arXiv subjects

Alexander Kubanek

Publications and source records attributed to Alexander Kubanek.

At least 19 recordsLinked to original sources

arXiv2026

Integration of hBN Single-Photon Emitters into a Hybrid Optomechanical Membrane-in-the-Middle Fiber-Cavity

The integration of membranes into optical resonators plays a key role in a variety of applications, including optomechanics. Membranes hosting single photon emitters, ideally with access to spin states, open new avenues in optomechanics, spin-mechanics and spin-optomechanics. Hexagonal boron nitride is among the most promising two-dimensional materials, showing excellent optical and mechanical properties combined with the ability to host optically active (spin) defects. The deterministic creation of optically active defect centers in hexagonal boron nitride membranes and their coupling to optomechanical systems is an outstanding challenge. Here, we explore an alternative hybrid approach to establish coupling between a single photon emitter in commercially available hexagonal boron nitride flakes and a fiber cavity mode. We address technical challenges, such as scattering losses arising from uncontrolled flake topography, and establish deterministic hexagonal boron nitride positioning on the cavity mirror. For the coupled system, we observe cavity-induced spectral enhancement by a factor of up to 100 at room temperature. We extend our work by positioning single photon emitters in hexagonal boron nitride flakes on a highly strained silicon nitride membrane in a membrane-in-the-middle configuration. The mechanical vibrational modes of the silicon nitride membrane and cavity-coupled emission from the single photon emitter are simultaneously observed. Our work is a first step towards the realization of a cavity optomechanical platform with an incorporated single photon emitter and provides a starting point to explore hybrid spin-optomechanics.

physics.optics↗

arXiv2026

Investigating Spectral Dynamics and Spin Signatures of a Mechanically Isolated Quantum Emitter in hBN

Mechanically isolated defect centers in hexagonal boron nitride are promising coherent quantum emitters, yet spectral instabilities persist, and their spin-related nature remains unclear. Here we investigate a single mechanically isolated quantum emitter in hBN integrated onto a coplanar waveguide. The emitter exhibits exceptionally bright resonant fluorescence with saturation count rates exceeding $10\,\mathrm{Mc/s}$. High-resolution spectroscopy reveals two closely spaced zero-phonon-line transitions originating from the same defect complex. Time-resolved spectroscopy shows that these transitions exhibit markedly different spectral diffusion dynamics, consistent with distinct donor-acceptor-pair-like recombination pathways with different sensitivities to local electrostatic fluctuations. Off-resonant blue illumination redistributes emission between the two transitions and increases the emission duty cycle without significantly modifying the dominant spectral diffusion rates at low temperature, indicating repumping from long-lived shelving states. Magnetic-field-dependent photoluminescence, optically detected magnetic resonance, and pump-probe measurements reveal millisecond-scale relaxation dynamics and magnetic-field-dependent fluorescence contrast, demonstrating spin-dependent population dynamics in the metastable shelving state. These results clarify how charge-driven spectral fluctuations and spin-dependent shelving jointly shape the optical cycling dynamics.

quant-ph↗

arXiv2026

Practical quantum tokens: challenges and perspectives

The concept of quantum tokens dates back alongside quantum cryptography to Stephen Wiesner's seminal work in 1983[1]. Already this initial work proposes society-relevant applications such as secure quantum banknotes, which can be exchanged between a bank and a customer. This quantum currency is based on various physical states that can be easily verified but is protected from being copied by the fundamental quantum laws. Four decades later, these ideas have flourished in the field of quantum information, and the concept of quantum banknotes has not only adopted many varying names, such as quantum money, quantum coins, quantum-digital payments, and quantum tokens, but also reached its first experimental demonstrations. In this perspective article, we discuss the current state-of-the-art of quantum tokens in the field of quantum information, as well as their future perspectives. We present a number of physical realizations of quantum tokens with integrated quantum memories and their applicability scenarios in detail. Finally, we discuss how quantum tokens fit into the information security ecosystem and consider their relationship to post-quantum cryptography.

quant-ph↗

arXiv2026

A Hybrid Jump-Diffusion Model for Coherent Optical Control of Quantum Emitters in hBN

Hexagonal boron nitride (hBN) has emerged as a promising two-dimensional host for stable single-photon emission owing to its wide bandgap, high photostability, and compatibility with nanophotonic integration. We present a simulation-based study of temperature-dependent spectral dynamics and optical coherence in a mechanically decoupled quantum emitter in hBN. Employing a hybrid stochastic framework that combines Ornstein--Uhlenbeck detuning fluctuations with temperature-dependent, Gaussian-distributed discrete frequency jumps, motivated by experimentally observed spectral diffusion and blinking, we reproduce the measured evolution of inhomogeneous linewidth broadening and the progressive degradation of photon coherence across the relevant cryogenic range (5-30K). The model captures phonon-related spectral diffusion with a cubic temperature dependence and the onset of jump-like spectral instabilities at higher temperatures. By calibrating the hybrid diffusion, jump parameters to the experimentally measured full width at half maximum (FWHM) of the emission line and analyzing the second-order correlation function $g^{(2)}(\tau)$ under resonant driving, we establish a unified phenomenological description that links stochastic detuning dynamics to the decay of optical coherence in a resonantly driven emitter. Analysis of $g^{(2)}(\tau)$ under resonant driving reveals an additional dephasing rate $\gamma_{\mathrm{sd+j}}$ that rises monotonically with temperature and drive strength, leading to a predicted critical crossover to overdamped dynamics at $T_{\mathrm{crit}} \approx 25.91$~K. This hybrid framework provides a quantitative connection between accessible spectroscopic observables and the dominant noise mechanisms limiting coherent optical control in mechanically decoupled quantum emitters, exemplified in hBN and generalizable to similar emitters in other materials.

quant-ph↗

arXiv2025

Extracting Membrane-like hexagonal Boron Nitride hosting single Defect Centers for Resonator Integration

The integration of membranes into optical resonators plays a key role in a variety of applications, including optomechanics. If such membranes host atom-like systems, ideally with access to spin states, new roads in quantum photonics and also in optomechanics can be taken. Layered, two-dimensional materials have emerged as candidates for membranes hosting atom-like quantum emitters. Hexagonal boron nitride (hBN) is among the most promising two-dimensional platforms showing good mechanical properties combined with the ability to host various kinds of optical active (spin-) defects. However, the determinisitc creation of optically active defect centers in hBN membranes is an outstanding challenge. Commercially available flakes of hBN host defect centers with promising optical properties, but the integration into optical resonators suffers from scattering losses due to the flakes topography and suitable transfer, handling and manipulation techniques need to be established. Here, we develope a toolset of nano-scaled manipulation techniques to extract membrane-like structures of commercially-available hBN containing spectrally narrow single photon emitters. We demonstrate the transfer and integration into photonic devices, by coupling a single photon emitter in membran-like hBN to the mode of an open Fabry-Perot fiber cavity (FPFC) and observe cavity induced spectral enhancement by a factor of up to 100 at room temperature. Overcoming hBN-induced scattering for extracted hBN membranes, which host single photon emitters, paves the way for future applications such as its use as an optomechanical system.

physics.optics↗

arXiv2025

Bipartite entanglement in a nuclear spin register mediated by a quasi-free electron spin

Quantum networks will rely on photons entangled to robust, local quantum registers for computation and error correction. We demonstrate control of and entanglement in a fully connected three-qubit $^{13}\mathrm{C}$ nuclear spin register in diamond. The register is coupled to a quasi-free electron spin-1/2 of a silicon-vacancy center (SiV). High strain decouples the SiVs electron spin from spin-orbit interaction reducing the susceptibility to phonons at liquid helium temperature. As a result, the electron spin lifetime of hundreds of milli seconds enables sensing of nuclear-nuclear couplings down to few hertz. To detect and control the register we leverage continuous decoupling using shaped, low-power microwave and direct radio frequency driving. Furthermore, we implement a nuclear spin conditional phase-gate on the electron spin to mediate bipartite entanglement. This approach presents an alternative to dynamically decoupled nuclear spin entanglement, not limited by the electron spin's 1/2 nature, opening up new avenues to an optically-accessible, solid-state quantum register.

quant-ph↗

arXiv2025

Tunable cavity coupling of a single SnV$^{-}$ center in nanodiamond across bad-emitter and bad-cavity regimes

Efficient coupling between quantum emitters and optical cavities is essential for scalable quantum photonic technologies. Group IV vacancy centers in diamond, particularly the negatively charged tin-vacancy center, have emerged as promising candidates due to their spectral stability, high Debye-Waller factor and large orbital splitting in ground-states. Here, we demonstrate controlled couplinag of a single negatively charged tin vacancy center hosted in a nanodiamond to a fully tunable Fabry-Perot microcavity. At cryogenic temperatures, we access the weak coupling regime and observe a transition from the bad-emitter to the bad-cavity regime as the optical transition of the color center narrows. At 4 K, a Purcell factor exceeding 1.7 is measured, confirming cavity-enhanced emission. The obtained results demonstrate the potential of SnV$^{-}$ centers in nanodiamonds as a coherent single-photon source for quantum networks.

quant-ph↗

arXiv2025

Fabrication of Customized, Low-Loss Optical Resonators by Combination of FIB-Milling and CO$_2$ Laser Ablation

Fabry-Perot cavities are essential tools for applications like precision metrology, optomechanics and quantum technologies. A major challenge is the creation of microscopic spherical mirror structures which allow the precise matching with the wavefront of a Gaussian beam, while providing high surface quality. We present a novel fabrication technique, enabling the creation of customized microscopic cavity mirror structures over a wide range of geometrical parameters, by combining focused ion beam milling (FIB) and CO$_2$ laser smoothing. While FIB milling allows us to imprint features on the mirror substrate with a resolution on the nanometer scale, the application of defocused CO$_2$ laser pulses consistently reduces remaining surface deformations down to a roughness of $σ_\text{RMS}=0.2\,$nm. The average deviation of the profile from a spherical shape is kept below a few nanometres. This technique enables the customized and repeatable fabrication of low loss optics on a wide range of optical substrates, including optical fibres. Thus, Fabry-Perot cavities can be fabricated with pre-defined modal volume, high finesse and tailored ellipticity. Since the structural shape of the mirror is created by FIB milling, its pixel-by-pixel nature of the writing process can be used to create arbitrary structures with a resolution given by the FIB milling. At the same time, the surface quality is given by the CO$_2$ laser smoothing processes, without degrading the FIB milled shape.

physics.optics↗

arXiv2025

Creation of Negatively Charged GeV and SnV centers in Nanodiamonds via Ion Implantation

Solid state quantum emitters, in particular group-IV vacancy centers in diamond, are at the forefront of research in quantum technologies due to their unique optical and spin properties. Reduction of the diamond host size to the nanoscale enables new opportunities in terms of integration and scalability. However, creating optically coherent quantum emitters in nanodiamonds remains a major challenge. Here, we present the fabrication of germanium- and tin- vacancy centers by means of ion implantation. We describe the fabrication process and present the optical properties of the created color centers. We achieve high purity single photon emission via resonant excitation and strong coherent drive of a SnV$^-$ center. The successful integration of heavier group-IV vacancy centers in nanodiamonds paves the way for further advances in fields like hybrid quantum photonics or sensing on the nanoscale.

quant-ph↗

arXiv2025

Vector Magnetometry Using Shallow Implanted NV Centers in Diamond with Waveguide-Assisted Dipole Excitation and Readout

On-chip magnetic field sensing with Nitrogen-Vacancy (NV) centers in diamond requires scalable integration of 3D waveguides into diamond substrates. Here, we develop a sensing array device with an ensemble of shallow implanted NV centers integrated with arrays of laser-written waveguides for excitation and readout of NV signals. Our approach enables an easy-to-operate on-chip magnetometer with a pixel size proportional to the Gaussian mode area of each waveguide. The performed continuous wave optically detected magnetic resonance on each waveguide gives an average dc-sensitivity value of $195 \pm 3 {nT}/\sqrt{Hz}$, which can be improved with lock-in-detection or pulsed-microwave sequences. We apply a magnetic field to separate the four NV crystallographic orientations of the magnetic resonance and then utilize a DC current through a straight wire antenna close to the waveguide to prove the sensor capabilities of our device. We reconstruct the complete vector magnetic field in the NV crystal frame using three different NV crystallographic orientations. By knowing the polarization axis of the waveguide mode, we project the magnetic field vector into the lab frame.

quant-ph↗

arXiv2024

Ultra-high strained diamond spin register with coherent optical link

Solid-state spin defects, such as color centers in diamond, are among the most promising candidates for scalable and integrated quantum technologies. In particular, the good optical properties of silicon-vacancy centers in diamond combined with naturally occurring and exceptionally coherent nuclear spins serve as a building block for quantum networking applications. Here, we show that leveraging an ultra-high strained silicon-vacancy center inside a nanodiamond allows us to coherently and efficiently control its electron spin, while mitigating phonon-induced dephasing at liquid helium temperature. Moreover, we indirectly control and characterize a 13C nuclear spin and establish a quantum register. We overcome limited nuclear spin initialization by implementing single-shot nuclear spin readout. Lastly, we demonstrate coherent optical control with GHz rates, thus connecting the register to the optical domain. Our work paves the way for future integration of quantum network registers into conventional, well-established photonics and hybrid quantum communication systems.

quant-ph↗

arXiv2024

Determining Strain Components in a Diamond Waveguide from Zero-Field ODMR Spectra of NV$^{-}$ Center Ensembles

The negatively charged nitrogen-vacancy (NV$^{-}$) center in diamond has shown great potential in nanoscale sensing and quantum information processing due to its rich spin physics. An efficient coupling with light, providing strong luminescence, is crucial for realizing these applications. Laser-written waveguides in diamond promote NV$^{-}$ creation and improve their coupling to light but, at the same time, induce strain in the crystal. The induced strain contributes to light guiding but also affects the energy levels of NV$^{-}$ centers. We probe NV$^{-}$ spin states experimentally with the commonly used continuous-wave zero-field optically detected magnetic resonance (ODMR). In our waveguides, the ODMR spectra are shifted, split, and consistently asymmetric, which we attribute to the impact of local strain. To understand these features, we model ensemble ODMR signals in the presence of strain. By fitting the model results to the experimentally collected ODMR data, we determine the strain tensor components at different positions, thus determining the strain profile across the waveguide. This shows that zero-field ODMR spectroscopy can be used as a strain imaging tool. The resulting strain within the waveguide is dominated by a compressive axial component transverse to the waveguide structure, with a smaller contribution from vertical and shear strain components.

cond-mat.mes-hall↗

arXiv2024

Cavity-Enhanced Emission and Absorption of Color Centers in a Diamond Membrane With Selectable Strain

Group IV color centers in diamond are among the most promising optically active spin systems with strong optical transitions and long spin coherences. The ground-state splitting of the center is particularly important to suppress the interaction with coherence-limiting phonons, which improves the coherence properties and sets the upper limit for the operating temperature. Negatively charged silicon-vacancy centers have an ordinary ground-state splitting of only 48GHz, resulting in required temperatures below one Kelvin, which can only be achieved by dilution refrigerators. Here, we increase the ground-state splitting by up to an order of magnitude by induced strain in a single-crystal diamond membrane. Furthermore, we demonstrate cavity-assisted spectroscopy enabled by coupling the emitter ensemble with a selectable strain to the mode of a Fabry-Perot microcavity. Calculation of the absorption cross-section yields $σ_{ens} = $4.9*10^-11 cm^2. Together with the Purcell-enhanced twofold reduction in emitter lifetime below 1ns, this makes the system a promising spin-photon interface at moderate temperatures of 4K.

quant-ph↗

arXiv2024

Laser-written waveguide-integrated coherent spins in diamond

Quantum emitters, such as the negatively charged nitrogen-vacancy center in diamond, are attractive for quantum technologies such as nano-sensing, quantum information processing, and as a non-classical light source. However, it is still challenging to position individual emitters in photonic structures whilst preserving the spin coherence properties of the defect. In this paper, we investigate single and ensemble waveguide-integrated nitrogen-vacancy centers in diamond fabricated by femtosecond laser writing followed by thermal annealing. Their spin coherence properties are systematically investigated and are shown to be comparable to native nitrogen-vacancy centers in diamond. This method paves the way for the fabrication of coherent spins integrated within photonic devices.

quant-ph↗

arXiv2024

Strongly Coupled Spins of Silicon-Vacancy Centers Inside a Nanodiamond with Sub-Megahertz Linewidth

The search for long-lived quantum memories, which can be efficiently interfaced with flying qubits is longstanding. One possible solution is to use the electron spin of a color center in diamond to mediate interaction between a long-lived nuclear spin and a photon. Realizing this in a nanodiamond furthermore facilitates the integration into photonic devices and enables the realization of hybrid quantum systems with access to quantum memories. Here, we investigated the spin environment of negatively-charged Silicon-Vacancy centers in a nanodiamond and demonstrate strong coupling of its electron spin, while the electron spin's decoherence rate remained below 1 MHz. We furthermore demonstrate multi-spin coupling with the potential to establish registers of quantum memories in nanodiamonds.

cond-mat.mes-hall↗

arXiv2023

Limits for coherent optical control of quantum emitters in layered materials

The coherent control of a two-level system is among the most essential challenges in modern quantum optics. Understanding its fundamental limitations is crucial, also for the realization of next generation quantum devices. The quantum coherence of a two level system is fragile in particular, when the two levels are connected via an optical transition. When such quantum emitters are located in solids the coherence suffers from the interaction of the optical transition with the solid state environment, which requires the sample to be cooled to temperatures of a few Kelvin or below. Here, we use a mechanically isolated quantum emitter in hexagonal boron nitride to explore the individual mechanisms which affect the coherence of an optical transition under resonant drive. We operate the system at the threshold where the mechanical isolation collapses in order to study the onset and temperature-dependence of dephasing and independently of spectral diffusion. The new insights on the underlying physical decoherence mechanisms reveals a limit in temperature until which coherent driving of the system is possible. This study enables to increase the operation temperature of quantum devices, therefore reducing the need for cryogenic cooling.

quant-ph↗

arXiv2023

Enhanced Spectral Density of a Single Germanium Vacancy Center in a Nanodiamond by Cavity-Integration

Color centers in diamond, among them the negatively-charged germanium vacancy (GeV$^-$), are promising candidates for many applications of quantum optics such as a quantum network. For efficient implementation, the optical transitions need to be coupled to a single optical mode. Here, we demonstrate the transfer of a nanodiamond containing a single ingrown GeV- center with excellent optical properties to an open Fabry-Pérot microcavity by nanomanipulation utilizing an atomic force microscope. Coupling of the GeV- defect to the cavity mode is achieved, while the optical resonator maintains a high finesse of F = 7,700 and a 48-fold spectral density enhancement is observed. This article demonstrates the integration of a GeV- defect with a Fabry-Pérot microcavity under ambient conditions with the potential to extend the experiments to cryogenic temperatures towards an efficient spin-photon platform.

physics.optics↗

arXiv2023

Controlling all Degrees of Freedom of the Optical Coupling in Hybrid Quantum Photonics

Nanophotonic quantum devices can significantly boost light-matter interaction which is important for applications such as quantum networks. Reaching a high interaction strength between an optical transition of a spin system and a single mode of light is an essential step which demands precise control over all degrees of freedom of the optical coupling. While current devices have reached a high accuracy of emitter positioning, the placement process remains overall statistically, reducing the device fabrication yield. Furthermore, not all degrees of freedom of the optical coupling can be controlled limiting the device performance. Here, we develop a hybrid approach based on negatively-charged silicon-vacancy center in nanodiamonds coupled to a mode of a Si$_3$N$_4$-photonic crystal cavity, where all terms of the coupling strength can be controlled individually. We use the frequency of coherent Rabi-oscillations and line-broadening as a measure of the device performance. This allows for iterative optimization of the position and the rotation of the dipole with respect to individual, preselected modes of light. Therefore, our work marks an important step for optimization of hybrid quantum photonics and enables to align device simulations with real device performance.

quant-ph↗