The implementation of a spin qubit in a quantum ring occupied by one or a few electrons is proposed. Quantum bit involves the Zeeman sublevels of the highest occupied orbital. Such a qubit can be initialized, addressed, manipulated, read out and coherently coupled to other quantum rings. An extensive discussion of relaxation and decoherence is presented. By analogy with quantum dots, the spin relaxation times due to spin-orbit interaction for experimentally accessible quantum ring architectures are calculated. The conditions are formulated under which qubits build on quantum rings can have long relaxation times of the order of seconds. Rapidly improving nanofabrication technology have made such ring devices experimentally feasible and thus promising for quantum state engineering.
Quantum information processing and spintronics have been major driving forces towards
full control of single-spin systems. In particular fascinating phenomena based on carrier confinement in ring shaped nanostructures have intrigued physicists for many years. It was found that nanorings with Radius less than 20nm can be considered as almost ideal quantum systems and thus can be, besides Quantum dots, excellent systems for spin studies. We have investigated quantum rings with a single or a few electrons and have shown that they can be treated as quantum bits fulfilling DiVincenzo criteria. We have shown that for both Quantum dots and Quantum Rings long relaxation times exceeding seconds at B = 1T are possible. It follows from our analysis that for singly occupied structures Quantum Dots have always longer relaxation time than Quantum rings but for relatively thin rings with higher occupation the relaxation times can exceed those for Quantum dots. However even singly occupied rings with radius less than 10nm can have relaxation times exceeding seconds. The single occupancy of rings makes the experiments and the analysis more transparent, however, there is an open question whether qubits with Ne over 1 can have some other advantages over those with Ne = 1. The presented considerations demonstrate the feasibility of operating single-electron spin in a quantum ring as the quantum bit. This is of big relevance for the use in quantum information processing devices.
Finally, it should be stressed that multiply connected ring geometry offers additional (orbital) degree of freedom to be used for quantum manipulations. It is possible to build a qubit also on the orbital degrees of freedom in some analogy to flux qubits on superconducting rings. Thus quantum carrier confinement in circular nanostructures can be the basis of many applications in quantum information processing devices.
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