identified by providing a comparison status, such as equality or larger or smaller, after performing a comparison between these states. In addition, this chapter shows that the quantum bit string comparator enables us to implement conditional statements in quantum computation, which is a basic structure for designing a comparison algorithm. However, the design requires a huge number of quantum gates, garbage outputs, and constant inputs. Moreover, the design does not show the area and power requirements of the circuit. A quantum comparator circuit is designed for a quantum comparator circuit using quantum dot cellular automata. The design requires a huge number of quantum gates and garbage outputs.

An ASIC implementation of a low power area efficient folded binary comparator circuit was invented in 2014. This comparator consists of a pre-computation unit and an encoder block. The basic principle is to group the binary inputs into digit sets. The digit sets are sent to the pre-computation unit starting from the most significant digit (MSD) to check for equality, and the computations in the pre-computation unit are stopped at the first digit set which produces a 1 as output. The corresponding digit set is then sent to the carry look-ahead (CLA) encoder block to find the greater of the two inputs. However, as this is an irreversible comparator circuit, this circuit has huge dynamic power dissipation.