Sergei Ya. Kilin

Single spin states in a defect center resolved by optical spectroscopy

Individual paramagnetic defect centers in diamond nanocrystals have been investigated by low-temperature high-resolution optical spectroscopy. Narrow fluorescence excitation spectral lines have been found, indicating transitions between individual spin sublevels. Spectral diffusion is explained by cross relaxation among spin sublevels and by the presence of excited electrons in the conduction band of diamond. The relaxation times are in the millisecond range. The system may be useful for quantum information processing with individual electron spins.

Spectroscopy of Single N-V Centers in Diamond

Over the past few years, the detection of single N-V centers in diamond has attracted much interest, since it is expected to lead to innovative applications in various domains of quantum information. The N-V center in diamond is a defectconsisting of a substitutional nitrogen atom adjacent to a carbon-atom vacancy. The optical transition between the 3A ground state and the 3E excited state has a very high quantum efficiency allowing single defect spectroscopy. Because of the paramagnetic nature of the ground electronic state, single N-V defects are believed to be promising candidates for solid state quantum computation. To date, however, the photophysics of the defect is not entirely understood. The existence of a singlet metastable state 1A was challenged in a series of papers reporting hole-burning, optically detected magnetic resonance and single molecule experiments. We present here an overview of recent low and room temperature data, indicating the important role of a metastable state of the defect in single center experiments.

Model systems and photo-kinetics of single N-V defect centers in diamond

Fluorescence excitation method of single molecule spectroscopy has been applied recently to study individual nitrogen-vacancy (N-V) defect centers in diamond. These objects are of great interest owing to the proposal to build quantum computer at /sup 13/C nuclear spins neighboring the N-V center. Based on new spectroscopic information we have developed simple models of the N-V center and used them to describe a wide range of experiments dealing both with single centers and with their ensembles. The first model emphasizes possible tunneling of N atom into vacancy and describes that in terms of double well potentials. The second model takes into account the spin-triplet character of the ground electronic state of the center and allows to describe a variety of fluorescence detected magnetic resonance phenomena.

Kramers-degenerated NV+113C spin systems in diamond: analytical description

Spin systems consisted of single electronic spin S=1 of the NV center and nearby nuclear spins I=1/2 of 13C atoms disposed in diamond lattice near the center can be used as a small register of a quantum computer or as a sensor of a magnetic field. At odd number of nuclear spins eigenvalues of the spin systems at zero external magnetic field are at least twofold degenerated (Kramers degeneration) due to time reversal invariance of the spin Hamiltonian. This degeneracy is lifted only by external magnetic field regardless of the presence of any electric (crystal) field which can also lift the degeneracy thus hindering measurement of the magnetic field. Therefore, the Kramers-degenerated spin systems can be very perspective for measurement of a local magnetic field by the NV-based single-spin quantum magnetometer. Here, we are considering analytically the simplest Kramers-degenerated spin system NV+113C consisting of a single electron spin S=1 of the NV сenter coupled by hyperfine interaction with a single nuclear spin I=1/2 of 13C atom disposed in arbitrary site of diamond lattice. Simple approximate analytical expressions are obtained for eigenvalues and eigenstates of the spin system.

Entanglement detection of GHZ states of electronic and two nuclear spins in NV center in diamond

We consider creation and detection of GHZ state of one electronic and two nuclear spins of carbon isotopes 13C in the first coordination shell of NV center in diamond. The method of entanglement detection is based on construction of an entanglement witness operator.

Towards scalable quantum computers: nano-design and simulations of quantum register

Quantum information technology (QIT) is extremely fast developing area strongly connected with achievements in modern physics. We present a review of recent achievements in implementation of solid-state scalable quantum processors with special emphasize on diamond-based quantum hardware.

Modeling of electronic and spin structure of single NV centers in nanostructured diamond: influence of nanodiamond surface

Modeling of atomic structure and distribution of spin density for the NV center formed close to the surface (111) of nano-diamond has been carried out using quantum-chemical PM3 and DFT methods. The case is considered where the nitrogen atom of NV center is located in the near-surface atomic layer of a face (111). The relaxation of surface atoms relative to the initial position results in N atom to be shifted from the cluster center parallel to the <111> direction by 0.16Å, and C atoms belonging to the surface layer are also shifted parallel to the <111> direction to the center by 0.18Å. As this takes place, C-C and C-N distances between relaxed atoms decrease and a graphite-like structure is formed on a (111) crystal face. In the structure, the N atom and C atoms nearest to it lay practically in the same plane. The formed CN bond can be considered as one-and-a-half bond. It has been found that unlike the NV center in bulk diamond for which the spin density is located mainly on the carbon atoms, being nearest neighbors to the vacancy of the NV center, in the case of the NV center located in immediate proximity to the surface, there is a redistribution of spin density resulting in its major allocation in three C atoms, the nearest neighbors to the N atom, that form the first atomic layer of a surface (111) of nano-crystal.

Quantum repeater on NV+ 13 C color centers in diamond

We suggest a concrete way to realize the quantum repeater protocol based on nitrogen-vacancy (NV) defects in diamond, feasible with present means of manipulating the defects.

Spectroscopy on single tunneling N-V defect centers in diamond

To describe experiments on fluorescence excitation of single N-V defect centers in diamond we have used the double well potential (DWP) model which incorporates the possibility of tunneling the nitrogen atom into the vacancy both in ground and excited electronic states of the center. Fluorescence and linear absorption spectra are calculated, DWPs parameter values for N-V centers are determined and manifestations of their variations due to local diamond lattice distortions in fluorescence spectra of various single N-V centers are studied.

Fluorescence-detected coherent phenomena on single triplet-state molecules

Fluorescence detected magnetic resonance (FDMR) coherent phenomena on single triplet-state chromophore guest molecule in low-temperature organic host matrix are analyzed within the stochastic approach to describe triplet electron spin dephasing due to frequency fluctuations Ut induced by host-matrix proton spins dynamics. Exact equations for density matrix of a molecule averaged over histories of the fluctuations Ut are constructed using the model of N random telegraph process. The equations are applied to calculate FDMR responses of a molecule to cw/pulsed MW field and to describe a wide range of available experimental data on (1) the power-broadened FDMR line shapes, (2) the FDMR nutations, (3) the FDMR Hahn echo for pentacene+p- terphenyl pair supposing the fluctuations Ut to be slow. The failure of the standard Bloch equations for this system is demonstrated and the effects of microwaves-suppressed dephasing are discussed.