arXiv subjects

Niels Gregersen

Publications and source records attributed to Niels Gregersen.

At least 19 recordsLinked to original sources

arXiv2026

Coherent dynamics of individual excitons in a quantum dot embedded in a nanopost

We measured coherent ultrafast dynamics of exciton complexes in a single strongly-confined InAs quantum dot embedded in a GaAs nanopost. Such a photonic structure combines a wave guiding with a cavity effect and assures an enhanced light-matter coupling. Coherence properties of an exciton-biexciton system hosted by a quantum dot are assessed with four-wave mixing microscopy. Our results show that this broad-band photonic structure is an excellent asset to probe coherent couplings in a small set of solid state quantum systems and to investigate the coherence dynamics within the level structure of their excited states.

cond-mat.mes-hall↗

arXiv2026

Industry-ready spin-photon interfaces for hybrid photonic quantum computing

Hybrid photonic quantum computers, combining stationary matter qubits and flying photonic qubits, offer an intrinsically networked and resource-efficient route to large-scale, error-corrected quantum computation. Their core components are cavity-coupled matter qubits that act as light--matter interfaces, enabling: high-efficiency on-demand single-photon generation, stable near-unity photon indistinguishability and spin--multi-photon entanglement. Semiconductor quantum dots in microcavities are a leading platform for realizing such devices. Yet reaching the performance, reproducibility and spin-coherence thresholds for large-scale error correction remains a major challenge requiring industrial fabrication and control. Here we report thousands of monolithic semiconductor quantum-dot devices fabricated using a III--V pilot production-line process compatible with large-scale deployment. Systematic control of source parameters yields state-of-the-art efficiency and supports a path to optical losses below fault-tolerance thresholds. Using field-quadrature state reconstruction as a stringent joint test of efficiency and indistinguishability, we observe near-unity photon quantum purity stable over tens of minutes and a record single-photon Wigner-function negativity. We further demonstrate seven-partite spin--multi-photon entanglement and spin coherence extendable to microsecond timescales in the low-magnetic-field regime. Finally, photons from distant sources are as indistinguishable as photons emitted successively by a single source. These results establish foundry-compatible III--V quantum dots as a scalable platform for hybrid photonic quantum computing.

quant-ph↗

arXiv2025

Integrated on-chip quantum light sources on a van der Waals platform

Scalable photonic quantum information technologies require a platform combining quantum light sources, waveguides, and detectors on a single chip. Here, we introduce a van der Waals platform comprising strain-engineered bilayer WSe$_2$ quantum emitters, integrated on multimode WS$_2$ waveguides with optimized grating couplers, enabling efficient on-chip quantum light sources. The emitters exhibit bright, highly polarized emission that couples efficiently into WS$_2$ waveguides. Under resonant p-shell excitation, we observe high-purity, waveguide-coupled single-photon emission, measured using both an off-chip Hanbury Brown-Twiss configuration ($g^{(2)}(0) = 0.003^{+0.030}_{-0.003}$) and an on-chip configuration ($g^{(2)}(0) = 0.076\pm0.023$). For a single output, the out-coupled single-photon count rate at the first lens reaches approximately 320 kHz under continuous-wave p-shell excitation, corresponding to an estimated waveguide-coupled rate of 1.7 MHz. These results demonstrate an efficient, integrated single-photon source and establish a pathway toward scalable photonic quantum information processing centered around nanoengineered van der Waals materials.

physics.optics↗

arXiv2025

Theory of single-photon emission from neutral and charged excitons in a polarization-selective cavity

Single-photon sources based on neutral or charged excitons in a semiconductor quantum dot are attractive resources for photonic quantum computers and simulators. To obtain indistinguishable photons, the source is pumped on resonance with polarized laser pulses, and the output is collected in orthogonal polarization. However, for sources featuring vertical emission of light, 50% of the emitted photons are unavoidably lost in this way. Here, we theoretically study the quantum dynamics of an exciton embedded in an asymmetric vertical cavity that favors emission in a specific polarization. We identify the configuration for optimal state initialization and demonstrate a path toward near-unity polarized efficiency. We also derive simple analytical formulas for the photon output in each polarization as a function of the Purcell-enhanced emission rates, which shed light on the physical mechanism behind our results.

cond-mat.mes-hall↗

arXiv2025

Exciton and biexciton preparation via coherent swing-up excitation in a GaAs quantum dot embedded in micropillar cavity

Coherent control of quantum emitters is essential for scalable quantum photonic technologies. The recently proposed swing-up of quantum emitter (SUPER) scheme allows efficient and coherent preparation of single photons via off-resonant, red-detuned laser pulses, simplifying laser suppression and enhancing photon collection. We present a systematic study of SUPER excitation applied to a single GaAs quantum dot in a low-Q micropillar cavity. We perform a comparison of the key figures of merit against the well-established two-photon excitation (TPE). Despite requiring higher excitation powers, SUPER achieves near-unity population inversion of the exciton state ($\sim$95%) and high single-photon purity ($g^{(2)}=0.03$) comparable to that under TPE, while also exhibiting a shortened decay time ($\sim$200 ps) reducing the time jitter in the exciton population. A polarization-resolved analysis reveals that when both excitation and collection are aligned with one of the exciton dipoles, SUPER results in polarized single-photon emission, exceeding the resonant TPE saturation by a factor of 1.45. Under optimized excitation conditions, we also observe biexciton preparation via a distinct SUPER resonance, confirmed by the appearance of the biexciton emission line, constituting the first experimental demonstration of biexciton preparation using SUPER. These findings are in good agreement with a proposed four-level theoretical model that incorporates the biexciton state. We also report that a slight misalignment of laser polarization induces an additional SUPER resonance that selectively populates the orthogonal exciton dipole, without altering the nominal excitation polarization. This unexpected behavior reveals a new degree of freedom for coherent state preparation. Our findings establish the SUPER scheme as a versatile tool for state-selective exciton and biexciton control.

physics.optics↗

arXiv2025

Toward triggered generation of indistinguishable single-photons from MoTe$_2$ quantum emitters

Single-photon sources operating in the telecom band are fundamental components for long-distance optical quantum communication and information processing. Two-dimensional (2D) transition metal dichalcogenides (TMDs) offer a promising platform for such sources, but their development has been hindered by limited spectral range and poor single-photon indistinguishability. Here, we demonstrate a reproducible and systematic approach for generating near-infrared (1090-1200 nm) quantum emitters in bilayer MoTe$_2$ using deterministic strain and defect engineering. These emitters exhibit strong linear polarization (DOLP $>70\%$), sub-nanosecond lifetimes ($\tau \leqslant$ ~450 ps), high single-photon purity ($g^{(2)}(0)<0.1$), and resolution-limited emission ($\sim$200 $\mu$eV). Electrostatic biasing enables Stark tuning over a $\sim$3 meV range, reduced photon bunching, and significantly shortened radiative lifetimes, yielding narrow emission with ratios of experimental to transform-limited linewidths as low as $R\sim55$. Most notably, two-photon interference measurements reveal a Hong-Ou-Mandel visibility of $V_\text{HOM}\sim $10$\%$, and up to $V_\text{HOM}\sim$ 40$\%$ with post-selection by temporal filtering, representing the highest reported indistinguishability for any TMD quantum emitters and the first such demonstration in the near-infrared regime. These results establish MoTe$_2$ as a viable platform for tunable, low-noise, high-purity single-photon sources with promising indistinguishability, paving the way for their integration into telecom-compatible quantum photonic technologies.

physics.optics↗

arXiv2025

Tunable and low-noise WSe$_2$ quantum emitters for quantum photonics

Low-noise and tunable single-photon sources are essential components of photonic quantum technologies. However, in WSe$_2$ quantum emitters, charge noise from fluctuations in their local electrostatic environment remains a major obstacle to achieving transform-limited single-photon emission and high photon indistinguishability. Here, we systematically investigate two noise mitigation strategies in hexagonal boron nitride (hBN) encapsulation and electrostatic biasing. We demonstrate that hBN encapsulation alone suppresses spectral wandering (from $\sim$170 $\mu$eV to $\sim$40 $\mu$eV) and narrows emission linewidths (from $\sim$500 $\mu$eV to $\sim$150 $\mu$eV), while applied bias enables stable Stark tuning over a 280 $\mu$eV range and further linewidth narrowing down to $\sim$100 $\mu$eV reaching the resolution-limited regime. Time-resolved and second-order correlation measurements confirm stable mono-exponential decay and high single-photon purity ($g^{(2)}(0) \approx 0.01$) with no observable blinking. To quantify progress toward the transform limit, we define two figures of merit: the linewidth ratio $R = W_{\text{exp}} / W_{\text{rad}}$ and total broadening $\Delta W = W_{\text{exp}} - W_{\text{rad}}$, with both being reduced more than five-fold in optimized devices. These results provide a robust framework for developing and evaluating low-noise, tunable WSe$_2$ quantum emitters, potentially realizing electrically controllable sources of indistinguishable single-photons for future photonic quantum technologies.

physics.optics↗

arXiv2025

Toward single-photon detection with superconducting niobium diselenide nanowires

We present superconducting nanowire single-photon detectors (SNSPDs) based on few-layer NbSe$_2$ fully encapsulated with hexagonal boron nitride (hBN), demonstrating single-photon sensitivity. Our fabrication process preserves the superconducting properties of NbSe$_2$ in nanowires, as confirmed by low-temperature transport measurements that show a critical temperature of $T_c \approx 6.5$ K, comparable to the reported values for unpatterned sheets, and it maintains a contact resistance of $\sim 50 \, \Omega$ at $T = 4$ K. Meandered NbSe$_2$ nanowires exhibit a responsivity of up to $4.9 \times 10^4$ V/W over a spectral range of 650-1550 nm in a closed-cycle cryostat at 4 K, outperforming planar and short-wire devices. The devices achieve a $1/e$ recovery time of $\tau = (135 \pm 36)$ ns, system timing jitter of $j_\text{sys} = (1103 \pm 7)$ ps, and detection efficiency of $\sim 0.01\%$ at $0.95I_c$, with a linear increase in detection probability confirming the single-photon operation. Furthermore, measurements under attenuated pulsed laser (1 MHz) indicate a success rate of up to $33\%$ in detecting individual optical pulses, establishing the platform as a promising candidate for developing efficient single-photon detectors.

physics.optics↗

arXiv2025

Deterministic fabrication of GaAs-quantum-dot micropillar single-photon sources

This study investigates the performance of droplet-etched GaAs quantum dots (QDs) integrated into micropillar structures using a deterministic fabrication technique. We demonstrate a unity QD positioning yield across 74 devices and consistent device performance. Under p-shell excitation, the QD decay dynamics within the micropillars exhibit biexponential behavior, accompanied by intensity fluctuations limiting the source efficiency to < 4.5%. Charge stabilization via low-power above-band LED excitation effectively reduces these fluctuations, doubling the source efficiency to $\sim$ 9%. Moreover, we introduce suppression of radiation modes by introducing cylindrical rings theoretically predicted to boost the collection efficiency by a factor of 4. Experimentally, only a modest improvement is obtained, underscoring the influence of even minor fabrication imperfections for this advanced design. Our findings demonstrate the reliability of our deterministic fabrication approach in producing high-yield, uniform devices, while offering detailed insights into the influence of charge noise and complex relaxation dynamics on the performance.

physics.optics↗

arXiv2024

Photonic "hourglass" design beyond the standard bulk model of phonon decoherence

We study the impact of mechanical vibrations on the performance of the photonic "hourglass" structure, which is predicted to emit single photons on-demand with near-unity efficiency and indistinguishability. Previous investigations neglected the impact of vibrational modes inherent to this quasi-1D geometry, relying instead on a three-dimensional bulk assumption for the phonon modes. However, it has been shown that phonon decoherence has a much stronger impact in 1D structures as compared with bulk media. Here, we surprisingly demonstrate the robustness of the photonic hourglass design, achieving close-to-unity indistinguishability even by incorporating a detailed description of the vibrational modes. We explain this unexpected result in terms of the large Purcell enhancement of the hourglass single-photon source, which eliminates the negative effect of phonons. Our findings highlight the key role of high-Q optical cavities in mitigating the detrimental effect of phonon decoherence, even for structures of reduced dimensionality.

quant-ph↗

arXiv2024

One-dimensional photonic wire as a single-photon source: Implications of cavity QED to a phonon bath of reduced dimensionality

While the semiconductor quantum dot placed in a solid-state material allows for deterministic emission of single photons, the photon indistinguishability is strongly influenced by the intrinsic coupling to lattice vibrations, phonons, of the solid-state environment. This work investigates the phonon-induced decoherence for a quantum dot placed in the one-dimensional system of a homogeneous cylindrical nanowire. Such a structure supports multiple longitudinal phonon branches, and we consider both a linear and a quadratic coupling of the emitter to these modes. Under a polaron approach, we initially derive an analytical expression for the 1D pure dephasing rate, which leads to a reduced pure dephasing rate compared with bulk. By implementing these results into a full cavity quantum electrodynamic model, we demonstrate that multimode coupling is necessary to correctly predict the indistinguishability in a 1D system, which may otherwise be significantly underestimated.

cond-mat.mes-hall↗

arXiv2024

SUPER excitation of quantum emitters is a multi-photon process

The swing-up of quantum emitter population (SUPER) scheme allows to populate the excited state of a quantum emitter with near-unity fidelity using two red-detuned laser pulses. Its off-resonant, yet fully coherent nature has attracted significant interest in quantum photonics as a valuable tool for preparing single-photon sources in their excited state on demand, while simultaneously ensuring straightforward spectral filtering of the laser. However, the physical understanding of this mechanism in terms of energy exchange between the electromagnetic field and the emitter is still lacking. Here, we present a fully quantized model of the swing-up excitation and demonstrate that it is in fact a multi-photon process, where one of the modes loses two or more photons while the other gains at least one. Our findings provide an unexpected physical interpretation of the SUPER scheme and unveil a new non-linear interaction between single emitters and multiple field modes.

quant-ph↗

arXiv2024

High-purity and stable single-photon emission in bilayer WSe$_2$ via phonon-assisted excitation

The excitation scheme is essential for single-photon sources, as it governs exciton preparation, decay dynamics, and the spectral diffusion of emitted photons. While phonon-assisted excitation has shown promise in other quantum emitter platforms, its proper implementation and systematic comparison with alternative excitation schemes have not yet been demonstrated in transition metal dichalcogenide (TMD) quantum emitters. Here, we investigate the impact of various optical excitation strategies on the single-photon emission properties of bilayer WSe$_2$ quantum emitters. Based on our theoretical predictions for the exciton preparation fidelity, we compare excitation via the longitudinal acoustic and breathing phonon modes to conventional above-band and near-resonance excitations. Under acoustic phonon-assisted excitation, we achieve narrow single-photon emission with a reduced spectral diffusion of 0.0129 nm, a 1.8-fold improvement over above-band excitation. Additionally, excitation through breathing-phonon mode yields a high purity of $ 0.947\pm 0.079\,$ and reduces the decay time by over an order of magnitude, reaching $(1.33 \pm 0.04)\,$ns. Our comprehensive study demonstrates the crucial role of phonon-assisted excitation in optimizing the performance of WSe$_2$-based quantum emitters, providing valuable insights for the development of single-photon sources for quantum photonics applications.

quant-ph↗

arXiv2024

Tailoring Polarization in WSe$_2$ Quantum Emitters through Deterministic Strain Engineering

Quantum emitters in transition metal dichalcogenides (TMDs) have recently emerged as a promising platform for generating single photons for optical quantum information processing. In this work, we present an approach for deterministically controlling the polarization of fabricated quantum emitters in a tungsten diselenide (WSe$_2$) monolayer. We employ novel nanopillar geometries with long and sharp tips to induce a controlled directional strain in the monolayer, and we report on fabricated WSe$_2$ emitters producing single photons with a high degree of polarization $(99\pm 4 \%)$ and high purity ($g^{(2)}(0) = 0.030 \pm 0.025$). Our work paves the way for the deterministic integration of TMD-based quantum emitters for future photonic quantum technologies.

physics.optics↗

arXiv2024

Single-photon emitters in WSe$_2$: Critical role of phonons on excitation schemes and indistinguishability

Within optical quantum information processing, single-photon sources based on a two-level system in a semiconductor material allow for on-demand generation of single photons. To initiate the spontaneous emission process, it is necessary to efficiently populate the excited state. However, reconciling the requirement for on-demand excitation with both high efficiency and high photon indistinguishability remains a challenge due to the presence of charge noise and phonon-induced decoherence in the solid-state environment. Here, we propose a method for reconstructing the phonon spectral density experienced by WSe$_{2}$ quantum emitters in the emission process. Using the reconstructed phonon spectral density, we analyze the performance of the resonant, phonon-assisted, and Swing-UP of the quantum EmitteR population (SUPER) swing-up excitation schemes. Under resonant excitation, we obtain an exciton preparation fidelity limited to $\sim$0.80 by the strong phonon coupling, which improves to 0.96 for the SUPER scheme (or 0.89, depending on the type of emitter considered). Under near-resonant phonon-assisted excitation, we observe near-unity excitation fidelity up to 0.976 (0.997). Additionally, we demonstrate that, assuming the suppression of the phonon sidebands, residual dephasing mechanisms such as charge/spin fluctuations are the dominating decoherence mechanisms undermining the photon indistinguishability.

quant-ph↗

arXiv2023

Assessing the alignment accuracy of state-of-the-art deterministic fabrication methods for single quantum dot devices

The realization of efficient quantum light sources relies on the integration of self-assembled quantum dots (QDs) into photonic nanostructures with high spatial positioning accuracy. In this work, we present a comprehensive investigation of the QD position accuracy, obtained using two marker-based QD positioning techniques, photoluminescence (PL) and cathodoluminescence (CL) imaging, as well as using a marker-free in-situ electron beam lithography (in-situ EBL) technique. We employ four PL imaging configurations with three different image processing approaches and compare them with CL imaging. We fabricate circular mesa structures based on the obtained QD coordinates from both PL and CL image processing to evaluate the final positioning accuracy. This yields final position offset of the QD relative to the mesa center of $\mu_x$ = (-40$\pm$58) nm and $\mu_y$ = (-39$\pm$85) nm with PL imaging and $\mu_x$ = (-39$\pm$30) nm and $\mu_y$ = (25$\pm$77) nm with CL imaging, which are comparable to the offset $\mu_x$ = (20$\pm$40) nm and $\mu_y$ = (-14$\pm$39) nm obtained using the in-situ EBL method. We discuss the possible causes of the observed offsets, which are significantly larger than the QD localization uncertainty obtained from simply imaging the QD light emission from an unstructured wafer. Our study highlights the influences of the image processing technique and the subsequent fabrication process on the final positioning accuracy for a QD placed inside a photonic nanostructure.

physics.optics↗

arXiv2023

Quantum dots for photonic quantum information technology

The generation, manipulation, storage, and detection of single photons play a central role in emerging photonic quantum information technology. Individual photons serve as flying qubits and transmit the quantum information at high speed and with low losses, for example between individual nodes of quantum networks. Due to the laws of quantum mechanics, quantum communication is fundamentally tap-proof, which explains the enormous interest in this modern information technology. On the other hand, stationary qubits or photonic states in quantum computers can potentially lead to enormous increases in performance through parallel data processing, to outperform classical computers in specific tasks when quantum advantage is achieved. Here, we discuss in depth the great potential of quantum dots (QDs) in photonic quantum information technology. In this context, QDs form a key resource for the implementation of quantum communication networks and photonic quantum computers because they can generate single photons on-demand. Moreover, QDs are compatible with the mature semiconductor technology, so that they can be integrated comparatively easily into nanophotonic structures, which form the basis for quantum light sources and integrated photonic quantum circuits. After a thematic introduction, we present modern numerical methods and theoretical approaches to device design and the physical description of quantum dot devices. We then present modern methods and technical solutions for the epitaxial growth and for the deterministic nanoprocessing of quantum devices based on QDs. Furthermore, we present the most promising concepts for quantum light sources and photonic quantum circuits that include single QDs as active elements and discuss applications of these novel devices in photonic quantum information technology. We close with an overview of open issues and an outlook on future developments.

quant-ph↗

arXiv2023

Enhancing the efficiency of quantum-dot-based single-photon source designs by suppressing background emission using concentric rings

In this paper, we theoretically demonstrate that a few-period circular Bragg reflector consisting of concentric rings placed around an infinite nanowire with an embedded quantum dot can increase the fraction of radiative emission into the fundamental $\mathrm{HE}_{11}$ mode ($\beta=\Gamma_{\rm HE_{11}}/\Gamma_{\rm Total}$) up to 0.999 due to enhanced suppression of the emission into radiation modes caused by a photonic bandgap effect. We then apply this strategy in the practically relevant case of the finite-sized single-photon source based on tapered nanowires and demonstrate that the collection efficiency can be improved. Additionally, we also show the beneficial effects of placing optimized rings around the micropillar single-photon source.

physics.optics↗