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Coherent storage of quantum information is crucial to many quantum technologies. Long coherence times have been demonstrated in trapped-ion qubits, typically using the hyperfine levels within the ground state of a single ion. However, recent research suggests qubits encoded in metastable states could provide architectural benefits for quantum information processing, such as the possibility of effective dual-species operation in a single-species system and erasure-error conversion for fault-tolerant quantum computing. Here we demonstrate long-lived encoding of a quantum state in the metastable states of a trapped ion. By sympathetically cooling with another ion of the same species and constantly monitoring for erasure errors, we demonstrate a coherence time of 136(42) seconds with a qubit encoded in the metastable $5D_{5/2}$ state of a single $^{137}$Ba$^+$ ion. In agreement with a model based on empirical results from dynamical-decoupling-based noise spectroscopy, we find that dephasing of the metastable levels is the dominant source of error once erasure errors are removed.

Radium-225 (nuclear spin $I=1/2$) ions possess electronic hyperfine transitions that are first-order insensitive to magnetic field noise, which is advantageous for optical clocks and quantum information science. We report on laser cooling and trapping of radium-225 ions and hyperfine splitting measurements of the ion's $7s$ $^2S_{1/2}$, $7p$ $^2P_{1/2}$, and $6d$ $^2D_{3/2}$ states. We measured the ground state hyperfine constant, $A(^2S_{1/2}) = -27.684511056(9)\ \mathrm{GHz}$, and the quadratic Zeeman coefficient, $C_2 = 142.3(10)\ \mathrm{Hz\ G}^{-2}$, of the $^2S_{1/2} (F=0, m_F = 0) \leftrightarrow~^2S_{1/2} (F=1, m_{F} = 0)$ transition. We also measured the hyperfine constants of the $^2P_{1/2}$ state, $A(^2P_{1/2}) = -5.447(4)\ \mathrm{GHz}$, and the $^2D_{3/2}$ state, $A(^2D_{3/2}) = -619.7(11)\ \mathrm{MHz}$., Comment: 5 pages, 4 figures

Excitons, the bosonic quasiparticle emerging from Coulomb interaction between electrons and holes, will undergo a Bose-Einstein condensation(BEC) and transition into a superfluid state with global phase coherence at low temperatures. An important platform to study such excitonic physics is built on double-layer quantum wells or recent two-dimensional material heterostructures, where two parallel planes of electrons and holes are separated by a thin insulating layer. Lowering this separation distance ($d$) enhances the interlayer Coulomb interaction thereby strengthens the exciton binding energy. However, an exceedingly small $d$ will lead to the undesired interlayer tunneling, which results the annihilation of excitons. Here, we report the observation of a sequences of robust exciton condensates(ECs) in double bilayer graphenes twisted to $\sim 10^\circ$ with no insulating mid-layer. The large momentum mismatch between the two graphene layers well suppress the interlayer tunneling, allowing us to reach the separation lower limit $\sim$ 0.334 nm and investigate ECs in the extreme coupling regime. Carrying out transport measurements on the bulk and edge of the devices, we find incompressible states corresponding to ECs when both layers are half-filled in the $N=0$ and $N=1$ Landau levels (LLs). The comparison between these ECs and theoretical calculations suggest that the low-energy charged excitation of ECs can be meron-antimeron or particle-hole pair, which relies on both LL index and carrier type. Our results establish large-angle twisted bilayers as an experimental platform with extreme coupling strength for studying quantum bosonic phase and its low-energy excitations.

Building a carbon-neutral world needs to remove the excess CO$_2$ that has already been dumped into the atmosphere. The sea, soil, vegetation, and rocks on Earth all naturally uptake CO$_2$ from the atmosphere. Human beings can accelerate these processes in specific ways. The review summarizes the present Direct Air Capture (DAC) technology that contribute to Negative Emissions. Research currently being done has suggested future perspectives and directions of various methods for Negative Emission. New generations of technologies have emerged as a result of recent advancements in surface chemistry, material synthesis, and engineering design. These technologies may influence the large-scale deployment of existing CO$_2$ capture technologies in the future.

The extra CO2 that has already been released into the atmosphere has to be removed in order to create a world that is carbon neutral. Technologies have been created to remove carbon dioxide from wet flue gas or even directly from ambient air, however these technologies are not widely deployed yet. New generations of creative CO2 capture sorbents have been produced as a consequence of recent improvements in material assembly and surface chemistry. We summarize recent progress on water-stable and encapsulated metal-organic frameworks (MOFs) for CO2 capture under a wide range of environmental and operating conditions. In particular, newly developed water-stable MOFs and hydrophobic coating technologies are discussed with insights into their materials discovery and the synergistic effects between different components of these hybrid sorbent systems. The future perspectives and directions of water-stable and encapsulated MOFs are also given for Direct Air Capture of CO2 and CO2 capture from wet flue gas.

The distributed consensus mechanism is the backbone of the rapidly developing blockchain network. Blockchain platforms consume vast amounts of electricity based on the current consensus mechanism of Proof of Work. Here, we point out an advanced consensus mechanism named Proof of Stake that can eliminate the extensive energy consumption of the current PoW-based blockchain. We comprehensively elucidate the current and projected energy consumption and carbon footprint of the PoW and PoS based Bitcoin and Ethereum blockchain platforms.

The self-assembly mechanism of one-end-open carbon nanotubes (CNTs) suspended in an aqueous solution was studied by molecular dynamics simulations. It was shown that two one-end-open CNTs with different diameters can coaxially self-assemble into a nanocapsule. The nanocapsules formed were stable in aqueous solution under ambient conditions, and the pressure inside the nanocapsule was much higher than the ambient pressure due to the van der Waals interactions between two parts of the nanocapsule. The effect of the normalized radius difference, normalized inter-tube distance and aspect ratio of the CNT pairs were systematically explored. The electric field response of nanocapsules was studied with ab initio molecular dynamics simulations, which shows that nanocapsules can be opened by applying an external electric field, due to the polarization of carbon atoms. This discovery not only sheds light on a simple yet robust nanocapsule self-assembly mechanism, but also underpins potential innovations in drug delivery, nano-reactors, etc.

A novel bilayer is introduced, consisting of a stiff film adhered to a soft substrate with patterned holes beneath the film and substrate interface. To uncover the transition of surface patterns, two dimensional plane strain simulations are performed on the defected bilayer subjected to uniaxial compression. Although the substrate is considered as the linear elastic material, the presence of defects can directly trigger the formation of locally ridged and then folding configurations from flat surface with a relatively small compressive strain. It is followed by the coexisting phases of folds and wrinkles under further overall compression. This phase transition reverses the traditional transition of wrinkle to ridge or fold for defect free substrates. It is also found that the onset of initial bifurcation is highly dependent on the spatial configuration and geometries of holes, since the interaction of defects allows more strain relief mechanisms beyond wrinkling. Furthermore, a rich diversity of periodic surface topologies, including overall waves, localizations, saw like and coexisting features of folds and wrinkles can be obtained by varying the diameter, depth and spacing of holes as well as compressive strain, which provides a potential approach to engineer various surface patterns for applications.

Pimple is one of the most common skin diseases for humans. The mechanical modeling of pimple growth is very limited. A finite element model is developed to quantify the deformation field with the expansion of follicle, and then the mechanical stimulus is related to the sensation of pain during the development of pimple. Through these models, parametric studies show the dependence of mechanical stimulus and pain level on the pimple-surrounded structures, follicle depth and mechanical properties of the epidermis. The findings in this paper may provide useful insights on prevention or pain relief of pimples, as well as those related to cosmetics and other tissue growth.

Based on extensive evolutionary algorithm driven structural search, we propose a new diphosphorus trisulfide (P2S3) 2D crystal, which is dynamically, thermally and chemically stable as confirmed by the computed phonon spectrum and ab initio molecular dynamics simulations. This 2D crystalline phase of P2S3 corresponds to the global minimum in the Born-Oppenheimer surface of the phosphorus sulfide monolayers with 2:3 stoichiometries. It is a wide band gap (4.55 eV) semiconductor with P-S {\sigma} bonds. The electronic properties of P2S3 structure can be modulated by stacking into multilayer P2S3 structures, forming P2S3 nanoribbons or rolling into P2S3 nanotubes, expanding its potential applications for the emerging field of 2D electronics.

Ion hydrations are ubiquitous in natural and fundamental processes. A quantitative analysis of a novel CO2 sorbent driven by ion hydrations was presented by molecular dynamics (MD). We explored the humidity effect on the diffusion and structure of ion hydrations in CO2 sorbent, as well as the working mechanism of the moisture-swing CO2 sorbent. The discovery of the diffusion coefficients of ions at different humidity levels can be applied to design a new CO2 sorbent with superior performance. The molecular structure analysis shows a higher precipitation rate of HCO3- ions than CO32- ions in a dry surrounding, which leads to a more alkaline surrounding on CO2 sorbent that may promote the absorption of CO2.

The hydration of ions in nanoscale hydrated clusters is ubiquitous and essential in many physical and chemical processes. Here we show that the hydrolysis reaction is strongly affected by relative humidity. The hydrolysis of CO32- with n = 1-8 water molecules is investigated by ab initio method. For n = 1-5 water molecules, all the reactants follow a stepwise pathway to the transition state. For n = 6-8 water molecules, all the reactants undergo a direct proton transfer to the transition state with overall lower activation free energy. The activation free energy of the reaction is dramatically reduced from 10.4 to 2.4 kcal/mol as the number of water molecules increases from 1 to 6. Meanwhile, the degree of the hydrolysis of CO32- is significantly increased compared to the bulk water solution scenario. The incomplete hydration shells facilitate the hydrolysis of CO32- with few water molecules (especially for n = 6) to be not only thermodynamically favorable but also kinetically favorable. This discovery provides valuable insights for designing efficient sorbents including that for CO2 air capture.

ReaxFF provides a method to model reactive chemical systems in large-scale molecular dynamics simulations. Here, we developed ReaxFF parameters for phosphorus and hydrogen to give a good description of the chemical and mechanical properties of pristine and defected black phosphorene. ReaxFF for P/H is transferable to a wide range of phosphorus and hydrogen containing systems including bulk black phosphorus, blue phosphorene, edge-hydrogenated phosphorene, phosphorus clusters and phosphorus hydride molecules. The potential parameters were obtained by conducting unbiased global optimization with respect to a set of reference data generated by extensive ab initio calculations. We extend ReaxFF by adding a 60{\deg} correction term which significantly improves the description of phosphorus clusters. Emphasis has been put on obtaining a good description of mechanical response of black phosphorene with different types of defects. Compared to nonreactive SW potential [1], ReaxFF for P/H systems provides a huge improvement in describing the mechanical properties the pristine and defected black phosphorene and the thermal stability of phosphorene nanotubes. A counterintuitive phenomenon is observed that single vacancies weaken the black phosphorene more than double vacancies with higher formation energy. Our results also show that mechanical response of black phosphorene is more sensitive to defects for the zigzag direction than for the armchair direction. Since ReaxFF allows straightforward extensions to the heterogeneous systems, such as oxides, nitrides, ReaxFF parameters for P/H systems build a solid foundation for the reactive force field description of heterogeneous P systems, including P-containing 2D van der Waals heterostructures, oxides, etc.

A novel system containing nanoporous materials and carbonate ions is proposed, which is capable to capture CO2 from ambient air simply by controlling the amount of water (humidity) in the system. The system absorbs CO2 from the air when the surrounding is dry, whereas desorbs CO2 when wet. A design of such a CO2 absorption/desorption system is investigated in this paper using molecular dynamics and quantum mechanics simulations, and also verified by experiments. Its working mechanism is revealed as the reduction of free energy of the carbonate ion hydrolysis with the decrease of the number of water molecules in confined nano-pores. The influences of pore size, spacing of cations, surface hydrophobicity and temperature on CO2 capture efficiency are elucidated. The study may help to design an efficient direct air capture system and contribute to the negative carbon emission.

Two-dimensional materials have attracted tremendous attention for their fascinating electronic, optical, chemical and mechanical properties. However, the band gaps of most 2D materials reported are smaller than 2.0 eV, which greatly restricted their optoelectronic applications in blue and ultraviolet range of the spectrum. Here, we propose a new stable sulfur nitride (S3N2) 2D crystal that is a covalent network composed solely of S-N {\sigma} bonds. S3N2 crystal is dynamically stable as confirmed by the computed phonon spectrum and ab initio molecular dynamics simulations in the NPT ensemble. Hybrid density functional calculations show that 2D S3N2 crystal is a wide, direct band-gap (3.17 eV) semiconductor with good hole mobility. These fascinating electronic properties could pave the way for its optoelectronic applications such as blue or ultra-violet light-emitting diodes (LEDs) and photodetectors.

Today, 2D semiconductor materials have been extended into the nitrogen group: phosphorene, arsenene, antimonene and even nitrogene. Motivated by them, based upon first-principles density functional calculations, we propose a new two-dimensional phosphorus nitride (PN) structure that is stable well above the room temperature, due to its extremely high cohesive energy. Unlike phosphorene, PN structure is resistant to high temperature oxidation. The structure is predicted to be a semiconductor with a wide, indirect band gap of 2.64 eV. More interestingly, the phosphorus nitride monolayer experiences an indirect-to-direct band-gap transition at a relatively small tensile strain. Such dramatic transformation in the electronic structure combined with structural stability and oxidation resistance at high temperature could pave the way for exciting innovations in high-speed ultrathin transistors, power electronic modules, ultra-high efficiency LEDs and semiconductor lasers.