SPINTRONICS BASED QUANTUM COMPUTING ARCHITECTURES

Abstract

A quantum computer performs computations on the principles of quantum mechanics that enables faster speed and higher security than classical computers, and also has the ability to process large amount of information due to its inherent ability of parallel processing. The important building blocks of the quantum computer are qubit, quantum register, quantum logic, quantum network, quantum reversibility, quantum teleportation, quantum data compression, quantum cryptography, universal quantum computing, and quantum algorithm. Quantum computers rely on basic quantum principles of superposition and entanglement. The time evolution of an arbitrary quantum state is computationally more powerful than evolution of a digital logic state. Theoretical quantum computing based on the rotation of the qubits has proved that there is a possibility of quantum devices to address the complex computing problems. However, presently, there is no computer in existence that can completely work on the principles of quantum mechanics. Therefore, the enormous advantages of quantum computing in comparison to its classical counterpart have forced researchers to explore the possibilities of physical realization with the help of emerging technologies. The basic requirements of Divincenzo criteria have to be fulfilled for the successful implementation of the quantum computing. This criteria suggest that the system realizing the quantum computing should have well characterized qubit; proper initial state of all qubits; enough isolation to the qubit(s); precise qubit state manipulation and facilitation of qubits interaction should be in time less than the qubit decoherence time; and the physical system should facilitate the measurement of each qubit to obtain the output of the quantum computation. Spintronics is one of the most efficient ways to physically realize quantum computing due to strong analogy of electron spin to the qubit. Spin-torque based on-chip qubit architecture paves the way for the research in spintronics based physical realization of quantum computer. However, the qubit decoherence is a critical issue in spin qubit architecture from the complex computing point of view. This issue can be dealt by two ways in this thesis; firstly, by utilizing the materials with spin qubits having very high spin coherence, and secondly, by reducing and optimizing the number of elementary quantum operations with the help of elementary quantum gate ii library. This thesis presents both ways in detail with demonstrations of reduction in number of elementary operations by elementary quantum gate library. A computing platform is realized using reduced elementary gates such as CNOT, SWAP, Toffoli, and Fredkin wherein the reduction in number of elementary operations is 36.36%, 36.36%, 35.44%, 35.64%, respectively. The optimization of the reduced number of operations for the quantum circuits representing the Boolean logics AND, OR, XOR, Hall Adder (HA), and Full Adder (FA), is also achieved with a reduction after optimization of 37.97%, 41.58%, 45.45%, 40%, and 40.55%, respectively. A quantum Fourier transform that is an integral part of the Shor's algorithm for the number factorization is also reduced and optimized. The reduction of 35.71% in number of elementary operations for the quantum Fourier transform is also demonstrated. Various other complex computing operations can be realized using the spin torque based qubit architecture. This thesis lays strong foundation for researchers aspiring to work in the area of quantum computing using spintronics platform and also discusses the associated challenges.