Two qubits in pure nuclear quadrupole resonance

Abstract

It is shown theoretically that by the use of two radio-frequency fieldsof the same resonance frequency but with the different phases anddirections the degeneracy of the energy spectrum of a spin system withI = 3/2is removed. This leads to four non-degenerate spin states which can be used as aplatform for quantum computing. The feasibility of quantum computing based ona pure (without DC magnetic fields) nuclear quadrupole resonance technique isinvestigated in detail. Various quantum logic gates can be constructedby using different excitation techniques allowing different manipulationswith the spin system states. Three realizations of quantum logic gatesare considered: the application of an additional magnetic field with theresonance frequency, the amplitude modulation of one of the applied RFfields by the resonance frequency field, and the level-crossing method.It is shown that the probabilities of the resonance transitions dependon the method of excitation and on the direction of the excitation field.Feasibility of quantum computing is demonstrated with the examples ofconstructing a controlled-NOT logic gate using the resonance excitationtechnique and SWAP and NOT2 logic gates using the level-crossing method.