On the formation of current sheets in response to the compression or expansion of a potential magnetic field

The compression or expansion of a magnetic field that is initially potential is considered. It was recently suggested by Janse & Low [2009, ApJ, 690, 1089] that, following the volumetric deformation, the relevant lowest energy state for the magnetic field is another potential magnetic field that in general contains tangential discontinuities (current sheets). Here we examine this scenario directly using a numerical relaxation method that exactly preserves the topology of the magnetic field. It is found that of the magnetic fields discussed by Janse & Low, only those containing magnetic null points develop current singularities during an ideal relaxation, while the magnetic fields without null points relax toward smooth force-free equilibria with finite non-zero current.Comment: Accepted for publication in Ap

Testing collapse models with levitated nanoparticles: the detection challenge

We consider a nanoparticle levitated in a Paul trap in ultrahigh cryogenic vacuum, and look for the conditions which allow for a stringent noninterferometric test of spontaneous collapse models. In particular we compare different possible techniques to detect the particle motion. Key conditions which need to be achieved are extremely low residual pressure and the ability to detect the particle at ultralow power. We compare three different detection approaches based respectively on a optical cavity, optical tweezer and a electrical readout, and for each one we assess advantages, drawbacks and technical challenges

Inhomogeneous mechanical losses in micro-oscillators with high reflectivity coating

We characterize the mechanical quality factor of micro-oscillators covered by a highly reflective coating. We test an approach to the reduction of mechanical losses, that consists in limiting the size of the coated area to reduce the strain and the consequent energy loss in this highly dissipative component. Moreover, a mechanical isolation stage is incorporated in the device. The results are discussed on the basis of an analysis of homogeneous and non-homogeneous losses in the device and validated by a set of Finite-Element models. The contributions of thermoelastic dissipation and coating losses are separated and the measured quality factors are found in agreement with the calculated values, while the absence of unmodeled losses confirms that the isolation element integrated in the device efficiently uncouples the dynamics of the mirror from the support system. Also the resonant frequencies evaluated by Finite-Element models are in good agreement with the experimental data, and allow the estimation of the Young modulus of the coating. The models that we have developed and validated are important for the design of oscillating micro-mirrors with high quality factor and, consequently, low thermal noise. Such devices are useful in general for high sensitivity sensors, and in particular for experiments of quantum opto-mechanics

Detection of weak stochastic force in a parametrically stabilized micro opto-mechanical system

Measuring a weak force is an important task for micro-mechanical systems, both when using devices as sensitive detectors and, particularly, in experiments of quantum mechanics. The optimal strategy for resolving a weak stochastic signal force on a huge background (typically given by thermal noise) is a crucial and debated topic, and the stability of the mechanical resonance is a further, related critical issue. We introduce and analyze the parametric control of the optical spring, that allows to stabilize the resonance and provides a phase reference for the oscillator motion, yet conserving a free evolution in one quadrature of the phase space. We also study quantitatively the characteristics of our micro opto-mechanical system as detector of stochastic force for short measurement times (for quick, high resolution monitoring) as well as for the longer term observations that optimize the sensitivity. We compare a simple, naive strategy based on the evaluation of the variance of the displacement (that is a widely used technique) with an optimal Wiener-Kolmogorov data analysis. We show that, thanks to the parametric stabilization of the effective susceptibility, we can more efficiently implement Wiener filtering, and we investigate how this strategy improves the performance of our system. We finally demonstrate the possibility to resolve stochastic force variations well below 1% of the thermal noise

An ultra-low dissipation micro-oscillator for quantum opto-mechanics

Generating non-classical states of light by opto-mechanical coupling depends critically on the mechanical and optical properties of micro-oscillators and on the minimization of thermal noise. We present an oscillating micro-mirror with a mechanical quality factor Q = 2.6x10^6 at cryogenic temperature and a Finesse of 65000, obtained thanks to an innovative approach to the design and the control of mechanical dissipation. Already at 4 K with an input laser power of 2 mW, the radiation-pressure quantum fluctuations become the main noise source, overcoming thermal noise. This feature makes our devices particularly suitable for the production of pondero-motive squeezing.Comment: 21 pages including Supplementary Informatio

Dynamical two-mode squeezing of thermal fluctuations in a cavity opto-mechanical system

We report the experimental observation of two-mode squeezing in the oscillation quadratures of a thermal micro-oscillator. This effect is obtained by parametric modulation of the optical spring in a cavity opto-mechanical system. In addition to stationary variance measurements, we describe the dynamic behavior in the regime of pulsed parametric excitation, showing enhanced squeezing effect surpassing the stationary 3dB limit. While the present experiment is in the classical regime, our technique can be exploited to produce entangled, macroscopic quantum opto-mechanical modes

Control of Recoil Losses in Nanomechanical SiN Membrane Resonators

In the context of a recoil damping analysis, we have designed and produced a membrane resonator equipped with a specific on-chip structure working as a "loss shield" for a circular membrane. In this device the vibrations of the membrane, with a quality factor of $10^7$, reach the limit set by the intrinsic dissipation in silicon nitride, for all the modes and regardless of the modal shape, also at low frequency. Guided by our theoretical model of the loss shield, we describe the design rationale of the device, which can be used as effective replacement of commercial membrane resonators in advanced optomechanical setups, also at cryogenic temperatures

Calibrated quantum thermometry in cavity optomechanics

Cavity optomechanics has achieved the major breakthrough of the preparation and observation of macroscopic mechanical oscillators in peculiarly quantum states. The development of reliable indicators of the oscillator properties in these conditions is important also for applications to quantum technologies. We compare two procedures to infer the oscillator occupation number, minimizing the necessity of system calibrations. The former starts from homodyne spectra, the latter is based on the measurement of the motional sidebands asymmetry in heterodyne spectra. Moreover, we describe and discuss a method to control the cavity detuning, that is a crucial parameter for the accuracy of the latter, intrinsically superior procedure

The role of social prescribers in wales:a consensus methods study

AIMS: Social prescribing continues to grow and change across healthcare services in Wales; however, research of the day-to-day performance of social prescribers is limited. This study aimed to explore which roles are perceived to be the most important and frequently used by social prescribers in Wales and compare these results to reports in studies of services in other countries in order to support future role development and potential standardisation. METHODS: This study used the Group Concept Mapping via the Concept Systems Global Max™ software to collect and analyse all data from both participants and literature. RESULTS: There was a total of 101 statements generated (119 participants, 84 literature) ranging from generic interpersonal skills to specialised training (cognitive behavioural therapy). These statements were then sorted by conceptual similarity into seven clusters (Providing a Specialist Service, Working in a person-centred way, Skills, Connecting Clients with Community, Collaborative Working, Evaluating and postprogramme duties, and Networking/Community). Statements were rated based on their perceived importance and frequency, with the ‘Skills’ cluster having the highest overall average and ‘Providing a Specialist Service’ having the lowest. CONCLUSIONS: Reports indicate that in general there is variation in the roles performed by individual participants in Wales; however, greater variation was observed between participants and literature suggesting geographical divergence in practice. In the top 12 highest rated statements for both frequency and importance, individualistic traits such as empathy and ‘being a listener’ are favoured over specialised methods such as cognitive behavioural therapy and behaviour change taxonomy. Results suggest that local need plays a part in the choices and performance of social prescribers and as such should be considered in future standardisation

Dynamic topology and flux rope evolution during non-linear tearing of 3D null point current sheets

In this work, the dynamic magnetic field within a tearing-unstable three-dimensional current sheet about a magnetic null point is described in detail. We focus on the evolution of the magnetic null points and flux ropes that are formed during the tearing process. Generally, we find that both magnetic structures are created prolifically within the layer and are non-trivially related. We examine how nulls are created and annihilated during bifurcation processes, and describe how they evolve within the current layer. The type of null bifurcation first observed is associated with the formation of pairs of flux ropes within the current layer. We also find that new nulls form within these flux ropes, both following internal reconnection and as adjacent flux ropes interact. The flux ropes exhibit a complex evolution, driven by a combination of ideal kinking and their interaction with the outflow jets from the main layer. The finite size of the unstable layer also allows us to consider the wider effects of flux rope generation. We find that the unstable current layer acts as a source of torsional magnetohydrodynamic waves and dynamic braiding of magnetic fields. The implications of these results to several areas of heliophysics are discussed