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Quantum Bits in Carbon Nanostructures

Final Report Summary - CARBONQUBITS (Quantum Bits in Carbon Nanostructures)

Quantum computation (QC) is expected to improve the efficiency of certain
computational tasks. QC schemes are formulated in terms of operations on quantum bits (qubits), whose values are superpositions of two quantum states. A natural candidate for the physical implementation of a qubit is the spin of an electron. Recent breakthrough experiments demonstrated the ability to control spin-based qubits using electrons confined in a solid state environment. New perspectives have been opened in solid-state QC by recent qubit proposals based on electronic states circulating around the circumference of a carbon nanotube (CNT). Such implementations are promising for various reasons, and are attempted in a number of experimental groups worldwide. To evaluate the potential in these qubit realizations, we proposed to develop a theoretical understanding of the effects relevant for QC in these devices.

During the project, our research focused on the description of the physical mechanisms that affect the functionality of QC with individual electrons. The questions we addressed are: What are the mechanisms allowing for initialization of the information carriers? How to read out the quantum bits? Under which conditions can we perform efficient quantum-logical operations on the qubits by using electric and magnetic fields? What are the unwanted physical processes that lead to the loss of information encoded in the quantum bits, and what are the corresponding qubit lifetimes? Our research provided answers to these questions, with a particular focus on the material-specific properties of carbon-based nanostructures, such as carbon nanotubes and graphene. To highlight the key results: (i) we have calculated how the speed of quantum-logical operations can be optimized in CNT quantum dots, (ii) we have shown that the interplay of mechanical and electronic degrees of freedom can facilitate certain QC-related tasks, (iii) we have elaborated how impurities (nuclear spins, charged impurities) limit the lifetimes of information carriers in nanotubes and graphene. Our results have already extended the understanding of recent state-of-the-art experiments. Furthermore, we expect that these results will enable the design of carbon nanostructures optimized for QC.

In the reporting period, the fellow held a tenured assistant professor position at Eotvos University, Budapest, Hungary. After completing the project, he started a tenured associate professor position at the Budapest University of Technology and Economics. The Marie Curie Career Integration Grant helped him to establish his own research group after his mobility period, to disseminate scientific results at international conferences, and to maintain co-operation with his international research partners. Thereby the EU contribution considerably improved the prospects of the fellow's permanent integration.