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Microwave resonators with a keyhole profile (KHRs) operating in the C band and the X band are designed and studied in numerical simulations and experiments. KHR structures concentrate a microwave magnetic field in a subwavelength volume, while suppressing microwave electric fields. This microwave magnetic field is focused at a finite working distance from KHR metal structures, allowing convenient optical excitation of the sample in both the Faraday geometry and the Voigt geometry. By means of room-temperature optically detected electron spin resonance on $\mathrm{Si}\mathrm{C}$ quantum defects, the conversion factor ${B}_{1}{P}_{\mathrm{MW}}^{\ensuremath{-}1/2}$ for conversion of microwave power into a microwave magnetic field is measured to be approximately $1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.2em}{0ex}}\mathrm{T}\phantom{\rule{0.1em}{0ex}}{\mathrm{W}}^{\ensuremath{-}1/2}$ at a frequency of approximately 7 GHz and a working distance of approximately 0.5 mm from the KHR structure. Numerical simulations match the experimental observations, and an example model code for use with the finite-element-method program elmer is provided. The KHR structures are most promising for fast coherent electron-spin control in solid-state spin qubits, where a large microwave magnetic field needs to be achieved with simultaneous suppression of microwave heating and electric fields, while permitting efficient optical spin initialization and readout.

Combining external control with long spin lifetime and coherence is a key challenge for solid state spin qubits. Tunnel coupling with electron Fermi reservoir provides robust charge state control in semiconductor quantum dots, but results in undesired relaxation of electron and nuclear spins through mechanisms that lack complete understanding. Here, we unravel the contributions of tunnelling-assisted and phonon-assisted spin relaxation mechanisms by systematically adjusting the tunnelling coupling in a wide range, including the limit of an isolated quantum dot. These experiments reveal fundamental limits and trade-offs of quantum dot spin dynamics: while reduced tunnelling can be used to achieve electron spin qubit lifetimes exceeding 1 s, the optical spin initialisation fidelity is reduced below 80%, limited by Auger recombination. Comprehensive understanding of electron-nuclear spin relaxation attained here provides a roadmap for design of the optimal operating conditions in quantum dot spin qubits.

Epitaxial quantum dots (QDs) have long been identified as promising charge spin qubits offering an efficient interface to quantum light and advanced semiconductor nanofabrication technologies. However, charge spin coherence is limited by interaction with the nanoscale ensemble of atomic nuclear spins, which is particularly problematic in strained self-assembled dots. Here, we use strain-free GaAs/AlGaAs QDs, demonstrating a fully functioning two-qubit quantum register using the nanoscale ensemble of arsenic quadrupolar nuclear spins as its hardware. Tailored radio-frequency pulses allow quantum state storage for up to 20 ms, and are used for few-microsecond single-qubit and two-qubit control gates with fidelities exceeding 97%. Combining long coherence and high-fidelity control with optical initialization and readout, we implement benchmark quantum computations such as Grover’s search and the Deutsch–Jozsa algorithm. Our results identify QD nuclei as a potential quantum information resource, which can complement charge spins and light particles in future QD circuits. Epitaxial quantum dot charge spin qubits offer efficient quantum light links, but their coherence is limited by interactions with the nanoscale ensemble of atomic nuclear spins. Employing nuclear spins instead as its hardware, strain-free GaAs/AlGaAs quantum dots can constitute a fully functional two-qubit quantum register.

Coherence of solid state spin qubits is limited by decoherence and random fluctuations in the spin bath environment. Here we develop spin bath control sequences which simultaneously suppress the fluctuations arising from intrabath interactions and inhomogeneity. Experiments on neutral self-assembled quantum dots yield up to a five-fold increase in coherence of a bare nuclear spin bath. Numerical simulations agree with experiments and reveal emergent thermodynamic behaviour where fluctuations are ultimately caused by irreversible conversion of coherence into many-body quantum entanglement. Simulations show that for homogeneous spin baths our sequences are efficient with non-ideal control pulses, while inhomogeneous bath coherence is inherently limited even under ideal-pulse control, especially for strongly correlated spin-9/2 baths. These results highlight the limitations of self-assembled quantum dots and advantages of strain-free dots, where our sequences can be used to control the fluctuations of a homogeneous nuclear spin bath and potentially improve electron spin qubit coherence., Fluctuating nuclear spin ensembles are a significant decoherence mechanism for solid-state spin qubits. Here the authors introduce an approach to controlling and extending the coherence of a nuclear spin bath around self-assembled quantum dots and gain insight into the many-body dynamics.

Recently, we have calibrated the diagonal components ${S}_{11}={S}_{xxxx}$ of the GaAs gradient elastic tensor ${S}_{ijkl}$ using nuclear magnetic resonance (NMR) and photoluminescence spectroscopy of GaAs/AlGaAs quantum dot structures in Faraday geometry [E. A. Chekhovich et al., Phys. Rev. B 97, 235311 (2018)]. Here we measure quantum dot NMR spectra in oblique magnetic fields giving access to the off-diagonal components ${S}_{yzyz}$. We find the ratios ${S}_{yzyz}/{S}_{xxxx}\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}+1.{98}_{\ensuremath{-}0.27}^{+0.21}$ for $^{75}\mathrm{As}$ and $\ensuremath{\approx}\ensuremath{-}0.{40}_{\ensuremath{-}0.31}^{+0.23}$ for $^{69}\mathrm{Ga}$. Combined with our previous results, we find all independent nonzero components of ${S}_{ijkl}$: $Q{S}_{xxxx}\ensuremath{\approx}+0.758\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ V, $Q{S}_{yzyz}\ensuremath{\approx}+1.51\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ V for $^{75}\mathrm{As}$ and $Q{S}_{xxxx}\ensuremath{\approx}\ensuremath{-}0.377\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ V, $Q{S}_{yzyz}\ensuremath{\approx}+0.151\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ V for $^{69}\mathrm{Ga}$, where $Q$ is the corresponding nuclear quadrupolar moment. Our results show that earlier nuclear acoustic resonance experiments [R. K. Sundfors, Phys. Rev. B 10, 4244 (1974)] most likely overestimated the GaAs gradient elastic tensors, especially for $^{69}\mathrm{Ga}$. We further use NMR spectroscopy in oblique fields for assumption-free measurement of the intrinsic strain in GaAs/AlGaAs quantum dots. We find deviations of the strain principle directions from the sample growth axis, which vary between individual quantum dots.

One of the key challenges in spectroscopy is the inhomogeneous broadening that masks the homogeneous spectral lineshape and the underlying coherent dynamics. Techniques such as four-wave mixing and spectral hole-burning are used in optical spectroscopy, and spin-echo in nuclear magnetic resonance (NMR). However, the high-power pulses used in spin-echo and other sequences often create spurious dynamics obscuring the subtle spin correlations important for quantum technologies. Here we develop NMR techniques to probe the correlation times of the fluctuations in a nuclear spin bath of individual quantum dots, using frequency-comb excitation, allowing for the homogeneous NMR lineshapes to be measured without high-power pulses. We find nuclear spin correlation times exceeding one second in self-assembled InGaAs quantum dots - four orders of magnitude longer than in strain-free III-V semiconductors. This observed freezing of the nuclear spin fluctuations suggests ways of designing quantum dot spin qubits with a well-understood, highly stable nuclear spin bath.

We achieve direct detection of electron hyperfine shifts in individual CdTe/ZnTe quantum dots. For the previously inaccessible regime of strong magnetic fields B_{z}≳0.1 T, we demonstrate robust polarization of a few-hundred-particle nuclear spin bath, with an optical initialization time of ∼1 ms and polarization lifetime exceeding ∼1 s. Nuclear magnetic resonance spectroscopy of individual dots reveals strong electron-nuclear interactions characterized by Knight fields |B_{e}|≳50 mT, an order of magnitude stronger than in III-V semiconductor quantum dots. Our studies confirm II-VI semiconductor quantum dots as a promising platform for hybrid electron-nuclear spin qubit registers, combining the excellent optical properties comparable to III-V dots and the dilute nuclear spin environment similar to group-IV semiconductors.

Lattice matched GaAs/AlGaAs epitaxial structures with quantum dots are studied at $T=4.2$ K under static uniaxial stress applied either along the [001] or [110] crystal directions. We conduct simultaneous measurements of the spectral shifts in the photoluminescence of the bulk GaAs substrate, which relate to strain via deformation potentials $a$ and $b$, and the quadrupolar shifts in the optically detected nuclear magnetic resonance spectra of the quantum dots, which relate to the same strain via the gradient-elastic tensor ${S}_{ijkl}$. Measurements in two uniaxial stress configurations are used to derive the ratio $b/a=0.242\ifmmode\pm\else\textpm\fi{}0.008$ in good agreement with previous studies on GaAs. Based on the previously estimated value of $a\ensuremath{\approx}\ensuremath{-}8.8$ eV we derive the product of the nuclear quadrupolar moment $Q$ and the $S$-tensor diagonal component in GaAs to be $Q{S}_{11}\ensuremath{\approx}+0.758\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ V for $^{75}\mathrm{As}$ and $Q{S}_{11}\ensuremath{\approx}\ensuremath{-}0.377\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ V for $^{69}\mathrm{Ga}$ nuclei. In our experiments the signs of ${S}_{11}$ are directly measurable, which was not possible in the earlier nuclear acoustic resonance studies. Our $Q{S}_{11}$ values are a factor of $\ensuremath{\sim}1.4$ smaller than those derived from the nuclear acoustic resonance experiments [Phys. Rev. B 10, 4244 (1974)]. The gradient-elastic tensor values measured in this work can be applied in structural analysis of strained III-V semiconductor nanostructures via accurate modeling of their magnetic resonance spectra.

Deep cooling of electron and nuclear spins is equivalent to achieving polarization degrees close to 100% and is a key requirement in solid-state quantum information technologies. While polarization of individual nuclear spins in diamond and SiC (ref. ) reaches 99% and beyond, it has been limited to 50-65% for the nuclei in quantum dots. Theoretical models have attributed this limit to formation of coherent 'dark' nuclear spin states but experimental verification is lacking, especially due to the poor accuracy of polarization degree measurements. Here we measure the nuclear polarization in GaAs/AlGaAs quantum dots with high accuracy using a new approach enabled by manipulation of the nuclear spin states with radiofrequency pulses. Polarizations up to 80% are observed-the highest reported so far for optical cooling in quantum dots. This value is still not limited by nuclear coherence effects. Instead we find that optically cooled nuclei are well described within a classical spin temperature framework. Our findings unlock a route for further progress towards quantum dot electron spin qubits where deep cooling of the mesoscopic nuclear spin ensemble is used to achieve long qubit coherence. Moreover, GaAs hyperfine material constants are measured here experimentally for the first time.

The interaction of an electronic spin with its nuclear environment, an issue known as the central spin problem, has been the subject of considerable attention due to its relevance for spin-based quantum computation using semiconductor quantum dots. Independent control of the nuclear spin bath using nuclear magnetic resonance techniques and dynamic nuclear polarization using the central spin itself offer unique possibilities for manipulating the nuclear bath with significant consequences for the coherence and controlled manipulation of the central spin. Here we review some of the recent optical and transport experiments that have explored this central spin problem using semiconductor quantum dots. We focus on the interaction between 10(4)-10(6) nuclear spins and a spin of a single electron or valence-band hole. We also review the experimental techniques as well as the key theoretical ideas and the implications for quantum information science.

Much new solid-state technology for single-photon sources, detectors, photovoltaics and quantum computation relies on the fabrication of strained semiconductor nanostructures. Development of such devices depends on techniques allowing structural analysis on the nanometer scale. However, commonly used microscopy methods are destructive, leading to the loss of the important link between the obtained structural information and the electronic and optical properties of the device. Nuclear magnetic resonance (NMR) is an attractive option for non-invasive structural analysis, but detecting NMR signals from few-nanometer sized structures is very challenging. The problem is further exacerbated in III-V semiconductors where all nuclei possess quadrupole moments resulting in extremely broad and weak NMR signals. Here I report on a recent development of high sensitivity techniques that solve these problems and move optically detected NMR to a new regime, allowing high-resolution spectroscopy of as few as 105 nuclei in highly strained nanostructures.

GaAs/AlGaAs quantum dots grown by in situ droplet etching and nanohole in-filling offer a combination of strong charge confinement, optical efficiency, and high spatial symmetry advantageous for polarization entanglement and spin-photon interface. Here, we study experimentally electron and nuclear spin properties of such dots. We find nearly vanishing electron $g$ factors $({g}_{e}l0.05)$, providing a potential route for electrically driven spin control schemes. Optical manipulation of the nuclear spin environment is demonstrated with nuclear spin polarization up to $65%$ achieved. Nuclear magnetic resonance spectroscopy reveals two distinct types of quantum dots: with tensile and with compressive strain along the growth axis. In both types of dots, the magnitude of strain ${\ensuremath{\epsilon}}_{b}l0.02%$ is nearly three orders of magnitude smaller than in self-assembled dots: On the one hand, this provides a route for eliminating a major source of electron spin decoherence arising from nuclear quadrupolar interactions, and on the other hand such strain is sufficient to suppress nuclear spin diffusion leading to a stable nuclear spin bath with nuclear spin lifetimes exceeding 500 s. The spin properties revealed in this work make this new type of quantum dot an attractive alternative to self-assembled dots for the applications in quantum information technologies.

Photoluminescence spectroscopy has been employed to study CdSe/ZnSe/ZnMnSe quantum dots. For most of the dots studied here luminescence comes in three spectrally separated features: neutral exciton (X), biexciton (XX), and charged exciton (X C ) states. Spectral properties of X and XX emission are well understood, however, in a marked contrast with previous studies, the observed fine structure of X C can not be explained within a commonly accepted model of a ground state trion luminescence. We find that at zero magnetic field luminescence from the charged state exhibits fine structure that varies gradually between different dots from a single unpolarized line to a quartet with the maximum splitting of 2 meV. Several models including magnetic polaron formation and double charging have been considered, but a plausible explanation can be given only if one considers the influence of a charge trapped in a nearby dot.

Low-temperature (T = 1.6 K) photoluminescence (PL) of individual CdSe/ZnSe/ZnMnSe quantum dots (QDs) with different magnitudes of the sp-d exchange interaction between the magnetic impurity ions and charge carriers has been studied in a magnetic field up to 12 T applied in the Faraday and Voigt geometry. The magnitude of the interaction was controlled by changing the fraction (ηe, h) of the squared wave function of charge carriers in the semimagnetic barrier by means of variation of the nonmagnetic (ZnSe) layer thickness. It is established that the sp-d exchange interaction leads to a change in the sign of the effective hole g factor even for ηe, h ∼ 5%, while further increase in the interaction magnitude is accompanied by a rapid growth in the magnitude of spin splitting for both electrons and holes. The quantum yield of PL exhibits a significant decrease due to nonradiative Auger recombination with the excitation of Mn ions only for ηe, h ∼ 12%, while the rate of the holes spin relaxation starts growing only for still higher ηe, h values. In a strong magnetic field perpendicular to the sample plane, the alignment of Mn spins leads to suppression of the Auger recombination only in the excited spin state. For a small rate of the hole spin relaxation, this leads to a rather unusual result: the emission from an excited trion state predominates in strong magnetic fields.

Interaction with nuclear spins leads to decoherence and information loss in solid-state electron-spin qubits. One particular, ineradicable source of electron decoherence arises from decoherence of the nuclear spin bath, driven by nuclear–nuclear dipolar interactions. Owing to its many-body nature nuclear decoherence is difficult to predict, especially for an important class of strained nanostructures where nuclear quadrupolar effects have a significant but largely unknown impact. Here, we report direct measurement of nuclear spin bath coherence in individual self-assembled InGaAs/GaAs quantum dots: spin-echo coherence times in the range 1.2–4.5 ms are found. Based on these values, we demonstrate that strain-induced quadrupolar interactions make nuclear spin fluctuations much slower compared with lattice-matched GaAs/AlGaAs structures. Our findings demonstrate that quadrupolar effects can potentially be used to engineer optically active III-V semiconductor spin-qubits with a nearly noise-free nuclear spin bath, previously achievable only in nuclear spin-0 semiconductors, where qubit network interconnection and scaling are challenging., Decoherence of the nuclear spin bath causes decoherence of electron spin qubits in the solid state. Here, Chekhovich et al. use spin-echo measurements to demonstrate the suppression of nuclear spin fluctuations in semiconductor quantum dots via strain-induced quadrupolar interactions.

Dynamics of nuclear spin decoherence and nuclear spin flip-flops in self-assembled InGaAs/GaAs quantum dots are studied experimentally using optically detected nuclear magnetic resonance (NMR). Nuclear spin-echo decay times are found to be in the range 1-4 ms. This is a factor of ~3 longer than in strain-free GaAs/AlGaAs structures and is shown to result from strain-induced quadrupolar effects that suppress nuclear spin flip-flops. The correlation times of the flip-flops are examined using a novel frequency-comb NMR technique and are found to exceed 1 s, a factor of ~1000 longer than in strain-free structures. These findings complement recent studies of electron spin coherence and reveal the paradoxical dual role of the quadrupolar effects in self-assembled quantum dots: large increase of the nuclear spin bath coherence and at the same time significant reduction of the electron spin-qubit coherence. Approaches to increasing electron spin coherence are discussed. In particular the nanohole filled GaAs/AlGaAs quantum dots are an attractive option: while their optical quality matches the self-assembled dots the quadrupolar effects measured in NMR spectra are a factor of 1000 smaller.

A wealth of atomistic information is contained within a self-assembled quantum dot (QD), associated with its chemical composition and the growth history. In the presence of quadrupolar nuclei, as in InGaAs QDs, much of this is inherited to nuclear spins via the coupling between the strain within the polar lattice and the electric quadrupole moments of the nuclei. Here, we present a computational study of the recently introduced inverse spectra nuclear magnetic resonance technique to assess its suitability for extracting such structural information. We observe marked spectral differences between the compound InAs and alloy InGaAs QDs. These are linked to the local biaxial and shear strains, and the local bonding configurations. The cation-alloying plays a crucial role especially for the arsenic nuclei. The isotopic line profiles also largely differ among nuclear species: While the central transition of the gallium isotopes have a narrow linewidth, those of arsenic and indium are much broader and oppositely skewed with respect to each other. The statistical distributions of electric field gradient (EFG) parameters of the nuclei within the QD are analyzed. The consequences of various EFG axial orientation characteristics are discussed. Finally, the possibility of suppressing the first-order quadrupolar shifts is demonstrated by simply tilting the sample with respect to the static magnetic field., Comment: Published version, 17 pages, 18 figures

Two-dimensional (2D) compounds provide unique building blocks for novel layered devices and hybrid photonic structures. However, large surface-to-volume ratio in thin films enhances the significance of surface interactions and charging effects requiring new understanding. Here we use micro-photoluminescence (PL) and ultrasonic force microscopy to explore the influence of the dielectric environment on optical properties of a few monolayer MoS2 films. PL spectra for MoS2 films deposited on SiO2 substrates are found to vary widely. This film-to-film variation is suppressed by additional capping of MoS2 with SiO2 and SiN, improving mechanical coupling of MoS2 with surrounding dielectrics. We show that the observed PL non-uniformities are related to strong variation in the local electron charging of MoS2 films. In completely encapsulated films, negative charging is enhanced leading to uniform optical properties. Observed great sensitivity of optical characteristics of 2D films to surface interactions has important implications for optoelectronics applications of layered materials., latest version

We study experimentally the dependence of dynamic nuclear spin polarization on the power of non-resonant optical excitation in two types of individual neutral semiconductor quantum dots: InGaAs/GaAs and GaAs/AlGaAs. We show that the mechanism of nuclear spin pumping via second order recombination of optically forbidden (''dark'') exciton states recently reported in InP/GaInP quantum dots [Phys. Rev. B 83, 125318 (2011)] is relevant for material systems considered in this work. In the InGaAs/GaAs dots this nuclear spin polarization mechanism is particularly pronounced, resulting in Overhauser shifts up to ~80 micro-eV achieved at optical excitation power ~1000 times smaller than the power required to saturate ground state excitons. The Overhauser shifts observed at low-power optical pumping in the interface GaAs/AlGaAs dots are generally found to be smaller (up to ~40 micro-eV). Furthermore in GaAs/AlGaAs we observe dot-to-dot variation and even sign reversal of the Overhauser shift which is attributed to dark-bright exciton mixing originating from electron-hole exchange interaction in dots with reduced symmetry. Nuclear spin polarization degrees reported in this work under ultra-low power optical pumping are comparable to those achieved by techniques such as resonant optical pumping or above-gap pumping with high power circularly polarized light. Dynamic nuclear polarization via second-order recombination of ''dark'' excitons may become a useful tool in single quantum dot applications, where manipulation of the nuclear spin environment or electron spin is required., Submitted to Phys. Rev. B

We realize growth of self-catalyzed core-shell GaAs/GaAsP nanowires (NWs) on Si substrates using molecular-beam epitaxy. Transmission electron microscopy (TEM) of single GaAs/GaAsP NWs confirms their high crystal quality and shows domination of the zinc-blende phase. This is further confirmed in optics of single NWs, studied using cw and time-resolved photoluminescence (PL). A detailed comparison with uncapped GaAs NWs emphasizes the effect of the GaAsP capping in suppressing the non-radiative surface states: significant PL enhancement in the core-shell structures exceeding 2000 times at 10K is observed; in uncapped NWs PL is quenched at 60K whereas single core-shell GaAs/GaAsP NWs exhibit bright emission even at room temperature. From analysis of the PL temperature dependence in both types of NW we are able to determine the main carrier escape mechanisms leading to the PL quench.

Strained semiconductor nanostructures can be used to make single-photon sources1, detectors2 and photovoltaic devices3, and could potentially be used to create quantum logic devices4, 5. The development of such applications requires techniques capable of nanoscale structural analysis, but the microscopy methods6, 7, 8 typically used to analyse these materials are destructive. NMR techniques can provide non-invasive structural analysis, but have been restricted to strain-free semiconductor nanostructures9, 10, 11 because of the significant strain-induced quadrupole broadening of the NMR spectra12, 13, 14. Here, we show that optically detected NMR spectroscopy can be used to analyse individual strained quantum dots. Our approach uses continuous-wave broadband radiofrequency excitation with a specially designed spectral pattern and can probe individual strained nanostructures containing only 1 × 105 quadrupole nuclear spins. With this technique, we are able to measure the strain distribution and chemical composition of quantum dots in the volume occupied by the single confined electron. The approach could also be used to address problems in quantum information processing such as the precise control of nuclear spins15, 16, 17 in the presence of strong quadrupole effects18, 19, 20, 21.

Control and understanding of the nuclear spin environment in nano-structures is of great importance in achieving robust coherence of spin-based qubits in the solid state. Recently, optical pumping of nuclear spins in quantum dots (QDs) has been demonstrated [1–8]. Dynamic nuclear polarization (DNP) due to the electron-nuclear hyperfine interaction (HI) occurs when non-equilibrium populations of electron spin states are created using resonant or quasi-resonant circularly polarized light of high intensity. Under such conditions, a direct correspondence between the sign of circular polarization of the exciting light and the direction of the nuclear spin alignment is observed [1–5, 9]. By contrast, suppression of the electron spin alignment under above barrier non-resonant excitation results in negligible nuclear spin polarization in a dot. Here we report measurements on individual neutral InP/GaInP quantum dots, which shed new light on the mechanisms of DNP in semiconductor nano-structures. They reveal previously unobserved phenomena: at low pumping levels, independent of light polarization and wavelength, optical pumping induces an effective nuclear field, which is always parallel to the external field applied along the growth axis of the structure. We show that at low power DNP in a neutral dot occurs via the second order recombination of ”dark” excitons accompanied by electron-nuclear spin flip-flop. This is revealed in photoluminescence (PL) measurements where optically inactive ”dark” excitons are observed due to weak mixing with the ”bright” states. Asymmetry in the energy splitting of the excitonic energy levels induced by electron-hole exchange interaction leads to a strong difference of DNP rates induced by ”dark” excitons with opposite spins [10]. As a result, the direction of nuclear polarization is independent of the average electron spin polarization on the dot. It is instead controlled by the direction of the external magnetic field experienced by the exciton. By contrast in the regime of higher powers, where the exciton states of the dot are saturated, we find a direct correspondence between the helicity of light and the direction of the nuclear spin alignment, as was observed

Quantum dots are a promising host for spin-based qubits. Whereas nuclear-field fluctuations adversely affect electron-spin coherence, the smaller hyperfine interaction between holes and nuclei makes holes a promising alternative. A sensitive measurement of the hyperfine constant of the holes in different quantum-dot material systems now demonstrates how this interaction can be tuned and perhaps further reduced. It has been proposed that valence-band holes can form robust spin qubits1,2,3,4 owing to their weaker hyperfine coupling compared with electrons5,6. However, it was demonstrated recently7,8,9,10,11 that the hole hyperfine interaction is not negligible, although a consistent picture of the mechanism controlling its magnitude is still lacking. Here we address this problem by measuring the hole hyperfine constant independently for each chemical element in InGaAs/GaAs, InP/GaInP and GaAs/AlGaAs quantum dots. Contrary to existing models10,11 we find that the hole hyperfine constant has opposite signs for cations and anions and ranges from −15% to +15% relative to that for electrons. We attribute such changes to the competing positive contributions of p-symmetry atomic orbitals and the negative contributions of d-orbitals. These findings yield information on the orbital composition of the valence band12 and enable a fundamentally new approach for verification of computed Bloch wavefunctions in semiconductor nanostructures13. Furthermore, we show that the contribution of cationic d-orbitals leads to a new mechanism of hole spin decoherence.

We demonstrate control by applied electric field of the charge states in single self-assembled InP quantum dots placed in GaInP Schottky structures grown by metalorganic vapor phase epitaxy. This has been enabled by growth optimization leading to suppression of formation of large dots uncontrollably accumulating charge. Using bias- and polarization-dependent micro-photoluminescence, we identify the exciton multi-particle states and carry out a systematic study of the neutral exciton state dipole moment and polarizability. This analysis allows for the characterization of the exciton wavefunction properties at the single dot level for this type of quantum dots. Photocurrent measurements allow further characterization of exciton properties by electrical means, opening new possibilities for resonant excitation studies for such system., Comment: 7 pages, 4 figures

We measure the hyperfine interaction of the valence band hole with nuclear spins in single $\mathrm{InP}/\mathrm{GaInP}$ semiconductor quantum dots. Detection of photoluminescence (PL) of both ``bright'' and ``dark'' excitons enables direct measurement of the Overhauser shift of states with the same electron but opposite hole spin projections. We find that the hole hyperfine constant is $\ensuremath{\approx}11%$ of that of the electron and has the opposite sign. By measuring the degree of circular polarization of the PL, an upper limit to the contribution of the heavy-light hole mixing to the measured value of the hole hyperfine constant is deduced. Our results imply that environment-independent hole spins are not realizable in III-V semiconductor, a result important for solid-state quantum information processing using hole spin qubits.

We report on dynamics of optically induced nuclear spin polarization in individual InP/GaInP quantum dots at $T=4.2\text{ }\text{K}$. Dots with different charge states arising from residual doping in a nominally undoped sample have been studied. In the same sample, we find strong dot-to-dot variation in the nuclear spin decay times in the dark from $\ensuremath{\sim}85$ to $\ensuremath{\sim}6000\text{ }\text{s}$. The longest decay times measured are comparable to those previously measured in bulk InP and correspond to almost complete suppression of nuclear spin diffusion out of the dot. In the negatively charged dots, the spin decay times exceed 300 s (with the slowest decay of $\ensuremath{\sim}6000\text{ }\text{s}$), about ${10}^{5}$ times longer than those reported previously in electron charged dots in gated structures. We discuss possible mechanisms responsible for suppression of nuclear spin diffusion, including inhomogeneous quadrupolar shifts and stabilizing effect of the hyperfine interaction with the electron confined in the dot.

We demonstrate that efficient optical pumping of nuclear spins in semiconductor quantum dots (QDs)\ud can be achieved by resonant pumping of optically forbidden transitions. This process corresponds to oneto-one\ud conversion of a photon absorbed by the dot into a polarized nuclear spin, and also has potential for\ud initialization of hole spin in QDs. We find that by employing this spin-forbidden process, nuclear\ud polarization of 65% can be achieved, markedly higher than from pumping the allowed transition, which\ud saturates due to the low probability of electron-nuclear spin flip-flop.\ud

We report optically detected nuclear magnetic resonance (ODNMR) measurements on small ensembles of nuclear spins in single GaAs quantum dots. Using ODNMR we make direct measurements of the inhomogeneous Knight field from a photo-excited electron which acts on the nuclei in the dot. The resulting shifts of the NMR peak can be optically controlled by varying the electron occupancy and its spin orientation, and lead to strongly asymmetric lineshapes at high optical excitation. The all-optical control of the NMR lineshape will enable position-selective control of small groups of nuclear spins in a dot. Our calculations also show that the asymmetric NMR peak lineshapes can provide information on the volume of the electron wave-function, and may be used for measurements of non-uniform distributions of atoms in nano-structures., Comment: submitted to PRL

Nuclear spin polarization dynamics are measured in optically pumped individual $\text{GaAs}/{\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}$ interface quantum dots by detecting the time dependence of the Overhauser shift in photoluminescence spectra. Long nuclear polarization decay times of $\ensuremath{\approx}1\text{ }\text{min}$ have been found indicating inefficient nuclear spin diffusion from the GaAs dot into the surrounding AlGaAs matrix in externally applied magnetic field. A spin-diffusion coefficient two orders lower than that previously found in bulk GaAs is deduced.

Sizable nuclear spin polarization is pumped in individual electron-charged $\mathrm{In}\mathrm{P}∕\mathrm{Ga}\mathrm{In}\mathrm{P}$ dots in a wide range of external magnetic fields ${B}_{Z}=0--5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ by circularly polarized optical excitation. We observe nuclear polarization of up to $\ensuremath{\approx}40%$ at ${B}_{Z}=1.5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ corresponding to an Overhauser field of $\ensuremath{\approx}1.2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. We find a strong feedback of the nuclear spin on the spin pumping efficiency. This feedback, which is produced by the Overhauser field, leads to nuclear spin bi-stability at low magnetic fields of ${B}_{Z}\ensuremath{\approx}0.3\ensuremath{-}1\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. We find that the splitting in magnetic field between the trion radiative recombination peaks markedly increases, when the Overhauser field in the dot cancels the external field. This counterintuitive result is shown to arise from the opposite contribution of the electron and hole Zeeman splittings to the optical transition energies.

Photoluminescence of excitons (X) and biexcitons (XX) from single neutral $\mathrm{CdSe}∕\mathrm{ZnSe}∕\mathrm{ZnMnSe}$ quantum dots (QDs) with various magnitudes of $sp\text{\ensuremath{-}}d$ exchange interaction has been investigated in magnetic fields $B$ up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ at $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The magnitude of the $sp\text{\ensuremath{-}}d$ interaction is varied by changing the penetration of the exciton wave function ${\ensuremath{\psi}}_{X}$ into the diluted magnetic semiconductor (DMS) barrier, $\ensuremath{\eta}$, by variation of the nonmagnetic $\mathrm{ZnSe}$ barrier layer thickness about a value of $1.75\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The small penetration of ${\ensuremath{\psi}}_{X}$ into the DMS barrier allowed a decrease of the X and XX emission line broadening caused by magnetic fluctuations and resolution of the fine structure of X states in a single QD. In contrast to bulk DMSs, the exciton line broadening when $B$ is normal to the QD plane was found to be a nonmonotonic function of magnetic field: at high $B$ it is suppressed due to Mn ion spin alignment, whereas at low $B$ it is decreased due to the mixing of ${J}_{z}=+1$ and $\ensuremath{-}1\phantom{\rule{0.3em}{0ex}}\mathrm{X}$ states in low-symmetry QDs by the electron-hole $(e\text{\ensuremath{-}}h)$ exchange interaction. In addition, magnetic fluctuations result in (i) emission depolarization and enhanced splitting of exciton emission lines compared to the $e\text{\ensuremath{-}}h$ exchange interaction at $B=0$ and (ii) strong enhancement of spin relaxation between ``bright'' $J=1$ exciton states in magnetic fields normal to the QD plane already at $\ensuremath{\eta}\ensuremath{\sim}1%$. The spin relaxation from bright to ``dark'' exciton states is negligible up to $\ensuremath{\eta}\ensuremath{\sim}4%$. Auger recombination of excitons with excitation of Mn ions markedly decreases the quantum efficiency of exciton emission at $B=0$ already at $\ensuremath{\eta}\ensuremath{\sim}4%$.

We achieve well-controlled and reproducible growth by low-pressure metalorganic vapor phase epitaxy (MOVPE) of low density InP/GaInP quantum dots optimized for single-dot physics and applications. We overcome the common occurrence of multi-modal distributions of quantum dot sizes by optimizing the growth on (100) GaAs substrates with a 3? misorientation towards 111. In contrast to other epitaxial techniques for quantum dot growth, very controllable dependence of the quantum dot sizes and densities on the nominal thickness of the InP layer is observed, enabling highly reproducible growth.

Photoluminescence spectra of CdSe/ZnSe/ZnMnSe semimagnetic quantum dots were studies in Faraday and Voigt geometries in the magnetic field up to 11 T at T=1.6 K. Incorporation of the nonmagnetic ZnSe layer between CdSe quantum dot layer and semimagnetic barriers reduced the strength of sp‐d exchange interaction and hence broadening of emission lines that obscures an observation of the fine structure of electron‐hole complexes. This allows one to identify exciton, biexciton and trion lines in the spectra of quantum dots and study their fine structure. An account of both sp‐d and electron‐hole exchange interactions are required in order to describe the Zeeman splitting and polarization behavior of the spectra.

Gallium chalcogenides are promising building blocks for novel van der Waals heterostructures. We report on the low-temperature micro-photoluminescence (PL) of GaTe and GaSe films with thicknesses ranging from 200 nm to a single unit cell. In both materials, PL shows a dramatic decrease by 10e4–10e5 when film thickness is reduced from 200 to 10 nm. Based on evidence from continuous-wave (cw) and time-resolved PL, we propose a model explaining the PL decrease as a result of non-radiative carrier escape via surface states. Our results emphasize the need for special passivation of two-dimensional films for optoelectronic applications.