Manipulation Of Electron Spins Research Articles

Precise Characterization of a Waveguide Fiber Interface in Silicon Carbide.

Spin-active optical emitters in silicon carbide are excellent candidates toward the development of scalable quantum technologies. However, efficient photon collection is challenged by undirected emission patterns from optical dipoles, as well as low total internal reflection angles due to the high refractive index of silicon carbide. Based on recent advances with emitters in silicon carbide waveguides, we now demonstrate a comprehensive study of nanophotonic waveguide-to-fiber interfaces in silicon carbide. We find that across a large range of fabrication parameters, our experimental collection efficiencies remain above 90%. Further, by integrating silicon vacancy color centers into these waveguides, we demonstrate an overall photon count rate of 181 kilo-counts per second, which is an order of magnitude higher compared to standard setups. We also quantify the shift of the ground state spin states due to strain fields, which can be introduced by waveguide fabrication techniques. Finally, we show coherent electron spin manipulation with waveguide-integrated emitters with state-of-the-art coherence times of T 2 ∼ 42 μs. The robustness of our methods is very promising for quantum networks based on multiple orchestrated emitters.

Real-space determination of the isolated magnetic skyrmion deformation under electric current flow

The manipulation and control of electron spins, the fundamental building blocks of magnetic domains and spin textures, are at the core of spintronics. Of particular interest is the effect of the electric current on topological magnetic skyrmions, such as the current-induced deformation of isolated skyrmions. The deformation has consequences ranging from perturbed dynamics to modified packing configurations. In this study, we measured the current-driven real-space deformation of isolated, pinned skyrmions within Co10Zn10 at room temperature. We observed that the skyrmions are surprisingly soft, readily deforming during electric current application into an elliptical shape with a well-defined deformation axis (semimajor axis). We found that this axis rotates unidirectionally toward the current direction irrespective of electric current polarity and that the elliptical deformation reverses back upon current termination. We quantified the average distortion δ, which increased by ∼90% during the largest applied current density |j| = 8.46 ×109 A/m2 when compared with the skyrmion's intrinsic shape ([Formula: see text]). Additionally, we demonstrated an approximately 120% average skyrmion core size expansion during current application, highlighting the skyrmions' inherent topological protection. This evaluation of in situ electric current-induced skyrmion deformation paints a clearer picture of spin-polarized electron-skyrmion interactions and may prove essential in designing spintronic devices.

Optically addressable universal holonomic quantum gates on diamond spins

The ability to individually control the numerous spins in a solid-state crystal is a promising technology for the development of large-scale quantum processors and memories. A localized laser field offers spatial selectivity for electron spin manipulation through spin–obit coupling, but it has been difficult to simultaneously achieve precise and universal manipulation. Here, we demonstrate microwave-driven holonomic quantum gates on an optically selected electron spin in a nitrogen-vacancy centre in diamond. The electron spin is precisely manipulated with global microwaves tuned to the frequency shift induced by the local optical Stark effect. We show the universality of the operations, including state initialization, preparation, readout and echo. We also generate optically addressable entanglement between the electron and adjacent nitrogen nuclear spin. High-fidelity operations are achieved by applying amplitude-alternating pulses, which are tolerant to fluctuations in microwave intensity and detuning. These techniques enable site-selective quantum teleportation transfer from a photon to a nuclear spin memory, paving the way for the realization of distributed quantum computers and the quantum Internet with large-scale quantum storage.

Robust ferroelectric-gating-dependent electronic and magnetic properties in a 1T-VSe2/BiAlO3(0001) multiferroic heterostructure

The modulation of magnetism and related electronic properties is highly desirable for advanced nanoscale electronic and spintronic applications, but volatility in electron spin manipulation is a major challenge that requires urgent attention. Here, we use density functional theory (DFT) calculations to show that the electronic and magnetic properties of a ferromagnetic [half-]metallic 1T-VSe2 monolayer can be effectively modulated via ferroelectric BiAlO3(0001) gating. In particular, the carry type in the 1T-VSe2 overlayer can be switched from being n-type to p-type or n+ -type through the reversal of the ferroelectric polarization. The magnetocrystalline anisotropy of the 1T-VSe2 monolayer can also be changed from easy-plane to easy-axis. In addition, the band offset, magnetic moments, exchange interaction, and Curie temperature (Tc) of the 1T-VSe2 overlayer show significant dependence on the electric polarization direction, with the calculated surface magnetoelectric coupling coefficients (αS) reaching 10−10 G cm [2]/V. This demonstration of robust ferroelectric-gating-dependent magnetism and related electronic properties holds great promise for the innovative design and implementation of next-generation nanoscale non-volatile ultra-low power devices.

Local breaking of the spin degeneracy in the vortex states of Ising superconductors: Induced antiphase ferromagnetic order

Ising spin-orbital coupling is usually easy to identify in the Ising superconductors via an in-plane critical field enhancement, but we show that the Ising spin-orbital coupling also manifests in the vortex physics for perpendicular magnetic fields. By self-consistently solving the Bogoliubov--de Gennes equations of a model Hamiltonian built on the honeycomb lattice with the Ising spin-orbital coupling pertinent to the transition metal dichalcogenides, we numerically investigate the local breaking of the spin and sublattice degeneracies in the presence of a perpendicular magnetic field. It is revealed that the ferromagnetic orders are induced inside the vortex core region by the Ising spin-orbital coupling. The induced magnetic orders are antiphase in terms of their opposite polarizations inside the two nearest-neighbor vortices with one of the two polarizations coming predominantly from one sublattice site, implying the local breaking of the spin and sublattice degeneracies. The finite-energy peaks of the local density of states for spin-up and spin-down in-gap states are split and shifted oppositely by the Ising spin-orbital coupling, and the relative shifts of them on sublattices $A$ and $B$ are also of opposite algebraic sign. The calculated results and the proposed scenario may not only serve as experimental signatures for identifying the Ising spin-orbital coupling in the Ising superconductors, but also be prospective in the manipulation of electron spins in motion through the orbital effect in the superconducting vortex states.

Electron-spin-resonance meanderlines for effective spin control in Si quantum dots for large-scale qubit applications

Coherent control of electron spins in multiple quantum dots (QDs) is critical for realizing large-scale spin qubits. The manipulation of electron spins in Si QDs can be achieved by electron spin resonance (ESR). While the effective spin control over a single qubit has been demonstrated, the architecture of ESR lines for large-scale spin qubits has yet to be demonstrated. In this paper, we propose an ESR meanderline for large-scale Si QDs. Simulation results show that magnetic fields can be effectively enhanced with low electric fields using a meanderline, enabling high-fidelity and low-noise control over electron spins in a 50-qubit system. Reflection coefficients of ESR meanderlines by on-wafer microwave measurements show low loss (−7 dB) for a 3-qubit device at a frequency range of 10 to 50 GHz.

Low dephasing and robust micromagnet designs for silicon spin qubits

Using micromagnets to enable electron spin manipulation in silicon qubits has emerged as a very popular method, enabling single-qubit gate fidelities larger than 99.9%. However, these micromagnets also apply stray magnetic field gradients onto the qubits, making the spin states susceptible to electric field noise and limiting their coherence times. We describe here a magnet design that minimizes qubit dephasing, while allowing for fast qubit control and addressability. Specifically, we design and optimize magnet dimensions and positions relative to the quantum dots, minimizing dephasing from magnetic field gradients. The micromagnet-induced dephasing rates with this design are up to three orders of magnitude lower than state-of-the-art implementations, allowing for long coherence times. This design is robust against fabrication errors and can be combined with a wide variety of silicon qubit device geometries, thereby allowing exploration of coherence limiting factors and novel upscaling approaches.

Room Temperature Ferromagnetism: Nonmagnetic Semiconductor Oxides and Nonmagnetic Dopants

Advances in room temperature ferromagnetic semiconductors increase the opportunity to commercialize full spintronic devices. The manipulation of electron spin in semiconductor materials has driven significant research activity with the goal of realizing their amazing technological potential. Coupling of magnetic and semiconducting properties could lead to a new generation of information and communication devices. During the past 20 years, the intensive research on magnetic semiconductor materials has led to discovery of two interesting facts. Room temperature ferromagnetism is observed in undoped semiconductor oxides with empty or completely filled d- or f-orbitals, and nonmagnetic dopants can induce or enhance the room temperature ferromagnetism in nonmagnetic semiconductor materials. The organized review which addresses this phenomenon and covers the large number of studies on this subject is rare. In this study, we firstly review the advantages aspects of spintronic devices as well as the materials suitable for these applications. Here, we tried to provide a systematic study on defects induced by room temperature ferromagnetism in undoped semiconductor oxides as well as the impact of nonmagnetic dopants on ferromagnetism. We hope this review assist researchers in creating a complete picture to develop future research activities to access innovative technological applications.

Resonant excitation of nanowire quantum dots

GaAs quantum dots in nanowires are one of the most promising candidates for scalable quantum photonics. They have excellent optical properties, can be frequency-tuned to atomic transitions, and offer a robust platform for fabrication of multi-qubit devices that promise to unlock the full technological potential of quantum dots. Coherent resonant excitation is necessary for virtually any practical application because it allows, for instance, for on-demand generation of single and entangled photons, photonic clusters states, and electron spin manipulation. However, emission from nanowire structures under this excitation scheme has never been demonstrated. Here we show, for the first time, biexciton–exciton cascaded emission via resonant two-photon excitation and resonance fluorescence from an epitaxially grown GaAs quantum dot in an AlGaAs nanowire. We also report that resonant excitation schemes, combined with above-bandgap excitation, can be used to clean and enhance the emission of nanowire quantum dots.

Piezotronic effect on Rashba spin–orbit coupling based on MAPbI3/ZnO heterostructures

Rashba spin–orbit coupling (SOC) is a core issue in semiconductor spintronics, which allows the manipulation of electron spin through an electric field rather than an external magnetic field, revealing a bright prospect for advanced electronic devices with ultra-high speed and integration. Conversely, the emerging piezotronic effect is the born characteristic for many semiconductors that have a non-central symmetric structure, such as ZnO and GaN. Here, we design three heterostructure devices, based on piezoelectric p-type (CH3NH3)PbI3 single crystals and n-type wurtzite-structured ZnO thin films, to theoretically study how the piezotronic effect can effectively work on the Rashba spin–orbit coupling. Benefiting from large piezoelectric charges at the interface when a vertical strain is applied, a high concentration of two-dimensional electron gas is induced in the plane of the heterostructure, which can tune the built-in electric field at the interface and further manipulate the Rashba SOC. With the increase in pressure, both the Rashba parameter and spin splitting are found to first vanish and then increase linearly for ZnO with doping densities of 1015 and 1016 cm−3. This work provides insight for manipulating electron spins via the introduction of piezocharges, showing great application potential of the piezotronic effect in tuning spintronic devices.

Tuning Magnetism of Metal Porphyrazine Molecules by a Ferroelectric In2Se3 Monolayer.

Electric field tuning of magnetism is highly desirable for nanoelectronics, but volatility in electron spin manipulation presents a major challenge that needs urgent resolution. Here, we show by first-principles calculations that magnetism of metal porphyrazine (MPz) molecules can be effectively tuned by switching ferroelectric polarization of an adjacent In2Se3 monolayer. The magnetic moments of TiPz and VPz (MnPz, FePz, and CoPz) decrease (increase) at one polarization but remain unchanged at reversed polarization. This intriguing phenomenon stems from distinct metal d-orbital occupation caused by electron transfer and energy-level shift associated with the polarization switch of the In2Se3 monolayer. Moreover, the ferroelectric switch also tunes the underlying electronic properties, producing a metallic, half-metallic, or semiconducting state depending on polarization. These findings of robust ferroelectric tuning of magnetism and related electronic properties in MPz-adsorbed In2Se3 hold great promise for innovative design and implementation in advanced magnetic memory storage, sensor, and spintronic devices.

Quadrupolar effect on nuclear spin depolarization in single self-assembled quantum dots

We investigate the nuclear spin polarization in single self-assembled InAlAs quantum dots under magnetic fields that are sufficiently weak to make the impacts of the quadrupole interaction apparent. The nuclear spin depolarization rate decreases as the magnitude of the applied longitudinal magnetic field increases. This trend can be explained by the dissolution of quadrupolar mixing that is coupled to the temporal fluctuations of the hyperfine interaction. In addition, the optically injected electron spin generates a considerable Overhauser field under a wide range of experimental conditions. These findings are important for nuclear spin engineering and electron spin manipulation in self-assembled III–V semiconductor quantum dots.

Optical control of room-temperature valley polaritons

Valley-polarized light–matter quasiparticles in two-dimensional semiconductor microcavities are demonstrated. Access to spin–valley physics may be useful for photonic quantum technologies. The formation of half-light half-matter quasiparticles under strong coupling results in properties unique from those of the constituent components. Fingerprints of both light and matter are imprinted on the new quasiparticles, called polaritons. In the context of two-dimensional (2D) materials, this opens up the possibility of exploiting the intriguing spin–valley physics of a bare semiconductor combined with the light mass of the photonic component for possible quantum technologies. Specifically, the valley degree of freedom1,2, which remained largely unexplored until the advent of these materials, is highly attractive in this context as it provides an optically accessible route for the control and manipulation of electron spin. Here, we report the observation of room-temperature strongly coupled light–matter quasiparticles that are valley polarized because of the coupling of photons with specific helicity to excitons that occupy quantum mechanically distinct valleys in momentum space. The realization of valley polaritons in 2D semiconductor microcavities presents the first step towards engineering valley-polaritonic devices.

Quantum engineering of matter from the laboratory to the market: an interview with Dieter Bimberg and Kang Wang

Abstract Quantum dots introduce a new form of quantum engineering of materials properties based purely on size and dimensionality. Whereas we traditionally think of materials as having intrinsic properties such as strength, electrical conductivity and light absorption or emission spectra due to their composition and atomic-scale structure, when their size is very small, such properties can be altered by quantum-mechanical effects, for example due to confinement of the quantum wavefunctions that determine the energies of charge carriers like electrons. As it became possible in the 1990s to control the size of material structures with a precision of less than a nanometre in all three dimensions of space, quantum effects began to be exploited for technological applications. Quantum dots are particles with a nanoscale extent in three dimensions—in effect, tiny fragments of material, often made of one type of semiconductor embedded in another. Quantum dots in which the wavelength of absorption and fluorescent emission of light is tuned by the nanoparticle size are being explored for uses ranging from colour displays to biomedical imaging. Films of semiconducting materials thin enough to exhibit one-dimensional quantum-confinement effects, meanwhile, are routinely used to make efficient optical devices such as lasers. Such applications have depended on the availability of techniques for precise control of particle size, composition and thickness. NSR spoke to two leading researchers in this field of quantum engineering about its development and prospects. Dieter Bimberg is founding director of the Center of Nanophotonics at the Technical University of Berlin in Germany. He pioneered the use of quantum dots in photonic devices such as lasers and optical amplifiers, as well as developing non-volatile ‘dynamic’ random-access memories (DRAMs), which retain their information when the power is switched off. Kang Wang is a professor of electrical engineering at the University of California at Los Angeles, whose work on nanoscale quantum devices has focused on electronic and magnetic properties, and in particular on the development of non-volative RAMs and the manipulation and control of electron spin as a new parameter for information processing (a technology called spintronics).

Multi-Frequency Pulsed Overhauser DNP at 1.2 Tesla

Abstract Dynamic nuclear polarization (DNP) is a methodology to increase the sensitivity of nuclear magnetic resonance (NMR) spectroscopy. It relies on the transfer of the electron spin polarization from a radical to coupled nuclear spins, driven by microwave excitation resonant with the electron spin transitions. In this work we explore the potential of pulsed multi-frequency microwave excitation in liquids. Here, the relevant DNP mechanism is the Overhauser effect. The experiments were performed with TEMPOL radicals in aqueous solution at room temperature using a Q-band frequency (1.2 T) electron paramagnetic resonance (EPR) spectrometer combined with a Minispec NMR spectrometer. A fast arbitrary waveform generator (AWG) enabled the generation of multi-frequency pulses used to either sequentially or simultaneously excite all three 14N-hyperfine lines of the nitroxide radical. The multi-frequency excitation resulted in a doubling of the observed DNP enhancements compared to single-frequency microwave excitation. Q-band free induction decay (FID) signals of TEMPOL were measured as a function of the excitation pulse length allowing the efficiency of the electron spin manipulation by the microwave pulses to be extracted. Based on this knowledge we could quantitatively model our pulsed DNP enhancements at 1.2 T by numerical solution of the Bloch equations, including electron spin relaxation and experimental parameters. Our results are in good agreement with theoretical predictions. Whereas for a narrow and homogeneous single EPR line continuous wave excitation leads to more efficient DNP enhancements compared to pulsed excitation for the same amount of averaged microwave power. The situation is different for radicals with several hyperfine lines or in the presence of inhomogeneous line broadening. In such cases pulsed single/multi-frequency excitation can lead to larger DNP enhancements.

Shaped electric fields for fast optimal manipulation of electron spin and position in a double quantum dot

We use quantum optimal control theory algorithms to design external electric fields that drive the coupled spin and orbital dynamics of an electron in a double quantum dot, subject to the spin-orbit interaction and Zeeman magnetic fields. We obtain time-profiles of multi-frequency electric pulses which increase the rate of spin-flip transitions by several orders of magnitude in comparison with monochromatic fields, where the spin Rabi oscillations were predicted to be very slow. This precise, with the accuracy higher than $10^{-4}$, and fast, at the timescale of the order of 0.1 ns, simultaneous control of the electron spin orientation and position is achieved while keeping the electron in the four lowest tunneling- and Zeeman-split levels through the duration of the pulse, facilitating high fidelity and suggesting useful applications in spintronics and quantum information devices.

Single-spin CCD.

Spin-based electronics or spintronics relies on the ability to store, transport and manipulate electron spin polarization with great precision. In its ultimate limit, information is stored in the spin state of a single electron, at which point quantum information processing also becomes a possibility. Here, we demonstrate the manipulation, transport and readout of individual electron spins in a linear array of three semiconductor quantum dots. First, we demonstrate single-shot readout of three spins with fidelities of 97% on average, using an approach analogous to the operation of a charge-coupled device (CCD). Next, we perform site-selective control of the three spins, thereby writing the content of each pixel of this 'single-spin charge-coupled device'. Finally, we show that shuttling an electron back and forth in the array hundreds of times, covering a cumulative distance of 80 μm, has negligible influence on its spin projection. Extrapolating these results to the case of much larger arrays points at a diverse range of potential applications, from quantum information to imaging and sensing.

(Invited) Spin-Based Silicon and CMOS-Compatible Devices

With CMOS feature size rapidly approaching ultimate scaling limits the electron spin attracts much attention as an alternative degree of freedom for low-power non-volatile applications [1]. Silicon, the main material of microelectronics, is perfectly suited for spin-driven applications, because it is mostly composed of nuclei without spin and is characterized by weak spin-orbit interaction [2]. Both factors favor a long electron spin lifetime in silicon. However, even a weak spin-orbit interaction causes the electron spin lifetime to decrease. The Elliot-Yafet spin relaxation due to phonons’ mediated scattering between the valleys along different crystallographic axes is the main spin relaxation mechanism in bulk silicon at room temperature [3,4]. This inter-valley scattering can be effectively eliminated by partially lifting the valley degeneracy either by stress or by confining an electron system. In strained silicon the spin lifetime can indeed be increased by a factor of two. In contrast, in silicon-on-insulator structures the spin lifetime is reduced; and methods to increase the spin lifetime are needed. Uniaxial stress dramatically reduces the spin relaxation and boosts the spin lifetime by an order of magnitude in thin silicon films [5]. The physical reason for the spin lifetime enhancement lies in the capability to lift completely the degeneracy between the two valleys in a confined electron systems by shear strain [6]. This degeneracy was a long-standing problem in silicon spintronics [7]; indeed, apart from causing a strong spin decoherence and relaxation, the degeneracy between the valleys introduces an unwanted competition between the quantum numbers of the valleys and electron spin, thus potentially threatening spin-based device operation. Therefore, lifting the valley degeneracy completely in a controllable way by means of standard stress techniques represents a major breakthrough in future silicon spin-driven applications. Despite impressive progress in spin injection in silicon at room temperature [2,8], the larger than predicted amplitude of the signal [2] is heavily debated prompting alternative explanations [9]. In addition, low efficiency of spin injection from ferromagnetic metal leads in silicon and the absence of a viable concept of electron spin manipulation in the channel by purely electrical means makes a practical realization of a device working on a principal similar to a MOSFET difficult. An experimental demonstration of the Datta-Das Spin FET [10] is pending for 25 years now. This motivates researchers to look into MOSFET and CMOS compatible spin-driven devices and applications. Commercially available CMOS compatible spin-transfer torque magnetic random access memory (MRAM) built on magnetic tunnel junctions (MTJs) possesses all properties characteristic to universal memory: fast operation, high density, and non-volatility. The reduction the critical current for magnetization switching is the main challenge. Perpendicular MTJs allow circumventing the problem at the expense of thermal stability [11]. The thermal stability is considerably increased in structures with a free layer composed of two vertically sandwiched layers [11]. An alternative path is to consider in-plane structures with composite free magnetic layers [12]. A substantial reduction of the critical current density is achieved, when the free layer is composed of two similar parts in the same plane next to each other [13]. The critical current reduction and/or switching is caused by the fact that the magnetization of the free layer remains in plane. MRAM can be used to build non-volatile logic-in-memory architectures with non-volatile storage elements on the top of logic circuits. Non-volatility and reduction of interconnects guarantee low-power consumption [14]. MRAM-CMOS hybrid ternary content-addressable memory and lock-up table circuits were demonstrated [14]. An alternative concept for non-volatile logic-in-memory circuits utilizes the same MRAM cells to store and process information simultaneously [15]. The work is supported by the European Research Council through the grant #247056 MOSILSPIN. 1. I.Zutic et al., Rev. Mod. Phys. 76 (2004) 323. 2. R.Jensen, Nature Materials 11 (2012) 400. 3. J.Ceng et al., Phys. Rev. Lett. 104 (2010) 016601. 4. P.Li and H.Dery, Phys. Rev. Lett. 107 (2011) 107203. 5. D.Osintsev et al., Solid-State Electron. 90 (2013) 34. 6. V.Sverdlov, Strain-induced effects in advanced MOSFETs, Springer 2011. 7. L.Zhang et al., Nature Commun. 4 (2013) 2396. 8. C.Li et al., Nature Commun. 2 (2011) 245. 9. Y.Song and H.Dery, Phys. Rev. Lett. 113 (2014) 047205. 10. S.Datta and B.Das, Appl. Phys. Lett. 56 (1990) 665. 11. S.Fukami et al., ASP-DAC 2014, p. 684. 12. A.Makarov et al., Phys. Stat. Solidi-RRL 5 (2011) 420. 13. A.Makarov et al., in: Future Trends in Microelectronics, J.Wiley&Sons, 2013, p.93. 14. T.Hanyu, ISCAS 2013, p.117. 15. H.Mahmoudi et al., in: Micro- and Nanoelectronics: Emerging Device Challenges and Solutions, CRC Press 2014, p.221.

Ultrafast single electron spin manipulation in 2D semiconductor quantum dots with optimally controlled time-dependent electric fields through spin-orbit coupling

We have studied theoretically the possibility of ultra-fast manipulation of a single electron spin in 2D semiconductor quantum dots, by means of high-frequency time-dependent electric fields. The electron spin degree of freedom is excited through spin-orbit coupling, and the procedure may be enhanced by the presence of a static magnetic field. We use quantum optimal control theory to tailor the temporal profile of the electric field in order to achieve the most effective manipulation. The scheme predicts significant control over spin operations in times of the order of picoseconds – an ultrafast time scale that permits to avoid the effects of decoherence if this scheme is to be used as a tool for quantum information processing.

All-electric all-semiconductor spin field-effect transistors.

The spin field-effect transistor envisioned by Datta and Das opens a gateway to spin information processing. Although the coherent manipulation of electron spins in semiconductors is now possible, the realization of a functional spin field-effect transistor for information processing has yet to be achieved, owing to several fundamental challenges such as the low spin-injection efficiency due to resistance mismatch, spin relaxation and the spread of spin precession angles. Alternative spin transistor designs have therefore been proposed, but these differ from the field-effect transistor concept and require the use of optical or magnetic elements, which pose difficulties for incorporation into integrated circuits. Here, we present an all-electric and all-semiconductor spin field-effect transistor in which these obstacles are overcome by using two quantum point contacts as spin injectors and detectors. Distinct engineering architectures of spin-orbit coupling are exploited for the quantum point contacts and the central semiconductor channel to achieve complete control of the electron spins (spin injection, manipulation and detection) in a purely electrical manner. Such a device is compatible with large-scale integration and holds promise for future spintronic devices for information processing.