arXiv subjects

Simone Luca Portalupi

Publications and source records attributed to Simone Luca Portalupi.

17 recordsLinked to original sources

arXiv2026

Rack-integrated quantum dot-based source of single and entangled photons at telecom C-band

For quantum light sources in everyday telecommunication networks, quantum science needs to be fully transformed into quantum technology. The first necessary step to move outside a well-controlled lab environment requires the use of quantum light sources operating in the technologically relevant telecom O- and C-band. This can be provided using epitaxial quantum dots as deterministic sources of quantum light. Particularly intriguing is that emitters operating at telecom wavelengths are rapidly catching up with their short wavelength counterparts in terms of performances. Here, we make a decisive step forward in the development of quantum communication networks: a state-of-the-art source of quantum light, a semiconductor quantum dot (QD), with record coincidence rate for entangled photon emission in the telecom C-band, is operated inside an optimized rack-based setup. This setup includes a tunable pulsed laser for the QD excitation (from quasi- to fully resonant excitation), all optics for the excitation filtering, and QD signal coupling into single-mode fibers. Overall, the setup allows for above $50\%$ transmission for both exciton and biexciton photons. These results show that quantum dots-based telecom light sources can now be transported and integrated into existing fiber infrastructures, an important step to demonstrate the feasibility of the upcoming quantum internet.

quant-ph↗

arXiv2026

A complete fs-laser-ablation route to miniaturized single-crystal PMN-PT piezoelectric actuators

This article presents a novel fabrication route for miniaturized piezoelectric actuators that relies exclusively on processes based on femtosecond (fs) laser ablation. Previous work has already demonstrated that fs-lasers are uniquely suited for the fabrication of piezoelectric actuators based on PMN-PT, which are required for multiaxial strain-tuning of quantum dots (QDs) to enable, e.g. the generation of highly entangled photon pairs. Building on these foundations, the present work advances actuator performance and capabilities by introducing a local thinning strategy. This approach allows the realization of smaller devices, which in turn enables lower operating voltages, while simultaneously offering the possibility of integrating multiple quantum light sources on a single chip. The article provides a detailed description of the full fabrication chain, entirely based on fs-laser processing steps, from substrate thinning to metal layer structuring and final device definition. A particular focus is placed on the final cutting process, where the implementation of a third-harmonic ultraviolet (UV) fs-laser wavelength significantly improves edge quality and shape definition compared to the second harmonic (SH) wavelength used in previous work. The device fabricated through the combination of local thinning and UV-based cutting promises not only to enhance the efficiency of strain transfer but also to ensure the mechanical stability required for practical applications. These results establish fs-laser-based fabrication as a versatile and scalable method for next-generation piezoelectric actuators, paving the way for advanced strain-engineering approaches in semiconductor quantum optics and integrated quantum photonics.

physics.app-ph↗

arXiv2026

Time-bin encoded quantum key distribution over 120 km with a telecom quantum dot source

Quantum key distribution (QKD) with deterministic single photon sources has been demonstrated over intercity fiber and free-space channels. The previous implementations relied mainly on polarization encoding schemes, which are susceptible to birefringence, polarization-mode dispersion and polarization-dependent loss in practical fiber networks. In contrast, time-bin encoding offers inherent robustness and has been widely adopted in mature QKD systems using weak coherent laser pulses. However, its feasibility in conjunction with a deterministic single-photon source has not yet been experimentally demonstrated. In this work, we construct a time-bin encoded QKD system employing a high-brightness quantum dot (QD) single-photon source operating at telecom wavelength. Our proof-of-concept experiment successfully demonstrates the possibility of secure key distribution over fiber link of 120 km, while maintaining extraordinary long-term stability over 6 hours of continuous operation. This work provides the first experimental validation of integrating a quantum dot single-photon source with time-bin encoding in a telecom-band QKD system. In addition, it demonstrates the highest secure key rate among the time-bin QKDs based on single-photon sources. This development signifies a substantial advancement in the establishment of a robust and scalable QKD network based on solid-state single-photon technology

quant-ph↗

arXiv2025

Ultrastable, low-error dynamic polarization encoding of deterministically generated single photons

The ability to inscribe information on single photons at high speeds is a crucial requirement for quantum applications such as quantum communication and measurement-based photonic quantum computation. Nowadays, most experimental implementations employ phase modulators in single-pass, Mach-Zehnder interferometer or Michelson interferometer configurations to encode information on photonic qubits. However, these approaches are intrinsically sensitive to environmental influences, limiting the achievable quantum error rates in practice. We report on the first demonstration of a polarization encoder for single-photon qubits based on a free-space Sagnac interferometer, showcasing inherent phase stability and overcoming previous error rate limitations. Telecom-wavelength single photons emitted by a quantum dot are modulated by the encoder under a repetition rate of 152 MHz. A quantum bit error rate of 0.69(2)% is achieved, marking the lowest error rate reported to date for high-speed information encoding on single photons. This work represents a key advance towards robust, scalable, and low-error quantum information processing with single photon sources.

quant-ph↗

arXiv2025

Efficient fiber coupling of telecom single-photons from circular Bragg gratings

Deterministic sources of quantum light are becoming increasingly relevant in the development of quantum communication, particularly in deployed fiber networks. Therefore, efficient fiber-coupled sources at telecom wavelength are highly sought after. With this goal in mind, we systematically investigate the fiber coupling performance of quantum dots in optical resonators under three experimental configurations. We quantify coupling efficiency and sensitivity to spatial displacement for single-mode fibers with 3D printed optics on their tip, and benchmark their behavior over a commercial cleaved-cut fiber and a standard optical setup. The reduction of the required optical elements when operating with a lensed or a bare fiber allows for an increased end-to-end efficiency by a factor of up to 3.0 +/- 0.2 over a standard setup. For the perspective of realizing a mechanically stable fiber-coupled source, we precisely quantify the spatial tolerance to fiber-cavity misalignment, observing less than 50 % count rate drop for several micrometers displacement. These results will play a key role in the future development of fiber-coupled sources of quantum light.

quant-ph↗

arXiv2024

Quantum Teleportation with Telecom Photons from Remote Quantum Emitters

The quest for a global quantum internet is based on the realization of a scalable network which requires quantum hardware with exceptional performance. Among them are quantum light sources providing deterministic, high brightness, high-fidelity entangled photons and quantum memories with coherence times in the millisecond range and above. To operate the network on a global scale, the quantum light source should emit at telecommunication wavelengths with minimum propagation losses. A cornerstone for the operation of such a quantum network is the demonstration of quantum teleportation. Here we realize full-photonic quantum teleportation employing one of the most promising platforms, i.e. semiconductor quantum dots, which can fulfill all the aforementioned requirements. Two remote quantum dots are used, one as a source of entangled photon pairs and the other as a single-photon source. The frequency mismatch between the triggered sources is erased using two polarization-preserving quantum frequency converters, enabling a Bell state measurement at telecommunication wavelengths. A post-selected teleportation fidelity of up to 0.721(33) is achieved, significantly above the classical limit, demonstrating successful quantum teleportation between light generated by distinct sources. These results mark a major advance for the semiconductor platform as a source of quantum light fulfilling a key requirement for a scalable quantum network. This becomes particularly relevant after the seminal breakthrough of addressing a nuclear spin in semiconductor quantum dots demonstrating long coherence times, thus fulfilling another crucial step towards a scalable quantum network.

quant-ph↗

arXiv2024

High-rate intercity quantum key distribution with a semiconductor single-photon source

Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution. Here we report on the first intercity QKD experiment using a bright deterministic single photon source. A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure. Utilising the 79 km long link with 25.49 dB loss (equivalent to 130 km for the direct-connected optical fibre) between the German cities of Hannover and Braunschweig, a record-high secret key bits per pulse of 4.8 * 10^{-5} with an average quantum bit error ratio of ~ 0.65 % are demonstrated. An asymptotic maximum tolerable loss of 28.11 dB is found, corresponding to a length of 144 km of standard telecommunication fibre. Deterministic semiconductor sources therefore challenge state-of-the-art QKD protocols and have the potential to excel in measurement device independent protocols and quantum repeater applications.

quant-ph↗

arXiv2023

Enhancing quantum cryptography with quantum dot single-photon sources

Quantum cryptography harnesses quantum light, in particular single photons, to provide security guarantees that cannot be reached by classical means. For each cryptographic task, the security feature of interest is directly related to the photons' non-classical properties. Quantum dot-based single-photon sources are remarkable candidates, as they can in principle emit deterministically, with high brightness and low multiphoton contribution. Here, we show that these sources provide additional security benefits, thanks to the tunability of coherence in the emitted photon-number states. We identify the optimal optical pumping scheme for the main quantum-cryptographic primitives, and benchmark their performance with respect to Poisson-distributed sources such as attenuated laser states and down-conversion sources. In particular, we elaborate on the advantage of using phonon-assisted and two-photon excitation rather than resonant excitation for quantum key distribution and other primitives. The presented results will guide future developments in solid-state and quantum information science for photon sources that are tailored to quantum communication tasks.

quant-ph↗

arXiv2022

Wavelength-tunable open double-microcavity to enhance two closely spaced optical transitions

Microcavities have long been recognized as indispensable elements in quantum photonic research due to their usefulness for enhanced light extraction and light-matter interaction. A conventional high-Q cavity structure typically allows only a single optical transition to be tuned into resonance with a specific mode. The transition to a more advanced double-cavity structure, however, introduces new and interesting possibilities such as enhancing two spectrally close optical transitions at the same time with two distinct cavity modes. Here, we investigate a cavity structure composed of a monolithic planar cavity enclosed between two semiconductor distributed Bragg reflectors (DBR) and a top dielectric mirror deposited on a fiber tip. While the bottom cavity is formed by the two DBRs, the mirror on the fiber tip and the top DBR of the semiconductor chip create a second tunable cavity. These coupled cavities exhibit mode hybridization when tuned into resonance and their splitting can be adjusted to match with the spectral separation of closely spaced optical transitions by a suitable sample design. Furthermore, we report on the simultaneous resonance tuning of the exciton and biexciton transition of a semiconductor quantum dot, each to a separate mode of the open fiber-based double cavity. Decay time measurements at simultaneous resonance showed a Purcell-factor of $F_P^X$=1.9$\pm$0.4 for the exciton transition.

physics.optics↗

arXiv2022

High emission rate from a Purcell-enhanced, triggered source of pure single photons in the telecom C-band

Several emission features mark semiconductor quantum dots as promising non-classical light sources for prospective quantum implementations. For long-distance transmission [1] and Si-based on-chip processing[2, 3], the possibility to match the telecom C-band [4] stands out, while source brightness and high single-photon purity are key features in virtually any quantum implementation [5, 6]. Here we present an InAs/InGaAs/GaAs quantum dot emitting in the telecom C-band coupled to a circular Bragg grating. The Purcell enhancement of the emission enables a simultaneously high brightness with a fiber-coupled single-photon count rate of 13.9MHz for an excitation repetition rate of 228MHz (first-lens collection efficiency ca. 17% for NA = 0.6), while maintaining a low multi-photon contribution of g(2)(0) = 0.0052. Moreover, the compatibility with temperatures of up to 40K attainable with compact cryo coolers, further underlines the suitability for out-of-the-lab implementations.

quant-ph↗

arXiv2021

Thin-Film InGaAs Metamorphic Buffer for telecom C-band InAs Quantum Dots and Optical Resonators on GaAs Platform

The GaAs-based material system is well-known for the implementation of InAs quantum dots (QDs) with outstanding optical properties. However, these dots typically emit at a wavelength of around 900nm. The insertion of a metamorphic buffer (MMB) can shift the emission to the technologically attractive telecom C-band range centered at 1550nm. However, the thickness of common MMB designs limits their compatibility with most photonic resonator types. Here we report on the MOVPE growth of a novel InGaAs MMB with a non-linear indium content grading profile designed to maximize plastic relaxation within minimal layer thickness. Single-photon emission at 1550nm from InAs QDs deposited on top of this thin-film MMB is demonstrated. The strength of the new design is proven by integrating it into a bullseye cavity via nano-structuring techniques. The presented advances in the epitaxial growth of QD/MMB structures form the basis for the fabrication of high-quality telecom non-classical light sources as a key component of photonic quantum technologies.

physics.optics↗

arXiv2020

3D printed micro-optics for quantum technology: Optimized coupling of single quantum dot emission into a single mode fiber

Future quantum technology relies crucially on building quantum networks with high fidelity. To achieve this challenging goal, it is of utmost importance to connect single quantum systems in a way such that their emitted single-photons overlap with the highest possible degree of coherence. This requires perfect mode overlap of the emitted light of different emitters, which necessitates the use of single mode fibers. Here we present an advanced manufacturing approach to accomplish this task: we combine 3D printed complex micro-optics such as hemispherical and Weierstrass solid immersion lenses as well as total internal reflection solid immersion lenses on top of single InAs quantum dots with 3D printed optics on single mode fibers and compare their key features. Interestingly, the use of hemispherical solid immersion lenses further increases the localization accuracy of the emitters to below 1 nm when acquiring micro-photoluminescence maps. The system can be joined together and permanently fixed. This integrated system can be cooled by dipping into liquid helium, by a Stirling cryocooler or by a closed-cycle helium cryostat without the necessity for optical windows, as all access is through the integrated single mode fiber. We identify the ideal optical designs and present experiments that prove excellent high-rate single-photon emission by high-contrast Hanbury Brown and Twiss experiments.

physics.app-ph↗

arXiv2020

Quantum dot single-photon emission coupled into single-mode fibers with 3D printed micro-objectives

User-friendly single-photon sources with high photon-extraction efficiency are crucial building blocks for photonic quantum applications. For many of these applications, such as long-distance quantum key distribution, the use of single-mode optical fibers is mandatory, which leads to stringent requirements regarding the device design and fabrication. We report on the on-chip integration of a quantum dot microlens with a 3D-printed micro-objective in combination with a single-mode on-chip fiber coupler. The practical quantum device is realized by deterministic fabrication of the QD-microlens via in-situ electron-beam lithography and 3D two-photon laser writing of the on-chip micro-objective and fiber-holder. The QD with microlens is an efficient single-photon source, whose emission is collimated by the on-chip micro-objective. A second polymer microlens is located at the end facet of the single-mode fiber and ensures that the collimated light is efficiently coupled into the fiber core. For this purpose, the fiber is placed in the on-chip fiber chuck, which is precisely aligned to the QD-microlens thanks to the sub-$μ$m processing accuracy of high-resolution two-photon direct laser writing. This way, we obtain a fully integrated high-quality quantum device with broadband photon extraction efficiency, a single-mode fiber-coupling efficiency of 26%, a single-photon flux of 1.5 MHz at single-mode fibre output and a multi-photon probability of 13 % under pulsed optical excitation. In addition, the stable design of the developed fiber-coupled quantum device makes it highly attractive for integration into user-friendly plug-and-play quantum applications.

physics.app-ph↗

arXiv2019

Delay and polarization routing of single photons

The full control of single photons is important in quantum information and quantum networking. A convenient storage device for photons is the key to memory assisted quantum communication and computing. While even a simple optical fiber can act as a convenient and reliable storage device, its storage time is tightly fixed and cannot be adapted. Therefore, the photon storage should ideally be actively controllable by external means, such as magnetic or electric control fields. In order to multiplex several photons, an active routing would also be desirable. Here we show that single photons of a semiconductor quantum dot can be deliberately delayed by an atomic vapor. Also, the output path can be selected, depending on an external magnetic field. By selecting the input polarization of the photons and by aligning the external magnetic field of the hot atomic vapor, the delay-based storage can be fine tuned to a deliberate value. With an overall delay of 25 ns, we are able to fine tune by more than 600 ps. Depending on the input polarization, the photons are routed into different output ports. The experimental data is fully resembled by a theoretical model, which describes the group velocity delay under consideration of spectral diffusion and considers the complex refractive index of the atomic vapor. The present results enable the use of an atomic vapor as a wavelength selective delay and allows for routing the single photons according to their polarization and an external magnetic field.

quant-ph↗

arXiv2019

Single-photon light emitting diodes based on pre-selected quantum dots using a deterministic lithography technique

In the present study, we developed a fabrication process of an electrically driven single-photon LED based on InP QDs emitting in the red spectral range, the wavelength of interest coinciding with the high efficiency window of Si APDs. A deterministic lithography technique allowed for the pre-selection of a suitable QD, here exclusively operated under electrical carrier injection. The final device was characterized under micro-electroluminescence in direct current, as well as in pulsed excitation mode. In particular, under pulsed excitation of one device, single-photon emission of a spectral line, identified as an exciton, has been observed with $g^{(2)}_\mathrm{raw}(0)=0.42\pm0.02$, where the non-zero $g^{(2)}$-value is mainly caused by background contribution in the spectrum and re-excitation processes due to the electrical pulse length. The obtained results constitute an important step forward in the fabrication of electrically driven single-photon sources, where deterministic lithography techniques can be used to sensibly improve the device performances. In principle, the developed process can be extended to any desired emitter wavelength above $600\,\mathrm{nm}$ up to the telecom bands.

cond-mat.mes-hall↗

arXiv2015

Cavity-enhanced simultaneous dressing of quantum dot exciton and biexciton states

We demonstrate the simultaneous dressing of both vacuum-to-exciton and exciton-to-biexciton transitions of a single semiconductor quantum dot in a high-Q micropillar cavity, using photoluminescence spectroscopy. Resonant two-photon excitation of the biexciton is achieved by spectrally tuning the quantum dot emission with respect to the cavity mode. The cavity couples to both transitions and amplifies the Rabi-frequency of the likewise resonant cw laser, driving the transitions. We observe strong-field splitting of the emission lines, which depend on the driving Rabi field amplitude and the cavity-laser detuning. A dressed state theory of a driven 4-level atom correctly predicts the distinct spectral transitions observed in the emission spectrum, and a detailed description of the emission spectra is further provided through a polaron master equation approach which accounts for cavity coupling and acoustic phonon interactions of the semiconductor medium.

quant-ph↗

arXiv2014

Phonon-tuned bright single-photon source

Pure and bright single photon sources have recently been obtained by inserting solid-state emitters in photonic nanowires or microcavities. The cavity approach presents the attractive possibility to greatly increase the source operation frequency. However, it is perceived as technologically demanding because the emitter resonance must match the cavity resonance. Here we show that the spectral matching requirement is actually strongly lifted by the intrinsic coupling of the emitter to its environment. A single photon source consisting of a single InGaAs quantum dot inserted in a micropillar cavity is studied. Phonon coupling results in a large Purcell effect even when the quantum dot is detuned from the cavity resonance. The phonon-assisted cavity enhanced emission is shown to be a good single-photon source, with a brightness exceeding $40$ \% for a detuning range covering 15 cavity linewidths.

cond-mat.mes-hall↗