Charge State Control Research Articles

Optical Ionization of Qubits and their Silent Charge States

Color centers in the technologically mature material silicon carbide are candidates for the implementation of quantum applications. Chargestate control and electrical read-out of qubits includes spin-to-charge conversion via optical excitation and subsequent ionization. In this work we address the dominant photoionization mechanism and the photophysical properties of the ionized silicon vacancy in 4H SiC using ab initio theory. We find that its nominally dark chargestates are infrared emitters.

Creation, Control, and Modeling of NV Centers in Nanodiamonds

AbstractSensing based on Nitrogen‐Vacancy (NV) centers in nanodiamonds (NDs) represents a potentially groundbreaking technology with broad applications. Nevertheless, the optimization of their quantum‐optical properties is still a challenging issue. The present work aims at enhancing and controlling NV centers optical properties in NDs by combining their surface chemistry tuning and proton beam irradiation. Systematic thermal oxidations are carried out to study the evolution of surface chemical groups (IR spectroscopy), as well as their influence on optical properties (photoluminescence spectroscopy, PL decay measurements). Proton irradiation is performed by exploring a wide range of fluences (1014 – 1017 cm−2) in order to precisely control the amount of NV centers, thus defining the conditions that maximize their creation and emission intensity. In addition, NV centers charge state control is achieved by assessing NV−/NV0 ratio upon different surface termination tuning and NV centers amount. Finally, a novel predictive mathematical model is developed, allowing for the evaluation of the efficiencies of the formation of both NV− and NV0. Although the model is tested in the specific case study with proton irradiated NDs, it offers a broad applicability, thus representing a key landmark in the prediction of the outcome of ion‐beam‐based color centers generation in diamond.

Super-Resolution Diamond Magnetic Microscopy of Superparamagnetic Nanoparticles.

Scanning-probe and wide-field magnetic microscopes based on nitrogen-vacancy (NV) centers in diamond have enabled advances in the study of biology and materials, but each method has drawbacks. Here, we implement an alternative method for nanoscale magnetic microscopy based on optical control of the charge state of NV centers in a dense layer near the diamond surface. By combining a donut-beam super-resolution technique with optically detected magnetic resonance spectroscopy, we imaged the magnetic fields produced by single 30 nm iron-oxide nanoparticles. The magnetic microscope has a lateral spatial resolution of ∼100 nm, and it resolves the individual magnetic dipole features from clusters of nanoparticles with interparticle spacings down to ∼190 nm. The magnetic feature amplitudes are more than an order of magnitude larger than those obtained by confocal magnetic microscopy due to the narrower optical point-spread function and the shallow depth of NV centers. We analyze the magnetic nanoparticle images and sensitivity as a function of the microscope's spatial resolution and show that the signal-to-noise ratio for nanoparticle detection does not degrade as the spatial resolution improves. We identify sources of background fluorescence that limit the present performance, including diamond second-order Raman emission and imperfect NV charge state control. Our method, which uses <10 mW laser power and can be parallelized by patterned illumination, introduces a promising format for nanoscale magnetic imaging.

Gate‐Based Protocol Simulations for Quantum Repeaters using Quantum‐Dot Molecules in Switchable Electric Fields

AbstractElectrically controllable quantum‐dot molecules (QDMs) are a promising platform for deterministic entanglement generation and, as such, a resource for quantum‐repeater networks. A microscopic open‐quantum‐systems approach based on a time‐dependent Bloch–Redfield equation is developed to model the generation of entangled spin states with high fidelity. The state preparation is a crucial step in a protocol for deterministic entangled‐photon‐pair generation that is proposed for quantum repeater applications. The theory takes into account the quantum‐dot molecules' electronic properties that are controlled by time‐dependent electric fields as well as dissipation due to electron–phonon interaction. The transition between adiabatic and non‐adiabatic regimes is quantified, which provides insights into the dynamics of adiabatic control of QDM charge states in the presence of dissipative processes. From this, the maximum speed of entangled‐state preparation is inferred under different experimental conditions, which serves as a first step toward simulation of attainable entangled photon‐pair generation rates. The developed formalism opens the possibility for device‐realistic descriptions of repeater protocol implementations.

Reversible optical data storage below the diffraction limit.

Colour centres in wide-bandgap semiconductors feature metastable charge states that can be interconverted with the help of optical excitation at select wavelengths. The distinct fluorescence and spin properties in each of these states have been exploited to show storage of classical information in three dimensions, but the memory capacity of these platforms has been thus far limited by optical diffraction. Here we leverage local heterogeneity in the optical transitions of colour centres in diamond (nitrogen vacancies) to demonstrate selective charge state control of individual point defects sharing the same diffraction-limited volume. Further, we apply this approach to dense colour centre ensembles, and show rewritable, multiplexed data storage with an areal density of 21 Gb inch-2 at cryogenic temperatures. These results highlight the advantages for developing alternative optical storage device concepts that can lead to increased storage capacity and reduced energy consumption per operation.

Heterogeneous integration of superconducting thin films and epitaxial semiconductor heterostructures with lithium niobate

We report on scalable heterointegration of superconducting electrodes and epitaxial semiconductor quantum dots (QDs) on strong piezoelectric and optically nonlinear lithium niobate. The implemented processes combine the sputter-deposited thin film superconductor niobium nitride and III–V compound semiconductor membranes onto the host substrate. The superconducting thin film is employed as a zero-resistivity electrode material for a surface acoustic wave resonator with internal quality factors representing a three-fold enhancement compared to identical devices with normal conducting electrodes. Superconducting operation of resonators is achieved to temperatures and electrical radio frequency powers . Heterogeneously integrated single QDs couple to the resonant phononic field of the surface acoustic wave resonator operated in the superconducting regime. Position and frequency selective coupling mediated by deformation potential coupling is validated using time-integrated and time-resolved optical spectroscopy. Furthermore, acoustoelectric charge state control is achieved in a modified device geometry harnessing large piezoelectric fields inside the resonator. The hybrid QD—surface acoustic wave resonator can be scaled to higher operation frequencies and smaller mode volumes for quantum phase modulation and transduction between photons and phonons via the QD. Finally, the employed materials allow for the realization of other types of optoelectronic devices, including superconducting single photon detectors and integrated photonic and phononic circuits.

Charge State Control over Point Defects in Sic Devices

Point defects in silicon carbide (SiC) are well positioned for integration with SiC based quantum photonic devices due to the maturity of SiC material and fabrication technology, the plethora of candidate quantum emitters that can be formed in SiC, and the potential for emission over a wide spectral range from the visible to the infrared. However, for each of the available color centers in SiC, only one of the charge states has displayed quantum emission, meaning that the emission strongly depends on the Fermi level and hence the doping concentration in the material. In this contribution, we discuss the methodology and mechanism for electrical charge-state control over point defects in SiC devices.

Identification of Exciton Complexes in Charge-Tunable Janus WSeS Monolayers.

Janus transition-metal dichalcogenide monolayers are artificial materials, where one plane of chalcogen atoms is replaced by chalcogen atoms of a different type. Theory predicts an in-built out-of-plane electric field, giving rise to long-lived, dipolar excitons, while preserving direct-bandgap optical transitions in a uniform potential landscape. Previous Janus studies had broad photoluminescence (>18 meV) spectra obfuscating their specific excitonic origin. Here, we identify the neutral and the negatively charged inter- and intravalley exciton transitions in Janus WSeS monolayers with ∼6 meV optical line widths. We integrate Janus monolayers into vertical heterostructures, allowing doping control. Magneto-optic measurements indicate that monolayer WSeS has a direct bandgap at the K points. Our results pave the way for applications such as nanoscale sensing, which relies on resolving excitonic energy shifts, and the development of Janus-based optoelectronic devices, which requires charge-state control and integration into vertical heterostructures.

Charge-State Control of a Host Metal–Organic Framework Enabled by Axially Coordinated Tripyridine Ligand Alternation

Charge-State Control of a Host Metal–Organic Framework Enabled by Axially Coordinated Tripyridine Ligand Alternation

Tunable Single-Atomic Charges on a Cleaved Intercalated Transition Metal Dichalcogenide.

Control of a single ionic charge state by altering the number of bound electrons has been considered as an ultimate testbed for atomic charge-induced interactions and manipulations, and such subject has been studied in artificially deposited objects on thin insulating layers. We demonstrate that an entire layer of controllable atomic charges on a periodic lattice can be obtained by cleaving metallic Co1/3NbS2, an intercalated transition metal dichalcogenide. We identified a metastable charge state of Co with a different valence and manipulated atomic charges to form a linear chain of the metastable charge state. Density functional theory investigation reveals that the charge state is stable due to a modified crystal field at the surface despite the coupling between NbS2 and Co via a1g orbitals. The idea can be generalized to other combinations of intercalants and base matrices, suggesting that they can be a new platform to explore single-atom-operational 2D electronics/spintronics.

Optical charge state manipulation of divacancy spins in silicon carbide under resonant excitation

Spin defects in silicon carbide (SiC) have attracted much attentions in various quantum technologies. In this work, we study the optical manipulation of charge state and coherent control of multifold divacancy spins ensemble in SiC under resonant excitation. The results reveal that the resonantly excited divacancy ensemble counts have dozens of enhancements by repumping a higher-energy laser. Moreover, it has a similar optimal repump laser wavelength of around 670 nm for multiple divacancies. On the basis of this, the optically detected magnetic resonance (ODMR) experiment shows that repump lasers with different wavelengths do not affect the ODMR contrast and line width. In addition, the repump lasers also do not change the divacancy spins’ coherence times. The experiments pave the way for using the optimal repump excitation method for SiC-based quantum information processing and quantum sensing.

Charge state control of the silicon vacancy and divacancy in silicon carbide

Color centers in silicon carbide (SiC), such as the negative silicon vacancy (VSi−) and neutral divacancy (VSiVC0), have recently been shown to be promising quantum bits (qubits) for a variety of applications in quantum communications and sensing. Considerable effort has been spent on improving the performance of these optical spin qubits, and the instability of their charge state is an important issue to be solved. Using electron paramagnetic resonance to monitor the charge state of dominant intrinsic defects in n-type, high-purity semi-insulating and p-type 4H-SiC, we reveal carrier compensation processes and the windows of the Fermi level that allow us to obtain stable VSi− and VSiVC0 in equilibrium. We show that stable VSi− and VSiVC0 ensembles can be obtained in n-type (p-type) via controlling the concentration of the Si vacancy (the C vacancy and the C antisite–vacancy pairs). The charge-state control of single VSi− and VSiVC0 emitters is expected to be possible in pure p-type layers by controlling the concentration of the C vacancy. In ultrapure materials, optical repumping is required for charge state control of single emitters.

Nanoscale electric-field imaging based on a quantum sensor and its charge-state control under ambient condition

Nitrogen-vacancy (NV) centers in diamond can be used as quantum sensors to image the magnetic field with nanoscale resolution. However, nanoscale electric-field mapping has not been achieved so far because of the relatively weak coupling strength between NV and electric field. Here, using individual shallow NVs, we quantitatively image electric field contours from a sharp tip of a qPlus-based atomic force microscope (AFM), and achieve a spatial resolution of ~10 nm. Through such local electric fields, we demonstrated electric control of NV’s charge state with sub-5 nm precision. This work represents the first step towards nanoscale scanning electrometry based on a single quantum sensor and may open up the possibility of quantitatively mapping local charge, electric polarization, and dielectric response in a broad spectrum of functional materials at nanoscale.

Exciton generation and recombination dynamics of quantum dots embedded in GaNAsP nanowires

Semiconductor quantum dots (QDs) acting as single-photon-emitters are potential building blocks for various applications in future quantum information technology. For such applications, a thorough understanding and precise control of charge states and capture/recombination dynamics of the QDs are vital. In this work, we study the dynamics of QDs spontaneously formed in GaNAsP nanowires, belonging to the dilute nitride material system. By using a random population model modified for these highly mismatched materials, we analyze the results from photoluminescence and photon correlation experiments and show a general trend of disparity in positive and negative trion populations and also a strong dependence of the capture/recombination dynamics and QD charge states on its surroundings. Specifically, we show that the presence of hole-trap defects in the proximity to some QDs facilitates formation of negative trions, which also causes a dramatic reduction of the neutral exciton lifetime. These findings underline the importance of proper understanding of the QD capture and recombination processes and demonstrate the possibility to use highly mismatched materials and defects for charge engineering of QDs.

A Label-Free Diamond Microfluidic DNA Sensor Based on Active Nitrogen-Vacancy Center Charge State Control.

We propose a label-free biosensor concept based on the charge state manipulation of nitrogen-vacancy (NV) quantum color centers in diamond, combined with an electrochemical microfluidic flow cell sensor, constructed on boron-doped diamond. This device can be set at a defined electrochemical potential, locking onto the particular chemical reaction, whilst the NV center provides the sensing function. The NV charge state occupation is initially prepared by applying a bias voltage on a gate electrode and then subsequently altered by exposure to detected charged molecules. We demonstrate the functionality of the device by performing label-free optical detection of DNA molecules. In this experiment, a monolayer of strongly cationic charged polymer polyethylenimine is used to shift the charge state of near surface NV centers from negatively charged NV- to neutral NV0 or dark positively charged NV+. Immobilization of negatively charged DNA molecules on the surface of the sensor restores the NV centers charge state back to the negatively charged NV-, which is detected using confocal photoluminescence microscopy. Biochemical reactions in the microfluidic channel are characterized by electrochemical impedance spectroscopy. The use of the developed electrochemical device can also be extended to nuclear magnetic resonance spin sensing.

Fundamental limits of electron and nuclear spin qubit lifetimes in an isolated self-assembled quantum dot

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

Study of ion-implanted nitrogen related defects in diamond Schottky barrier diode by transient photocapacitance and photoluminescence spectroscopy

The study of nitrogen-vacancy (NV) centers in diamond are growing attractive in the application of quantum devices. Here, electrical control of NV charge state and defects induced by nitrogen ions implantation in diamond were investigated by transient photocapacitance (TPC) spectroscopy and photoluminescence (PL) spectroscopy. The experiments show that thresholds of 1.2 eV appeared in TPC spectra are probably due to the presence of excited defect energy levels related to vacancy or NV center. Alternatively, the 2.2 eV defect observed in the TPC spectrum is probably attributed to NV centers. The variation of TPC and PL spectra with different applied voltages suggests that bias voltages control the charge state of NV centers since their effect on the Fermi level shifting in the depletion region. Furthermore, the steady-state photocapacitance indicates that the 2.2 eV deep trap slows down the process of photocapacitance rise and fall, and these processes can be enhanced by a higher electrical field.

Ionicity Diagrams for Electron-Donor and -Acceptor Metal-Organic Frameworks: DA Chains and D2A Layers Obtained from Paddlewheel-Type Diruthenium(II,II) Complexes and Polycyano-Organic Acceptors.

Recent developments in research concerning metal-organic frameworks and coordination polymers have provided the successful design of charge-variable molecular frameworks. However, few comprehensive studies exist that investigate the control of charge states in series of molecular frameworks such as these. Herein, we discuss the ionicity diagrams of two series containing electron-donor (D) and -acceptor (A) units: one-dimensional DA chains and two-dimensional D2A layers. The series were obtained by reacting paddlewheel-type diruthenium(II,II) complexes ([Ru2II,II]), which served as D units, with the polycyano-organic acceptors N,N'-dicyanoquinodiimine (DCNQI) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ), which served as A units. Fifteen novel members of neutral charged DA chains were fabricated in this study to characterize the ionicity diagrams for DA and D2A systems.

0D/2D Heterostructures Vertical Single Electron Transistor

AbstractMixed‐dimensional heterostructures formed by the stacking of 2D materials with nanostructures of distinct dimensionality constitute a new class of nanomaterials that offers multifunctionality that goes beyond those of single dimensional systems. An unexplored architecture of single electron transistor (SET) is developed that employs heterostructures made of nanoclusters (0D) grown on a 2D molybdenum disulfide (MoS2) channel. Combining the large Coulomb energy of the nanoclusters with the electronic capabilities of the 2D layer, the concept of 0D–2D vertical SET is unveiled. The MoS2 underneath serves both as a charge tunable channel interconnecting the electrode, and as bottom electrode for each v‐SET cell. In addition, its atomic thickness makes it thinner than the Debye screening length, providing electric field transparency functionality that allows for an efficient electric back gate control of the nanoclusters charge state. The Coulomb diamond pattern characteristics of SET are reported, with specific doping dependent nonlinear features arising from the 0D/2D geometry that are elucidated by theoretical modeling. These results hold promise for multifunctional single electron device taking advantage of the versatility of the 2D materials library, with as example envisioned spintronics applications while coupling quantum dots to magnetic 2D material, or to ferroelectric layers for neuromorphic devices.

A theoretical study of de-charging excitations of the NV-center in diamond involving a nitrogen donor * *Dedicated to the memory of Sven Öberg.

The negatively charged nitrogen vacancy centre in diamond is a promising candidate for future nanoscale quantum applications. For its operation it is important to have control of the centres charge state, and to avoid temporary disappearance of the NV-center’s functionality, termed photo-blinking. In this work, we use density functional theory simulations to investigate excitations that result in loss of an electron from NV− to a nearby nitrogen donor (donor-N+), leading to NV0 and donor-N0 charge state, and the corresponding deexcitation. Since these processes involve two different localized defect centres in the diamond lattice (the NV-center and the donor-N) they are non-local excitations. We have studied the de-charging both as a one-photon process and through a sequential two-photon process via the NV-center excited state. We propose de-charging directly from the NV-center to the donor-N as a possible mechanism for photo-blinking of the NV-center that involve an additional electron spin resonance active defect, the donor-N0. We have found that the excitation energies are converged when the distance between the two is larger than 10.4 Å. We also compute excitations to the conduction band edge from NV− (to NV0) and from donor-N0 (to donor-N+) using G0W0 + BSE.