Browsing by Author "Popa, Iulian"

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Pulsed magnetic resonance on single defect centers in diamond
(2006) Popa, Iulian; Wrachtrup, Jörg (Prof. Dr.)
In the present work, the spin properties of single Nitrogen-Vacancy (NV) defect centers in diamond are investigated, in perspective to their application to quantum computing. One of the actual approaches to quantum computing is based on spins, and, therefore, their behavior and properties related to manipulation and readout are very important. Using conventional NMR, an ensemble of spins can be used as a qubit implementation. However, the method has limitations in regards to the number of qubits that can be achieved. Conventional magnetic resonance methods cannot be applied to a single spin because of their low sensitivity. Instead, an alternative method for detection of magnetic resonance, Optical Detection of Magnetic Resonance (ODMR) was proved to be suitable for detection of single spins. The method of choice for studying single NV defects combines single molecule detection techniques with magnetic resonance. The NV center in diamond is paramagnetic ( S = 1). The energy level scheme consists of a triplet ground state and a triplet excited state as well as a singlet metastable state. The center can be optically detected due to the fluorescent transition between the excited and ground triplets. In zero magnetic field, the ground state splits into three components, (X and Y, m$_S = \pm 1$) and Z (m$_S = 0$), separated by 2.88 GHz. The ground state can be used as a spin qubit implementation. In order to determine the properties associated with the NV triplet ground state, continuous wave (cw) and pulsed ODMR methods have been applied. The coherent evolution in time of the electron spin in a microwave field is probed by observing transient nutations. The decoherence time of the single electron spin is measured by the Hahn echo method. ESEEM experiments prove the hyperfine structure of the neighboring nuclei, i.e., for this case, $^{14}$N nucleus. Simulations of the energy level scheme and spin dynamics of a single electron spin confirm the experimental findings. Furthermore, the influence of the optical readout on the evolution of the system is studied, and the possibility of observing quantum Zeno effect in a single electron spin is addressed. The hyperfine coupling of the single electron spin to a single neighboring $^{13}$C gives the possibility of scaling up the number of spin qubits. The $^{13}$C nucleus has a spin 1/2. The nuclear spin states can be readout via the electron spin states. The hyperfine energy level scheme was probed experimentally by cw ODMR. Calculations confirmed the hyperfine structure. A Hahn echo sequence, adapted to nuclear spins was employed for determining the decoherence time for the single nuclear spin. The state of the setup at that time did not allow the determination of the hyperfine tensor. All calculation assume an isotropic value for the hyperfine constant. The issue of dipolar coupling between two NV centers is addressed, in the context of its eventual use towards increasing the degree of qubit scalability. The magnetic dipolar coupling depends upon the distance between the centers, according to the known rule of dipolar coupling interaction energy. The resulting energy level scheme of two coupled NVs was probed via cw ODMR. Two theoretical models for the coupling between centers result in a good agreement with experimental data.