Dressed states of a strain-driven spin in diamond

Emerging quantum technologies, such as quantum information processing and quantum metrology, require quantum systems that provide reliable toolsets for initialization, readout, and coherent manipulation as well as long coherence times. The coherence of these systems, however, is usually limited by uncontrolled interactions with the surrounding environment. In particular, innovations building on solid-state spin systems like the Nitrogen-Vacancy (NV) center in diamond ordinarily involve the use of magnetic field-sensitive states. In this case, ambient magnetic field fluctuations constitute a serious impediment that shortens the coherence time considerably. Thus, the protection of individual quantum systems from environmental perturbations constitutes a fundamentally important but also a challenging task for the further development of quantum appliances. In this thesis, we address this challenge by extending the widely used approach of dynamical decoupling to protect a quantum system from decoherence. Specifically, we study three-level dressed states that emerge under continuous, `closed-contour' interaction driving. To implement and investigate these dressed states, we exploit well-established methods for coherent microwave and strain manipulation of the NV center spin in a hybrid spin-mechanical system. Our results reveal that this novel continuous decoupling mechanism can overcome external magnetic fluctuations in a robust way. We demonstrate experimentally that the dressed states we created are long-lived and find coherence times nearly two orders of magnitude longer than the inhomogeneous dephasing time of the NV spin, even for moderate driving strengths. To realize direct and efficient access to the coherence-protected dressed states under closed-contour driving, we further demonstrate the use of state transfer protocols for their initialization and readout. In addition to an adiabatic approach, we apply recently developed protocols based on `shortcuts to adiabaticity' to accomplish the initialization process, which ultimately accelerates the transfer speed by a factor of $2.6$ compared to the fastest adiabatic protocol with similar fidelity. Moreover, we show bidirectionality of the accelerated state transfer, which allows us to directly read out the dressed state population and to quantify the transfer fidelity of $\approx$$\,99\,\%$. By employing the methods to prepare and read out the dressed states, we lay the foundation to meet the remaining key requirement for quantum systems -- coherent quantum control. We present direct, coherent manipulation of the dressed states in their own manifold and exploit this for extensive characterization of the dressed states' properties. Thus, our results constitute an elementary step to establish the dressed states as a powerful resource in prospective quantum sensing applications. Harnessing quantum systems like the dressed states as nanoscale sensors of external fields requires the detailed characterization of the local internal environment. In the final part of this thesis, we report on the determination of intrinsic effective fields of individual NV center spins. We study single NVs in high purity diamond and find that local strain dominates over local electric fields. In addition, we experimentally demonstrate and theoretically describe a new technique for performing single spin-based polarization analysis of microwave fields in a tunable, linear basis

Spin-stress and spin-strain coupling in diamond-based hybrid spin oscillator systems

Hybrid quantum systems, which combine quantum-mechanical systems with macroscopic mechanical oscillators, have attracted increasing interest as they are well suited as high-performance sensors or transducers in quantum computers. A promising candidate is based on diamond cantilevers, whose motion is coupled to embedded Nitrogen-Vacancy (NV) centers through crystal deformation. Even though this type of coupling has been investigated intensively in the past, several inconsistencies exist in available literature, and no complete and consistent theoretical description has been given thus far. To clarify and resolve these issues, we here develop a complete and consistent formalism to describe the coupling between the NV spin degree of freedom and crystal deformation in terms of stress, defined in the crystal coordinate system XYZ, and strain, defined in the four individual NV reference frames. We find that the stress-based approach is straightforward, yields compact expressions for stress-induced level shifts and therefore constitutes the preferred approach to be used in future advances in the field. In contrast, the strain-based formalism is much more complicated and requires extra care when transforming into the employed NV reference frames. Furthermore, we illustrate how the developed formalism can be employed to extract values for the spin-stress and spin-strain coupling constants from data published by Teissier et al..Comment: 14 pages, 3 figures; SOM available for download under https://quantum-sensing.physik.unibas.ch/publications/research-articles.htm

Finite time St\"uckelberg interferometry with nanomechanical modes

St\"uckelberg interferometry describes the interference of two strongly coupled modes during a double passage through an avoided energy level crossing. In this work, we experimentally investigate finite time effects in St\"uckelberg interference and provide an exact analytical solution of the St\"uckelberg problem. Approximating this solution in distinct limits reveals uncharted parameter regimes of St\"uckelberg interferometry. Experimentally, we study these regimes using a purely classical, strongly coupled nanomechanical two-mode system of high quality factor. The classical two-mode system consists of the in-plane and out-of-plane fundamental flexural mode of a high stress silicon nitride string resonator, coupled via electric gradient fields. The dielectric control and microwave cavity enhanced universal transduction of the nanoelectromechanical system allows for the experimental access to all theoretically predicted St\"uckelberg parameter regimes. We exploit our experimental and theoretical findings by studying the onset of St\"uckelberg interference in dependence of the characteristic system control parameters and obtain characteristic excitation oscillations between the two modes even without the explicit need of traversing the avoided crossing. The presented theory is not limited to classical mechanical two-mode systems but can be applied to every strongly coupled (quantum) two-level system, for example a spin-1/2 system or superconducting qubit

Non-reciprocal coherent dynamics of a single spin under closed-contour interaction

Three-level quantum systems have formed a cornerstone of quantum optics since the discovery of coherent population trapping (CPT) and electromagnetically induced transparency. Key to these phenomena is quantum interference, which arises if two of the three available transitions are coherently driven at well-controlled amplitudes and phases. The additional coherent driving of the third available transition would form a closed-contour interaction (CCI) from which fundamentally new phenomena would emerge, including phase-controlled CPT and one atom interferometry. However, due to the difficulty in experimentally realising a fully coherent CCI, such aspects of three-level systems remain unexplored as of now. Here, we exploit recently developed methods for coherent driving of single Nitrogen-Vacancy (NV) electronic spins to implement highly coherent CCI driving. Our experiments reveal phase-controlled, single spin quantum interference fringes, reminiscent of electron dynamics on a triangular lattice, with the driving field phases playing the role of a synthetic magnetic flux. We find that for suitable values of this phase, CCI driving leads to efficient coherence protection of the NV spin, yielding a nearly two orders of magnitude improvement of the coherence time, even for moderate drive strengths <~1MHz. Our results establish CCI driving as a novel paradigm in coherent control of few-level systems that offers attractive perspectives for applications in quantum sensing or quantum information processing.Comment: 18 pages, 11 figures. Including supplementary material. Comments are welcome. For further information visit https://quantum-sensing.physik.unibas.ch/news.htm

Normas internacionales y adaptación nacional: la terminología de las normas de la Global Reporting Initiative (GRI) en español peninsular y catalán (International standards and national adaptation: the terminology of the GRI in Spanish and Catalan)

Aquest article analitza l&rsquo;exist&egrave;ncia de difer&egrave;ncies (especialment terminol&ograve;giques) en l&rsquo;adaptaci&oacute; dels est&agrave;ndards de sostenibilitat de la Global Reporting Initiative (GRI) entre empreses que publiquen els seus informes en castell&agrave; i en catal&agrave;, aix&iacute; com l&rsquo;&uacute;s&nbsp;de sinon&iacute;mia d&rsquo;unitats l&egrave;xiques especialitzades. Els resultats indiquen que, en aquest &agrave;mbit, les empreses mostren poca propensi&oacute; a modificar un model proposat.Este art&iacute;culo analiza la existencia de diferencias (especialmente terminol&oacute;gicas) en la adaptaci&oacute;n de los est&aacute;ndares de sostenibilidad de la Global Reporting Initiative (GRI) entre empresas que publican sus informes en castellano y en catal&aacute;n, as&iacute; como el uso de sinonimia de unidades l&eacute;xicas especializadas. Los resultados indican que, en este &aacute;mbito, las empresas muestran poca propensi&oacute;n a modificar un modelo propuesto.This article analyzes the existence of differences (especially terminological ones) in the adaptation of the Global Reporting Initiative (GRI) sustainability standards between companies that publish their reports in Spanish and Catalan, as well as the use of synonymy of specialized lexical units. The results indicate that, in this area, companies show little propensity to modify a proposed model

Quasi-Dynamic Estimation of OD Flows From Traffic Counts Without Prior OD Matrix

Bauer D, Richter G, Asamer J, Heilmann B, Lenz G, Kölbl R. Quasi-Dynamic Estimation of OD Flows From Traffic Counts Without Prior OD Matrix. IEEE Transactions on Intelligent Transportation Systems. 2018;19(6):2025-2034.This paper proposes a fully specified statistical model for the quasi-dynamic estimation of origin–destination (OD) flows from traffic counts for highway stretches and networks or for urban areas where the path choice is of minor importance. Hereby, the approach (E. Cascetta et al., Transp. Res. B, Methodol., vol. 55, pp. 171–187, 2013) is extended by eliminating the need for supplying a historic OD matrix. This is done by a combination of least squares estimation for replicating measured link flows with maximum entropy methods to fill in the non-observable part of the distribution across paths. Additionally, it is stressed that the quasi-dynamic assumption of constant path choice proportions over time-of-day-intervals for days of the same day category can be used in order to enhance estimation by including multi-day observations. Jointly one obtains a statistical framework with an explicit estimation algorithm that can be used to test the quasi-dynamic assumption. The approach is demonstrated to provide accurate results in a small-scale simulation study as well as two real-world case studies, one dealing with a highway segment where taxi floating car data provides the true OD flows for the taxis, and the other one dealing with an urban area with a very limited number of alternative paths allowing for explicit path enumeration

Projekt TDSA: Verbesserung der Konstrollsysteme der beiden Dish-Stirling-Solarmodule im Solar Village, Riyadh, Saudi-Arabien Abschlussbericht

With 4 refs., 1 tab.SIGLECopy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Detecting squeezing from the fluctuation spectrum of a driven nanomechanical mode

Squeezing of quantum and classical fluctuations of one of the quadratures of a vibrational mode enables using this quadrature for high precision measurements. Conventionally squeezing is detected by mixing the mode vibrations with a known signal in homodyne detection. In this paper we demonstrate a different approach to revealing and characterizing squeezing. Using a resonantly driven nonlinear nanomechanical resonator with a high quality factor, we show that classical fluctuations about the stable states of forced vibrations are squeezed and that the squeezing can be measured directly by studying the power spectrum of these fluctuations. The measurement does not require any additional signal. Our experimental and theoretical results are in excellent agreement. They directly extend to the quantum domain and demonstrate an unconventional aspect of squeezing.publishe

Spectral evidence of squeezing of a weakly damped driven nanomechanical mode

Because of the broken time-translation symmetry, in periodically driven vibrational systems fluctuations of different vibration components have different intensities. Fluctuations of one of the components are often squeezed, whereas fluctutions of the other component, which is shifted in phase by π/2, are increased. Squeezing is a multi-faceted phenomenon, it attracts much attention from the perspective of high-precision measurements. Here we demonstrate a new and hitherto unappreciated side of squeezing: its direct manifestation in the spectra of driven vibrational systems. With a weakly damped nanomechanical resonator, we study the spectrum of thermal fluctuations of a resonantly driven nonlinear mode. In the attained sideband-resolved regime, we show that the asymmetry of the spectrum directly characterizes the squeezing. This opens a way to deduce squeezing of thermal fluctuations in strongly underdamped resonators, for which a direct determination by a standard homodyne measurement is impeded by frequency fluctuations. The experimental and theoretical results are in excellent agreement. We further extend the theory to also describe the spectral manifestation of squeezing of quantum fluctuations.publishe

Squeezing of thermal fluctuations in a driven nanomechanical resonator

Classical squeezing allows manipulating the noise distribution of thermal fluctuations by suppressing the noise along one of the two quadratures. Squeezing of thermal fluctuations has mostly been studied in parametric amplifiers, including realizations based on microwave [1,2] or mechanical [3,4] resonators as well as trapped ions [5], and is typically detected in a homodyne measurement. Here we show thermal squeezing of a nanomechanical resonator of ultra-high quality factor at room temperature. Squeezing is accomplished by driving the resonator in the nonlinear Duffing regime. Contrary to previous studies [6-8], auxilliary noise injection into the system is not required. Remarkably, the squeezing manifests itself directly in the power spectrum without the need of a homodyne measurement. Classical noise squeezing promises to reduce heating in computers [9], and represents an important asset for precision sensing [5,10,11] enabling the advent of a new generation of nanomechanical detectors at room temperature.publishe