S. Ya. Kilin

Quantum computation using the 13C nuclear spins near the single NV defect center in diamond

We discuss the possibility of realizing quantum computation on the basis of a cluster of single interacting nuclear spins in solids. This idea seems to be feasible because of the combination of two techniques—Single Molecule Spectroscopy and Optically Detected Electron Nuclear Double Resonance. Compared to the well-known bulk Nuclear Magnetic Resonance (NMR), the proposed method of quantum computation has the advantage that quantum computation is performed with pure spin states and the quantum processor is more easily scalable. At the same time, the advantages of NMR quantum computation are kept: long coherence time and easy construction of quantum gates. As a specific system to implement the above idea, we discuss the 13C-nuclear spins in the nearest vicinity of a single nitrogen-vacancy (NV) defect center in diamond, which can be optically detected using the technique of scanning confocal microscopy. Owing to the hyperfine coupling of the ground state electron paramagnetic spin S=1 of the center to 13C nuclear spins in a diamond lattice, the states of nuclear spins in the vicinity of the defect-center can be addressed individually. Preliminary consideration shows that it should be possible to address up to 12 individual 13C nuclear spins. The dephasing time of the nuclear spin states at low temperatures allows realization up to 105 gates.

A quantum computer based on NV centers in diamond : Optically detected nutations of single electron and nuclear spins

In an effort to realize a two-bit processor for a quantum computer on the basis of single nitrogen-vacancy defect centers (NV centers) in diamond, the optically detected nutations of the electron spin of a single NV center in the ground state and of the nuclear spin of a 13C atom located at a diamond lattice site nearest to the NV center are studied. The photodynamics of NV and NV + 13C centers under different temperatures and optical excitation conditions is discussed. A seven-level model of a center excited by radiation from an Ar+ laser at room temperature is proposed. On the basis of this model, the experimental spectra of optically detected electron paramagnetic and electron-nuclear double resonances of single NV and NV + 13C centers in diamond nanocrystals, as well as experimental data on the optically detected nutations of the electron and nuclear spins of these centers caused by the actions of pulsed microwave and radiofrequency fields, respectively, are interpreted.

Spin-selective low temperature spectroscopy on single molecules with a triplet-triplet optical transition: Application to the NV defect center in diamond

The spin-selective photokinetics of a single matrix-isolated impurity molecule with a triplet-triplet optical transition, T0–T1, is considered and the manifestations of the photokinetics in the fluorescence excitation spectra and intensity autocorrelation functions g(2)(τ) of the molecule undergoing narrow-band optical excitation is studied to resolve the fine structure of the transition. The rates of intersystem crossings (ISCs) T1→S→T0 to and from a nonradiating singlet state S of the molecule and the rate of population relaxation among the ground (T0) state sublevels can be obtained from the spectra and g(2)(τ) using the analytical expressions obtained. New experiments on an individual NV defect center in nanocrystals of diamond, where, for the first time, the fine structure of its triplet-triplet 3A-3E zero-phonon optical transition (~637 nm) at 1.4 K was resolved, are interpreted. It is concluded that the rate of the ISC transition from the mS=0 sublevel of the excited 3E state to the singlet 1A state (~1 kHz) is much slower than the rates from the mS=±1 substates, while the rates of ISC transitions to different mS substates of the ground 3A state are close to each other (~1 Hz). As a result, only the optical transition between mS=0 sublevels in the 3A-3E manifold contributes strongly to the fluorescence. The experimentally observed double-exponential decay of the g(2)(τ) function is explained by the two pathways available to the center for it to leave the S state: (i) the S→ T0(mS)=0) transition and (ii) the S→T0(mS=±1) transitions followed by the slow spin-lattice relaxation T0(mS=±1)→T0(mS=0) (rate ~0.1 Hz). The work is important for studies where the NV center is used as a single photon source or for quantum information processing.

Theoretical study of hyperfine interactions and optically detected magnetic resonance spectra by simulation of the C291[NV]-H172 diamond cluster hosting nitrogen-vacancy center

Single nitrogen-vacancy (NV) centers in diamond coupled to neighboring nuclear spins are promising candidates for room-temperature applications in quantum information processing, quantum sensing and metrology. Here we report on a systematic density functional theory simulation of hyperfine coupling of the electronic spin of the NV center to individual 13C nuclear spins arbitrarily disposed in the H-terminated C291[NV]-H172 cluster hosting the NV center. For the ‘families’ of equivalent positions of the 13C atom in diamond lattices around the NV center we calculated hyperfine characteristics. For the first time the data are given for a system where the 13C atom is located on the NV center symmetry axis. Electron paramagnetic resonance transitions in the coupled electron–nuclear spin system 14NV-13C are analyzed as a function of the external magnetic field. Previously reported experimental data from Dreau et al (2012 Phys. Rev. B 85 134107) are described using simulated hyperfine coupling parameters.

Fiber-optic magnetic-field imaging

We demonstrate a scanning fiber-optic probe for magnetic-field imaging where nitrogen-vacancy (NV) centers are coupled to an optical fiber integrated with a two-wire microwave transmission line. The electron spin of NV centers in a diamond microcrystal attached to the tip of the fiber probe is manipulated by a frequency-modulated microwave field and is initialized by laser radiation transmitted through the optical tract of the fiber probe. The two-dimensional profile of the magnetic field is imaged with a high speed and high sensitivity using the photoluminescence spin-readout return from NV centers, captured and delivered by the same optical fiber.

Spectroscopy on single N–V defect centers in diamond: tunneling of nitrogen atoms into vacancies and fluorescence spectra

Abstract Experiments on fluorescence excitation of single N–V defect centers in diamond are described in terms of 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.

Ab initio modeling of the electronic and spin properties of the [NV] centers in diamond nanocrystals

The electronic and spin properties of different nanocrystals of carbon are studied. The properties of these cluster systems are modeled in terms of the ab initio (Hartree-Fock) and semiempirical (PM3, AM1) quantum-chemical methods. The calculations are performed for different carbon nanocluster systems: defect-free and with [NV]− centers, hydrogen passivated (C38H42, C71H84, C86H78), and with a free (unpassivated) surface (C38, C71, C86). The spin properties of unhydrated nanoclusters were studied for the first time. The structure of all the clusters under study was optimized using the total energy minimization principle. It is shown that, in the case of hydrated carbon nanocrystals passivated by hydrogen atoms, diamond-like clusters are formed. The atomic structure of an unpassivated nanocrystal depends on the number of atoms in the cluster, as well as on its initial geometrical parameters. In some cases, clusters with a fullerene-like surface are formed. In hydrogenpassivated diamond nanocrystals with [NV]− centers, the spin density is localized at the nuclei of C atoms nearest to the center vacancies. For the unpassivated counterparts, the spin density is localized at the nuclei of C atoms forming the surface of the corresponding nanocrystal.

On “anomalous” free induction decay rate

Abstract Generalized non-markovian master equations, which take into account the nonvanishing bath correlation time τ c , are used to explain the “anomalous” (which cannot be described by the well-known Bloch master equations) behaviour of the optical free induction decay recently observed by DeVoe and Brewer [Phys. Rev. Lett. 50 (1983) 1269] for the Pr 3+ : LaF 3 at T ≃ 1.6 K. We found τ c = 5 μs to be the best fit of our theory to the above experiment provided that T 1 = 100 μs, T 2 = 24 μs. New experimental tests of the non-markovian relaxation theory are proposed.

Single-atom laser: Coherent and nonclassical effects in the regime of a strong atom-field correlation

Based on the approximation of strong correlations between an atom and an intracavity field, which implies the equal probabilities of finding the atom in the ground state and n photons in the field and of finding the atom in an excited state and n−1 photons in the field, it is shown that the conditional states of a field generated by a single-atom laser are described by the diagonal part of the generalized coherent Mittag-Leffler state. The quasi-distributions P and Q of the intracavity-field probability amplitude are found, and the boundedness of the Glauber function on a segment is shown. The possibility of inversionless lasing is demonstrated, and the absence of a lasing threshold is found for some region of parameters. The regimes of generation of the amplitude-squeezed states of the field are studied and the parameters of the system providing the maximum squeezing are determined. It is shown that the atom-field states are entangled at weak pump intensities.

Guided-wave-coupled nitrogen vacancies in nanodiamond-doped photonic-crystal fibers

Zero-phonon-line (ZPL) emission of nitrogen vacancies (NVs) is coupled to the guided modes of solid- and hollow-core nanodiamond-doped photonic-crystal fibers (PCFs). Both types of PCFs are tailored toward enhancing ZPL emission coupling to the fiber modes. In solid-core PCFs, this involves enhancing the evanescent field of the waveguide modes supported by an ultrasmall fiber core. In hollow-core PCFs, the NV emission spectrum is matched with the transmission band of the fiber, controlled by the photonic bands of the fiber cladding.