A gate defined quantum dot on the two-dimensional transition metal dichalcogenide semiconductor WSe2

Excited state spectroscopy and spin splitting in single layer MoS2 quantum dots

Semiconducting transition metal dichalcogenides (TMDCs) are very promising materials for quantum dots and spin-qubit implementation. Reliable operation of spin qubits requires the knowledge of the Landé g-factor, which can be measured by exploiting the discrete energy spectrum on a quantum dot. However, the quantum dots realized in TMDCs are yet to reach the required control and quality for reliable measurement of excited state spectroscopy and the g-factor, particularly in atomically thin layers. Quantum dot sizes reported in TMDCs so far are not small enough to observe discrete energy levels on them. Here, we report on electron transport through discrete energy levels of quantum dots in a single layer MoS2 isolated from its environment using a dual gate geometry. The quantum dot energy levels are separated by a few (5-6) meV such that the ground state and the first excited state transitions are clearly visible, thanks to the low contact resistance of ∼700 Ω and relatively low gate voltages. This well-resolved energy separation allowed us to accurately measure the ground state g-factor of ∼5 in MoS2 quantum dots. We observed a spin-filling sequence in our quantum dots under a perpendicular magnetic field. Such a system offers an excellent testbed to measure the key parameters for evaluation and implementation of spin-valley qubits in TMDCs, thus accelerating the development of quantum systems in two-dimensional semiconducting TMDCs.

Dielectrics for Two-Dimensional Transition-Metal Dichalcogenide Applications

Despite over a decade of intense research efforts, the full potential of two-dimensional transition-metal dichalcogenides continues to be limited by major challenges. The lack of compatible and scalable dielectric materials and integration techniques restrict device performances and their commercial applications. Conventional dielectric integration techniques for bulk semiconductors are difficult to adapt for atomically thin two-dimensional materials. This review provides a brief introduction into various common and emerging dielectric synthesis and integration techniques and discusses their applicability for 2D transition metal dichalcogenides. Dielectric integration for various applications is reviewed in subsequent sections including nanoelectronics, optoelectronics, flexible electronics, valleytronics, biosensing, quantum information processing, and quantum sensing. For each application, we introduce basic device working principles, discuss the specific dielectric requirements, review current progress, present key challenges, and offer insights into future prospects and opportunities.

Non-Destructive Low-Temperature Contacts to MoS2 Nanoribbon and Nanotube Quantum Dots

Molybdenum disulfide nanoribbons and nanotubes are quasi-1D semiconductors with strong spin-orbit interaction, a nanomaterial highly promising for quantum electronic applications. Here, it is demonstrated that a bismuth semimetal layer between the contact metal and this nanomaterial strongly improves the properties of the contacts. Two-point resistances on the order of 100 kΩ are observed at room temperature. At cryogenic temperature, Coulomb blockade is visible. The resulting stability diagrams indicate a marked absence of trap states at the contacts and the corresponding disorder, compared to previous devices that use low-work-function metals as contacts. Single-level quantum transport is observed at temperatures below 100 mK.

Geometry-modulated dipole polarizability of the two-dimensional Mott-Wannier excitons in gate-defined anisotropic quantum dot

A theoretical investigation on neutral excitons confined to a mono-layer (ML) semiconductor transition metal dichalcogenide (TMDC) materials under the influence of elliptically deformed gate induced confining potential is presented. It has been shown that the anisotropy of the confinement induces the anisotropy of linear response of the system on in-plane external electric field. The linear response is expressed in terms of principal moments of the static dipole polarizability tensor. In this manner the direction-dependent polarizability of the system can be fully controlled by tuning the parameters of gate-induced confining potential. The components of the polarizability tensor are determined using finite-field method based on the exact diagonalization of the electron-hole Hamiltonian including confining potential, Coulomb electron-hole interaction and an external electric field, within effective mass approximation, close to the K-points of the first Brillouin zone of a single-layer MX\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 material. The useful scaling relations for energies and dipole polarizabilities as functions of material parameters have been found. The influence of the anisotropy of the confining potential on correlated behavior of charge distribution inside the neutral system has also been demonstrated.

Enhanced Deep Ultraviolet Photoresponse in Ga doped ZnMgO Thin Film

High Mg content (60%) ZnMgO samples with and without Ga dope were grown by an RF magnetron sputtering system. The effect of Ga dope on the ZnMgO sample and the respective ultraviolet photodetectors (UVPD) device’s performance were carefully studied by various experimental methods. The investigations of the structure and optical properties of the ZnMgO sample established that the Ga doped sample has a better crystal quality and larger band gap (5.54 eV). The current-voltage characteristics indicate that both the photocurrent and dark current were enhanced after Ga dope. Under 12 V bias, the undoped UVPD show two spectral response peaks at 244 nm and 271 nm with a responsivity of 1.9 A/W and 0.38 A/W, respectively. While the Ga doped UVPD showed only one response peak at 241 nm and the deep UV responsibility up to 8.9 A/W;, as the bias increased from 12 V to 60 V, the responsiveness raised to 52 A/W, with a signal to noise ratio (241 nm/700 nm) as high as 10⁵. Combining the results of XRD, PL spectrum and XPS, the enhanced ultraviolet photoresponse of the Ga dope device contributed to improving the crystal quality and “dopant-defect pairing effect” caused by Ga doping, which led to a considerable reduction in the number of ionized impurities in the scatting centers, and enhanced the carrier’s mobility. Our work demonstrates that even a high Mg content ZnMgO can exhibit enhanced UV performance after a Ga dope due to the dopant-defect pairing effect, which confirmed the advantage of the use of ZnMgO in the deep-UV region.

Gate-Defined Quantum Confinement in CVD 2D WS2

Temperature-dependent transport measurements are performed on the same set of chemical vapor deposition (CVD)-grown WS₂ single- and bilayer devices before and after atomic layer deposition (ALD) of HfO₂ . This isolates the influence of HfO₂ deposition on low-temperature carrier transport and shows that carrier mobility is not charge impurity limited as commonly thought, but due to another important but commonly overlooked factor: interface roughness. This finding is corroborated by circular dichroic photoluminescence spectroscopy, X-ray photoemission spectroscopy, cross-sectional scanning transmission electron microscopy, carrier-transport modeling, and density functional modeling. Finally, electrostatic gate-defined quantum confinement is demonstrated using a scalable approach of large-area CVD-grown bilayer WS₂ and ALD-grown HfO₂ . The high dielectric constant and low leakage current enabled by HfO₂ allows an estimated quantum dot size as small as 58 nm. The ability to lithographically define increasingly smaller devices is especially important for transition metal dichalcogenides due to their large effective masses, and should pave the way toward their use in quantum information processing applications.

Direct Patterning of Optoelectronic Nanostructures Using Encapsulated Layered Transition Metal Dichalcogenides

Direct top-down nanopatterning of semiconductors is a powerful tool for engineering properties of optoelectronic devices. Translating this approach to two-dimensional semiconductors such as monolayer transition metal dichalcogenides (TMDs) is challenging because of both the small scales required for confinement and the degradation of electronic and optical properties caused by high-energy and high-dose electron radiation used for high-resolution top-down direct electron beam patterning. We show that encapsulating a TMD monolayer with hexagonal boron nitride preserves the narrow exciton linewidths and emission intensity typical in such heterostructures after electron beam lithography, allowing direct patterning of functional optical monolayer nanostructures on scales of a few tens of nanometers. We leverage this fabrication method to study size-dependent effects on nanodot arrays of MoS2 and MoSe2 as well as laterally confined electrical transport devices, demonstrating the potential of top-down lithography for nanoscale TMD optoelectronics.

Quantum-Engineered Devices Based on 2D Materials for Next-Generation Information Processing and Storage

As an approximation to the quantum state of solids, the band theory, developed nearly seven decades ago, fostered the advance of modern integrated solid-state electronics, one of the most successful technologies in the history of human civilization. Nonetheless, their rapidly growing energy consumption and accompanied environmental issues call for more energy-efficient electronics and optoelectronics, which necessitate the exploration of more advanced quantum mechanical effects, such as band-to-band tunneling, spin-orbit coupling, spin-valley locking, and quantum entanglement. The emerging 2D layered materials, featured by their exotic electrical, magnetic, optical, and structural properties, provide a revolutionary low-dimensional and manufacture-friendly platform (and many more opportunities) to implement these quantum-engineered devices, compared to the traditional electronic materials system. Here, the progress in quantum-engineered devices is reviewed and the opportunities/challenges of exploiting 2D materials are analyzed to highlight their unique quantum properties that enable novel energy-efficient devices, and useful insights to quantum device engineers and 2D-material scientists are provided.

Quantum emitters and detectors based on 2D van der Waals materials

Light plays an essential role in our world, with several technologies relying on it. Photons will also play an important role in the emerging quantum technologies, which are primed to have a transformative effect on our society. The development of single-photon sources and ultra-sensitive photon detectors is crucial. Solid-state emitters are being heavily pursued for developing truly single-photon sources for scalable technology. On the detectors' side, the main challenge lies in inventing sensitive detectors operating at sub-optical frequencies. This review highlights the promising research being conducted for the development of quantum emitters and detectors based on two-dimensional van der Waals (2D-vdW) materials. Several 2D-vdW materials, from canonical graphene to transition metal dichalcogenides and their heterostructures, have generated a lot of excitement due to their tunable emission and detection properties. The recent developments in the creation, fabrication and control of quantum emitters hosted by 2D-vdW materials and their potential applications in integrated photonic devices are discussed. Furthermore, the progress in enhancing the photon-counting potential of 2D material-based detectors, viz. 2D photodetectors, bolometers and superconducting single-photon detectors functioning at various wavelengths is also reported.

Enhanced light-matter interaction in two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS₂, WS₂, MoSe₂, and WSe₂, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research.

Nanomaterials for Quantum Information Science and Engineering

Quantum information science and engineering (QISE)-which entails the use of quantum mechanical states for information processing, communications, and sensing-and the area of nanoscience and nanotechnology have dominated condensed matter physics and materials science research in the 21st century. Solid-state devices for QISE have, to this point, predominantly been designed with bulk materials as their constituents. This review considers how nanomaterials (i.e., materials with intrinsic quantum confinement) may offer inherent advantages over conventional materials for QISE. The materials challenges for specific types of qubits, along with how emerging nanomaterials may overcome these challenges, are identified. Challenges for and progress toward nanomaterials-based quantum devices are condidered. The overall aim of the review is to help close the gap between the nanotechnology and quantum information communities and inspire research that will lead to next-generation quantum devices for scalable and practical quantum applications.

Nickel particle-enabled width-controlled growth of bilayer molybdenum disulfide nanoribbons

Transition metal dichalcogenides exhibit a variety of electronic behaviors depending on the number of layers and width. Therefore, developing facile methods for their controllable synthesis is of central importance. We found that nickel nanoparticles promote both heterogeneous nucleation of the first layer of molybdenum disulfide and simultaneously catalyzes homoepitaxial tip growth of a second layer via a vapor-liquid-solid (VLS) mechanism, resulting in bilayer nanoribbons with width controlled by the nanoparticle diameter. Simulations further confirm the VLS growth mechanism toward nanoribbons and its orders of magnitude higher growth speed compared to the conventional noncatalytic growth of flakes. Width-dependent Coulomb blockade oscillation observed in the transfer characteristics of the nanoribbons at temperatures up to 60 K evidences the value of this proposed synthesis strategy for future nanoelectronics.

Point Defects in Two-Dimensional Indium Selenide as Tunable Single-Photon Sources

In the past few years remarkable interest has been kindled by the development of nonclassical light sources and, in particular, of single-photon emitters (SPE), which represent fundamental building blocks for optical quantum technology. In this Letter, we analyze the stability and electronic properties of an InSe monolayer with point defects with the aim of demonstrating its applicability as an SPE. The presence of deep defect states within the InSe band gap is verified when considering substitutional defects with atoms belonging to group IV, V, and VI. In particular, the optical properties of Ge as substitution impurity of Se predicted by solving the Bethe-Salpeter equation on top of the GW corrected electronic states show that transitions between the valence band maximum and the defect state are responsible for the absorption and spontaneous emission processes, so that the latter results in a strongly peaked spectrum in the near-infrared. These properties, together with a high localization of the involved electronic states, appear encouraging in the quest for novel SPE materials.

Signature of Spin-Resolved Quantum Point Contact in p-Type Trilayer WSe2 van der Waals Heterostructure

In this study, an electrostatically induced quantum confinement structure, so-called quantum point contact, has been realized in a p-type trilayer tungsten diselenide-based van der Waals heterostructure with modified van der Waals contact method with degenerately doped transition metal dichalcogenide crystals. Clear quantized conductance and pinch-off state through the one-dimensional confinement were observed by dual-gating of split gate electrodes and top gate. Conductance plateaus were observed at a step of e²/h in addition to quarter plateaus such as 0.25 × 2e²/h at a finite bias voltage condition indicating the signature of intrinsic spin-polarized quantum point contact.

A Roadmap for Disruptive Applications and Heterogeneous Integration Using Two-Dimensional Materials: State-of-the-Art and Technological Challenges

This Mini Review attempts to establish a roadmap for two-dimensional (2D) material-based microelectronic technologies for future/disruptive applications with a vision for the semiconductor industry to enable a universal technology platform for heterogeneous integration. The heterogeneous integration would involve integrating orthogonal capabilities, such as different forms of computing (classical, neuromorphic, and quantum), all forms of sensing, digital and analog memories, energy harvesting, and so forth, all in a single chip using a universal technology platform. We have reviewed the state-of-the-art 2D materials such as graphene, transition metal dichalcogenides, phosphorene and hexagonal boron nitride, and so forth, and how they offer unique possibilities for a range of futuristic/disruptive applications. Besides, we have discussed the technological and fundamental challenges in enabling such a universal technology platform, where the world stands today, and what gaps are required to be filled.

Nano-chevron quantum dot for spin-qubit applications

We study the theoretical properties of a parabolic hBN/MoS₂/hBN heterostructure quantum dot potential generated via electrostatic gates and its interaction with a cobalt nano chevron. We demonstrate that such an example system can undergo electric dipole spin resonance for a single electron isolated to the K' valley within the MoS₂ monolayer, and such a system can achieve pi-rotation times of approximately 5.5 ns under the influence of a 20.89 GHz driving field. Our proposed system requires operating conditions easily achievable with current experimental methods and would allow for the all-electrical control of a spin-qubit within an MoS₂ device. Our results show that such a system is experimentally feasible and would have comparable properties to that of more traditional silicon based spin-qubits. Furthermore, the design of the device can be applied to other material systems beyond MoS₂ and cobalt. In theory, the proposed structure could make use of any 2D material that experiences strong proximity exchange interactions with other magnetic materials, which makes our proposed design highly general.

Nanoscale Trapping of Interlayer Excitons in a 2D Semiconductor Heterostructure

For quantum technologies based on single excitons and spins, the deterministic placement and control of a single exciton is a longstanding goal. MoSe₂-WSe₂ heterostructures host spatially indirect interlayer excitons (IXs) that exhibit highly tunable energies and unique spin-valley physics, making them promising candidates for quantum information processing. Previous IX trapping approaches involving moiré superlattices and nanopillars do not meet the quantum technology requirements of deterministic placement and energy tunability. Here, we use a nanopatterned graphene gate to create a sharply varying electric field in close proximity to a MoSe₂-WSe₂ heterostructure. The dipole interaction between the IX and the electric field creates an ∼20 nm trap. The trapped IXs show the predicted electric-field-dependent energy, saturation at low excitation power, and increased lifetime, all signatures of strong spatial confinement. The demonstrated architecture is a crucial step toward the deterministic trapping of single IXs, which has broad applications to scalable quantum technologies.

Gate-tunable quantum dot formation between localized-resonant states in a few-layer MoS2

We demonstrate a gate-tunable quantum dot (QD) located between two potential barriers defined in a few-layer MoS₂. Although both local gates used to tune the potential barriers have disorder-induced QDs, we observe diagonal current stripes in current resonant islands formed by the alignment of the Fermi levels of the electrodes and the energy levels of the disorder-induced QDs, as evidence of the gate-tunable QD. We demonstrate that the charging energy of the designed QD can be tuned in the range of 2-6 meV by changing the local-gate voltages in ∼1 V.

Theory of valley-resolved spectroscopy of a Si triple quantum dot coupled to a microwave resonator

We theoretically study a silicon triple quantum dot (TQD) system coupled to a superconducting microwave resonator. The response signal of an injected probe signal can be used to extract information about the level structure by measuring the transmission and phase shift of the output field. This information can further be used to gain knowledge about the valley splittings and valley phases in the individual dots. Since relevant valley states are typically split by several [Formula: see text], a finite temperature or an applied external bias voltage is required to populate energetically excited states. The theoretical methods in this paper include a capacitor model to fit experimental charging energies, an extended Hubbard model to describe the tunneling dynamics, a rate equation model to find the occupation probabilities, and an input-output model to determine the response signal of the resonator.

Carrier control in 2D transition metal dichalcogenides with Al2O3 dielectric

We report transport measurements of dual gated MoS₂ and WSe₂ devices using atomic layer deposition grown Al₂O₃ as gate dielectrics. We are able to achieve current pinch-off using independent split gates and observe current steps suggesting possible carrier confinement. We also investigated the impact of gate geometry and used electrostatic potential simulations to explain the observed device physics.

Preparation and Characterization of Chiral Transition-Metal Dichalcogenide Quantum Dots and Their Enantioselective Catalysis

Two-dimensional transition-metal dichalcogenides (TMDs) had attracted enormous interests owing to their extraordinary optical, physical, and chemical properties. Herein, we prepared for the first time a series of chiral TMD quantum dots (QDs) from MoS₂ and WS₂ bulk crystals by covalent modification with chiral ligands cysteine and penicillamine. The chiral TMD QDs were carefully investigated by spectroscopic and microscopic techniques. Their chiral optical activity was confirmed by distinct circular dichroism signals different to those of the chiral ligands. Interestingly, with the assistance of copper ions, the chiral QDs displayed strong and chiral selective peroxidase-like activity. Up to now, inorganic nanomaterials with peroxidase-like activity were tremendous but seldom examples with enantioselectivity. The enantioselectivity of our chiral TMD QDs toward chiral substrates d- and l-tyrosinol was highly up to 6.77, which was almost the best performance ever reported. The mechanisms of enantioselectivity was further investigated by quartz crystal microbalance assays. We believed that because of the extraordinary electronic and optical properties, the chiral TMD QDs should be useful for nonlinear optical materials, asymmetric catalysis, chiral and biological sensors, and so on.

Gate-Defined Quantum Confinement in InSe-Based van der Waals Heterostructures

Indium selenide, a post-transition metal chalcogenide, is a novel two-dimensional (2D) semiconductor with interesting electronic properties. Its tunable band gap and high electron mobility have already attracted considerable research interest. Here we demonstrate strong quantum confinement and manipulation of single electrons in devices made from few-layer crystals of InSe using electrostatic gating. We report on gate-controlled quantum dots in the Coulomb blockade regime as well as one-dimensional quantization in point contacts, revealing multiple plateaus. The work represents an important milestone in the development of quality devices based on 2D materials and makes InSe a prime candidate for relevant electronic and optoelectronic applications.

Quantum transport through MoS2 constrictions defined by photodoping

We present a device scheme to explore mesoscopic transport through molybdenum disulfide (MoS₂) constrictions using photodoping. The devices are based on van-der-Waals heterostructures where few-layer MoS₂ flakes are partially encapsulated by hexagonal boron nitride (hBN) and covered by a few-layer graphene flake to fabricate electrical contacts. Since the as-fabricated devices are insulating at low temperatures, we use photo-induced remote doping in the hBN substrate to create free charge carriers in the MoS₂ layer. On top of the device, we place additional metal structures, which define the shape of the constriction and act as shadow masks during photodoping of the underlying MoS₂/hBN heterostructure. Low temperature two- and four-terminal transport measurements show evidence of quantum confinement effects.

Electrical control of charged carriers and excitons in atomically thin materials

Electrical confinement and manipulation of charge carriers in semiconducting nanostructures are essential for realizing functional quantum electronic devices^1-3. The unique band structure^4-7 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 devices^9-11 due to their large exciton binding energy^12,13. 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 couplings^14-16. 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.

Three-electron spin qubits

The goal of this article is to review the progress of three-electron spin qubits from their inception to the state of the art. We direct the main focus towards the exchange-only qubit (Bacon et al 2000 Phys. Rev. Lett. 85 1758-61, DiVincenzo et al 2000 Nature 408 339) and its derived versions, e.g. the resonant exchange (RX) qubit, but we also discuss other qubit implementations using three electron spins. For each three-spin qubit we describe the qubit model, the envisioned physical realization, the implementations of single-qubit operations, as well as the read-out and initialization schemes. Two-qubit gates and decoherence properties are discussed for the RX qubit and the exchange-only qubit, thereby completing the list of requirements for quantum computation for a viable candidate qubit implementation. We start by describing the full system of three electrons in a triple quantum dot, then discuss the charge-stability diagram, restricting ourselves to the relevant subsystem, introduce the qubit states, and discuss important transitions to other charge states (Russ et al 2016 Phys. Rev. B 94 165411). Introducing the various qubit implementations, we begin with the exchange-only qubit (DiVincenzo et al 2000 Nature 408 339, Laird et al 2010 Phys. Rev. B 82 075403), followed by the RX qubit (Medford et al 2013 Phys. Rev. Lett. 111 050501, Taylor et al 2013 Phys. Rev. Lett. 111 050502), the spin-charge qubit (Kyriakidis and Burkard 2007 Phys. Rev. B 75 115324), and the hybrid qubit (Shi et al 2012 Phys. Rev. Lett. 108 140503, Koh et al 2012 Phys. Rev. Lett. 109 250503, Cao et al 2016 Phys. Rev. Lett. 116 086801, Thorgrimsson et al 2016 arXiv:1611.04945). The main focus will be on the exchange-only qubit and its modification, the RX qubit, whose single-qubit operations are realized by driving the qubit at its resonant frequency in the microwave range similar to electron spin resonance. Two different types of two-qubit operations are presented for the exchange-only qubits which can be divided into short-ranged and long-ranged interactions. Both of these interaction types are expected to be necessary in a large-scale quantum computer. The short-ranged interactions use the exchange coupling by placing qubits next to each other and applying exchange-pulses (DiVincenzo et al 2000 Nature 408 339, Fong and Wandzura 2011 Quantum Inf. Comput. 11 1003, Setiawan et al 2014 Phys. Rev. B 89 085314, Zeuch et al 2014 Phys. Rev. B 90 045306, Doherty and Wardrop 2013 Phys. Rev. Lett. 111 050503, Shim and Tahan 2016 Phys. Rev. B 93 121410), while the long-ranged interactions use the photons of a superconducting microwave cavity as a mediator in order to couple two qubits over long distances (Russ and Burkard 2015 Phys. Rev. B 92 205412, Srinivasa et al 2016 Phys. Rev. B 94 205421). The nature of the three-electron qubit states each having the same total spin and total spin in z-direction (same Zeeman energy) provides a natural protection against several sources of noise (DiVincenzo et al 2000 Nature 408 339, Taylor et al 2013 Phys. Rev. Lett. 111 050502, Kempe et al 2001 Phys. Rev. A 63 042307, Russ and Burkard 2015 Phys. Rev. B 91 235411). The price to pay for this advantage is an increase in gate complexity. We also take into account the decoherence of the qubit through the influence of magnetic noise (Ladd 2012 Phys. Rev. B 86 125408, Mehl and DiVincenzo 2013 Phys. Rev. B 87 195309, Hung et al 2014 Phys. Rev. B 90 045308), in particular dephasing due to the presence of nuclear spins, as well as dephasing due to charge noise (Medford et al 2013 Phys. Rev. Lett. 111 050501, Taylor et al 2013 Phys. Rev. Lett. 111 050502, Shim and Tahan 2016 Phys. Rev. B 93 121410, Russ and Burkard 2015 Phys. Rev. B 91 235411, Fei et al 2015 Phys. Rev. B 91 205434), fluctuations of the energy levels on each dot due to noisy gate voltages or the environment. Several techniques are discussed which partly decouple the qubit from magnetic noise (Setiawan et al 2014 Phys. Rev. B 89 085314, West and Fong 2012 New J. Phys. 14 083002, Rohling and Burkard 2016 Phys. Rev. B 93 205434) while for charge noise it is shown that it is favorable to operate the qubit on the so-called '(double) sweet spots' (Taylor et al 2013 Phys. Rev. Lett. 111 050502, Shim and Tahan 2016 Phys. Rev. B 93 121410, Russ and Burkard 2015 Phys. Rev. B 91 235411, Fei et al 2015 Phys. Rev. B 91 205434, Malinowski et al 2017 arXiv: 1704.01298), which are least susceptible to noise, thus providing a longer lifetime of the qubit.

Electrotunable artificial molecules based on van der Waals heterostructures

Quantum confinement has made it possible to detect and manipulate single-electron charge and spin states. The recent focus on two-dimensional (2D) materials has attracted significant interests on possible applications to quantum devices, including detecting and manipulating either single-electron charging behavior or spin and valley degrees of freedom. However, the most popular model systems, consisting of tunable double-quantum-dot molecules, are still extremely difficult to realize in these materials. We show that an artificial molecule can be reversibly formed in atomically thin MoS₂ sandwiched in hexagonal boron nitride, with each artificial atom controlled separately by electrostatic gating. The extracted values for coupling energies at different regimes indicate a single-electron transport behavior, with the coupling strength between the quantum dots tuned monotonically. Moreover, in the low-density regime, we observe a decrease of the conductance with magnetic field, suggesting the observation of Coulomb blockade weak anti-localization. Our experiments demonstrate for the first time the realization of an artificial quantum-dot molecule in a gated MoS₂ van der Waals heterostructure, which could be used to investigate spin-valley physics. The compatibility with large-scale production, gate controllability, electron-hole bipolarity, and new quantum degrees of freedom in the family of 2D materials opens new possibilities for quantum electronics and its applications.

Gate-Defined One-Dimensional Channel and Broken Symmetry States in MoS2 van der Waals Heterostructures

We have realized encapsulated trilayer MoS₂ devices with gated graphene contacts. In the bulk, we observe an electron mobility as high as 7000 cm²/(V s) at a density of 3 × 10¹² cm^-2 at a temperature of 1.9 K. Shubnikov-de Haas oscillations start at magnetic fields as low as 0.9 T. The observed 3-fold Landau level degeneracy can be understood based on the valley Zeeman effect. Negatively biased split gate electrodes allow us to form a channel that can be completely pinched off for sufficiently large gate voltages. The measured conductance displays plateau-like features.

Single-Shot Ternary Readout of Two-Electron Spin States in a Quantum Dot Using Spin Filtering by Quantum Hall Edge States

We report on the single-shot readout of three two-electron spin states-a singlet and two triplet substates-whose z components of spin angular momentum are 0 and +1, in a gate-defined GaAs single quantum dot. The three spin states are distinguished by detecting spin-dependent tunnel rates that arise from two mechanisms: spin filtering by spin-resolved edge states and spin-orbital correlation with orbital-dependent tunneling. The three states form one ground state and two excited states, and we observe the spin relaxation dynamics among the three spin states.

Improving the luminescence enhancement of hybrid Au nanoparticle-monolayer MoS2 by focusing radially-polarized beams

Monolayer transition-metal dichalcogenides (TMDs) have grown as fantastic building blocks for optoelectronic applications, owing to their direct band gap, transparency, and mechanical flexibility. Since the luminescence of monolayer TMDs suffers from low light absorption and emission, surface plasmons, which confine light at subwavelength and enhance the local electric field, are utilized to boost both excitation and emission fields of TMDs, enabling strong light-matter interaction at the nano-scale. Meanwhile, radially-polarized beams (RPBs) as new and attractive excitation source have found many applications in surface plasmon polaritons, optical tweezer and so on. Here, by using RPBs, we demonstrate the photoluminescence (PL) enhancement of monolayer molybdenum disulfide (MoS₂) hybridized with 210 nm-diameter gold nanoparticle (AuNP) is improved by about 1.37-fold compared with linearly-polarized beams (LPBs). Besides, the PL enhancement with RPBs depends on the size of AuNP as well. With 210nm-diameter AuNP, the PL enhancement is more than 1.5-fold higher than that with 60nm-diameter AuNP. This study highlights that RPBs are superior to LPBs for tuning the near-field system response and shows that RPBs drive a valuable avenue to further study the emerging two-dimentional materials.

Coulomb blockade in monolayer MoS2 single electron transistor

Substantial effort has been dedicated to understand the intrinsic electronic properties of molybdenum disulfide (MoS2). However, electron transport study on monolayer MoS2 has been challenging to date, especially at low temperatures due to large metal/semiconductor junction barriers. Herein, we report the fabrication and characterization of the monolayer MoS2 single-electron transistor. High performance devices are obtained through the use of low work function metal (zinc) contact and a rapid thermal annealing step. Coulomb blockade is observed at low temperatures and is attributed to single-electron tunneling via two tunnel junction barriers. The nature of Coulomb blockade is also investigated by temperature-dependent conductance oscillation measurement. Our results hold promise for the study of novel quantum transport phenomena in 2D semiconducting atomic layer crystals.

Temperature dependence of Coulomb oscillations in a few-layer two-dimensional WS2 quantum dot

Standard semiconductor fabrication techniques are used to fabricate a quantum dot (QD) made of WS₂, where Coulomb oscillations were found. The full-width-at-half-maximum of the Coulomb peaks increases linearly with temperature while the height of the peaks remains almost independent of temperature, which is consistent with standard semiconductor QD theory. Unlike graphene etched QDs, where Coulomb peaks belonging to the same QD can have different temperature dependences, these results indicate the absence of the disordered confining potential. This difference in the potential-forming mechanism between graphene etched QDs and WS₂ QDs may be the reason for the larger potential fluctuation found in graphene QDs.