Quantum information processing by NMR using strongly coupled spins

The enormous theoretical potential of quantum information processing (QIP) is driving the pursuit for its practical realization by various physical techniques. Currently, nuclear magnetic resonance (NMR) has been the forerunner by demonstrating a majority of quantum algorithms. In NMR, spin-systems consisting of coupled nuclear spins are utilized as qubits. In order to carry out QIP, a spin-system has to meet two major requirements: (i) qubit addressability and (ii) mutual coupling among the. qubits. It has been demonstrated that the magnitude of the mutual coupling among qubits can be increased by orienting the spin-systems in a liquid crystal matrix and utilizing the residual dipolar couplings. While utilizing residual dipolar couplings may be useful to increase the number of qubits, nuclei of the same species (homonuclei) might become strongly coupled. In strongly coupled spin-systems, spins lose their individual identity of being qubits. We propose that even such strongly coupled spin-systems can be used for QIP and the qubit-manipulation can be achieved by transition-selective pulses. We demonstrate experimental preparation of pseudopure states, creation of maximally entangled states, implementation of logic gates and implementation of Deutsch-Jozsa (DJ) algorithm in strongly coupled 2, 3 and 4 spin-systems. The energy levels of the strongly coupled 3 and 4 spin-systems were obtained using a Z-COSY experiment.