8th Symposium on Frequency Standards and Metrology 2015

The eighth Symposium on Frequency Standards and Metrology [1] has been organized from 12-16 October in the Seminaris SeeHotel near Lake Templin at Potsdam, Germany. The symposium included a keynote lecture by David Wineland, 48 invited oral presentations and a session with 111 invited poster contributions. The symposium is the most recent one of a series that was initially organized by Jacques Vanier in 1971 in Forét Montmorency in Quebec, Canada. The next ones took place in 1976 in Copper Mountain, USA, organized by Helmut Hellwig, 1981 in Aussois, France (Claude Audoin) [2], 1988 in Ancona, Italy (Andrea De Marchi) [3], 1995 in Woods Hole, USA (James Bergquist) [4], 2001 in St Andrews, UK (Patrick Gill) [5], and 2008 in Pacific Grove, USA (Lute Maleki) [6]. The symposium series serves as an international discussion forum on precision frequency standards throughout the electromagnetic spectrum and associated metrology. It focuses on the fundamental scientific aspects of the latest ideas, results and applications in relation to these frequency standards. During the seven years after the last symposium very significant progress has occurred in various associated fields. Carrying forward the historical sketch by David Wineland [7] it is interesting to wrap up the new ideas, the novel technologies, developments and inventions that have been created and implemented in the seven years since the last symposium. In the microwave domain the accuracy of caesium fountain clocks as the primary standards of time and frequency approaches 10 -16 [8] and small scale, miniaturized atomic clocks [9] and sensors are further developed with respect to accuracy with ample applications in science, technology [10] and life sciences [11]. Today, optical clocks are about to make the vision of Hans Dehmelt [12] a reality. Several optical clocks capable of estimated fractional uncertainties in the 10 -18 regime, both based on neutral atoms in a lattice and trapped single ions, have been presented at the symposium. In the latter case this was only possible with newly developed schemes for the interrogation that are capable to remove the frequency shifts associated with strong light field necessary for the interrogation [13]. The achieved accuracy already asks for a new definition of the unit of time [14]. The verification of those breath-taking uncertainties by comparison of clocks at different laboratories is one of the challenges today. Conventional methods of Two-Way Satellite Time and Frequency Transfer have already been used to make intercontinental comparisons [15] with fractional uncertainty in the low 10 -15 range. The uncertainty associated with the transfer via the microwave links may be reduced e.g. in the ACES project. Novel methods that can achieve this goal better use transportable frequency standards [16], the evaluation of direct frequency ratios between optical frequency standards [17,18], or long haul fiber links for remote frequency comparisons that have been implemented [19]. Optical atomic clocks approaching 10 -18 fractional uncertainty, in conjunction with accurate links, can serve as accurate measuring devices for the geopotential with height resolution in the cm range, thereby entering the era of a relativistic geodesy [20]. Besides accuracy, long-lived optical transitions hold the promise of optical clocks with unprecedented low instability if the transitions can be interrogated by lasers of adequate stability and phase coherence. Recent years have experienced tremendous progress on various fronts. The more classical approach stabilizes the interrogating laser to a high finesse optical cavity. In the past decade the different methods of vibration-insensitive mounting have made lasers with about one Hertz linewidth available in virtually every laboratory when required. There was, however, a limit given by the Brownian motion of the spacer [21], the mirror substrates and the mirror coatings. Use of long cavities [22], crystalline spacers and substrates, and cryogenic operating temperatures [23] have led to instabilities below the 10 -16 regime in 1 to 100 seconds. The recent development of crystalline mirrors [24] and other achievements promise instabilities in the low 10 -17 regime and below. Unconventional other approaches rely on spectral-hole burning [25] or emitting photons on an ultra-narrow clock transition into the mode of a high Q resonator [26]. This progress was paralleled by a better understanding of the relevant noise processes and their experimental characterization [27] and theoretical modelling [28]. Spectroscopy on highly charged ions together with the associated technology now enter the field of frequency metrology and deliver exciting results [29]. They have the prospect to lead to alternative frequency standards and to push the field of precision tests of theoretical approaches. Clocks and oscillators in the optical and microwave domain are by far the most accurate measuring instruments and therefore are also utilized for investigating fundamental questions like possible variations of fundamental constants [30] or the effects of dark energy [31]. Nuclear transitions might extend these opportunities [32]. Femtosecond combs [33] have been further miniaturized, have been tested in microgravity and have found novel applications as astro-combs. Finally, I would like to express my cordial thanks to the sponsors, the Governing Board, the International Steering Committee, the Local Organizing Committee, the session chairs and the reviewers who helped with the realization of the symposium and the preparation of these proceedings.

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