Desired Charge State Research Articles

Heralded initialization of charge state and optical-transition frequency of diamond tin-vacancy centers

Diamond tin-vacancy centers have emerged as a promising platform for quantum information science and technology. A key challenge for their use in more-complex quantum experiments and scalable applications is the ability to prepare the center in the desired charge state with the optical transition at a predefined frequency. Here we report on heralding such successful preparation using a combination of laser excitation, photon detection, and real-time logic. We first show that fluorescence photon counts collected during an optimized resonant probe pulse strongly correlate with the subsequent charge state and optical-transition frequency, enabling real-time heralding of the desired state through threshold photon counting. We then implement and apply this heralding technique to photoluminescence-excitation measurements, coherent optical driving, and an optical Ramsey experiment, finding strongly increased optical coherence with increasing threshold. Finally, we demonstrate that the prepared optical frequency follows the probe laser across the inhomogeneous linewidth, enabling tuning of the transition frequency over multiple homogeneous linewidths. Published by the American Physical Society 2024

First Experiments with CRYRING@ESR

The low-energy heavy ion storage ring CRYRING was transported from Stockholm to Darmstadt, modernized and reconfigured, and recommissioned as CRYRING@ESR. The machine is now in operation with all installations in service and is available as a user facility for experiments proposed through the SPARC collaboration. During the 2020–2022 period, we brought a number of experimental installations into service and used them to measure first data: the ultra-cold electron cooler for merged-beam electron–ion collisions, the gas jet target for atomic collisions, a next-generation microcalorimeter-based X-ray spectroscopy setup, and others. Ions can be injected either in low charge states from a local ion source through a 300 keV/u RFQ linac, or in high charge states from the GSI accelerator chain through ESR. This allows for very broad access to ions across the entire periodic table. CRYRING@ESR is able to de- or accelerate ions and cool and store beams of isotopically pure species in a desired charge state. While the analysis is still largely ongoing, the first experimental data already show that the machine reached its expected performance level, and our high expectations regarding achievable resolution in spectroscopy experiments have been fulfilled. With access to new classes of ions available through ESR injection and a new generation of experimental instrumentation, CRYRING@ESR is a unique facility for experiments with heavy, highly charged ions. Here, we will review our present setup and machine performance, discuss the data from our first commissioning experiments and briefly preview the upcoming new installations for the coming years.

Experimental Demonstration of the Thin-Film Liquid-Metal Jet as a Charge Stripper.

For high-power heavy ion accelerators, the development of a suitable charge stripper, which can handle intense beams, is essential. This Letter describes the first experimental demonstration of a heavy ion liquid lithium charge stripper. A 10-20 μm thick liquid lithium jet flowing at >50 m/s was formed and confirmed stable when bombarded by various heavy ion beams, while increasing the charge state of the incoming beams to the desired charge state range. This demonstration proved the existing power limitation with the conventional strippers can be overcome by the liquid-metal stripper, opening completely new possibilities in high-power accelerator development.

Coherence of a charge stabilised tin-vacancy spin in diamond

Quantum information processing (QIP) with solid state spin qubits strongly depends on the efficient initialisation of the qubit’s desired charge state. While the negatively charged tin-vacancy (SnV−) centre in diamond has emerged as an excellent platform for realising QIP protocols due to long spin coherence times at liquid helium temperature and lifetime limited optical transitions, its usefulness is severely limited by termination of the fluorescence under resonant excitation. Here, we unveil the underlying charge cycle, potentially applicable to all group IV-vacancy (G4V) centres, and exploit it to demonstrate highly efficient and rapid initialisation of the desired negative charge state of single SnV centres while preserving long term stable optical resonances. In addition to investigating the optical coherence, we all-optically probe the coherence of the ground state spins by means of coherent population trapping and find a spin dephasing time of 5(1) μs. Furthermore, we demonstrate proof-of-principle single shot spin state readout without the necessity of a magnetic field aligned to the symmetry axis of the defect.

Miniaturizing neural networks for charge state autotuning in quantum dots

A key challenge in scaling quantum computers is the calibration and control of multiple qubits. In solid-state quantum dots (QDs), the gate voltages required to stabilize quantized charges are unique for each individual qubit, resulting in a high-dimensional control parameter space that must be tuned automatically. Machine learning techniques are capable of processing high-dimensional data—provided that an appropriate training set is available—and have been successfully used for autotuning in the past. In this paper, we develop extremely small feed-forward neural networks that can be used to detect charge-state transitions in QD stability diagrams. We demonstrate that these neural networks can be trained on synthetic data produced by computer simulations, and robustly transferred to the task of tuning an experimental device into a desired charge state. The neural networks required for this task are sufficiently small as to enable an implementation in existing memristor crossbar arrays in the near future. This opens up the possibility of miniaturizing powerful control elements on low-power hardware, a significant step towards on-chip autotuning in future QD computers.

Charge Detection in an Array of CMOS Quantum Dots

The recent development of arrays of quantum dots in semiconductor nanostructures highlights the progress of quantum devices toward large scale. However, how to realize such arrays on a scalable platform such as silicon is still an open question. One of the main challenge resides in the detection of charges within the array. It is a prerequisite functionality to initialize a desired charge state and readout spins through spin-to-charge conversion mechanisms. In this paper, we use two methods based on either a single-lead charge detector, or a reprogrammable single electron transistor. Thanks to these methods, we study the charge dynamics and sensitivity by performing single shot detection of the charge. Finally, we can probe the charge stability at any node of a linear array and assess the Coulomb disorder in the structure. We find an electrochemical potential fluctuation induced by charge noise comparable to that reported in other silicon quantum dots.

Enhancement of charge breeding efficiency for rare isotope beam with the control of magnetic field profile and electron beam energy in EBIS

The magnetic field in the ion trap of electron beam ion source (EBIS) determines the current density of electron beam and thus the depth of trap potential, which affects the charge breeding efficiency to desired charge state. An EBIS charge breeder has been constructed to be used for the Rare Isotope Science Project in Korea. A 6 T superconducting solenoid is used for the trap, and uniform magnetic field is extended with correction coils at the ends. The effect of field uniformity on the electron current density is evaluated using TRAK, and it is shown sizable improvement in the charge breeding efficiency for rare isotope beams can be obtained by elaborate magnetic design. Furthermore, the electron beam energy affects the ionization efficiency. The electron energy is often reduced in the trap for optimal matching with charge striping cross section. However, virtual cathode formation can appear in the process of energy reduction. Maximum beam currents limited by electron energy are studied analytically and by TRAK simulation in the view of improving the charge breeding efficiency.

Charge stripping at high energy heavy ion Linacs

For each heavy-ion accelerator facility charge stripping is a key technology – the stripping charge state, its efficiency to produce ions in the selected charge state, and the beam quality after stripping substantially determine the entire accelerator performance. Modern heavy ion accelerator facilities such as the future Facility for Antiproton and Ion Research (FAIR) at GSI, Darmstadt, Germany provide for high-intensity heavy-ion beams beyond 200 MeV/u. Heavy ions generated in an ion source at comparatively low charge states are pre-accelerated in a Linac to a few MeV/u, after charge stripping the average charge state is increased and one (or several) ion charge states are selected for further acceleration. This enables more efficient acceleration up to the final beam energy, compared to acceleration of ions with a low charge state. C-foil stripping allows for highest mean charge state and best stripping efficiency into the desired charge state. Therefore minimum acceleration voltage could be expected utilizing C-foil stripping. Due to the high power deposited by the ions in the stripping media and radiation damages if solids are used, self-recovering stripper media must be used in any case. First layout scenarios for a 200 MeV/u heavy ion Linac considering efficient heavy ion stripping will be presented.

An electrostatic in-line charge-state purification system for multicharged ions in the kiloelectronvolt energy range.

The performance of a newly built omega type electrostatic analyzer designed to act as an in-line charge-state purification system for ions in the kiloelectronvolt energy range is reported. The analyzer consists of a set of four consecutive electrostatic 140° concentric cylindrical electrodes enclosed by Matsuda electrodes. This setup was recently tested and validated using O5+, Ar9+, and Xe20+ ion beams at an energy of 14 qkeV at the ARIBE facility. A resolving power of 10.5 and a transmission of 100% of the desired charge state are measured allowing a good purification of incoming ion beams with charge states up to 10+ and a fairly good purification for charge states at least up to 20+. In comparison with other in-line solutions such as the Wien filter, our system has the advantage of being purely electrostatic and therefore lacking common drawbacks as, for example, hysteresis.

Charged dopants in neutral supercells through substitutional donor (acceptor): nitrogen donor charging of the nitrogen-vacancy center in diamond

Charged defects are traditionally computed by adding (subtracting) electrons for negative (positive) impurities. When using periodic boundary conditions this results in artificially charged supercells that also require a compensating background charge of the opposite sign, which makes slab supercells problematic because of an arbitrary dependence on the vacuum thickness. In this work, we test the method of using neutral supercells through the use of a substitutional electron donor (acceptor) to describe charged systems. We use density functional theory (DFT) to compare the effects of charging the well-studied NV-center in diamond by a substitutional donor nitrogen. We investigate the influence of the donor-N on the NV-center properties as a function of the distance between them, and find that they converge toward those obtained when adding an electron. We analyze the spin density and conclude that the donor-N has a zero magnetic moment, and thus, will not be seen in electron spin resonance. We validate our DFT energies through comparison to GW simulations. Charging the NV-center with a substitutional donor-N enables accurate calculations of slabs, without the ambiguity of using charged supercells. Implantation of donor-N atoms opens up the possibility to engineer NV-centers with the desired charge state for future ICT and sensor applications.

Effects of external stress field on the charge stability of nitrogen vacancy centers in diamond

The interaction of the atom-like defects in semiconductors with external fields provides an avenue to quantum information processing and nanoscale sensors. Meanwhile, external fields may induce instability of the desired charge state of the defects. It is essential to understand how the charge state of a defect is affected by external fields that introduced in diverse applications. In this letter, we explore the stability of the negatively charged state (NV–) and the neutral state (NV0) of the nitrogen vacancy (NV) center in diamond under stress by first-principles calculations. We find that the relative stability of NV– to NV0 is always reduced by the stress if the NV center is free to relax its orientation. Once the NV center has formed and retains its orientation, however, the relative stability of NV– can be always enhanced by compressive stress along its trigonal symmetry axis. We believe that the results are not only significant for control of the charge stability of NV center but also enlightening for applications based on specific charge states of other kinds of defects in the stress field.

Production of high intensity 48Ca for the 88-Inch Cyclotron and other updates

Recently the Versatile ECR for NUclear Science (VENUS) ion source was engaged in a 60-day long campaign to deliver high intensity (48)Ca(11+) beam to the 88-Inch Cyclotron. As the first long term use of VENUS for multi-week heavy-element research, new methods were developed to maximize oven to target efficiency. First, the tuning parameters of VENUS for injection into the cyclotron proved to be very different than those used to tune VENUS for maximum beam output of the desired charge state immediately following its bending magnet. Second, helium with no oxygen support gas was used to maximize the efficiency. The performance of VENUS and its low temperature oven used to produce the stable requested 75 eμA of (48)Ca(11+) beam current was impressive. The consumption of (48)Ca in VENUS using the low temperature oven was checked roughly weekly, and was found to be on average 0.27 mg/h with an ionization efficiency into the 11+ charge state of 5.0%. No degradation in performance was noted over time. In addition, with the successful operation of VENUS the 88-Inch cyclotron was able to extract a record 2 pμA of (48)Ca(11+), with a VENUS output beam current of 219 eμA. The paper describes the characteristics of the VENUS tune used for maximum transport efficiency into the cyclotron as well as ongoing efforts to improve the transport efficiency from VENUS into the cyclotron. In addition, we briefly present details regarding the recent successful repair of the cryostat vacuum system.

Direct plasma injection scheme with various ion beams

The laser ion source is one of the most powerful heavy ion sources. However, it is difficult to obtain good stability and to control its intense current. To overcome these difficulties, we proposed a new beam injection scheme called “direct plasma injection scheme”. Following this it was established to provide various species with desired charge state as an intense accelerated beam. Carbon, aluminum and iron beams have been tested.

The ECRIS charge state breeding project at TRIUMF

The performance of charge state breeding with an electron cyclotron resonance (ECR) ion source intended to increase the charge state of online produced radioactive ions at the ISAC facility at TRIUMF has been investigated. A 14 GHz PHOENIX from PANTECHNIK has been setup on a test bench. Singly charged ions have been produced with several ion sources typical for the on-line operation and were injected into the charge breeder. The main purpose of the tests has been the optimization of the efficiency for the charge breeding into the desired charge state. Maximum efficiencies reached so far with the standard one step deceleration of the ions in front of the plasma are up to about 6% for noble gas ions and about 3.5% for alkalines. As ion optics simulations show, the acceptance can be increased by a two step deceleration. In order to meet the velocity acceptance of the accelerator at different A/q values a similar two gap acceleration system for the highly charged ions has been installed to allow the source to run at different voltages. For the further beam transport to the accelerator, cross sections for charge exchange of the highly charged ions with the residual gas have been determined.

Stable Ag(II) Coordination Complexes in the Gas Phase

The preparation and study of gas-phase transition-metal complexes in their higher oxidation states, i.e., Cu(II), Cr(III), Fe(II), etc., presents a considerable technical challenge. Charge transfer prevents such species from being “grown” as cluster ions and techniques, such as electrospray, do not always produce the desired charge state or allow for experiments to be performed on a broad range of ligands. Discussed here are new results from a technique which promises to overcome some of these problems, and appears capable of producing complexes from a wide variety of metals and ligands. Data are presented for complexes based on silver(II) in association with a broad range of ligands, including pyridine, tetrahydrofuran, and benzene. For each [AgLn]2+system, two important quantities are identified: (i) the minimum number of ligands required to form a stable unit and (ii) the value of n for which the intensity distribution reaches a maximum. For nitrogen-containing ligands these numbers are 2 and 4, respectively, and for oxygen-containing ligands 4 and 5. A series of aromatic ligands all exhibit coordination numbers of 2. For several of the nitrogen-based ligands the most stable combinations correspond to those identified in the condensed phase, and [Ag(pyridine)4]2+ is a very good example of such behavior. In the case of the oxygen-containing ligands, there are no direct condensed-phase analogues, but some of the more stable combinations identified may offer prospects for future preparative work. Within the latter group, not only was the presence of stable silver(II)/CO2 complexes very unexpected, but with [Ag(CO2)4]2+ being the most stable combination, the pattern of behavior is markedly different from that of other oxygen-containing ligands. The composition and charge states of many of the stable complexes were confirmed via collisional activation, where both ligand loss and charge-transfer processes could be identified. Only one example of a chemical reaction could be clearly identified as being initiated by the presence of silver(II).

Developments at the CERN laser ion source

The high current, high charge-state ion beam which can be extracted from a laser produced plasma is well suited, after initial acceleration, for injection into synchrotrons. At CERN, the production of a heavy ion beam using a CO2 laser ion source is studied. The latest results of experiments with a tantalum ion beam with charge states up to 23+ and accelerated by a radio frequency quadrupole from 6.9 to 100 keV/u, are presented along with simulations of the low energy beam transport. The ion yield at the desired charge state, the pulse to pulse stability of the ion beam, and the system reliability are all of major interest. Work is under way to replace the low repetition rate free-running laser oscillator by a master oscillator and power amplifier system. The master oscillator is operational and the first results of measurements of its beam quality and stability are presented.

Production of microbunched beams of very highly charged ions with an electron beam ion source

Electron beam ion sources produce very highly charged ions most efficiently in a batch mode as the confinement time can be directly optimized for the production of the desired charge state. If, after confinement, the voltage of the ion-confining downstream dam is lowered rapidly, all ions escape and form an ion beam pulse with a length of a few tens of μs. Raising the main trap voltage while maintaining a constant dam voltage in a “spill-over expulsion” reduces the energy spread of the expelled ions. The longer time periods of “slow-,” “leaky batch mode-,” and “direct current (dc) batch mode-” expulsions allow for increasing the ion beam duty cycle. Combining the rapid expulsion with one of the latter methods allows for the expulsion of the ions of a single batch in many small microbunches with variable intervals, maintaining the low energy spread and the increased duty cycle of slow expulsions. Combining the “microbunching” with “dc batch mode production” and a multitrap operation will eventually allow for the production of equally intense ion bunches over a wide range of frequencies without any deadtime, and with minimal compromise on the most efficient production parameters.

A case study for charge state breeding of heavy ions in the storage ring, ESR

Utilizing a multi-charge state mode for the operation of the heavy ion storage ring ESR, heavy ions in the charge state q can be bred from stored ions with an original charge state q + 1 via radiative recombination in the electron cooler. Using additionally the internal gas target of the ESR, also ionization and electron capture have to be taken into account. This method was tested with Pb ions, injected as bare species (Pb82+) at 277 MeV/u into the ring. The observed evolution of the intensity for the coasting beams with different charge states is described by simple rate equations applying proper rates for the charge changing processes in the cooler and in the gas jet. Using this model, a comparison of the efficiencies of the two different methods is made, i.e. the direct accumulation of the desired charge state and the breeding technique. In the latter case, projectiles ionized in the gas target are transferred back to the Pb81+ charge state via radiative recombination in the cooler and can thus efficiently be reused.

Charge-state dependence of M-shell x-ray production in 67Ho by 2-12-MeV carbon ions.

Charge-state dependence of M-shell x-ray production cross sections of $_{67}\mathrm{Ho}$ bombarded by 2--12-MeV carbon ions, with and without K-shell vacancies, were measured using a windowless Si(Li) x-ray detector with a full-width-at-half-maximum resolution of 135 eV at 5.9 keV. Carbon ions of different charge states were produced using a postacceleration, nitrogen gas stripping cell. The carbon ions were then magnetically analyzed to select the desired charge state and energy before entering the target chamber. The total M-shell and M\ensuremath{\zeta}, M\ensuremath{\alpha},\ensuremath{\beta}, and M\ensuremath{\gamma} x-ray cross sections were measured. The electron-capture (EC) contributions as well as the direct-ionization (DI) contributions can be determined by making a comprehensive study of the projectile-charge-state dependence of the target x-ray production cross sections for targets in which the single-collision realm is maintained. In this paper, both EC and DI contributions and the total M-shell x-ray production cross sections are compared to both the first Born theory and to the perturbed-stationary-state theory with energy-loss, Coulomb-deflection, and relativistic corrections.

Charge exchange of very heavy ions in carbon foils and in the residual gas of GANIL cyclotrons

The results of several years of operation with heavy to very heavy ions in GANIL accelerators, along with specific measurements concerning stripping through carbon foils, led to a set of valuable recipes for predicting the characteristics required to obtain a desired charge state distribution. The ion species under study range from Ar to U and the energies from 3.8 to 10.6 MeV/nucleon. The experimental method and the fits of the results are presented. A series of measurements was also set up to evaluate the beam losses during acceleration in the three cyclotrons; with the help of existing models for charge exchange cross-sections, it is shown that vacuum requirements can be fairly accurately determined.