Electron Spin Lifetime Research Articles

Hole and electron spin lifetime in lightly n-doped silicon at low temperatures

We report on photoinduced inverse spin-Hall effect (ISHE) measurements as a function of the incident photon energy in the 4–50 K temperature range for a Pt/n-doped Si junction. Optical spin injection allows generating a spin-oriented population of electrons and holes around the Δ valleys and Γ point of the Si Brillouin zone, respectively. Spin-polarized carriers cross the Pt/Si contact and then enter the Pt overlayer, where spin-to-charge conversion occurs by means of spin-dependent scattering with Pt nuclei. For temperatures T up to 20 K, most of the dopants are not ionized, so that the electric field, stemming from the contact potential between Pt and Si, extends to the whole Si substrate, which becomes insulating, and only spin-oriented holes reach the Pt layer and contribute to the ISHE spectra. For T>20 K, donors are partially ionized, and the resulting space charge close to the Pt/Si interface leads to the formation of a Schottky contact where the electric field rapidly vanishes within a few micrometers. As a consequence, also spin-polarized electrons enter Pt by means of thermionic emission, contributing to the ISHE signal. We numerically solve the one-dimensional spin drift-diffusion equations for holes and electrons and estimate the temperature dependence of the spin lifetime in Si for both populations, demonstrating that Si may serve as a versatile platform for spintronic applications, able to leverage both electrons and holes.

(Invited) Fullerenes for Bio-Applications: The Way of Water

Nano-materials are emerging as important components for a series of bio-applications. In this context, carbon nanostructures such as graphene, carbon nanotubes and fullerenes have unique chemical and physical properties that make them attractive for such applications. Carbon nanomaterials have been used in tissue engineering, biosensing and drug delivery. They have also shown anti-microbial activity. They can be loaded with drugs for controlled local delivery. More importantly, the surface of carbon nanomaterials can be functionalised to enhance their biocompatibility by increasing their water-solubility.In this talk, I will present an update on our work in using fullerenes and their derivatives for developing bio-nanomaterials. I shall focus on the combination of carbon nanostructures and polymeric materials which can result in supramolecular hydrogels with interesting properties. Hydrogels are emerging as important materials in applications such as cellular scaffoldings in regenerative medicine/tissue engineering, tissue replacements, wound dressings, etc.I will present the synthesis of supramolecular hydrogels made from fullerenes of different sizes (C60, C84 and C90–92) that have been hydroxylated and functionalised with hydrophilic oligoethylene glycol chains. To the best of our knowledge, there have been no attempts made at incorporating higher fullerenes and their derivatives into such extended supramolecular structures, before now.I will also show possibilities for extending our approach to endohedral fullerenes such as N@C60. I will show how we can use advanced techniques including AFM to characterise the elasticity of the hydrogels formed. The ability to incorporate endohedral fullerenes in hydrogels may be important in extending the possibility for using these materials as sensors in vitro or even in vivo, exploiting the quantum properties of such molecular systems such as their extended electron and nuclear spin lifetimes.

Dielectric and Wavefunction Engineering of Electron Spin Lifetime in Colloidal Nanoplatelet Heterostructures.

Colloidal semiconductor nanoplatelets (NPLs) have emerged as low-cost and free-standing alternates of traditional quantum wells. The giant heavy- and light-hole splitting in NPLs allows for efficient optical spin injection. However, the electron spin lifetimes for prototypical CdSe NPLs are within a few picoseconds, likely limited by strong electron-hole exchange in these quantum- and dielectric-confined materials. Here how this hurdle can be overcome with engineered NPL-heterostructures is demonstrated. By constructing type-I CdSe/ZnS core/shell NPLs, dielectric screening inside the core is strongly enhanced, prolonging the electron spin polarization time (τesp) to over 30ps (or 60ps electron spin-flip time). Alternatively, by growing type-II CdSe/CdTe core/crown NPLs to spatially separate electron and hole wavefunctions, the electron-hole exchange is strongly suppressed, resulting in τesp as long as 300ps at room temperature. This study not only exemplifies how the well-established synthetic chemistry of colloidal heterostructures can aid in spin dynamics control but also establishes the feasibility of room-temperature coherent spin manipulation in colloidal NPLs.

High-resolution nanoscale NMR for arbitrary magnetic fields

Nitrogen vacancy (NV) centers are a major platform for the detection of nuclear magnetic resonance (NMR) signals at the nanoscale. To overcome the intrinsic electron spin lifetime limit in spectral resolution, a heterodyne detection approach is widely used. However, application of this technique at high magnetic fields is yet an unsolved problem. Here, we introduce a heterodyne detection method utilizing a series of phase coherent electron nuclear double resonance sensing blocks, thus eliminating the numerous Rabi microwave pulses required in the detection. Our detection protocol can be extended to high magnetic fields, allowing chemical shift resolution in NMR experiments. We demonstrate this principle on a weakly coupled 13C nuclear spin in the bath surrounding single NV centers, and compare the results to existing heterodyne protocols. Additionally, we identify the combination of NV-spin-initialization infidelity and strong sensor-target-coupling as linewidth-limiting decoherence source, paving the way towards high-field heterodyne NMR protocols with chemical resolution.

Coherence protection of spin qubits in hexagonal boron nitride

Spin defects in foils of hexagonal boron nitride are an attractive platform for magnetic field imaging, since the probe can be placed in close proximity to the target. However, as a III-V material the electron spin coherence is limited by the nuclear spin environment, with spin echo coherence times of ∽100 ns at room temperature accessible magnetic fields. We use a strong continuous microwave drive with a modulation in order to stabilize a Rabi oscillation, extending the coherence time up to ∽ 4μs, which is close to the 10 μs electron spin lifetime in our sample. We then define a protected qubit basis, and show full control of the protected qubit. The coherence times of a superposition of the protected qubit can be as high as 0.8 μs. This work establishes that boron vacancies in hexagonal boron nitride can have electron spin coherence times that are competitive with typical nitrogen vacancy centres in small nanodiamonds under ambient conditions.

Fast electron and slow hole spin relaxation in CsPbI3 nanocrystals

Spin-dependent properties of lead halide perovskites (LHPs) have recently gained significant attention paving their way toward spin-optoelectronic applications. However, separate measurements of the electron and hole spin relaxation rates are so far missing in LHPs. The knowledge of the electron and hole spin relaxation timescales is necessary to understand the spin-dependent properties of LHPs. Here, we report on the spin polarization dynamics in CsPbI3 nanocrystals (NCs). We employ polarization dependent ultrafast differential transmission spectroscopy (DTS) at room temperature to study the spin polarization dynamics in this system. In the case of pure CsPbI3 NCs, it is not possible to measure separately electron and hole spin relaxation rates from the polarization dependent DTS. Here, we introduce the soluble fullerene derivative PC60BM as an electron acceptor along with CsPbI3 to create an imbalance between the photoexcited electrons and holes in the NCs and, thus, affecting their spin-dependent carrier distribution. CsPbI3:PC60BM blend sample shows a distinct difference in the spin dependent kinetics of the DTS spectra as compared to the NCs-only sample. With the help of a kinetic model for the spin-dependent charge carrier distributions, we separately determine the electron and hole spin relaxation times in CsPbI3 NCs. We find that the room temperature hole spin lifetime (τh ∼ 5 ps) is ∼13 times longer than the electron spin lifetime (τe ∼ 0.4 ps). We ascribe the fast electron spin relaxation to the presence of strong spin–orbit coupling in the conduction band, which is ineffective for holes in the s-type valence band.

(Digital Presentation) Evaluation of Crack Propagation in Strained Sige on Ge(111) Patterned with Various Etching Thickness

Recently, it has been shown that Ge and SiGe with (111) orientation are very attractive for applications to spintronic devices because high quality ferromagnetic materials can be epitaxially grown on the Ge(111)[1]. Moreover strain introduction is expected to improve the spin lifetime of electrons. Previously we studied surface morphologies and strain states for the strained SiGe(111) layers grown on Ge(111), and found that cracks and related ridge roughness appear on the surfaces[2]. Moreover, we have demonstrated that such crack formation can be drastically suppressed by patterning of Ge-on-Si substrates [3]. In this work, we study mechanisms of the crack generation, propagation and network formation by evaluating strained SiGe layers grown on the patterned Ge substrates with various etching thicknesses.In experiments, the crystal growth was carried out with solid source molecular beam epitaxy. The Ge-on-Si(111) was fabricated using the so-called two-step growth method. 40 and 650 nm thick Ge layers were subsequently grown on a Si(111) substrate at 400 and 700 °C, respectively, followed by annealing at 800 °C for 10 min. Subsequently, the 80 µm × 80 µm square mesa-patterning of the Ge-on-Si(111) and Ge(111) substrates were performed by the standard photolithography process. Depending on etched depth, partially etched Ge-on-Si (PE-GOS), fully etched Ge-on-Si (FE-GOS) and partially etched Ge substrates (PE-Ge) were prepared where the etched depth outside of mesa was from 350 to 600 nm for PE-GOS and 350 nm to 1 µm for PE-Ge. Then strained Si0.2Ge0.8 layers with thicknesses of 250 nm were grown on these templates at 350 °C.It is found that completely no crack is formed on the surface of the SiGe layers on FE-GOS, whereas high density crack network appears both inside and outside of the mesa for the SiGe on PE-GOS. We consider that the cracks are firstly generated in the SiGe grown on Ge outside of the mesa, and the generated cracks propagate into the mesa, leading to the crack network on the mesa. It is also demonstrated that the crack generation does not occur in the strained SiGe grown on Si, that is, fully etched region outside of the mesa, and that the density of the crack generation sources are so low that the mesa area (80 µm × 80 µm square) is free from the crack generation. Thus, we can say that the Ge layer has to be completely etched down in order to suppress crack formation.A behavior of the crack propagation across the mesa boundary was next investigated in more detail in terms of the mesa etching thickness, that is, the step height of the mesa. We compared several samples of the strained SiGe layers that were grown on the PE-Ge substrates with various etching thickness. As a result, it is found that the crack network appears both inside and outside of the mesa in the case of the etching thickness below 1 µm. By contrast, for the sample with the etching thickness of 1 µm, the crack network arises only outside of the mesa and a crack-free SiGe is formed on the mesa. This observation indicates that the crack propagation across the mesa boundary takes place when the step height is below 1 µm. Since the strained SiGe layer is not continuous between inside and outside of the mesa for all samples studied here, not only the crack within the SiGe layer but also the crack penetrating into the Ge substrate is speculated to contribute to the crack propagation behavior.This work was supported in part by Grant-in-Aid for Scientific Research (Nos. 19H02175, 19H05616, 20K21009) from MEXT, Japan[1] K. Hamaya et al., J. Phys. D: Appl. Phys. 51, 393001(2018). [2] Md. M. Alam et al., Appl. Phys. Express 12, 081005 (2019). [3] Y. Wagatsuma et al., Appl. Phys. Express14 025502(2021). Figure 1

Spin helices in GaAs quantum wells: Interplay of electron density, spin diffusion, and spin lifetime

To establish a correlation between spin diffusion, spin lifetime, and electron density, we study spin polarization evolution in low-dimensional GaAs semiconductors hosting two-dimensional electron gases by employing time-resolved magneto-optical Kerr effect microscopy. It is shown that for the establishment of the longest spin-lifetime, the variation in the scattering rate with electron density is of more importance than fulfilling the persistent spin helix condition when the Rashba α and Dresselhaus β parameters are balanced. More specifically, regardless of α and β linear dependencies on the electron density, the spin relaxation rate is determined by the spin diffusion coefficient that depends on electron density nonmonotonously. The longest experimental spin-lifetime occurs at an electron density, corresponding to transition from Boltzmann to Fermi–Dirac statistics, which is several times higher than that when the persistent spin helix is expected. These facts highlight the role the electron density may play when considering applications for spintronic devices.

Electric fields and substrates dramatically accelerate spin relaxation in graphene

Electrons in graphene are theoretically expected to retain spin states much longer than most materials, making graphene a promising platform for spintronics and quantum information technologies. Here, we use first-principles density-matrix (FPDM) dynamics simulations to show that interaction with electric fields and substrates strongly enhance spin relaxation through scattering with phonons. Consequently, the relaxation time at room temperature reduces from microseconds in free-standing graphene to nanoseconds in graphene on hexagonal boron nitride (hBN) substrate, the order of magnitude typically measured in experiments. Further, inversion symmetry breaking by hBN introduces a stronger asymmetry in electron and hole spin lifetimes, than predicted by the conventional D'yakonov-Perel' (DP) model for spin relaxation. Deviations from the conventional DP model are stronger for in-plane spin relaxation, resulting in out-of-plane to in-plane lifetime ratios much greater than 1/2 with a maximum close to the Dirac point. These FPDM results, independent of symmetry-specific assumptions or material-dependent parameters, also validate recent modifications of the DP model to explain such deviations. Overall, our results indicate that spin-phonon relaxation in the presence of substrates may be more important in graphene than typically assumed, requiring consideration for graphene-based spin technologies at room temperature.

Photoluminescence of Double Quantum Wells: Asymmetry and Excitation Laser Wavelength Effects

Circularly polarized photoluminescence (PL) spectroscopy measured at 19 K on GaAs/AlGaAs symmetric and asymmetric double quantum wells (DQW) is reported. The PL is obtained by exciting the sample with a circularly polarized (left or right) laser in order to create an initial unbalanced distribution of electron spins in the conduction band and, in this way, obtain the electron spin lifetime . The effects of the excitation laser wavelength are estimated by exciting with laser wavelengths of 701.0, 787.0, 801.5, and 806.5 nm. The increase of with the excitation wavelength is attributed to the lower initial quasimomentum k of the excited carriers, which also reduces spin–orbit relaxation processes. is found to be higher in asymmetric DQWs: this is attributed to the wider QWs in these samples, which reduces spin relaxation due to the Dresselhaus mechanism. In addition, a smaller contribution from the Rashba mechanism is also detected by comparing samples with built‐in electric fields of different orientations defined by doped barrier layers.

Deterministic Shallow Dopant Implantation in Silicon with Detection Confidence Upper-Bound to 99.85% by Ion-Solid Interactions.

Silicon chips containing arrays of single dopant atoms can be the material of choice for classical and quantum devices that exploit single donor spins. For example, group-V donors implanted in isotopically purified 28 Si crystals are attractive for large-scale quantum computers. Useful attributes include long nuclear and electron spin lifetimes of 31 P, hyperfine clock transitions in 209 Bi or electrically controllable 123 Sb nuclear spins. Promising architectures require the ability to fabricate arrays of individual near-surface dopant atoms with high yield. Here, an on-chip detector electrode system with 70eV root-mean-square noise (≈20 electrons) is employed to demonstrate near-room-temperature implantation of single 14keV 31 P+ ions. The physics model for the ion-solid interaction shows an unprecedented upper-bound single-ion-detection confidence of 99.85 ± 0.02% for near-surface implants. As a result, the practical controlled silicon doping yield is limited by materials engineering factors including surface gate oxides in which detected ions may stop. For a device with 6nm gate oxide and 14keV 31 P+ implants, a yield limit of 98.1% is demonstrated. Thinner gate oxides allow this limit to converge to the upper-bound. Deterministic single-ion implantation can therefore be a viable materials engineering strategy for scalable dopant architectures in silicon devices.

Generating T centres in photonic silicon-on-insulator material by ion implantation

Global quantum networks will benefit from the reliable fabrication and control of high-performance solid-state telecom photon-spin interfaces. T radiation damage centres in silicon provide a promising photon-spin interface due to their narrow O-band optical transition near 1326 nm and long-lived electron and nuclear spin lifetimes. To date, these defect centres have only been studied as ensembles in bulk silicon. Here, we fabricate high concentration T centre ensembles in the 220 nm device layer of silicon-on-insulator wafers by ion implantation and subsequent annealing. We then develop a method that uses spin-dependent optical transitions to benchmark the characteristic optical spectral diffusion within these T centre ensembles. Using this new technique, we show that with minimal optimization to the fabrication process high densities of implanted T centres localized ≲100 nm from an interface display ∼1 GHz characteristic levels of total spectral diffusion.

Experimental extraction of donor-driven spin relaxation in n -type nondegenerate germanium

Using pure spin current transport measurements in lateral spin-valve devices, we study the spin relaxation in an $n$-type nondegenerate Ge layer, which is moderately doped Ge (P: $\ensuremath{\sim}{10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$). The obtained spin diffusion length $({\ensuremath{\lambda}}_{\mathrm{Ge}})$ of the nondegenerate Ge is two to three times greater than that of heavily doped degenerate Ge (P: $\ensuremath{\sim}{10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) in the temperature range from 8 to 100 K. We find that the electron spin lifetime ($\ensuremath{\tau}$) for the nondegenerate Ge is monotonically increased with decreasing temperature $(T)$. The increase in $\ensuremath{\tau}$ at temperatures less than 50 K can be interpreted in terms of the donor-driven spin relaxation mechanism including the 1/$\sqrt{T}$ behavior in multivalley semiconductors, proposed by Song et al. [Y. Song, O. Chalaev, and H. Dery, Phys. Rev. Lett. 113, 167201 (2014)]. We note that it is important for the $\ensuremath{\tau}$ of the moderately doped nondegenerate Ge to partly consider the $T$-independent component of spin relaxation in addition to the 1/$\sqrt{T}$ component.

Electron and Hole Spin Relaxation in CdSe Colloidal Nanoplatelets.

Solution-processed quantum-confined nanocrystals are important building blocks for scalable implementation of quantum information science. Extensive studies on colloidal quantum dots (QDs) have revealed subpicosecond hole spin relaxation, whereas the electron spin dynamics remains difficult to probe. Here we study electron and hole spin dynamics in CdSe colloidal nanoplatelets (also called quantum wells) of varying thicknesses using circularly polarized transient absorption spectroscopy at room temperature. The clear spectroscopic features of transition bands associated with heavy, light, and spin-orbit split-off holes enabled separate probes of electron and hole dynamics. The hole spin-flip occurred within ∼200 fs, arising from strong spin-orbit coupling in the valence band. The electron spin lifetime decreased from 6.2 to 2.2 ps as the platelet thickness is reduced from 6 to 4 monolayers, reflecting an exchange interaction between the electron and the hole and/or surface dangling bond spins enhanced by quantum confinement.

Increased Critical Thickness for Strained SiGe on Ge-on-Si(111)

Since a Ge has been recognized as one of the promising alternative materials to Si, many researches have been pursued mostly based on a bulk Ge with a (100) surface orientation. Recently, it has been shown that a Ge with a (111) orientation is more favorable for applications to spintronics devices because high quality ferromagnetic materials can be epitaxially grown on the Ge(111)[1]. Also a strained SiGe(111) is expected to improve the spin lifetime of electrons. So far, however, epitaxial growth of tensile-strained SiGe on Ge has not been explored sufficiently.Recently we systematically investigated critical thickness of the strained SiGe/Ge-on-Si(111) [2], and found that unusual line-shaped ridge roughness appears on the SiGe surface at the initial stage of strain relaxation. Moreover, we observed generation of cracks extending deep into underlying Ge layer in the ridge region, imposing essential requirements to suppress the formation of cracks and related ridges. For this purpose, we attempted the growth of a strained SiGe on a Ge-on-Si(111) that was patterned in line & space mesa and succeeded in suppression of the ridge roughness formation [3]. Toward device applications, such as spintronics devices, we need to widen the area of the crack-free strained SiGe and to increase critical thickness of the strained SiGe/Ge-on-Si.In this work, we employ much larger (~ 50 um) square-shaped mesa-pattern of the Ge-on-Si and succeed in the growth of crack-free strained SiGe with the thickness which largely exceeds the expected critical thickness.In experiments, the crystal growth was carried out with solid source molecular beam epitaxy. The Ge-on-Si(111) was fabricated using the so-called two-step growth method. The 40 and 650 nm thick Ge layers were subsequently grown on a Si(111) substrate at 400 and 700 °C, respectively, followed by annealing at 800 °C for 10 min. For experiments of the SiGe growth on the patterned Ge(111) and Ge-on-Si(111), one of the Ge(111) samples and grown Ge-on-Si(111) samples was subjected to photolithography process[3]. Subsequently the 200 to 250 nm thick SiGe layer was grown on the patterned Ge(111) and Ge-on-Si(111) substrate at 350 °C.Figures 1(a)-1(d) show laser microscope (LM) surface images of 200 or 250 nm thick strained SiGe layers grown on (a) the un-patterned Ge-on-Si(111), (b) the patterned Ge-on-Si(111), (c) the un-patterned Ge(111) substrate and (d) patterned Ge(111) substrate. For the 250 nm thick Si0.2Ge0.8 on the un-patterned GOS [Fig. 1(a)], we can clearly observe high-density ridge roughness running along three equivalent [1-10], [10-1] and [01-1] directions, which are corresponding to intersections of surface and (111) slip planes. The ridge roughness involves cracks along the plane. In contrast, for the 250 nm thick Si0.2Ge0.8 on the patterned GOS [Fig. 1(b)], we can find the surface is very flat without such ridge roughness. This result implies that the strain energy accommodated in the strained SiGe is effectively released at mesa edges, which can eliminate a driving force for the ridge roughness formation, that is, the crack formation. It is very interesting to compare these results with those obtained for the SiGe on Ge substrates. Figs. 1(c) and (d) shows ridge roughness appears for both 200 nm thick SiGe layers grown on the patterned and un-patterned Ge(111) substrates. This means that the patterning cannot suppress the ridge formation for the case of the strained SiGe on the Ge(111) substrate. Note that the ridge roughness is seen to be connected between the mesa patterned and etched areas, implying that the SiGe film grown on the etched area affects the ridge formation mechanism.In conclusion, we can say that the critical thickness of the strained SiGe can be significantly increased by means of growing the SiGe layer on the patterned Ge-on-Si(111) and the 250 nm thick strained Si0.2Ge0.8 can be grown without any roughness and crack formation. This thickness largely exceeds an expected critical thickness for the Si0.2Ge0.8/Ge(111) [2]. It can be said that this method can highly widen the applicability of the strained SiGe(111) heterostructures to spintronics and high performance electronic devices.This work was supported in part by MEXT Supported Program for theStrategic Research Foundation at Private Universities 2015–2019 and by Grant-in-Aid for Scientific Research (Nos. 16H02333, 19H02175 and 19H05616) from MEXT, Japan[1] K. Hamaya et al., J. Phys. D: Appl. Phys. 51, 393001 (2018).[2] Md. M. Alam et al., Appl. Phys. Express 12, 081005 (2019).[3] Y. Wagatsuma et al., Mater. Sci. Semicond. Process. 117, 105153 (2020). Figure 1

Oblique Nuclear Quadrupole Interaction in Self-Assembled Structures Based on Semiconductor Quantum Dots

Dynamic nuclear polarization (DNP) is well recognized as being important in spintronics and quantum-information processing. DNP gives rise to high nuclear spin polarization that not only can prolong electron-spin lifetime by generating an Overhauser field (OHF), but also has fertilized the idea to explore nuclear spin qubits. In strained quantum-dot structures (QDSs), a nuclear spin is coupled to a strain field via its quadrupole moment. It has been shown that such nuclear quadrupole interaction (NQI) can be used to achieve appreciable DNP and hence electron-spin polarization. Here, we uncover magneto-optical anomalies from a series of laterally arranged $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ QDSs that arise from the NQI and DNP in these nanostructures. We find that the principal axis of NQI in symmetry-lowered QDSs significantly deviates from the growth direction, resulting in tilting of OHF with an angle exceeding 37\ifmmode^\circ\else\textdegree\fi{}. The resulting transverse component of OHF is probed with respect to the crystallographic orientations and its influence on the DNP and ensemble spin dephasing is analyzed. We show that a high-symmetry electronic confinement potential for excitons does not guarantee a high-symmetry NQI for nuclei within the same nano-object, thereby calling for correlated optimization in the symmetry of the electronic confinement potential and that of the nuclear spin bath. Our results underline the role of oblique NQI in electron-spin decoherence and depolarization, which has so far largely been overlooked. This work thus sheds light on design rules for engineering the electronic and spin landscape of QDSs for better performance of DNP desirable for applications in spintronics and quantum computation.

Silicon-Integrated Telecommunications Photon-Spin Interface

Long-distance entanglement distribution is a vital capability for quantum technologies. An outstanding practical milestone towards this aim is the identification of a suitable matter-photon interface which possesses, simultaneously, long coherence lifetimes and efficient telecommunications-band optical access. In this work, alongside its sister publication, we report upon the T center, a silicon defect with spin-selective optical transitions at 1326 nm in the telecommunications O-band. Here we show that the T center in $^{28}$Si offers electron and nuclear spin lifetimes beyond a millisecond and second respectively, as well as optical lifetimes of 0.94(1) $\mu$s and a Debye-Waller factor of 0.23(1). This work represents a significant step towards coherent photonic interconnects between long-lived silicon spins, spin-entangled telecom single-photon emitters, and spin-dependent silicon-integrated photonic nonlinearities for future global quantum technologies.

Low-noise GaAs quantum dots for quantum photonics

Quantum dots are both excellent single-photon sources and hosts for single spins. This combination enables the deterministic generation of Raman-photons—bandwidth-matched to an atomic quantum-memory—and the generation of photon cluster states, a resource in quantum communication and measurement-based quantum computing. GaAs quantum dots in AlGaAs can be matched in frequency to a rubidium-based photon memory, and have potentially improved electron spin coherence compared to the widely used InGaAs quantum dots. However, their charge stability and optical linewidths are typically much worse than for their InGaAs counterparts. Here, we embed GaAs quantum dots into an n-i-p-diode specially designed for low-temperature operation. We demonstrate ultra-low noise behaviour: charge control via Coulomb blockade, close-to lifetime-limited linewidths, and no blinking. We observe high-fidelity optical electron-spin initialisation and long electron-spin lifetimes for these quantum dots. Our work establishes a materials platform for low-noise quantum photonics close to the red part of the spectrum.

Suppression of thermally excited electron-spin relaxation in InGaAs quantum dots using p-doped capping layers toward enhanced room-temperature spin polarization

The suppression of a thermally excited electron-spin relaxation in InGaAs quantum dots (QDs) using p-doped capping layers toward enhanced room-temperature (RT) spin polarization has been demonstrated, in which the electron-spin polarization in QD excited states (ESs) was measured through time-resolved spin-dependent photoluminescence. We revealed that the p-doping of QDs can enhance the emission intensity of QD-ES by approximately twofold to threefold over a wide temperature range. An electron-spin relaxation time of 106 ps was observed at 293 K for p-doped QDs, which is approximately three times longer than the radiative lifetime of 36 ps, relative to the shorter electron-spin relaxation time of 71 ps for undoped QDs. The increased electron-spin lifetime was mainly attributed to the suppressed relaxation of the electron spin reinjected from the p-doped capping barrier after thermal escape from an ES, where the D'yakonov-Perel' spin relaxation in the barrier was potentially weakened through impurity scattering. These results suggest that InGaAs QDs with p-doped capping layers have a significant advantage for use in spin-functional optical active layers with a higher spin polarization toward RT.

Spin noise at electron paramagnetic resonance

We develop a microscopic theory of spin noise in solid-state systems at electron paramagnetic resonance, when the spin dynamics is driven by static and radio-frequency (rf) magnetic fields and the stochastic effective magnetic field stemming from the interaction with environment. The rf field splits the peaks in the power spectrum of spin noise into the Mollow-type triplets and also gives rise to additional spin-spin correlations which oscillate in the absolute time at the rf frequency and the double frequency. Even in systems with strong inhomogeneous broadening, the spin-noise spectrum contains narrow lines insensitive to the dispersion of the effective $g$ factors. Thus, the measurements of spin noise at electron paramagnetic resonance provide an access to the intrinsic spin lifetime of electrons.