Massimo Zucco

Geodesy and metrology with a transportable optical clock

Optical atomic clocks, due to their unprecedented stability1–3 and uncertainty3–6, are already being used to test physical theories7,8 and herald a revision of the International System of Units9,10. However, to unlock their potential for cross-disciplinary applications such as relativistic geodesy11, a major challenge remains: their transformation from highly specialized instruments restricted to national metrology laboratories into flexible devices deployable in different locations12–14. Here, we report the first field measurement campaign with a transportable 87Sr optical lattice clock12. We use it to determine the gravity potential difference between the middle of a mountain and a location 90 km away, exploiting both local and remote clock comparisons to eliminate potential clock errors. A local comparison with a 171Yb lattice clock15 also serves as an important check on the international consistency of independently developed optical clocks. This campaign demonstrates the exciting prospects for transportable optical clocks.An atomic clock has been deployed on a field measurement campaign to determine the height of a mountain location 1,000 m above sea level, returning a value that is in good agreement with state-of-the-art geodesy.

A VLBI experiment using a remote atomic clock via a coherent fibre link

We describe a VLBI experiment in which, for the first time, the clock reference is delivered from a National Metrology Institute to a radio telescope using a coherent fibre link 550 km long. The experiment consisted of a 24-hours long geodetic campaign, performed by a network of European telescopes; in one of those (Medicina, Italy) the local clock was alternated with a signal generated from an optical comb slaved to a fibre-disseminated optical signal. The quality of the results obtained with this facility and with the local clock is similar: interferometric fringes were detected throughout the whole 24-hours period and it was possible to obtain a solution whose residuals are comparable to those obtained with the local clock. These results encourage further investigation of the ultimate VLBI performances achievable using fibre dissemination at the highest precision of state-of-the-art atomic clocks.

A coherent fiber link for very long baseline interferometry

We realize a coherent fiber link for application in very long baseline interferometry (VLBI) for radio astronomy and geodesy. A 550-km optical fiber connects the Italian National Metrological Institute (INRIM) to a radio telescope in Italy and is used for the primary Cs fountain clock stability and accuracy dissemination. We use an ultrastable laser frequency- referenced to the primary standard as a transfer oscillator; at the radio telescope, an RF signal is generated from the laser by using an optical frequency comb. This scheme now provides the traceability of the local maser to the SI second, realized by the Cs fountain at the 1.7 × 10-16 accuracy. The fiber link never limits the experiment and is robust enough to sustain radio astronomical campaigns. This experiment opens the possibility of replacing the local hydrogen masers at the VLBI sites with optically-synthesized RF signals. This could improve VLBI resolution by providing more accurate and stable frequency references and, in perspective, by enabling common- clock VLBI based on a network of telescopes connected by fiber links.

Optical frequency transfer with a 1284 km coherent fiber link

We realized a coherent optical fiber link in Italy, with a fiber haul of 642 km. To characterize the link, we doubled the link to 1284 km, demonstrating a characterization technique based on the double round trip on a single fiber. The link is intended to significantly improve the frequency references used in radio-astronomy and precision measurements in atomic physics. The data analysis is based on the Allan deviation, whose expression is theoretically derived from the noise power spectrum. The demonstrated resolution is 3×10-19 in 1000 s and the uncertainty of the transfer is 5×10-19. The arming of a second fiber is avoided, and this could be rather beneficial to long hauls realizations for a continental fiber network for frequency and time metrology.

LIFT-the Italian link for time and frequency

We report on the realization of the coherent optical link for time and frequency dissemination developed in Italy. The fiber backbone connects scientific laboratories that need accurate time and frequency measurements located in Torino, Milano, Bologna and Firenze. We briefly describe the technique, and report the status of installation and preliminary characterization.

Optical frequency link between Torino and Firenze for remote comparison between Yb and Sr optical clocks

The Italian Institute of Metrology (INRIM), the European Laboratory for NonLinear Spectroscopy (LENS), the Physics and Astronomy Department of the University of Firenze (UNIFI) and the Politecnico of Torino started a project to establish a 650 km optical fiber link that will enable remote frequency comparisons between high accuracy microwave and optical clocks of INRIM and UNIFI-LENS and an absolute measurement of the UNIFI-LENS Sr clock versus INRIM Cesium fountains. We report the overall architecture of the project, the present status of the experiment and the preliminary tests performed on the equipments.

Iodine Absorption Cells Purity Testing

This article deals with the evaluation of the chemical purity of iodine-filled absorption cells and the optical frequency references used for the frequency locking of laser standards. We summarize the recent trends and progress in absorption cell technology and we focus on methods for iodine cell purity testing. We compare two independent experimental systems based on the laser-induced fluorescence method, showing an improvement of measurement uncertainty by introducing a compensation system reducing unwanted influences. We show the advantages of this technique, which is relatively simple and does not require extensive hardware equipment. As an alternative to the traditionally used methods we propose an approach of hyperfine transitions’ spectral linewidth measurement. The key characteristic of this method is demonstrated on a set of testing iodine cells. The relationship between laser-induced fluorescence and transition linewidth methods will be presented as well as a summary of the advantages and disadvantages of the proposed technique (in comparison with traditional measurement approaches).

Iodine absorption cells quality measurements

This work is oriented to comparison of methods for iodine absorption cells quality evaluation. Optical frequency references based on molecular iodine represent one of the most used references for stabilizion of laser standards working at visible spectral range. Unfortunately iodine is a media with very high sensitivity to contamination so the chemical purity of iodine cells must be preciselly controlled. Traditional methods for iodine absorption cells quality checking have several difficulties and disadvantages, these problems complicate their common using in labs. Due to this reason we propose an alternative method of spectral linewidths measurement, which overcomes these difficulties of traditionally used approaches and which serves as a tool for iodine cells quality evaluation. In this work we present the results of comparison of two laser induced fluorescence setups (with and without the compensation for the laser source spectral mode-hops) with proposed method of selected hyperfine transition linewidth measurements and we discuss advantages and limitations of these methods for practical using.

Iodine absorption cells quality evaluation

This paper is oriented towards comparison of measurement methods for iodine absorption cells quality evaluation. Traditionally used method of laser induced fluorescence with evaluation of Stern-Volmer coefficients and method of measurement of absolute frequency shifts of hyperfine spectras have several limitations and sensitivity limits. Due to this reason an alternative technique of hyperfine spectra linewidth measurement is proposed and summarization of all methods advantages and drawbacks are described.

Iodine absorption cells quality evaluation methods

The absorption cells represent an unique tool for the laser frequency stabilization. They serve as irreplaceable optical frequency references in realization of high-stable laser standards and laser sources for different brands of optical measurements, including the most precise frequency and dimensional measurement systems. One of the most often used absorption media covering visible and near IR spectral range is molecular iodine. It offers rich atlas of very strong and narrow spectral transitions which allow realization of laser systems with ultimate frequency stabilities in or below 10-14 order level. One of the most often disccussed disadvantage of the iodine cells is iodine’s corrosivity and sensitivity to presence of foreign substances. The impurities react with absorption media and cause spectral shifts of absorption spectra, spectral broadening of the transitions and decrease achievable signal-to-noise ratio of the detected spectra. All of these unwanted effects directly influence frequency stability of the realized laser standard and due to this fact, the quality of iodine cells must be precisely controlled. We present a comparison of traditionally used method of laser induced fluorescence (LIF) with novel technique based on hyperfine transitions linewidths measurement. The results summarize advantages and drawbacks of these techniques and give a recommendation for their practical usage.