We investigate the spin properties of InAs/InGaAs/InP quantum dots grown by metalorganic vapor-phase epitaxy (MOVPE) deposition using droplet epitaxy, which emits in the telecom C band. Using pump-probe Faraday ellipticity measurements, we determine electron and hole g factors of | g e | = 0.93 and | g h | = 0.47 , with the electron g factor being nearly twice as low as typical molecular beam epitaxy Stranski-Krastanov (SK) grown samples. Most significantly, we measure a longitudinal spin relaxation time T 1 = 3.0 µ s , representing an order of magnitude improvement over comparable MBE SK grown samples. Despite significant electron g -factor anisotropy, we observed that it is reduced relative to similar material composition samples grown with MBE or MOVPE SK methods. We attribute this g -factor anisotropy and spin lifetime improvements to the enhanced structural symmetry achieved via MOVPE droplet epitaxy, which mitigates the inherent structural asymmetry in strain-driven growth approaches for InAs/InP quantum dots. These results demonstrate that MOVPE droplet epitaxy-grown InAs/InGaAs/InP quantum dots exhibit favorable spin properties for potential implementation in quantum information applications.
We investigate the spin properties of InAs/InGaAs/InP quantum dots grown by metalorganic vapor-phase epitaxy (MOVPE) deposition using droplet epitaxy, which emits in the telecom C band. Using pump-probe Faraday ellipticity measurements, we determine electron and hole g factors of | g e | = 0.93 and | g h | = 0.47 , with the electron g factor being nearly twice as low as typical molecular beam epitaxy Stranski-Krastanov (SK) grown samples. Most significantly, we measure a longitudinal spin relaxation time T 1 = 3.0 µ s , representing an order of magnitude improvement over comparable MBE SK grown samples. Despite significant electron g -factor anisotropy, we observed that it is reduced relative to similar material composition samples grown with MBE or MOVPE SK methods. We attribute this g -factor anisotropy and spin lifetime improvements to the enhanced structural symmetry achieved via MOVPE droplet epitaxy, which mitigates the inherent structural asymmetry in strain-driven growth approaches for InAs/InP quantum dots. These results demonstrate that MOVPE droplet epitaxy-grown InAs/InGaAs/InP quantum dots exhibit favorable spin properties for potential implementation in quantum information applications.
We investigate the spin properties of charge carriers in vertically coupled InAs/InAlGaAs quantum dots grown by molecular beam epitaxy, emitting at telecom C-band wavelengths, with a silicon δ -doped layer. Using time-resolved pump-probe Faraday ellipticity measurements, we systematically study single-, two-, and four-layer quantum dot (QD) configurations to quantify how vertical coupling affects key spin-coherence parameters. Our measurements reveal distinct layer-dependent effects: (i) Adding a second QD layer flips the resident charge from electrons to holes, consistent with optically induced electron tunneling into lower-energy dots and resultant hole charging. (ii) Starting from the four-layer sample, the pump-probe signal develops an additional nonoscillating, decaying component absent in single- and two-layer samples, attributed to multiple layer growth changing the strain environment, which reduces heavy-hole and light-hole mixing. (iii) With four layers or more, hole spin mode locking (SML) can be observed, enabling quantitative extraction of the hole coherence time T 2 ≈ 13 ns from SML amplitude saturation. We also extract longitudinal spin relaxation ( T 1 ) and transverse ( T 2 * ) spin dephasing times, g-factors, and inhomogeneous dephasing parameters for both electrons and holes across all layer configurations. The hole spin dephasing times T 2 * remain relatively constant (2.3–2.7 ns) across layer counts, while longitudinal relaxation times T 1 decrease with increasing layers (from 0.9 µ s for single-layer to 0.32 µ s for four-layer samples). These findings provide potential design guidelines for engineering spin coherence in telecom-band QDs for quantum information applications.
The development of quantum materials for single-photon emission is crucial for the advancement of quantum information technology. Although significant advancement has been witnessed in recent years for single photon sources in near infrared band (λ~700-1000 nm), several challenges have yet to be addressed for ideal single photon emission at the telecommunication band. In this study, we present a droplet-epitaxy strategy for O-band to C-band single-photon source based semiconductor quantum dots (QDs) using metal-organic vapor-phase epitaxy (MOVPE). Via investigating the growth conditions of the epitaxial process, we have successfully synthesized InAs/InP QDs with narrow emission lines spanning a broad spectral range of λ~1200-1600 nm. The morphological and optical properties of the samples were characterized using atomic force microscopy and micro photoluminescence spectroscopy. The recorded single-photon purity of a plain QD structure reaches (g(2)(0) = 0.16), with a radiative recombination lifetime as short as 1.5 ns. This work provides a crucial platform for future research on integrated microcavity enhancement techniques and coupled QDs with other quantum photonics in the telecom bands, offering significant prospects for quantum network applications.
This thesis investigates two complementary quantum technologies for secure optical communication: spin dynamics in telecom C-band quantum dots and a high-bandwidth source-device-independent quantum random number generator. Together, they address two central requirements of quantum networks: long-lived spin memories for quantum repeaters and secure, high-rate entropy generation for quantum key distribution. The first part studies the spin dynamics of InAs/InGaAs/InP quantum dots grown by strain-free droplet epitaxy using time-resolved Faraday ellipticity. Compared to Stranski-Krastanov grown telecom dots, droplet epitaxy shows improved spin properties, including an electron spin relaxation time of 3.0 us and reduced in-plane electron g-factor anisotropy, indicating enhanced structural symmetry. Polarization recovery and Hanle measurements reveal nuclear spin lifetimes comparable to electron spin lifetimes, attributed to quadrupolar interactions arising from Ga-In interdiffusion. These results demonstrate the advantages of droplet epitaxy over strain-based growth and indicate further potential through optimized material compositions. The second part investigates one-, two-, and four-layer InAs/InAlGaAs Stranski-Krastanov quantum dots to assess layer-dependent carrier type and spin coherence. Increasing layer number induces a transition from electron to hole resident carriers due to interlayer tunnelling into lower-energy dots, the emergence of spin-mode locking for four or more layers, and the appearance of a non-oscillating spin component likely associated with reduced heavy-hole/light-hole mixing. Although droplet epitaxy samples exhibit superior spin parameters overall, no spin-mode locking is observed, potentially due to shorter transverse coherence or electron-dominated charging. The final part presents a source-device-independent quantum random number generator based on heterodyne detection of vacuum fluctuations. Using a 90 degree optical hybrid, balanced photodetectors, a dual-channel 12-bit ADC operating at 3.2 GS/s per channel, and FPGA-implemented Toeplitz hashing with PCIe transfer, a total extracted bandwidth of 33.91 Gbit/s is achieved. The system assumes no trust in the optical source and performs all extraction in hardware, maintaining security even under adversarial state preparation.
The ability of two photons to interfere lies at the heart of many photonic quantum networking concepts and requires that the photons are indistinguishable with sufficient coherence times to resolve the interference signals. However, for solid-state quantum light sources, this can be challenging to achieve as they are in constant interaction with noise sources in their environment. Here, we investigate the noise sources that affect InAs/InP quantum dots emitting in the telecom C-band by comparing their behavior on a wetting layer for Stranski–Krastanov grown quantum dots with a nearly wetting layer-free environment achieved with the droplet epitaxy growth mode. We show that the droplet epitaxy growth mode is beneficial for a quiet environment, leading to 96% of exciton transitions having a coherence time longer than the typical detector resolution of 100 ps, even under non-resonant excitation. We also show that the decay profile indicates the presence of slow dephasing processes, which can be compensated for experimentally. We finally conduct Hong–Ou–Mandel interference measurements between subsequently emitted photons and find a corrected two-photon interference visibility of 98.6 ± 1.6% for droplet-epitaxy grown quantum dots. The understanding of the influence of their surroundings on the quantum optical properties of these emitters is important for their optimization and use in future quantum networking applications.
High-quality InAs quantum wells grown on InP are a promising platform for topological quantum information processing due to their large g-factor, strong Rashba spin-orbit interaction, and their compatibility with in-situ-deposited superconductors. In this work, we investigate InAs/InGaAs quantum wells grown on InP (001) wafers, focusing on how the layer structure and strain influence the electronic properties and surface morphology. By combining quantum transport measurements with atomic force microscopy, we show that the layer design predominantly affects the mobility anisotropy, which aligns well with the surface morphology. Surface characterization further reveals the mechanism of quantum well collapse when the layer thickness exceeds the strain limit. In addition, transport measurements demonstrate that quantum confinement has a clear impact on band nonparabolicity.
Non-classical photon sources made of semiconductor quantum dots (QDs) emitting in the telecommunication C-band are crucial components for low-loss, long-distance photonic quantum communication networks. Here we designed and fabricated strain--free In0.7Ga0.3As/In0.7Al0.3As QDs grown on GaAs(111)A substrates working as single-photon emitters in the 1550 nm window. The QDs were grown via local droplet etching method in a molecular beam epitaxy environment, employing a thin In0.7Al0.3As metamorphic buffer layer with the same lattice constant of the QD material, thus allowing for a completely strain--free self-assembly of the QDs. The QDs exhibit a C3v symmetry with a ground state emission in the 1400--1600 nm range. The exciton lifetimes of ≈ 1.3--1.9 ns and linewidths as low as ≈ 300 μeV show the good quality of the fabricated QDs. Second-order autocorrelation measurements under pulsed excitation confirmed the single-photon purity of the emitters, yielding a g(2)(0) value of 0.141±0.027