arXiv subjects

Tim Schröder

Publications and source records attributed to Tim Schröder.

At least 37 records · Page 2Linked to original sources

arXiv2024

Ambiguous Resonances in Multipulse Quantum Sensing with Nitrogen Vacancy Centers

Dynamical decoupling multipulse sequences can be applied to solid state spins for sensing weak oscillating fields from nearby single nuclear spins. By periodically reversing the probing system's evolution, other noises are counteracted and filtered out over the total evolution. However, the technique is subject to intricate interactions resulting in additional resonant responses, which can be misinterpreted with the actual signal intended to be measured. We experimentally characterized three of these effects present in single nitrogen vacancy centers in diamond, where we also developed a numerical simulations model without rotating wave approximation, showing robust correlation to the experimental data. Regarding centers with the $^{15}$N nitrogen isotope, we observed that a small misalignment in the bias magnetic field causes the precession of the nitrogen nuclear spin to be sensed by the electronic spin of the center. Another studied case of ambiguous resonances comes from the coupling with lattice $^{13}$C nuclei, where we used the echo modulation frequencies to obtain the interaction Hamiltonian and then utilized the latter to simulate multipulse sequences. Finally, we also measured and simulated the effects from the free evolution of the quantum system during finite pulse durations. Due to the large data volume and the strong dependency of these ambiguous resonances with specific experimental parameters, we provide a simulations dataset with a user-friendly graphical interface, where users can compare simulations with their own experimental data for spectral disambiguation. Although focused with nitrogen vacancy centers and dynamical decoupling sequences, these results and the developed model can potentially be applied to other solid state spins and quantum sensing techniques.

quant-ph↗

arXiv2024

AlGaN/AlN heterostructures: an emerging platform for nonlinear integrated photonics

In the rapidly evolving area of integrated photonics, there is a growing need for materials that satisfy the particular requirements of increasingly complex and specialized devices and applications. Present photonic material platforms have made significant progress over the past years; however, each platform still faces specific material and performance challenges. We introduce a novel material for integrated photonics: Aluminum Gallium Nitride (AlGaN) on Aluminum Nitride (AlN) as a platform for developing reconfigurable and nonlinear on-chip optical systems. AlGaN combines compatibility with standard semiconductor fabrication technologies, high electro-optic modulation capabilities, and large nonlinear coefficients while providing a broad and low-loss spectral transmission range, making it a viable material for advanced photonic applications. In this work, we design and grow AlGaN/AlN heterostructures and integrate fundamental photonic building blocks into these chips. In particular, we fabricate edge couplers, low-loss waveguides, directional couplers, and tunable high-quality factor ring resonators to enable nonlinear light-matter interaction and quantum functionality. The comprehensive platform we present in this work paves the way for nonlinear photon-pair generation applications, on-chip nonlinear quantum frequency conversion, and fast electro-optic modulation for switching and routing classical and quantum light fields.

physics.optics↗

arXiv2024

Efficient Microwave Spin Control of Negatively Charged Group-IV Color Centers in Diamond

In this work, we provide a comprehensive overview of the microwave-induced manipulation of electronic spin states in negatively charged group-IV color centers in diamond with a particular emphasis on the influence of strain. Central to our investigation is the consideration of the full vectorial attributes of the magnetic fields involved, which are a dc field for lifting the degeneracy of the spin levels and an ac field for microwave control between two spin levels. We observe an intricate interdependence between their spatial orientations, the externally applied strain, and the resultant efficacy in spin state control. In most work to date the ac and dc magnetic field orientations have been insufficiently addressed, which has led to the conclusion that strain is indispensable for the effective microwave control of heavier group-IV vacancies, such as tin- and lead-vacancy color centers. In contrast, we find that the alignment of the dc magnetic field orthogonal to the symmetry axis and the ac field parallel to it can make the application of strain obsolete for effective spin manipulation. Furthermore, we explore the implications of this field configuration on the spin's optical initialization, readout, and gate fidelities.

quant-ph↗

arXiv2023

Coherent microwave, optical, and mechanical quantum control of spin qubits in diamond

Diamond has emerged as a highly promising platform for quantum network applications. Color centers in diamond fulfill the fundamental requirements for quantum nodes: they constitute optically accessible quantum systems with long-lived spin qubits. Furthermore, they provide access to a quantum register of electronic and nuclear spin qubits and they mediate entanglement between spins and photons. All these operations require coherent control of the color center's spin state. This review provides a comprehensive overview of the state-of-the-art, challenges, and prospects of such schemes, including, high fidelity initialization, coherent manipulation, and readout of spin states. Established microwave and optical control techniques are reviewed, and moreover, emerging methods such as cavity-mediated spin-photon interactions and mechanical control based on spin-phonon interactions are summarized. For different types of color centers, namely, nitrogen-vacancy and group-IV color centers, distinct challenges persist that are subject of ongoing research. Beyond fundamental coherent spin qubit control techniques, advanced demonstrations in quantum network applications are outlined, for example, the integration of individual color centers for accessing (nuclear) multi-qubit registers. Finally, we describe the role of diamond spin qubits in the realization of future quantum information applications.

quant-ph↗

arXiv2023

Deterministic Creation of Large Photonic Multipartite Entangled States with Group-IV Color Centers in Diamond

Measurement-based quantum computation relies on single qubit measurements of large multipartite entangled states, so-called lattice-graph or cluster states. Graph states are also an important resource for quantum communication, where tree cluster states are a key resource for one-way quantum repeaters. A photonic realization of this kind of state would inherit many of the benefits of photonic platforms, such as very little dephasing due to weak environmental interactions and the well-developed infrastructure to route and measure photonic qubits. In this work, a linear cluster state and GHZ state generation scheme is developed for group-IV color centers. In particular, this article focuses on an in-depth investigation of the required control operations, including the coherent spin and excitation gates. We choose an off-resonant Raman scheme for the spin gates, which can be much faster than microwave control. We do not rely on a reduced level scheme and use efficient approximations to design high-fidelity Raman gates. We benchmark the spin-control and excitation scheme using the tin vacancy color center coupled to a cavity, assuming a realistic experimental setting. Additionally, the article investigates the fidelities of the Raman and excitation gates in the presence of radiative and non-radiative decay mechanisms. Finally, a quality measure is devised, which emphasizes the importance of fast and high-fidelity spin gates in the creation of large entangled photonic states.

quant-ph↗

arXiv2023

Diamond-on-chip infrared absorption magnetic field camera

Integrated and fiber-packaged magnetic field sensors with a sensitivity sufficient to sense electric pulses propagating along nerves in life science applications and with a spatial resolution fine enough to resolve their propagation directions will trigger a tremendous step ahead not only in medical diagnostics, but in understanding neural processes. Nitrogen-vacancy centers in diamond represent the leading platform for such sensing tasks under ambient conditions. Current research on uniting a good sensitivity and a high spatial resolution is facilitated by scanning or imaging techniques. However, these techniques employ moving parts or bulky microscope setups. Despite being far developed, both approaches cannot be integrated and fiber-packaged to build a robust, adjustment-free hand-held device. In this work, we introduce novel concepts for spatially resolved magnetic field sensing and 2-D gradiometry with an integrated magnetic field camera. The camera is based on infrared absorption optically detected magnetic resonance (IRA-ODMR) mediated by perpendicularly intersecting infrared and pump laser beams forming a pixel matrix. We demonstrate our 3-by-3 pixel sensor's capability to reconstruct the position of an electromagnet in space. Furthermore, we identify routes to enhance the magnetic field camera's sensitivity and spatial resolution as required for complex sensing applications.

physics.app-ph↗

arXiv2023

Fabrication of Sawfish photonic crystal cavities in bulk diamond

Color centers in diamond are quantum systems with optically active spin-states that show long coherence times and are therefore a promising candidate for the development of efficient spin-photon interfaces. However, only a small portion of the emitted photons is generated by the coherent optical transition of the zero-phonon line (ZPL), which limits the overall performance of the system. Embedding these emitters in photonic crystal cavities improves the coupling to the ZPL photons and increases their emission rate. Here, we demonstrate the fabrication process of "Sawfish" cavities, a design recently proposed that has the experimentally-realistic potential to simultaneously enhance the emission rate by a factor of 46 and couple photons into a single-mode fiber with an efficiency of 88%. The presented process allows for the fabrication of fully suspended devices with a total length of 20.5 $μ$m and features size as small as 40 nm. The optical characterization shows fundamental mode resonances that follow the behavior expected from the corresponding design parameters and quality (Q) factors as high as 3825. Finally, we investigate the effects of nanofabrication on the devices and show that, despite a noticeable erosion of the fine features, the measured cavity resonances deviate by only 0.9 (1.2)% from the corresponding simulated values. This proves that the Sawfish design is robust against fabrication imperfections, which makes it an attractive choice for the development of quantum photonic networks.

physics.optics↗

arXiv2023

Resource-efficient fault-tolerant one-way quantum repeater with code concatenation

One-way quantum repeaters where loss and operational errors are counteracted by quantum error correcting codes can ensure fast and reliable qubit transmission in quantum networks. It is crucial that the resource requirements of such repeaters, for example, the number of qubits per repeater node and the complexity of the quantum error correcting operations are kept to a minimum to allow for near-future implementations. To this end, we propose a one-way quantum repeater that targets both the loss and operational error rates in a communication channel in a resource-efficient manner using code concatenation. Specifically, we consider a tree-cluster code as an inner loss-tolerant code concatenated with an outer 5-qubit code for protection against Pauli errors. Adopting flag-based stabilizer measurements, we show that intercontinental distances of up to 10,000 km can be bridged with a minimal resource overhead by interspersing repeater nodes that each specializes in suppressing either loss or operational errors. Our work demonstrates how tailored error-correcting codes can significantly lower the experimental requirements for long-distance quantum communication.

quant-ph↗

arXiv2022

'Sawfish' Photonic Crystal Cavity for Near-Unity Emitter-to-Fiber Interfacing in Quantum Network Applications

Photon loss is one of the key challenges to overcome in complex photonic quantum applications. Photon collection efficiencies directly impact the amount of resources required for measurement-based quantum computation and communication networks. Promising resources include solid-state quantum light sources, however, efficiently coupling light from a single quantum emitter to a guided mode remains demanding. In this work, we eliminate photon losses by maximizing coupling efficiencies in an emitter-to-fiber interface. We develop a waveguide-integrated 'Sawfish' photonic crystal cavity and use finite element simulations to demonstrate that our system transfers, with 97.4% efficiency, the zero-phonon line emission of a negatively-charged tin vacancy center in diamond adiabatically to a single-mode fiber. A surrogate model trained by machine learning provides quantitative estimates of sensitivities to fabrication tolerances. Our corrugation-based design proves robust under state-of-the-art nanofabrication parameters, maintaining an emitter-to-fiber coupling efficiency of 88.6%. To demonstrate its potential in reducing resource requirements, we apply the Sawfish cavity to a recent one-way quantum repeater protocol.

quant-ph↗

arXiv2022

Optically coherent nitrogen-vacancy defect centers in diamond nanostructures

Optically active solid-state spin defects have the potential to become a versatile resource for quantum information processing applications. Nitrogen-vacancy defect centers (NV) in diamond act as quantum memories and can be interfaced by coherent photons as demonstrated in entanglement protocols. However, in particular in diamond nanostructures, the effect of spectral diffusion leads to optical decoherence hindering entanglement generation. In this work, we present strategies to significantly reduce the electric noise in diamond nanostructures. We demonstrate single NVs in nanopillars exhibiting lifetime-limited linewidth on the time scale of one second and long-term spectral stability with inhomogeneous linewidth as low as 150 MHz over three minutes. Excitation power and energy-dependent measurements in combination with nanoscopic Monte Carlo simulations contribute to a better understanding of the impact of bulk and surface defects on the NV's spectral properties. Finally, we propose an entanglement protocol for nanostructure-coupled NVs providing entanglement generation rates up to hundreds of kHz.

quant-ph↗

arXiv2021

Optimized diamond inverted nanocones for enhanced color center to fiber coupling

Nanostructures can be used for boosting the light outcoupling of color centers in diamond; however, the fiber coupling performance of these nanostructures is rarely investigated. Here, we use a finite element method for computing the emission from color centers in inverted nanocones and the overlap of this emission with the propagation mode in a single-mode fiber. Using different figures of merit, the inverted nanocone parameters are optimized to obtain maximal fiber coupling efficiency, free-space collection efficiency, or rate enhancement. The optimized inverted nanocone designs show promising results with 66% fiber coupling or 83% free-space coupling efficiency at the tin-vacancy center zero-phonon line wavelength of 619 nm. Moreover, when evaluated for broadband performance, the optimized designs show 55% and 76% for fiber coupling and free-space efficiencies respectively, for collecting the full tin-vacancy emission spectrum at room temperature. An analysis of fabrication insensitivity indicates that these nanostructures are robust against imperfections. For maximum emission rate into a fiber mode, a design with a Purcell factor of 2.34 is identified. Finally, possible improvements offered by a hybrid inverted nanocone, formed by patterning into two different materials, are investigated, and increases the achievable fiber coupling efficiency to 71%.

quant-ph↗

arXiv2021

Hidden Silicon-Vacancy Centers in Diamond

We characterize a high-density sample of negatively charged silicon-vacancy (SiV$^-$) centers in diamond using collinear optical multidimensional coherent spectroscopy. By comparing the results of complementary signal detection schemes, we identify a hidden population of \ce{SiV^-} centers that is not typically observed in photoluminescence, and which exhibits significant spectral inhomogeneity and extended electronic $T_2$ times. The phenomenon is likely caused by strain, indicating a potential mechanism for controlling electric coherence in color-center-based quantum devices.

quant-ph↗

arXiv2021

Coherent Interactions Between Silicon-Vacancy Centers in Diamond

We report coherent interactions within an ensemble of silicon-vacancy color centers in diamond. The interactions are ascribed to resonant dipole-dipole coupling. Further, we demonstrate control over resonant center pairs by using a driving optical pulse to induce collective, interaction-enabled Rabi-oscillations in the ensemble. Non-resonant center pairs do not undergo collective oscillations.

quant-ph↗

arXiv2021

Using silicon-vacancy centers in diamond to probe the full strain tensor

An ensemble of silicon vacancy centers in diamond (\ce{SiV-}) is probed using two coherent spectroscopy techniques. Two main distinct families of \ce{SiV-} centers are identified using multidimensional coherent spectroscopy, and these families are paired with two orientation groups by comparing spectra from different linear polarizations of the incident laser. By tracking the peak centers in the measured spectra, the full diamond strain tensor is calculated local to the laser spot. Such measurements are made at multiple points on the sample surface and variations in the strain tensor are observed.

cond-mat.mtrl-sci↗

arXiv2021

Bright and photostable single-photon emitter in silicon carbide

Single-photon sources are of paramount importance in quantum communication, quantum computation, and quantum metrology. In particular, there is great interest in realizing scalable solid-state platforms that can emit triggered photons on demand to achieve scalable nanophotonic networks. We report on a visible-spectrum single-photon emitter in 4H silicon carbide (SiC). The emitter is photostable at room and low temperatures, enabling photon counts per second in excess of 2$\times$10$^6$ from unpatterned bulk SiC. It exists in two orthogonally polarized states, which have parallel absorption and emission dipole orientations. Low-temperature measurements reveal a narrow zero phonon line (linewidth $<0.1~$nm) that accounts for $>30$% of the total photoluminescence spectrum.

quant-ph↗

arXiv2020

Roadmap for gallium arsenide spin qubits

Gate-defined quantum dots in gallium arsenide (GaAs) have been used extensively for pioneering spin qubit devices due to the relative simplicity of fabrication and favourable electronic properties such as a single conduction band valley, a small effective mass, and stable dopants. GaAs spin qubits are readily produced in many labs and are currently studied for various applications, including entanglement, quantum non-demolition measurements, automatic tuning, multi-dot arrays, coherent exchange coupling, and teleportation. Even while much attention is shifting to other materials, GaAs devices will likely remain a workhorse for proof-of-concept quantum information processing and solid-state experiments.

cond-mat.mes-hall↗

arXiv2020

Picosecond time-resolved antibunching measures nanoscale exciton motion, annihilation, and true number of chromophores

The particle-like nature of light becomes evident in the photon statistics of fluorescence of single quantum systems as photon antibunching. In multichromophoric systems, exciton diffusion and subsequent annihilation occurs. These processes also yield photon antibunching but cannot be interpreted reliably. Here, we develop picosecond time-resolved antibunching (psTRAB) to identify and decode such processes. We use psTRAB to measure the true number of chromophores on well-defined multichromophoric DNA-origami structures, and precisely determine the distance-dependent rates of annihilation between excitons. Further, psTRAB allows us to measure exciton diffusion in mesoscopic H- and J-type conjugated-polymer aggregates. We distinguish between one-dimensional intra-chain and three-dimensional inter-chain exciton diffusion at different times after excitation and determine the disorder-dependent diffusion lengths. Our method provides a new lens through which excitons can be studied at the single-particle level, enabling the rational design of improved excitonic probes such as ultra-bright fluorescent nanoparticles, and materials for optoelectronic devices.

physics.chem-ph↗

arXiv2020

Deterministic positioning of nanophotonic waveguides around single self-assembled quantum dots

The capability to embed self-assembled quantum dots (QDs) at predefined positions in nanophotonic structures is key to the development of complex quantum photonic architectures. Here, we demonstrate that QDs can be deterministically positioned in nanophotonic waveguides by pre-locating QDs relative to a global reference frame using micro-photoluminescence ($μ$PL) spectroscopy. After nanofabrication, $μ$PL images reveal misalignments between the central axis of the waveguide and the embedded QD of only $(9\pm46$) nm and $(1\pm33$) nm, for QDs embedded in undoped and doped membranes, respectively. A priori knowledge of the QD positions allows us to study the spectral changes introduced by nanofabrication. We record average spectral shifts ranging from 0.1 to 1.1 nm, indicating that the fabrication-induced shifts can generally be compensated by electrical or thermal tuning of the QDs. Finally, we quantify the effects of the nanofabrication on the polarizability, the permanent dipole moment and the emission frequency at vanishing electric field of different QD charge states, finding that these changes are constant down to QD-surface separations of only 70 nm. Consequently, our approach deterministically integrates QDs into nanophotonic waveguides whose light-fields contain nanoscale structure and whose group index varies at the nanometer level.

quant-ph↗