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Silicon spin qubits are among the most promising candidates for large scale quantum computers, due to their excellent coherence and compatibility with CMOS technology for upscaling. Advanced industrial CMOS process flows allow wafer-scale uniformity and high device yield, but off the shelf transistor processes cannot be directly transferred to qubit structures due to the different designs and operation conditions. To therefore leverage the know-how of the micro-electronics industry, we customize a 300mm wafer fabrication line for silicon MOS qubit integration. With careful optimization and engineering of the MOS gate stack, we report stable and uniform quantum dot operation at the Si/SiOx interface at milli-Kelvin temperature. We extract the charge noise in different devices and under various operation conditions, demonstrating a record-low average noise level of 0.61 ${\mu}$eV/${\sqrt{Hz}}$ at 1 Hz and even below 0.1 ${\mu}$eV/${\sqrt{Hz}}$ for some devices and operating conditions. By statistical analysis of the charge noise with different operation and device parameters, we show that the noise source can indeed be well described by a two-level fluctuator model. This reproducible low noise level, in combination with uniform operation of our quantum dots, marks CMOS manufactured MOS spin qubits as a mature and highly scalable platform for high fidelity qubits., Comment: 22 pages, 13 figures

Large-scale superconducting quantum computing systems entail high-fidelity control and readout of large numbers of qubits at millikelvin temperatures, resulting in a massive input-output bottleneck. Cryo-electronics, based on complementary metal-oxide-semiconductor (CMOS) technology, may offer a scalable and versatile solution to overcome this bottleneck. However, detrimental effects due to cross-coupling between the electronic and thermal noise generated during cryo-electronics operation and the qubits need to be avoided. Here we present an ultra-low power radio-frequency (RF) multiplexing cryo-electronics solution operating below 15 mK that allows for control and interfacing of superconducting qubits with minimal cross-coupling. We benchmark its performance by interfacing it with a superconducting qubit and observe that the qubit's relaxation times ($T_1$) are unaffected, while the coherence times ($T_2$) are only minimally affected in both static and dynamic operation. Using the multiplexer, single qubit gate fidelities above 99.9%, i.e., well above the threshold for surface-code based quantum error-correction, can be achieved with appropriate thermal filtering. In addition, we demonstrate the capability of time-division-multiplexed qubit control by dynamically windowing calibrated qubit control pulses. Our results show that cryo-CMOS multiplexers could be used to significantly reduce the wiring resources for large-scale qubit device characterization, large-scale quantum processor control and quantum error correction protocols., 16+6 pages, 4+1+5 figures, 1 table

Moire superlattices in twisted van der Waals materials have recently emerged as a promising platform for engineering electronic and optical properties. A major obstacle to fully understanding these systems and harnessing their potential is the limited ability to correlate direct imaging of the moire structure with optical and electronic properties. Here we develop a secondary electron microscope technique to directly image stacking domains in fully functional van der Waals heterostructure devices. After demonstrating the imaging of AB/BA and ABA/ABC domains in multilayer graphene, we employ this technique to investigate reconstructed moire patterns in twisted WSe2/WSe2 bilayers and directly correlate the increasing moire periodicity with the emergence of two distinct exciton species in photoluminescence measurements. These states can be tuned individually through electrostatic gating and feature different valley coherence properties. We attribute our observations to the formation of an array of two intralayer exciton species that reside in alternating locations in the superlattice, and open up new avenues to realize tunable exciton arrays in twisted van der Waals heterostructures, with applications in quantum optoelectronics and explorations of novel many-body systems. Scanning electron microscopy is used to image stacking domains in few-layer graphene, as well as moire patterns in twisted van der Waals heterostructures, allowing for the correlation of the local structure with their excitonic properties.

Transition metal dichalcogenide (TMD) semiconductor heterostructures are actively explored as a new platform for quantum optoelectronic systems. Most state of the art devices make use of insulating hexagonal boron nitride (hBN) that acts as a wide-bandgap dielectric encapsulating layer that also provides an atomically smooth and clean interface that is paramount for proper device operation. We report the observation of large, through-hBN photocurrents that are generated upon optical excitation of hBN encapsulated MoSe2 and WSe2 monolayer devices. We attribute these effects to Auger recombination in the TMDs, in combination with an asymmetric band offset between the TMD and the hBN. We present experimental investigation of these effects and compare our observations with detailed, ab-initio modeling. Our observations have important implications for the design of optoelectronic devices based on encapsulated TMD devices. In systems where precise charge-state control is desired, the out-of-plane current path presents both a challenge and an opportunity for optical doping control. Since the current directly depends on Auger recombination, it can act as a local, direct probe of both the efficiency of the Auger process as well as its dependence on the local density of states in integrated devices.

A single-photon source based on a superconducting qubit as an artificial atom and nonlinear optical readout via sum-frequency generation is presented. The analysis done via master equation and Quitp suggest efficiencies of 98% and g(2)(0)=5e-5.

This work studies the performance of a Ge-based plasmonic detector with carrier mobility and lifetime variation. The performance is evaluated in terms of the detector's dark current, photocurrent, and detection bandwidth.

Quantum computers based on superconducting qubits have emerged as a leading candidate for a scalable quantum processor architecture. The core of a quantum processor consists of quantum devices that are manipulated using classical electronic circuits, which need to be co-designed for optimal performance and operation. As the principles governing the behavior of the classical circuits and the quantum devices are different, this presents a unique challenge in terms of the simulation, design and optimization of the joint system. A methodology is presented to transform the behavior of small-scale quantum processors to equivalent circuit models that are usable with classical circuits in a generic electrical simulator, enabling the detailed analysis of the impact of many important non-idealities. The methodology has specifically been employed to derive a circuit model of a superconducting qubit interacting with the quantized electromagnetic field of a superconducting resonator. Based on this technique, a comprehensive analysis of the qubit operation is performed, including the coherent control and readout of the qubit using electrical signals. Furthermore, the effect of several non-idealities in the system such as qubit relaxation, decoherence and leakage out of the computational subspace are captured, in contrast to previous works. As the presented method enables the co-simulation of the control electronics with the quantum system, it facilitates the design and optimization of near-term superconducting quantum processors.

Excitons are composite bosons that can feature spin singlet and triplet states. In usual semiconductors, without an additional spin-flip mechanism, triplet excitons are extremely inefficient optical emitters. Transition metal dichalcogenides (TMDs), with their large spin-orbit coupling, have been of special interest for valleytronic applications for their coupling of circularly polarized light to excitons with selective valley and spin$^{1-4}$. In atomically thin MoSe$_2$/WSe$_2$ TMD van der Waals (vdW) heterostructures, the unique atomic registry of vdW layers provides a quasi-angular momentum to interlayer excitons$^{5,6}$, enabling emission from otherwise dark spin triplet excitons. Here, we report electrically tunable spin singlet and triplet exciton emission from atomically aligned TMD heterostructures. We confirm the spin configurations of the light-emitting excitons employing magnetic fields to measure effective exciton g-factors. The interlayer tunneling current across the TMD vdW heterostructure enables the electrical generation of singlet and triplet exciton emission in this atomically thin PN junction. We demonstrate electrically tunability between the singlet and triplet excitons that are generated by charge injection. Atomically thin TMD heterostructure light emitting diodes thus enables a route for optoelectronic devices that can configure spin and valley quantum states independently by controlling the atomic stacking registry., Comment: Extended work of previous study: arXiv:1912.07678

The nitrogen vacancy (NV) center in diamond exhibits spin-dependent fluorescence and long spin coherence times under ambient conditions, enabling applications in quantum information processing and sensing. NV centers near the surface can have strong interactions with external materials and spins, enabling new forms of nanoscale spectroscopy. However, NV spin coherence degrades within 100 nanometers of the surface, suggesting that diamond surfaces are plagued with ubiquitous defects. Prior work on characterizing near-surface noise has primarily relied on using NV centers themselves as probes; while this has the advantage of exquisite sensitivity, it provides only indirect information about the origin of the noise. Here we demonstrate that surface spectroscopy methods and single spin measurements can be used as complementary diagnostics to understand sources of noise. We find that surface morphology is crucial for realizing reproducible chemical termination, and use these insights to achieve a highly ordered, oxygen-terminated surface with suppressed noise. We observe NV centers within 10 nm of the surface with coherence times extended by an order of magnitude.

We demonstrate a new approach for dynamically manipulating the optical response of an atomically thin semiconductor, a monolayer of MoSe2, by suspending it over a metallic mirror. First, we show that suspended van der Waals heterostructures incorporating a MoSe2 monolayer host spatially homogeneous, lifetime-broadened excitons. Then, we interface this nearly ideal excitonic system with a metallic mirror and demonstrate control over the exciton-photon coupling. Specifically, by electromechanically changing the distance between the heterostructure and the mirror, thereby changing the local photonic density of states in a controllable and reversible fashion, we show that both the absorption and emission properties of the excitons can be dynamically modulated. This electromechanical control over exciton dynamics in a mechanically flexible, atomically thin semiconductor opens up new avenues in cavity quantum optomechanics, nonlinear quantum optics, and topological photonics., accepted version

We demonstrate that a single layer of MoSe_{2} encapsulated by hexagonal boron nitride can act as an electrically switchable mirror at cryogenic temperatures, reflecting up to 85% of incident light at the excitonic resonance. This high reflectance is a direct consequence of the excellent coherence properties of excitons in this atomically thin semiconductor. We show that the MoSe_{2} monolayer exhibits power-and wavelength-dependent nonlinearities that stem from exciton-based lattice heating in the case of continuous-wave excitation and exciton-exciton interactions when fast, pulsed laser excitation is used.

Taking electrical control Excitons—bound pairs of electrons and holes in a solid—can, in principle, be used as information carriers. However, their lifetime is limited because the electrons and holes tend to quickly recombine. One way to extend this lifetime is to physically separate electrons and holes—for example, by having them reside in different layers of a van der Waals heterostructure. Jauregui et al. used this strategy to form long-lived interlayer excitons in a heterostructure made out of monolayers of molybdenum diselenide (MoSe 2 ) and tungsten diselenide (WSe 2 ). Through electrical control of the layers in the heterostructure, the researchers further increased exciton lifetime and formed and manipulated charged excitons. Science , this issue p. 870

We demonstrate that a single layer of MoSe_{2} encapsulated by hexagonal boron nitride can act as an electrically switchable mirror at cryogenic temperatures, reflecting up to 85% of incident light at the excitonic resonance. This high reflectance is a direct consequence of the excellent coherence properties of excitons in this atomically thin semiconductor. We show that the MoSe_{2} monolayer exhibits power-and wavelength-dependent nonlinearities that stem from exciton-based lattice heating in the case of continuous-wave excitation and exciton-exciton interactions when fast, pulsed laser excitation is used.

Transition metal dichalcogenide (TMD) monolayers are direct bandgap semiconductors that feature tightly bound excitons, strong spin-orbit coupling, and spin-valley degrees of freedom. Depending on the spin configuration of the electron-hole pairs, intra-valley excitons of TMD monolayers can be either optically bright or dark. Dark excitons involve nominally spin-forbidden optical transitions with zero in-plane transition dipole moment, making their detection with conventional far-field optical techniques challenging. Here, we introduce a new method for probing the optical properties of two-dimensional (2D) materials via near-field coupling to surface plasmon polaritons (SPPs), which selectively enhances optical transitions with dipole moments normal to the 2D plane. We utilize this method to directly detect dark excitons in monolayer TMDs. When a WSe2 monolayer is placed on top of a single-crystal silver film, its emission into near-field-coupled SPPs displays new spectral features whose energies and dipole orientations are consistent with dark neutral and charged excitons. The SPP-based near-field spectroscopy significantly enhances experimental capabilities for probing and manipulating exciton dynamics of atomically thin materials., Comment: 4 figures

Electrical confinement and manipulation of charge carriers in semiconducting nanostructures are essential for realizing functional quantum electronic devices1–3. The unique band structure4–7 of atomically thin transition metal dichalcogenides (TMDs) offers a new route towards realizing novel 2D quantum electronic devices, such as valleytronic devices and valley–spin qubits 8 . 2D TMDs also provide a platform for novel quantum optoelectronic devices9–11 due to their large exciton binding energy12,13. However, controlled confinement and manipulation of electronic and excitonic excitations in TMD nanostructures have been technically challenging due to the prevailing disorder in the material, preventing accurate experimental control of local confinement and tunnel couplings14–16. Here we demonstrate a novel method for creating high-quality heterostructures composed of atomically thin materials that allows for efficient electrical control of excitations. Specifically, we demonstrate quantum transport in the gate-defined, quantum-confined region, observing spin–valley locked quantized conductance in quantum point contacts. We also realize gate-controlled Coulomb blockade associated with confinement of electrons and demonstrate electrical control over charged excitons with tunable local confinement potentials and tunnel couplings. Our work provides a basis for novel quantum opto-electronic devices based on manipulation of charged carriers and excitons. Formation of a homogeneous two-dimensional electron gas in transition metal dichalcogenide heterostructures allows for efficient electrical control of charge carriers and excitons.

Electron spin in quantum dots are extensively studied as a qubit for quantum information processing. However, the coherence of electron spin is deleteriously influenced by nuclear spin. Quantum-dot holes are a potential alternative. Full control over hole-spin qubits is now achieved using picosecond lasers.

Many proposed photonic quantum networks rely on matter qubits to serve as memory elements1,2. The spin of a single electron confined in a semiconductor quantum dot forms a promising matter qubit that may be interfaced with a photonic network3. Ultrafast optical spin control allows gate operations to be performed on the spin within a picosecond timescale4,5,6,7,8,9,10,11,12,13,14, orders of magnitude faster than microwave or electrical control15,16. One obstacle to storing quantum information in a single quantum dot spin is the apparent nanosecond-timescale dephasing due to slow variations in the background nuclear magnetic field15,16,17. Here we use an ultrafast, all-optical spin echo technique to increase the decoherence time of a single quantum dot electron spin from nanoseconds to several microseconds. The ratio of decoherence time to gate time exceeds 105, suggesting strong promise for future photonic quantum information processors18 and repeater networks1,2. An ultrafast, all-optical spin echo technique is used to increase the decoherence time of a single quantum dot electron spin from nanoseconds to several microseconds. The ratio of decoherence time to gate time exceeds 105, suggesting strong promise for future photonic quantum information processors and repeater networks.

‘In this chapter, we introduced and analyze techniques that allow truly secure secret key sharing over long distances, using public, open channels, where the laws of quantum mechanics ensure the security of the long distance key sharing – an idea generally referred to as the essence of a quantum repeater. We describe several proof-of-principle experiments where technology based on self-assembled quantum dots is used as the backbone of a future quantum repeater.’

De-Twinning a Superconductor Insight into the mechanism of electrical transport in a solid can often be gained by measuring its resistivity along different spatial directions. However, iron-based superconductors form numerous twin boundaries where two different orientations of a crystal meet, and so the measured resistivity along any in-plane direction will be averaged over these orientations. Chu et al. (p. 824 ) were able to “de-twin” the compound Ba(Fe 1− x Co x ) 2 As 2 , enabling unambiguous measurements of its normal-state resistivity along the in-plane lattice axes. Differences were observed in the resistivity values along the two axes, which suggests that the breaking of the symmetry of the lattice and electron subsystems occur simultaneously.

Semiconductor quantum dots can be utilized to capture single electron or hole spins and they have therewith promise for various applications in fields like spintronics, spin based quantum information processing and chiral photonics. We integrate quantum dots into semiconductor microcavities to enhance light-matter interaction for ultrafast optical manipulation and read-out. Single electron and single hole spins can be statistically or deterministically loaded into the quantum dots and coherently controlled. Within the about μs-coherence times of the spins about 105 complete single qubit rotations can be performed with ultrafast optical pulses. By utilizing a Λ-type energy level system of a single quantum-dot electron spin in a magnetic field and ultrafast non-linear frequency conversion, quantum-dot spin-photon entanglement is observed.

Single spins in semiconductor quantum dots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantum dot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantum dot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a 'flying' photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantum dot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantum dots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantum dot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantum dot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is entangled with the spin, and these photons are then interfered. We review recent work demonstrating entanglement between a stationary spin qubit and a flying photonic qubit. These experiments utilize the polarization- and frequency-dependent spontaneous emission from the lowest charged exciton state to single spin Zeeman sublevels.

Entanglement between stationary quantum memories and photonic qubits is crucial for future quantum communication networks. Although high-fidelity spin-photon entanglement was demonstrated in well-isolated atomic and ionic systems, in the solid-state, where massively parallel, scalable networks are most realistically conceivable, entanglement fidelities are typically limited due to intrinsic environmental interactions. Distilling high-fidelity entangled pairs from lower-fidelity precursors can act as a remedy, but the required overhead scales unfavourably with the initial entanglement fidelity. With spin-photon entanglement as a crucial building block for entangling quantum network nodes, obtaining high-fidelity entangled pairs becomes imperative for practical realization of such networks. Here we report the first results of complete state tomography of a solid-state spin-photon-polarization-entangled qubit pair, using a single electron-charged indium arsenide quantum dot. We demonstrate record-high fidelity in the solid-state of well over 90%, and the first (99.9%-confidence) achievement of a fidelity that will unambiguously allow for entanglement distribution in solid-state quantum repeater networks.

Individual electron and hole quantum dot spin qubits can be coherently manipulated using picosecond modelocked laser pulses; an all-optical spin-echo was implemented that decouples slow environmental changes. While dephasing and decoherence mechanisms for electrons and holes are intrinsically different, similar qualitative results are obtained, except for dynamic nuclear polarization effects that affect the controllability of electrons. In addition, we demonstrate spin-photon entanglement in a charged InAs quantum dot, using an ultrafast downconversion technique that converts a single, spontaneously emitted photon at 900 nm into a 1560 nm photon with picosecond timing resolution. This ultrafast conversion technique allows quantum erasure of which-path frequency information in the spontaneous emission process.

In this chapter, we shall derive how ultrafast optical control can be used in combination with Larmor precession for full SU(2) control of a single quantum dot electron spin. Such ultrafast electron spin control was first reported in Refs. [1–3]. In combination with accurate timing control over the optical fields used to realize this SU(2) control, arbitrary pulse (control) patterns can be applied to the spin [3]. As we will show in Sect. 3.3.2, such pulse sequences can be used to overcome the effects of slowly varying Larmor precession due to variations in the spin’s solid state environment. As a particular example, we will study the effects of the hyperfine interaction between an electron spin and the nuclear spins inside the quantum dot [4–7] – the latter will be discussed in detail in Sect. 3.3.1.

While fast, complete SU(2) control of a long-lived qubit is an essential ingredient of any quantum information system, our focus on quantum communication systems, and quantum repeaters in particular, requires a means of interfacing the stationary, matter and memory qubits, with flying, photonic qubits. As we derived in Sects. 1.1.3.1 and 1.4, entanglement between a spin-qubit and a photonic qubit [1–5] provides exactly the type of interface required in order to generate remotely-entangled spin-pairs and implement the quantum teleportation and entanglement swapping schemes that are at the heart of quantum repeaters [6–11].

The strong contact hyperfine interaction between a single electron and the nuclear spins in a quantum dot [1–5] represents a severe limitation on its use as a qubit, as we discussed in Sect. 3.3.1. In particular, it results in non-linear, non-Markovian dynamics of the electron-nuclear system [6–9], making the use of the Larmor-precession as a single qubit gate cumbersome. While dynamical decoupling, in essence an extension of the simple spin echo presented in Sect. 3.3.2 to multiple refocusing pulses, could be used to decouple the effects of the nuclear spin bath [10–14], avoiding the contact hyperfine interaction altogether would be a much simpler solution. For some materials, this could be achieved by careful isotopic engineering: examples for group-IV and II–VI materials can be found in Refs. [15–19]. However, In, Ga and As do not possess any spin-0 isotopes, making this approach impossible for the self-assembled quantum dots used in this work.

Quantum Information Processing [1] (QIP), roughly defined as that branch of physics, engineering and computer science that attempts to incorporate fundamental concepts from quantum mechanics in order to augment and improve on existing information processing capabilities, was initially proposed as an answer to a fundamental question in both theoretical physics and quantum chemistry: how to keep track of the gigantic state space that is present in large-scale quantum mechanical systems [2]? Such quantum simulation has since become the subject of an entire subfield of study [3], building on the intrinsic state space provided by quantum systems to understand fundamental properties of nature, especially in solid-state and many-body physics – properties and studies that would be intractable using classical mathematical tools based on digital computing power.

In the previous chapters, detuned optical pulses were used to generate qubit rotations. While deriving the effect of the laser pulses on the spin, we generally ignored overall, global phases. However, in this chapter, we shall report on experiments were the global (geometric) phases of a (detuned) 2-level interaction can be visualized in Ramsey interferometry, by beating the phase against the Larmor precession. In the context of 2-qubit interactions, geometric phases play a crucial role for the realization of entangling gates [1, 2]; the realization of a fast, pulsed global phase for single quantum dot electron spins is essential for this task.

The quantum bits used in the remainder of this work, are individual electron (Chaps. 3–5 and 7) or hole spins (Chap. 6) in self-assembled quantum dots [1, 2]. Spins, either as direct spin-1/2 particles or as pseudospin-submanifolds of larger systems, are generally considered as good candidate-qubits due to their relatively limited interaction with the environment [3]. In addition, and as we shall show below, the confinement of the spin to quantum dots provides an additional protection of the spin degree of freedom.

The ultrafast optical spin control techniques described in Chap. 3 were derived under the assumption of the pulses being short compared to the Larmor precession frequency. In practice, however, picosecond-timescale pulses are used, combined with Larmor precession periods of several tens of picoseconds [1, 2]. While lowering the magnetic field and increasing the Larmor precession period would render the picosecond pulses (relatively) ‘shorter’, this would come at the expense of the speed of elementary quantum gates (the Larmor precession acts as one of the fundamental 1-qubit gates). In addition, decreasing the magnetic field lowers the spin readout fidelity, as the filtering of the optical pumping laser becomes more troublesome – we refer to Sect. 3.2.1 and Appendix A.1.1.

Long-distance quantum communication networks require appropriate interfaces between matter qubit-based nodes and low-loss photonic quantum channels. We implement a downconversion quantum interface, where the single photons emitted from a semiconductor quantum dot at 910 nm are downconverted to 1560 nm using a fiber-coupled periodically poled lithium niobate waveguide and a 2.2-$\mu$m pulsed pump laser. The single-photon character of the quantum dot emission is preserved during the downconversion process: we measure a cross-correlation $g^{(2)}(\tau = 0) = 0.17$ using resonant excitation of the quantum dot. We show that the downconversion interface is fully compatible with coherent optical control of the quantum dot electron spin through the observation of Rabi oscillations in the downconverted photon counts. These results represent a critical step towards a long-distance hybrid quantum network in which subsystems operating at different wavelengths are connected through quantum frequency conversion devices and 1.5-$\mu$m quantum channels.

We present a proposal for a quantum repeater which uses fault-tolerant quantum error correction to overcome decoherence limitations to creating long-distance entanglement between quantum dot spin qubits.

The in-plane resistivity anisotropy has been measured for detwinned single crystals of Ba(Fe1−xNix)2As2 and Ba(Fe1−xCux)2As2. The data reveal a nonmonotonic doping dependence, similar to previous observations for Ba(Fe1−xCox)2As2. Magnetotransport measurements of the parent compound reveal a nonlinear Hall coefficient and a large linear term in the transverse magnetoresistance. Both effects are rapidly suppressed with chemical substitution over a similar compositional range as the onset of the large in-plane resistivity anisotropy. This suggests that the relatively small in-plane anisotropy of the parent compound in the spin-density wave state is due to the presence of an isotropic, high mobility pocket of the reconstructed Fermi surface. Progressive suppression of the contribution to the conductivity arising from this isotropic pocket with chemical substitution eventually reveals the underlying in-plane anisotropy associated with the remaining Fermi surface pockets.

Electron spins in quantum dots under coherent control exhibit a number of novel feedback processes. Here, we present experimental and theoretical evidence of a feedback process between nuclear spins and a single electron spin in a single charged InAs quantum dot, controlled by the coherently modified probability of exciting a trion state. We present a mathematical model describing competition between optical nuclear pumping and nuclear spin-diffusion inside the quantum dot. The model correctly postdicts the observation of a hysteretic sawtooth pattern in the free-induction-decay of the single electron spin, hysteresis while scanning a narrowband laser through the quantum dot's optical resonance frequency, and non-sinusoidal fringes in the spin echo. Both the coherent electron-spin rotations, implemented with off-resonant ultrafast laser pulses, and the resonant narrow-band optical pumping for spin initialization interspersed between ultrafast pulses, play a role in the observed behavior. This effect allows dynamic tuning of the electron Larmor frequency to a value determined by the pulse timing, potentially allowing more complex coherent control operations., Comment: 15 pages, 7 figures. Corrected and expanded discussion. Now includes analysis of spin-echo and optical pumping experiments, in addition to FID