流动VLBI站氢原子钟的研制与性能测试

氢脉泽是至今为止除极短时间测量间隔之外最稳定的频率标准 ,它是射电天文所必需的频率标准。本文讨论了为流动VLBI站研制的新一代氢脉泽的物理结构特点 ,包括对腔泡结构、磁屏蔽、原子束光学系统等采用的新技术。性能测试表明 ,在 80s到 1d之间其频率稳定度优于 1× 10 - 1 4 ,有了较大的提高。     

‘Senses and Hands to the Same Degree as Thought’—Ole Rømer's Mechanical Astronomy

The astronomer Ole Rømer emphasized the mechanical nature of the practice of astronomy and this paper attempts to unravel what Rømer meant by the close association between mechanics and astronomy. The point of departure is Rømer's work with Tycho Brahe's observations and his stay at the Royal Academy of the Sciences in Paris. Analyses of Rømer's letters and treatises show that he not only focused on direct presentations of observations and instruments, but demanded an independence of his results that went beyond the rhetoric of first‐hand knowledge. Rømer wanted his observations and instruments to demonstrate the physical theories themselves, rather than asking for theories to be accepted on his authority. His invention of the meridian circle with its facility for cross‐checking measurements is seen in connection with this drive to make what is observed ‘deserve credit in its own right rather than because of the observer.’     

Römer,Flamsteed, Cassini and the Speed of Light

One of Ole Römer's most influential contributions to astronomy was the theory that light has a finite speed, which he calculated from inequalities in the motion of a satellite of Jupiter. The English astronomer John Flamsteed, first director of the Royal Observatory at Greenwich, is credited with being an early and influential supporter of the theory. This article examines how he came to be so, taking issue with the claim that he was instantly converted to the idea by a personal encounter with Römer. The publication of Flamsteed's Correspondence has made illuminating material newly accessible; through letters it is possible to track the gradual development of his thinking and how it related to his dealings with Römer and with Giovanni Domenico Cassini (director of the Paris Observatory). The resulting account offers new insights into their attempts to share information, in a situation where motives for collaborating were counterbalanced by a potential for rivalry and conflict, and into the interlinked roles played by written correspondence, personal meetings and empirical investigation in the spreading of new ideas.     

Exchange Collisions, Wall Interactions, and Resettability of the Hydrogen Maser

Experiments were performed in order to verify the resettability of the hydrogen maser. The method consisted of measuring the output frequency of one maser against the hydrogen pressure. It was found that at a given tuning of the cavity no shift larger than 2.1 parts in 1013 was observed for a change of 4 to 1 in pressure. This experiment also showed that the pressure shift due to exchange collisions, predicted by Bender, could not be observed for the field-independent transition in the hydrogen maser. Two masers, having the same storage bulb design and the same wall coating, were tuned by this technique and were found to have a frequency difference of 7.6 parts in 1013. Experiments on the wall coating of the hydrogen maser storage bulb were made. Relaxation and decorrelation times of various materials were measured. The hyperfine splitting of the ground state of hydrogen measured against cesium is also given.     

Bayesian image restoration in astronomy: Application to images of the recent collision of comet shoemaker-levy 9 with jupiter

In this work, we examine simple to complex methods proposed within the Bayesian paradigm to perform image restoration in astronomy. We start by describing the classical conditional and simultaneous autoregressions, then we move on to study how to incorporate smoothness constraints to the classical Richardson-Lucy restoration method and also how to modify the image scale and define prior models on other scales than the linear one. Finally, we compare those models on images of Jupiter after the impacts of the fragments of the comet Shoemaker-Levy 9 at two wavelengths.     

Frequency domain control of quantum state singlet/triplet character and prospects for an all-optical spin switch

In recent years, quantum optics has been expanded from atomic to molecular systems despite much weaker oscillator strengths and complex relaxation pathways that have presented serious challenges in the past. The richness of molecular excitation pathways and the variety of molecular interactions has made it possible to develop novel applications in this field. We have demonstrated how the Autler–Townes effect can be used to control molecular angular momentum alignment and how the Autler–Townes split line shape, combined with accurate control-laser electric field amplitude measurement, can be used to map the absolute magnitude of the molecular internuclear distance dependent electronic transition dipole moment function. In addition, the electric field amplitude in the control laser Rabi frequency can be used as a ‘tuning’ mechanism for the mixing coefficients of energy levels that are weakly perturbed by the spin–orbit interaction, i.e. to control the valence electron spin polarization and the spin multiplicity of molecular quantum. We propose an extension of this control mechanism to an all-optical spin switch.     

一种基于星载氢钟误差信号提取的实现方法

针对中国新一代卫星导航系统的高精度定位要求,星载氢原子钟以极其优异的长期稳定性、漂移率以及温度特性,作为频率基准大规模应用于该系统.双频方案的被动型星载氢原子钟由物理部分及电路部分两大部分组成,微波激励信号对物理部分微波腔激励后产生了腔频误差信号及高稳晶振误差信号.对于如何提取并处理这两种误差信号,提出了一种对调幅信号...     

Intensity dependence suppression and enhancement of four-wave mixing in a micrometric thin vapour

We theoretically investigate dressed-four-wave mixing (dressed-FWM) spectroscopy of rubidium atoms in a micrometric thin vapour. It is found that Dike-narrowing type Autler–Townes (AT) spectroscopy with high resolution can be achieved in a reverse Y-type four-level atomic system due to the phase-conjugated configuration of laser beams and the transient effects of atom–wall collision in the thin vapour. We also show that controllable suppression and enhancement of the dressed-FWM signal due to the evolution of atomic coherence can be obtained by selecting different coupling field intensities at the proper detuning of the probe and the coupling fields. This control of FWM processes can be interpreted by dressed state analysis and probably used in the design of optical switch and the enhancement of FWM processes for frequency conversion.     

Things of Darkness: Genetics,Melanins and the Regime of Salazar (1936–1952)

This article discusses the interaction between genetics and politics during the early phase of Salazar's regime. In particular it focuses on the work of the Portuguese biologist José A. Serra who investigated the genetics of hair pigmentation at the University of Coimbra. The first part of the article describes how Serra's research benefitted from the ideological and political context in Coimbra before and during WWII, and how his work on melanins was a clear response to a new project initiated at the German Kaiser Wilhelm Institut für Anthropologie. The second part shows how his expertise in the inheritance and composition of hair colour was required by the regime in the post‐war period, when wool became a priority of the corporatist State. The ‘things of darkness’ are melanins, dark biological pigments responsible for pigmentation in mammalian tissues, used in this historical investigation to connect Serra's rather obscure field of research to the political context of his time.     

守时用小型氢原子钟的研制

本文报告了一种守时用小型氢原子钟的设计和研制。该钟应用了一个精密的电容式谐振腔，由于其体积很小，从而使钟的整体体积和重量较通常的氢钟大大减小。腔的Ｑ值采用正反馈获得增益，从而能维持氢脉泽振荡并改善信号的信噪比。由于采用了腔频自动调谐系统，钟的长期稳定度可与通常的氢钟相比。     

Experimental Study of the Frequency Stability of a Maser Oscillator Operated with an External Feedback Loop

In this paper, we report the results of an experimental study of the short-and mid-term frequency stability for both a hydrogen maser and a rubidium maser operated with an external feedback loop to modify the cavity quality factor. A revised version of the theoretical expression of the frequency stability for this type of maser is given and a numerical solution for various maser parameters is calculated. The predicted frequency stability exhibits an optimum when the cavity Q is varied. The experimental results presented in this study agree with this conclusion.     

Atomic interference in the micromaser and statistical properties of the micromaser field

Abstract

In this paper we report on atomic interferometry in the micromaser. The atomic inversion is recorded while scanning the cavity frequency across the atomic resonance. For high pump rates, interference patterns are observed on the low-frequency wing of the maser line. The interferences are due to the non-adiabatic mixing of dressed states of the atoms at the entrance and exit holes of the maser cavity, leading to a Ramsey-type two-field interaction. Furthermore, statistical properties of the maser field are investigated via a measurement of the statistics of the pump atoms that leave the maser cavity. Theoretical expressions for the time dependence of the Fano–Mandel parameter Q _A(t) are compared with the experimental data. We demonstrate, that metastability of the maser field and atomic interference strongly influence the approach to a steady-state value of Q _A(t).     

Turing's three philosophical lessons and the philosophy of information

In this article, I outline the three main philosophical lessons that we may learn from Turing's work, and how they lead to a new philosophy of information. After a brief introduction, I discuss his work on the method of levels of abstraction (LoA), and his insistence that questions could be meaningfully asked only by specifying the correct LoA. I then look at his second lesson, about the sort of philosophical questions that seem to be most pressing today. Finally, I focus on the third lesson, concerning the new philosophical anthropology that owes so much to Turing's work. I then show how the lessons are learned by the philosophy of information. In the conclusion, I draw a general synthesis of the points made, in view of the development of the philosophy of information itself as a continuation of Turing's work.     

Measurement of the Effect of Thermal Noise on the Amplitude of Oscillation of a Maser Oscillator

Experimental verification of the effect of thermal noise on the oscillation amplitude of a maser is described. Two different masers are considered, the H maser with an homogeneous atomic line and the Rb maser with an inhomogeneous line. The effect is characterized by the spectral density of the relative amplitude fluctuations and this parameter is measured with a conventional superheterodyne AM receiver for various maser saturation factors. Experimental results show good agreement with theoretical predicted values. At high saturation factors, a peak appears with a maximum at a frequency close to the Rabi frequency. Observed values at high Fourier frequencies allow a measurement of the atomic power by the knowledge of the absolute temperature of the cavity and the prediction of the oscillator short-term frequency stability. Furthermore, the effect on amplitude and on frequency noise of an external feedback loop used to increase the cavity quality factor is discussed.     

Heron's Dioptra 35 and Analemma Methods: An Astronomical Determination of the Distance between Two Cities

Abstract.   Heron's Dioptra 35 is the unique witness of an ancient mathematical procedure for finding the great arc distance between two cities using methods of ancient spherical astronomy and simultaneous observations of a lunar eclipse. This paper provides a new study of the text, with mathematical and historical commentary. I argue that Heron's account is a summary of some longer work of mathematical astronomy or geography, which made extensive use of the analemma, an ancient model of the celestial sphere. Heron's text can be used to show the utility of the analemma model, both as a theoretical device and as a computational tool.     

Normative,descriptive and formative approaches to describing normal work area

This paper examines normative and formative approaches toward describing normal work area. Squires’ model of normal work area is an example of a normative approach, which prescribes the shape of normal work area. Squires’ model has been taken as the starting point for more recent attempts to delineate reach boundaries in the workplace. However, his original paper does not present the derivation for his model or his claim that the distal boundary of his normal work area can be described by a prolate epicycloid. This paper presents the derivation for Squires’ model and shows that the resulting curve is not a prolate epicycloid, but a related form, a hypotrochoid. It then identifies shortcomings of normative models, like Squires’, notably the viability of the reach assumptions on which such normative models are based and their inability to deal with context-conditioned variability in the workplace. Finally, it examines the application of formative models, which attempt to identify constraints on the types of reach actions that people use. Data on how people actually reach may constitute a more appropriate foundation for constructing work area models needed to develop workplace standards.     

An Electronic System for the Tuning of Masers

The automatic tuning of frequency standards gives a method of removing drifts due to cavity pulling. The paper describes an apparatus designed primarily for tuning the hydrogen maser. The apparatus can also be used for tuning other masers such as the rubidium maser. The system incorporates several of the best features of similar devices previously described in the literature. It includes a digital averager which makes possible the use of a reference oscillator less stable than the maser to be tuned. It includes also a digital integrator, solving the usual problem of offsets in standard integrators. Frequency corrections are made either proportionally or by unit steps through a varactor coupled to the maser cavity. The electronics are made almost entirely with TTL digital integrated circuits (type 7400) for low cost, reliability, and availability. Results on the tuning of rubidium and hydrogen masers are given.     

Short-Term Frequency Stability of the Rb87 Maser

Measurements of the short-term stability of the Rb87 maser are reported here. The measurements were made as a function of the maser power output and of the receiver cutoff frequency. The experimental data are compared to theoretical results obtained from an approximate theory. In this theory the transfer function of the maser for thermal noise is derived, and the spectral density of the phase fluctuations is calculated. An analytical expression for the "Allan variance" is also given. A comparison of the stability of the Rb87 maser with existing frequency standards shows its superiority for averaging times less than 1 s. We obtain ?f/f ? 1.3 × 10-3 ?-1. A stability of 5 × 10-12 for ? ?1000 s is also reported.