Nanodiamonds in levitated optomechanics

This thesis reports on research undertaken to explore the viability of using nanodiamonds containing nitrogen vacancy (NV) centres in optical dipole traps. The impact of illuminating single NV centres with 1550 nm, a common dipole trap wavelength, is investigated. A reduction of 7% in the fluorescence intensity is observed using 20-30 mW of illumination, whilst the NV centre’s optically detected magnetic resonance (ODMR) signal contrast and electron spin T2 time remain unaffected. These results are better than those of similar experiments with 1064 nm. A method for creating and characterising pure type IIa nanodiamonds containing NV centres is presented. Bulk chemical vapour deposition (CVD) diamonds are electron irradiated and annealed, before being ball milled into nanodiamonds. The bulk purity is determined by quantitative electron paramagnetic resonance. The nanodiamonds are characterised by Raman spectroscopy, electron microscopy and energy dispersive X-ray spectroscopy (EDX). Small quantities of contamination by the silicon nitride milling material could be found using EDX. A confocal microscope was constructed and single NV centres inside the nanodiamonds were found to be photostable and ODMR shows an average ODMR contrast of 9%. An optical dipole trap was constructed and CVD derived nanodiamonds were levitated. Measurements of the centre-of-mass motion show that unlike commercial type 1b nanodiamonds, they mostly remain at or close to thermal equilibrium in moderate vacuum where commercial material was previously reported to burn and/or graphitise, even when the optical intensity is raised above 700 GW/m². Nanodiamonds are observed to be suddenly ejected from the trap at ~1 mbar

Burning and graphitization of optically levitated nanodiamonds in vacuum

A nitrogen-vacancy (NV−) centre in a nanodiamond, levitated in high vacuum, has recently been proposed as a probe for demonstrating mesoscopic centre-of-mass superpositions and for testing quantum gravity. Here, we study the behaviour of optically levitated nanodiamonds containing NV− centres at sub-atmospheric pressures and show that while they burn in air, this can be prevented by replacing the air with nitrogen. However, in nitrogen the nanodiamonds graphitize below ≈10 mB. Exploiting the Brownian motion of a levitated nanodiamond, we extract its internal temperature (Ti) and find that it would be detrimental to the NV− centre’s spin coherence time. These values of Ti make it clear that the diamond is not melting, contradicting a recent suggestion. Additionally, using the measured damping rate of a levitated nanoparticle at a given pressure, we propose a new way of determining its size

Tolerance in the Ramsey interference of a trapped nanodiamond

In the scheme recently proposed by M. Scala et al. [Phys. Rev. Lett. 111, 180403 (2013)], a gravity-dependent phase shift is induced on the spin of a nitrogen-vacancy (NV) center in a trapped nanodiamond by the interaction between its magnetic moment and the quantized motion of the particle. This provides a way to detect spatial quantum superpositions by means of only spin measurements. Here, the effect of unwanted coupling with other motional degrees of freedom is considered, and we show that it does not affect the validity of the scheme. Both this coupling and the additional error source due to misalignment between the quantization axis of the NV center spin and the trapping axis are shown not to change the qualitative behavior of the system, so that a proof-of-principle experiment can be neatly performed. Our analysis, which shows that the scheme retains the important features of not requiring ground-state cooling and of being resistant to thermal fluctuations, can be useful for several schemes which have been proposed recently for testing macroscopic superpositions in trapped microsystems

Deep three-dimensional solid-state qubit arrays with long-lived spin coherence

Nitrogen-vacancy centers (NVCs) in diamond show promise for quantum computing, communication, and sensing. However, the best current method for entangling two NVCs requires that each one is in a separate cryostat, which is not scalable. We show that single NVCs can be laser written 6–15-µm deep inside of a diamond with spin coherence times that are an order of magnitude longer than previous laser-written NVCs and at least as long as naturally occurring NVCs. This depth is suitable for integration with solid immersion lenses or optical cavities and we present depth-dependent T2 measurements. 200 000 of these NVCs would fit into one diamond

Pure nanodiamonds for levitated optomechanics in vacuum

Optical trapping at high vacuum of a nanodiamond containing a nitrogen vacancy centre would provide a test bed for several new phenomena in fundamental physics. However, the nanodiamonds used so far have absorbed too much of the trapping light, heating them to destruction (above 800 K) except at pressures above ~10 mbar where air molecules dissipate the excess heat. Here we show that milling diamond of 1000 times greater purity creates nanodiamonds that do not heat up even when the optical intensity is raised above 700 GW m−2 below 5 mbar of pressure

Imaging damage in steel using a diamond magnetometer

We demonstrate a simple, robust, and contactless method for nondestructive testing of magnetic materials such as steel. This uses a fiber-coupled magnetic sensor based on nitrogen-vacancy centers (NVCs) in diamond without magnetic shielding. Previous NVC magnetometry has sought a homogeneous bias magnetic field on the diamond to improve the sensitivity. In contrast, here we show that the spatial resolution for imaging is improved by applying an inhomogeneous magnetic field to the steel even though this leads to an inhomogeneous magnetic field on the diamond. Structural damage in the steel distorts the inhomogeneous magnetic field and by detecting this distortion we reconstruct the damage profile through quantifying the shifts in the NVC Zeeman splitting. With a 1-mm magnet as the source of our inhomogeneous magnetic field, we achieve a high spatial resolution of 1 mm in the plane parallel and 0.1 mm in the direction perpendicular to the surface of the steel. This works even when the steel is covered by a nonmagnetic material. The lift-off distance of our sensor head from the surface of 316 stainless steel is up to 3 mm

Subnanotesla magnetometry with a fiber-coupled diamond sensor

Nitrogen-vacancy centers (NVCs) in diamond are being explored for future quantum technologies, and in particular ensembles of NVC are the basis for sensitive magnetometers. We present a fiber-coupled NVC magnetometer with an unshielded sensitivity of (310±20)pT/√Hz in the frequency range of 10–150 Hz at room temperature. This takes advantage of low-strain 12C diamond, lenses for fiber coupling and optimization of microwave modulation frequency, modulation amplitude, and power. Fiber coupling means the sensor can be conveniently brought within 2 mm of the object under study

Laser writing of coherent colour centres in diamond

Optically active point defects in crystals have gained widespread attention as photonic systems that can find use in quantum information technologies [1,2]. However challenges remain in the placing of individual defects at desired locations, an essential element of device fabrication. Here we report the controlled generation of single nitrogen-vacancy (NV) centres in diamond using laser writing [3]. The use of aberration correction in the writing optics allows precise positioning of vacancies within the diamond crystal, and subsequent annealing produces single NV centres with up to 45% success probability, within about 200 nm of the desired position. Selected NV centres fabricated by this method display stable, coherent optical transitions at cryogenic temperatures, a pre-requisite for the creation of distributed quantum networks of solid-state qubits. The results illustrate the potential of laser writing as a new tool for defect engineering in quantum technologies

Laser writing of coherent colour centres in diamond

Optically active point defects in crystals have gained widespread attention as photonic systems that can find use in quantum information technologies. However challenges remain in the placing of individual defects at desired locations, an essential element of device fabrication. Here we report the controlled generation of single negatively charged nitrogen-vacancy (NV-) centres in diamond using laser writing. Aberration correction in the writing optics allows precise positioning of vacancies within the diamond crystal, and subsequent annealing produces single NV- centres with up to (45 ± 15)% success probability, within about 200 nm of the desired position in the transverse plane. Selected NV- centres display stable, coherent optical transitions at cryogenic temperatures, a pre-requisite for the creation of distributed quantum networks of solid-state qubits. The results illustrate the potential of laser writing as a new tool for defect engineering in quantum technologies, and extend laser processing to the single defect domain

An analytical model for the detection of levitated nanoparticles in optomechanics

Interferometric position detection of levitated particles is crucial for the centre-of-mass (CM) motion cooling and manipulation of levitated particles. In combination with balanced detection and feedback cooling, this system has provided picometer scale position sensitivity, zeptonewton force detection, and sub-millikelvin CM temperatures. In this article, we develop an analytical model of this detection system and compare its performance with experimental results allowing us to explain the presence of spurious frequencies in the spectra