Dr Lachlan Rogers

Single quantum light emitters are valuable resources for engineered quantum systems. They can function as robust single-photon generators, allow optical control of single spins, provide readout capabilities for atomic-scale sensors, and provide interfaces between stationary and flying qubits. Environmental factors can lead to single emitters exhibiting 'blinking', whereby the fluorescence level switches between on and off states. Detailed characterisation of this blinking behaviour including determining the switching rates is often a powerful way to gain understanding about the underlying physical mechanisms. While simple thresholds can be used to identify the on and off intervals and thus extract the rates from the time-series of counts for bright emitters with low background noise, such approaches become difficult for emitters fluorescing at low levels, high noise, or switching at fast rates. We develop a Bayesian approach capable of inferring switching rates directly from the time-series. This is able to deal with high levels of noise and fast switching in fluorescence traces. Moreover, the Bayesian inference also yields a robust picture of the parameter uncertainties, providing a benefit also for bright emitters in low-noise settings. The technique can be adapted to identify the underlying states as well as extracting the rates of switching. Finally, our method is applicable to a broad range of systems that show behaviour analogous to a blinking emitter.

The ST1 center is a point defect in diamond with bright fluorescence and a mechanism for optical spin initialization and readout. The center has impressive potential for applications in diamond quantum computing as a quantum bus to a register of nuclear spins. This is because it has an exceptionally high readout contrast, and unlike the well-known nitrogen-vacancy center, it does not have a ground state electronic spin that decoheres the nuclear spins. However, its chemical structure is unknown, and there are large gaps in our understanding of its properties. We present the discovery of ST1 centers in natural diamond. Our experiments identify interesting power dependence of the center¿s optical dynamics and reveal new electronic structure. We also present a theory of its electron-phonon interactions, which we combine with previous experiments, to shortlist likely candidates for its chemical structure.

We report on the isolation of single negatively-charged-silicon-vacancy (Si-V-) centers in nanodiamonds. We observe the fine structure of single Si-V-centers with reduced inhomogeneous ensemble linewidth below the excited-state splitting, stable optical transitions, good polarization contrast, and excellent spectral stability under resonant excitation. On the basis of our experimental results, we develop an analytical strain model where we extract the ratio between strain coefficients of excited and ground states as well the intrinsic zero-strain spin-orbit splittings. The observed strain values are as low as the best values in low-strain bulk diamond. We achieve our results by means of H-plasma treatment of the diamond surface and in combination with resonant and off-resonant excitation. Our work paves the way for indistinguishable, single-photon emission. Furthermore, we demonstrate controlled nanomanipulation by an atomic-force-microscope cantilever of one-A nd two-dimensional alignments with an accuracy of about 10 nm, as well as new tools including dipole rotation and cluster decomposition. Combined, our results show the potential to utilize Si-V-centers in nanodiamonds for controlled interfacing via optical coupling of individually-well-isolated atoms for bottom-up assemblies of complex quantum systems.

We demonstrate a quantum nanophotonics platform based on germanium-vacancy (GeV) color centers in fiber-coupled diamond nanophotonic waveguides. We show that GeV optical transitions have a high quantum efficiency and are nearly lifetime broadened in such nanophotonic structures. These properties yield an efficient interface between waveguide photons and a single GeV center without the use of a cavity or slow-light waveguide. As a result, a single GeV center reduces waveguide transmission by 18±1% on resonance in a single pass. We use a nanophotonic interferometer to perform homodyne detection of GeV resonance fluorescence. By probing the photon statistics of the output field, we demonstrate that the GeV-waveguide system is nonlinear at the single-photon level.

A solid-state system combining a stable spin degree of freedom with an efficient optical interface is highly desirable as an element for integrated quantum-optical and quantum-information systems. We demonstrate a bright color center in diamond with excellent optical properties and controllable electronic spin states. Specifically, we carry out detailed optical spectroscopy of a germanium-vacancy (GeV) color center demonstrating optical spectral stability. Using an external magnetic field to lift the electronic spin degeneracy, we explore the spin degree of freedom as a controllable qubit. Spin polarization is achieved using optical pumping, and a spin relaxation time in excess of 20µs is demonstrated. We report resonant microwave control of spin transitions, and use this as a probe to measure the Autler-Townes effect in a microwave-optical double-resonance experiment. Superposition spin states were prepared using coherent population trapping, and a pure dephasing time of about 19ns was observed at a temperature of 2.0 K.

Qudi is a general, modular, multi-operating system suite written in Python 3 for controlling laboratory experiments. It provides a structured environment by separating functionality into hardware abstraction, experiment logic and user interface layers. The core feature set comprises a graphical user interface, live data visualization, distributed execution over networks, rapid prototyping via Jupyter notebooks, configuration management, and data recording. Currently, the included modules are focused on confocal microscopy, quantum optics and quantum information experiments, but an expansion into other fields is possible and encouraged.

Colour centres in nanodiamonds are an important resource for applications in quantum sensing, biological imaging, and quantum optics. Here we report unprecedented narrow optical transitions for individual colour centres in nanodiamonds smaller than 200 nm. This demonstration has been achieved using the negatively charged silicon vacancy centre, which has recently received considerable attention due to its superb optical properties in bulk diamond. We have measured an ensemble of silicon-vacancy centres across numerous nanodiamonds to have an inhomogeneous distribution of 1.05 nm at 5 K. Individual spectral lines as narrower than 360 MHz were measured in photoluminescence excitation, and correcting for apparent spectral diffusion yielded an homogeneous linewidth of about 200 MHz which is close to the lifetime limit. These results indicate the high crystalline quality achieved in these nanodiamond samples, and advance the applicability of nanodiamond-hosted colour centres for quantum optics applications.

We report on the noise spectrum experienced by few nanometer deep nitrogen-vacancy centers in diamond as a function of depth, surface coating, magnetic field and temperature. Analysis reveals a double-Lorentzian noise spectrum consistent with a surface electronic spin bath in the low frequency regime, along with a faster noise source attributed to surface-modified phononic coupling. These results shed new light on the mechanisms responsible for surface noise affecting shallow spins at semiconductor interfaces, and suggests possible directions for further studies. We demonstrate dynamical decoupling from the surface noise, paving the way to applications ranging from nanoscale NMR to quantum networks.

We investigate phonon induced electronic dynamics in the ground and excited states of the negatively charged silicon-vacancy (SiV-) centre in diamond. Optical transition linewidths, transition wavelength and excited state lifetimes are measured for the temperature range 4 K-350 K. The ground state orbital relaxation rates are measured using time-resolved fluorescence techniques. Amicroscopic model of the thermal broadening in the excited and ground states of the SiV- centre is developed. A vibronic process involving single-phonon transitions is found to determine orbital relaxation rates for both the ground and the excited states at cryogenic temperatures.Wediscuss the implications of our findings for coherence of qubits in the ground states and propose methods to extend coherence times of SiV- qubits.

Luminescence properties of nitrogen-vacancy (NV) and silicon-vacancy (SiV) centers were investigated for the series of O-terminated composite nanodiamonds consisting of a high-pressure¿high-temperature (HPHT) diamond core and a chemical-vapor-deposition (CVD) diamond outer layer of different thickness. It was found that emission of NV and SiV centers cease to ¿feel¿ the diamond surface at a distance of 12 and 4 nm, respectively, from it. This finding determines minimum sizes of O-terminated nanodiamonds in which stable single photon emitters could be formed based on NV and SiV centers. Suggested composite diamond nanostructure are optimal for design of two-color luminescent markers. The studied diamond nanoparticles present composite structure of ¿core-outer layer¿ type. The core is 20 nm HPHT diamond containing NV centers, whereas outer diamond layer containing SiV centers is formed by CVD synthesis.

With optical/electronic devices of the next generation in mind, we provide a guideline for the growth of homoepitaxial diamond films that possess higher crystalline quality, higher chemical purity, and a higher carbon isotopic ratio. A custom-built microwave plasma-assisted chemical vapor deposition system was constructed to achieve these requirements. To improve both the purity and crystalline quality of homoepitaxial diamond films, an advanced growth condition was applied: higher oxygen concentration in the growth ambient. Under this growth condition for high-quality diamond, a thick diamond film of =30 µm was deposited reproducibly while maintaining high purity and a flat surface. Then, combining this advanced growth condition for non-doped diamond with a unique doping technique that provides parts-per-billion order doping, single-color centers of either nitrogen-vacancy or silicon-vacancy centers that show excellent properties were formed. The new idea of using these color centers as a probe for detecting tiny amounts of impurities was presented. These advanced growth and characterization techniques are expected to open up new fields of diamond research that require extremely low-impurity concentration, for use in power devices and quantum information devices.

Atomic-sized fluorescent defects in diamond are widely recognized as a promising solid state platform for quantum cryptography and quantum information processing. For these applications, single photon sources with a high intensity and reproducible fabrication methods are required. In this study, we report a novel color center in diamond, composed of a germanium (Ge) and a vacancy (V) and named the GeV center, which has a sharp and strong photoluminescence band with a zero-phonon line at 602 nm at room temperature. We demonstrate this new color center works as a single photon source. Both ion implantation and chemical vapor deposition techniques enabled fabrication of GeV centers in diamond. A first-principles calculation revealed the atomic crystal structure and energy levels of the GeV center.

We demonstrate that silicon-vacancy (SiV) centers in diamond can be used to efficiently generate coherent optical photons with excellent spectral properties. We show that these features are due to the inversion symmetry associated with SiV centers. The generation of indistinguishable single photons from separated emitters at 5 K is demonstrated in a Hong-Ou-Mandel interference experiment. Prospects for realizing efficient quantum network nodes using SiV centers are discussed.

The silicon-vacancy (SiV-) color center in diamond has attracted attention because of its unique optical properties. It exhibits spectral stability and indistinguishability that facilitate efficient generation of photons capable of demonstrating quantum interference. Here we show optical initialization and readout of electronic spin in a single SiV- center with a spin relaxation time of T1=2.4±0.2ms. Coherent population trapping (CPT) is used to demonstrate coherent preparation of dark superposition states with a spin coherence time of T2=35±3ns. This is fundamentally limited by orbital relaxation, and an understanding of this process opens the way to extend coherence by engineering interactions with phonons. Hyperfine structure is observed in CPT measurements with the Si29 isotope which allows access to nuclear spin. These results establish the SiV- center as a solid-state spin-photon interface.

The negatively charged silicon-vacancy (SiVâ) center in diamond is a promising single-photon source for quantum communications and information processing. However, the center's implementation in such quantum technologies is hindered by contention surrounding its fundamental properties. Here we present optical polarization measurements of single centers in bulk diamond that resolve this state of contention and establish that the center has a â¿©111â¿ aligned split-vacancy structure with D3d symmetry. Furthermore, we identify an additional electronic level and evidence for the presence of dynamic Jahn-Teller effects in the center's 738-nm optical resonance.

The silicon-vacancy centre (SiV-) in diamond has exceptional spectral properties for single-emitter quantum information applications. Most of the fluorescence is concentrated in a strong zero phonon line (ZPL), with a weak phonon sideband extending for 100 nm that contains several clear features. We demonstrate that the ZPL position can be used to reliably identify the silicon isotope present in a single SiV- centre. This is of interest for quantum information applications since only the 29Si isotope has nuclear spin. In addition, we show that the sharp 64 meV phonon peak is due to a local vibrational mode of the silicon atom. The presence of a local mode suggests a plausible origin of the measured isotopic shift of the ZPL.

Among promising color centers for single-photon sources in diamond, the negatively charged silicon-vacancy (SiV%) has 70% of its emission to the zero-phonon line (ZPL), in contrast to the negatively charged nitrogen vacancy (NV-), which has a broad spectrum. Fabricating single centers of useful defect complexes with high yield and excellent grown-in defect properties by ion implantation has proven to be challenging. We have fabricated bright single SiV- centers by 60-keV focused ion beam implantation and subsequent annealing at 1000 °C with high positioning accuracy and a high yield of 15%.

Emitters of indistinguishable single photons are crucial for the growing field of quantum technologies. To realize scalability and increase the complexity of quantum optics technologies, multiple independent yet identical single-photon emitters are required. However, typical solid-state single-photon sources are inherently dissimilar, necessitating the use of electrical feedback or optical cavities to improve spectral overlap between distinct emitters. Here we demonstrate bright silicon vacancy (SiV') centres in low-strain bulk diamond, which show spectral overlap of up to 91% and nearly transform-limited excitation linewidths. This is the first time that distinct single-photon emitters in the solid state have shown intrinsically identical spectral properties. Our results have impact on the application of single-photon sources for quantum optics and cryptography. © 2014 Macmillan Publishers Limited. All rights reserved.

We report a study of the E3 excited-state structure of single negatively charged nitrogen-vacancy (NV) defects in diamond, combining resonant excitation at cryogenic temperatures and optically detected magnetic resonance. A theoretical model is developed and shows excellent agreement with experimental observations. In addition, we show that the two orbital branches associated with the E3 excited state are averaged when operating at room temperature. This study leads to an improved physical understanding of the NV defect electronic structure, which is invaluable for the development of diamond-based quantum information processing. © 2009 The American Physical Society.

The optical transition linewidth and emission polarization of single nitrogen-vacancy (NV) centers are measured from 5 K to room temperature. Interexcited state population relaxation is shown to broaden the zero-phonon line and both the relaxation and linewidth are found to follow a T5 dependence for T<100K. This dependence indicates that the dynamic Jahn-Teller effect is the dominant dephasing mechanism for the NV optical transitions at low temperatures. © 2009 The American Physical Society.

Using pulsed optically detected magnetic resonance techniques, we directly probe electron-spin resonance transitions in the excited-state of single nitrogen-vacancy (NV) color centers in diamond. Unambiguous assignment of excited state fine structure is made, based on changes of NV defect photoluminescence lifetime. This study provides significant insight into the structure of the emitting 3E excited state, which is invaluable for the development of diamond-based quantum information processing. © IOP Publishing Ltd and Deutsche Physikalische Gesellschan.

The emission intensity of diamond samples containing negatively charged nitrogen-vacancy centres are measured as a function of magnetic field along the <111> direction for various temperatures. At low temperatures the responses are sample and stress dependent and can be modelled in terms of the previous understanding of the 3E excited state fine structure which is strain dependent. At room temperature the responses are largely sample and stress independent, and modelling involves invoking a strain independent excited state with a single zero field spin-level splitting of 1.42 GHz. The change in behaviour is attributed to a temperature dependent averaging process over the components of the excited state orbital doublet. It decouples orbit and spin and at high temperature the spin levels become independent of any orbit splitting. One significant implication of this averaging is that it simplifies the development of room temperature applications. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

An emission band in the infrared (IR) is shown to be associated with a transition within the negative nitrogen-vacancy centre in diamond. The band has a zero-phonon line at 1046 nm, and uniaxial stress and magnetic field measurements indicate that the emission is associated with a transition between 1E and 1A 1 singlet levels. Inter-system crossing to these singlets causes the spin polarization that makes the NV - centre attractive for quantum information processing, and the IR emission band provides a new avenue for using the centre in such applications. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Future CMOS will require the placement of dopants in a predetermined location, namely, a single atom control. Deterministic doping method, i.e. single-ion implantation, realizes ordered arrays of single-atoms in silicon, diamond and other materials, which might provide opportunities to single-dopant transport or single-photon source beneficial to quantum processing.

The understanding of the coherence properties of photons emitted from negatively charged nitrogen-vacancy (NV) centers in diamond is essential for the success of quantum information applications based on indistinguishable photons. Here we study both the polarization of photons emitted from and the linewidth of photons absorbed by single NV centers as a function of temperature T. We find that for T < 100 K the main dephasing mechanism contributing to the linewidth broadening is phonon-mediated population transfer between the two excited orbital states. The observed T5 temperature dependence of the population transfer rate and linewidth is experimental evidence of a dynamic Jahn-Teller effect in the excited states. © 2010 Copyright SPIE - The International Society for Optical Engineering.

The object is to summarise our understanding of the negative nitrogen-vacancy center in diamond and also to highlight difficulties with current models. © 2010 Copyright SPIE - The International Society for Optical Engineering.

The negative nitrogen-vacancy centre in diamond is used to illustrate electromagnetically induced transparency features within a coherent hole. A hole is created in an inhomogeneously broadened electron spin transition and its spectrum modified by driving hyperfine transitions. The modified spectrum is discussed in terms of dressed states of the system, and the spectrum is shown to be in good correspondence with numerical solutions of the density matrix for the doubly driven system. © 2009 Elsevier B.V.

Under optical illumination in the blue or green, negatively charged nitrogen vacancy centres in diamond emit in the red. The intensity of this emission varies slightly depending on spin state occupation. Optical transitions occur predominantly without change of spin projection. However, excited m s = ±1 states can decay non-radiatively to the ground m s = 0 state via intermediary singlet states.With continuous excitation, this effect transfers most of the population to the ms = 0 state resulting in decay becoming almost entirely radiative so that optical emission is stronger than when all spin states are occupied equally. However, under optical illumination the ms = 0 polarization is reduced when an applied magnetic field induces avoided crossings between energy levels. The spin states are mixed and some population is diverted into the ms = ±1 states with consequent reduction in optical emission. The change of emission can be calculated from a rate equation model involving the spin states of the ground and excited levels plus one singlet level [5]. The spin states of the excited levels are also affected by strain and in this work we calculate the variation in optical emission with changing magnetic field for various fixed values of strain.

Infrared emission has been observed for the nitrogen-vacancy (NV -) colour centre in diamond, in addition to the characteristic red emission of this centre [1]. However, this infrared emission zero-phonon line at 1046 nm is about four orders of magnitude weaker than the red emission. This is somewhat surprising, as a third of the population is known to decay without the red emission and the infrared emission is considered to be associated with the alternative decay path. The most obvious explanation is a competing efficient non-radiative decay. There are few reports of diamond emitting at wavelengths longer than 1000 nm. Colour centres in diamond have strong electron-phonon coupling and, of course, the phonon energies in diamond are high. These properties suggest that non-radiative decay could dominate the transition whenever the electronic energy lies in the infrared. This would be a general phenomena and would account for the weak IR emission observed for this centre. It would also account for why there are so few reports of emission from diamond in the infrared.

Polarization and photoluminescence excitation spectroscopy are used to measure the nitrogenvacancy center optical transition polarization and linewidth as a function of temperature. Finite relaxation and line-broadening is observed even at temperatures below 25 K. © 2008 Optical Society of America.