Creating diamond color centers for quantum optical applications

Nitrogen vacancy (NV) centers in diamond have distinct promise as solid-state qubits. This is because of their large dipole moment, convenient level structure and very long room-temperature coherence times. In general, a combination of ion irradiation and subsequent annealing is used to create the centers, however for the rigorous demands of quantum computing all processes need to be optimized, and decoherence due to the residual damage caused by the implantation process itself must be mitigated. To that end we have studied photoluminescence (PL) from NV$^-$, NV$^0$ and GR1 centers formed by ion implantation of 2MeV He ions over a wide range of fluences. The sample was annealed at $600^{\circ}$C to minimize residual vacancy diffusion, allowing for the concurrent analysis of PL from NV centers and irradiation induced vacancies (GR1). We find non-monotic PL intensities with increasing ion fluence, monotonic increasing PL in NV$^0$/NV$^-$ and GR1/(NV$^0$ + NV$^1$) ratios, and increasing inhomogeneous broadening of the zero-phonon lines with increasing ion fluence. All these results shed important light on the optimal formation conditions for NV qubits. We apply our findings to an off-resonant photonic quantum memory scheme using vibronic sidebands

Photo induced ionization dynamics of the nitrogen vacancy defect in diamond investigated by single shot charge state detection

The nitrogen-vacancy centre (NV) has drawn much attention for over a decade, yet detailed knowledge of the photophysics needs to be established. Under typical conditions, the NV can have two stable charge states, negative (NV-) or neutral (NV0), with photo induced interconversion of these two states. Here, we present detailed studies of the ionization dynamics of single NV centres in bulk diamond at room temperature during illumination in dependence of the excitation wavelength and power. We apply a recent method which allows us to directly measure the charge state of a single NV centre, and observe its temporal evolution. Results of this work are the steady state NV- population, which was found to be always < 75% for 450 to 610 nm excitation wavelength, the relative absorption cross-section of NV- for 540 to 610 nm, and the energy of the NV- ground state of 2.6 eV below the conduction band. These results will help to further understand the photo-physics of the NV centre.Comment: 9 pages, 7 figure

Single photon emitters based on Ni/Si related defects in single crystalline diamond

We present investigations on single Ni/Si related color centers produced via ion implantation into single crystalline type IIa CVD diamond. Testing different ion dose combinations we show that there is an upper limit for both the Ni and the Si dose 10^12/cm^2 and 10^10/cm^2 resp.) due to creation of excess fluorescent background. We demonstrate creation of Ni/Si related centers showing emission in the spectral range between 767nm and 775nm and narrow line-widths of 2nm FWHM at room temperature. Measurements of the intensity auto-correlation functions prove single-photon emission. The investigated color centers can be coarsely divided into two groups: Drawing from photon statistics and the degree of polarization in excitation and emission we find that some color centers behave as two-level, single-dipole systems whereas other centers exhibit three levels and contributions from two orthogonal dipoles. In addition, some color centers feature stable and bright emission with saturation count rates up to 78kcounts/s whereas others show fluctuating count rates and three-level blinking.Comment: 7 pages, submitted to Applied Physics B, revised versio

Is manganese-doped diamond a ferromagnetic semiconductor?

We use density-functional theoretical methods to examine the recent prediction, based on a mean-field solution of the Zener model, that diamond doped by Mn (with spin S=5/2) would be a dilute magnetic semiconductor that remains ferromagnetic well above room temperature. Our findings suggest this to be unlikely, for four reasons: (1) substitutional Mn in diamond has a low-spin S=1/2 ground state; (2) the substitutional site is energetically unfavorable relative to the much larger "divacancy" site; 3) Mn in the divacancy site is an acceptor, but with only hyperdeep levels, and hence the holes are likely to remain localized; (4) the calculated Heisenberg couplings between Mn in nearby divacancy sites are two orders of magnitude smaller than for substitutional Mn in germanium.Comment: 5 pages, 5 figure

Photochromism in single nitrogen-vacancy defect in diamond

Photochromism in single nitrogen-vacancy optical centers in diamond is demonstrated. Time-resolved optical spectroscopy shows that intense irradiation at 514 nm switches the nitrogen-vacancy defects to the negative form. This defect state relaxes back to the neutral form under dark conditions. Temporal anticorrelation of photons emitted by the different charge states of the optical center unambiguously indicates that the nitrogen-vacancy defect accounts for both 575 nm and 638 nm emission bands. Possible mechanism of photochromism involving nitrogen donors is discussed.Comment: 11 pages, 3 figures, submitted to Applied Physics B: Lasers and Optic

Prediction and measurement of the size-dependent stability of fluorescence in diamond over the entire nanoscale

Fluorescent defects in non-cytotoxic diamond nanoparticles are candidates for qubits in quantum computing, optical labels in biomedical imaging and sensors in magnetometry. For each application these defects need to be optically and thermodynamically stable, and included in individual particles at suitable concentrations (singly or in large numbers). In this letter, we combine simulations, theory and experiment to provide the first comprehensive and generic prediction of the size, temperature and nitrogen-concentration dependent stability of optically active NV defects in nanodiamonds.Comment: Published in Nano Letters August 2009 24 pages, 6 figure

Bromination of double-walled carbon nanotubes

Double-walled carbon nanotubes (DWCNTs) synthesized by catalytic chemical vapor deposition (CCVD) have been functionalized by bromine vapor at room temperature. At least two different bromine species were detected in the product using X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis. The primary form is negatively charged Br2 molecules exhibiting an intense resonance at ∼238 cm−1 in the Raman spectrum. The electron transfer from the nanotubes to the adsorbed molecules is detected from C 1s XPS and near-edge X-ray absorption fine structure spectra. The optical absorption spectra reveal that although the metallic nanotubes are more reactive to Br2, the outer semiconducting nanotubes also readily interact with Br2 adsorbates. The secondary bromine form is attributed to covalent C-Br bonding, and its possible sources are discussed in the light of quantum-chemical calculations. Analysis of the XPS, Raman, and optical absorption spectra of the Br-DWCNTs annealed at 100-170 ° C indicates preservation of a part of bromine molecules in samples that affects the electronic and vibration properties of nanotubes

Diamond as a magnetic field calibration probe

An optical method is proposed for the calibration of pulsed magnetic fields using photoluminescence from synthetic diamond. Generally, the pulsed magnetic field profile is reconstructed by measuring the pick-up voltage in a small coil with an effective area that needs to be known accurately. A useful method to calibrate this area is presented using the 1.4040 eV optical transition at the 1.4 eV Ni-related centre in diamond. The field value is calculated from the Zeeman splitting of the optical lines using the g-factor previously characterized by electron spin resonance. Numerous advantages of the method presented are discussed