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arXiv:2504.20497 · Physical review. B./Physical review. B

Initialization of neutral and charged exciton spin states in a telecom-emitting quantum dot

Giora Peniakov, Johannes Michl, Mohamed Helal, Raphael Joos, Michael Jetter, Simone Luca Portalupi, Peter Michler, Sven Höfling, Tobias Huber

Published Jul 8, 2025• 4 citations
arXivHTMLPDF

Abstract

Photonic cluster states are highly entangled states that allow for photonic quantum computing and memory-less quantum repeaters. Their generation has been recently demonstrated using semiconductor quantum dots emitting at the 900 nm wavelength range. However, a similar demonstration at the communication-optimal telecom range has remained elusive. A key ingredient that is still missing is an appropriate optical excitation method. A central requirement of such a method is to allow an arbitrary spin initialization of quantum-dot excitonic complexes. In this work, we report on developing such a method based on a quasiresonant p -shell excitation for an InAs/InGaAs quantum dot emitting at 1535 nm. We show qubit writing of a neutral exciton and spin-preserving excitation of a negative trion. Using the Larmor precession of the negative trion under an externally applied magnetic field, we determine the in-plane g factors of both the electron and the hole in the investigated quantum dot. In addition, we measure a lower bound on the hole coherence time, T 2 * > 6.4 ns , boosting its candidacy as a sound photon entangler for more advanced quantum photonic schemes.

DOI
10.1103/3qh7-b696

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Photonic cluster states are highly entangled states that allow for photonic quantum computing and memory-less quantum repeaters. Their generation has been recently demonstrated using semiconductor quantum dots emitting at the 900 nm wavelength range. However, a similar demonstration at the communication-optimal telecom range has remained elusive. A key ingredient that is still missing is an appropriate optical excitation method. A central requirement of such a method is to allow an arbitrary spin initialization of quantum-dot excitonic complexes. In this work, we report on developing such a method based on a quasiresonant p -shell excitation for an InAs/InGaAs quantum dot emitting at 1535 nm. We show qubit writing of a neutral exciton and spin-preserving excitation of a negative trion. Using the Larmor precession of the negative trion under an externally applied magnetic field, we determine the in-plane g factors of both the electron and the hole in the investigated quantum dot. In addition, we measure a lower bound on the hole coherence time, T 2 * > 6.4 ns , boosting its candidacy as a sound photon entangler for more advanced quantum photonic schemes.
Photonic cluster states are a key resource for photonic quantum information processing. So far, deterministic generation of these states has been limited to the near-infrared wavelength range. To achieve quantum advantage in communication while maintaining compatibility with silicon photonics, operation in the telecom wavelength range is required. In this work, we demonstrate deterministic cluster state generation directly in the telecom C-band. This is achieved through repetitive excitation of a hole spin confined in an indium-arsenide quantum dot subjected to an external magnetic field. We characterize the quantum process that generates the cluster state by measuring its process map, obtaining a fidelity of F=0.71±0.01\mathrm{F} = 0.71 \pm 0.01F=0.71±0.01 to the ideal case. As part of this characterization, we observe spin--photon polarization entanglement with a negativity of N=0.27±0.02\mathrm{N} = 0.27 \pm 0.02N=0.27±0.02. The emitted photons exhibit indistinguishability of at least 83%, demonstrating the potential for future fusion gates necessary for photonic cluster state generation beyond linear connectivity.
Matter qubits that maintain coherence over extended timescales are essential for many pursued applications in quantum communication and quantum computing. Significant progress has already been made on extending coherence times of spins in semiconductor quantum dots while interfacing them with photons in the near-infrared wavelength range. However, similar results for quantum dots emitting at the telecom range, crucial for many applications, have so far lagged behind. Here, we report on InAs/InAlGaAs quantum dots integrated in a deterministically placed circular Bragg grating emitting at 1.55 μm1.55\,μ\mathrm{m}1.55μm. We quantify the g-factors of electrons and holes from polarization-resolved measurements of a positive trion in an in-plane magnetic field and study the dynamics of the ground-state hole spin qubit. We then herald the hole spin in a pulsed two-photon correlation measurement and determine its inhomogeneous dephasing time to T2∗=(15.9±1.7)T_{2}^{*}=(15.9 \pm 1.7)T2∗​=(15.9±1.7) ns.
Quantum networks, relying on the distribution of quantum entanglement between remote locations, have the potential to transform quantum computation and secure long-distance quantum communication. However, a fundamental ingredient for fibre-based implementations of such networks, namely entanglement between a single spin and a photon directly emitted at telecom wavelengths, has been unattainable so far. Here, we use a negatively charged exciton in an InAs/InP quantum dot to implement an optically active spin qubit taking advantage of the lowest-loss transmission window, the telecom C-band. We investigate the coherent interactions of the spin-qubit system under resonant excitation, demonstrating high fidelity spin initialisation and coherent control using picosecond pulses. We further use these tools to measure the coherence of a single, undisturbed electron spin in our system. Finally, we demonstrate spin-photon entanglement in a solid-state system with entanglement fidelity F = 80.07 ± 2.9%, more than 10 standard deviations above the classical limit.
A central challenge for scalable quantum networks is the realization of coherent interfaces between stationary qubits and telecom-band photonic qubits for long-distance entanglement distribution. Semiconductor quantum dots emitting at telecom wavelengths present a promising spin-photon platform, and a precise understanding of the properties of the confined spin is crucial for optimizing its interplay with the photonic qubit. Here, we simultaneously benchmark the electron and hole g-factors and coherence properties of a droplet epitaxy QD, solely from time and polarization resolved photon correlations. These measurements identify the hole as the preferable qubit for spin-photon entanglement in quantum network nodes. We then perform full state tomography of the confined hole ground state to reveal subtle anisotropies in the spin precession, providing essential diagnostics for minimizing phase errors critical for deterministic multiphoton entanglement generation.
A central challenge for scalable quantum networks is the realization of coherent interfaces between stationary qubits and telecom-band photonic qubits for long-distance entanglement distribution. Semiconductor quantum dots emitting at telecom wavelengths present a promising spin–photon platform, and a precise understanding of the properties of the confined spin is crucial for optimizing its interplay with the photonic qubit. Here, we simultaneously benchmark the electron and hole g -factors and coherence properties of a droplet epitaxy quantum dot, solely from time- and polarization-resolved photon correlations. These measurements identify the hole as the preferable qubit for spin–photon entanglement in quantum network nodes. We then perform full state tomography of the confined hole ground state to reveal subtle anisotropies in the spin precession, providing essential diagnostics for minimizing phase errors critical for deterministic multiphoton entanglement generation.
Photonic graph states are key resource states for measurement based quantum information processing. As semiconductor quantum dots are excellent deterministic photon emitters, several protocols using them for the generation of linear cluster states have been proposed, either based on constant precession of a hole or electron spin in a weak magnetic field, or based on optical spin control, in a stronger magnetic field. We theoretically compare four such schemes, using polarization or time-bin encoding, respectively, for a range of cavity environments and spin coherence times. In particular we study how different error mechanisms affect the different schemes, using a microscopic model of the spin control, the excitation and emission dynamics, and of the phonon bath. We find the spin-precession based schemes to scale well with strong cavity enhancement and to be naturally robust against phonon-induced decoherence, while the schemes using optical spin control can perform well for lower spin coherence times and are strongly dependent on the cooperativity of the cavity induced cycling transition. Our results provide a regime map for choosing between magnetic-field-driven and optically controlled protocols depending on spin coherence time, Purcell enhancement, and suppression of unwanted decay channels.