X Zhang

In this paper, the turbulent transport properties of ST40 hot ion plasmas are examined and fully predictive time evolving modelling of a hot ion plasma pulse was performed. Understanding turbulent transport on spherical tokamaks (STs) is challenging due to their unique geometry characteristics. ST40 hot ion plasmas are typically unstable to ion scale Trapped Electron Modes (TEMs) and Ubiquitous Modes (UMs), driven from the kinetic response of trapped particles and passing ions, and electron scale Electron Temperature Gradient Modes (ETGs) at the edge of the plasma. A comparison between the linear unstable modes of the gyro-kinetic code GS2 and the gyro-fluid code TGLF showed that both models agree to a satisfactory level. However, some discrepancy was observed at the core of the plasma where a large fraction of beams ions exists, and electromagnetic effects are potentially important. Turbulent fluxes were also observed to be somewhat overpredicted with TGLF. The core heat ion transport is observed to be close to neoclassical levels due to turbulence suppression from high rotation and fast ion stabilisation, while the edge region is dominated by anomalous transport in both ions and electrons. As a result, enhanced energy confinement is observed in those plasmas driven by the reduced turbulent core region and the confined beam ions. Fully predictive simulations using the ASTRA transport solver coupled with SPIDER, NUBEAM, NCLASS and TGLF together with a novel reduced scrape of layer (SOL) model for the simulation of the last closed flux surface (LCFS) boundary conditions was attempted. Agreement in global quantities but also kinetic profiles between the predictive and interpretative modelling as well as experimental measurements was observed.

Dissipation is often considered as a detrimental effect in quantum systems for unitary quantum operations. However, it has been shown that suitable dissipation can be useful resources both in quantum information and quantum simulation. Here, we propose and experimentally simulate a dissipative phase transition (DPT) model using a single trapped ion with an engineered reservoir. We show that the ion's spatial oscillation mode reaches a steady state after the alternating application of unitary evolution under a quantum Rabi model Hamiltonian and sideband cooling of the oscillator. The average phonon number of the oscillation mode is used as the order parameter to provide evidence for the DPT. Our work highlights the suitability of trapped ions for simulating open quantum systems and shall facilitate further investigations of DPT with various dissipation terms.

Quantum phase transitions (QPTs) are usually associated with many-body systems with large degrees of freedom approaching the thermodynamic limit. In such systems, the many-body ground state shows abrupt changes at zero temperature when the control parameter of the Hamiltonian is scanned across a quantum critical point. Recently it has been realized that a QPT can also occur in a simple system composed of only a two-level atom and a single-mode bosonic field, described by the quantum Rabi model (QRM). Here we report the first experimental demonstration of a QPT in the QRM using a single trapped ion. We measure the average spin-up state population of the ion and the average phonon number in its spatial oscillation mode as two order parameters and observe the clear evidences of the phase transition via slow quench of the coupling between the ion and its spatial motion. An experimental probe of the phase transitions in a fundamental quantum optics model without imposing the thermodynamic limit opens up a new window for the controlled study of QPTs and quantum critical phenomena.

The exquisite control exhibited over quantum states of individual particles has revolutionized the field of precision measurement, as exemplified by the most accurate atomic clock realized in single trapped ions. Whereas many-atom lattice clocks have shown advantages in measurement precision over trapped-ion clocks, their accuracy has remained 20 times worse. Here we demonstrate, for the first time, that a many-atom system achieves accuracy (6x10^{-18}) better than a single ion-based clock, with vastly reduced averaging times (3000 s). This is the first time a single clock has achieved the best performance in all three key ingredients necessary for consideration as a primary standard - stability, reproducibility, and accuracy. This work paves the way for future experiments to integrate many-body quantum state engineering into the frontiers of quantum metrology, creating exciting opportunities to advance precision beyond the standard quantum limit. Improved frequency standards will have impact to a wide range of fields from the realization of the SI units, the development of quantum sensors, to precision tests of the fundamental laws of nature.