Realization of a pulsed optically pumped Rb clock with a frequency stability below [Formula: see text].

We present the frequency stability performances of a vapor cell Rb clock based on the pulsed optically pumping (POP) technique. The clock has been developed in the frame of a collaboration between INRIM and Leonardo SpA, aiming to realize a space-qualified POP frequency standard. The results here reported were obtained with an engineered physics package, specifically designed for space applications, joint to laboratory-grade optics and electronics.

Coherent phase transfer for real-world twin-field quantum key distribution.

Quantum mechanics allows distribution of intrinsically secure encryption keys by optical means. Twin-field quantum key distribution is one of the most promising techniques for its implementation on long-distance fiber networks, but requires stabilizing the optical length of the communication channels between parties. In proof-of-principle experiments based on spooled fibers, this was achieved by interleaving the quantum communication with periodical stabilization frames.

Extensive cosmic showers detection: the importance of timing and the role of GPS in the EEE experiment.

Extreme Energy Events (EEE) is an extended Cosmic Rays (CRs) Observatory, composed of about 60 tracking telescopes spread over more than 10 degrees in Latitude and Longitude. We present the metrological characterization of a representative set of actually installed EEE GPS receivers, their calibration and their comparison with respect to dual-frequency receivers for timing applications, as well as plans for a transportable measurement system to calibrate the currently deployed GPS receivers. Finally, the realization of an INRIM Laboratory dedicated to EEE, aimed at hosting reference telescopes and allowing timing studies for Particle Physics/Astrophysics experiments, is presented, as well as the possibility of synchronizing already deployed telescopes utilizing White Rabbit Technique, over optical fiber links, directly with the Universal Time Coordinated time scale, as realized by INRIM (UTC(IT)).

Kr-Based Buffer Gas for Rb Vapor-Cell Clocks.

Optically pumped Rb vapor cell clocks are by far the most used devices for timekeeping in all ground and space applications. The compactness and the robustness of this technology make Rb clocks extremely well fit to a large number of applications, including GNSS, telecommunication, and network synchronization. Many efforts are devoted to improve the stability of Rb clocks and reduce their environmental sensitivity.

Loaded Microwave Cavity for Compact Vapor-Cell Clocks.

Vapor-cell devices based on microwave interrogation provide a stable frequency reference with a compact and robust setup. Further miniaturization must focus on optimizing the physics package, containing the microwave cavity and atomic reservoir. In this article, we present a compact cavity-cell assembly based on a dielectric-loaded cylindrical resonator.

Tunable UV spectrometer for Doppler broadening thermometry of mercury.

We realized a UV laser spectrometer at 253.7 nm for Doppler broadening thermometry on the - intercombination line in mercury vapors. Our setup is based on the two-stage duplication of a 1014.

Intensity Detection Noise in Pulsed Vapor-Cell Frequency Standards.

Laser intensity noise is currently recognized as one of the main factors limiting the short-term stability of vapor-cell clocks. In this article, we propose a signal theory approach to estimate the contribution of the laser intensity fluctuations to the short-term stability of vapor-cell clocks working in a pulsed regime. Specifically, given the laser intensity noise spectrum, an analytical expression is derived to evaluate its impact on the clock Allan deviation (ADEV).

Reducing Cavity-Pulling Shift in Ramsey-Operated Compact Clocks.

We describe a method to stabilize the amplitude of the interrogating microwave field in compact atomic clocks working in a Ramsey approach. In this technique, we take advantage of the pulsed regime to use the atoms themselves as microwave amplitude discriminators. Specifically, in addition to the dependence on the microwave detuning, the atomic signal after the Ramsey interrogation acquires a dependence on the microwave pulse area (amplitude times duration) that can be exploited to implement an active stabilization of the microwave field amplitude, in a similar way in which the Ramsey clock signal is used to lock the local oscillator frequency to the atomic reference.

Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables.

Detecting ocean-floor seismic activity is crucial for our understanding of the interior structure and dynamic behavior of Earth. However, 70% of the planet's surface is covered by water, and seismometer coverage is limited to a handful of permanent ocean bottom stations. We show that existing telecommunication optical fiber cables can detect seismic events when combined with state-of-the-art frequency metrology techniques by using the fiber itself as the sensing element.

Multiple wavelength stabilization on a single optical cavity using the offset sideband locking technique.

We implemented a compact, robust, and stable device for simultaneous frequency stabilization of lasers with different wavelengths used for the cooling and trapping of Yb atoms in an optical lattice clock. The lasers at 399, 556, and 759 nm are locked to a single ultra-stable cavity using the offset sideband locking technique, a modified version of the Pound-Drever-Hall method. For the most demanding stabilization here, the 556 nm laser, this system exhibits a 300 Hz linewidth for an integration time of 80 ms.

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.

Measuring absolute frequencies beyond the GPS limit via long-haul optical frequency dissemination.

Global Positioning System (GPS) dissemination of frequency standards is ubiquitous at present, providing the most widespread time and frequency reference for the majority of industrial and research applications worldwide. On the other hand, the ultimate limits of the GPS presently curb further advances in high-precision, scientific and industrial applications relying on this dissemination scheme. Here, we demonstrate that these limits can be reliably overcome even in laboratories without a local atomic clock by replacing the GPS with a 642-km-long optical fiber link to a remote primary caesium frequency standard.

Comb-assisted cavity ring-down spectroscopy of a buffer-gas-cooled molecular beam.

We demonstrate continuous-wave cavity ring-down spectroscopy of a partially hydrodynamic molecular beam emerging from a buffer-gas-cooling source. Specifically, the (ν1 + ν3) vibrational overtone band of acetylene (C2H2) around 1.5 μm is accessed using a narrow-linewidth diode laser stabilized against a GPS-disciplined rubidium clock via an optical frequency comb synthesizer.

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.

Yellow laser performance of Dy³⁺ in co-doped Dy,Tb:LiLuF₄.

We present laser results obtained from a Dy³⁺-Tb³⁺ co-doped LiLuF₄ crystal, pumped by a blue emitting InGaN laser diode, aiming for generation of a compact 578 nm source. We exploit the yellow Dy³⁺ transition ⁴F(9/2)⇒⁶H(13/2) to generate yellow laser emission. The lifetime of the lower laser level is quenched, via energy transfer, to co-doped Tb³⁺ ions in the fluoride crystal.

Efficient frequency doubling at 399 nm.

We describe a reliable, high-power, and narrow-linewidth laser source at 399 nm, which is useful for cooling and trapping of ytterbium atoms. A continuous-wave titanium-sapphire laser at 798 nm is frequency doubled using a lithium triborate crystal in an enhancement cavity. Up to 1.

Active disturbance rejection control of temperature for ultrastable optical cavities.

This paper describes the application of a novel active disturbance rejection control (ADRC) to the stabilization of the temperature of two ultra-stable Fabry-Perot cavities. The cavities are 10 cm long and entirely made of ultralow- expansion glass. The control is based on a linear extended state observer that estimates and compensates the disturbance in the system in real time.

Enhanced temperature sensitivity in vapor-cell frequency standards.

We report on the measurement of an anomalously large temperature sensitivity of the clock frequency in a Rb cell with buffer gas. The effect is observed in a prototype of pulsed optically pumped frequency standard which allows high resolution measurements because of its frequency stability at the level 1.7 × 10(-13) for 1 s of measurement time.

Pulsed optically pumped rubidium clock with high frequency-stability performance.

In this paper, we present the performance of a vapor-cell rubidium frequency standard working in the pulsed regime, in which the clock signal is represented by a Ramsey pattern observed on an optically detected laser absorption signal. The main experimental results agree with previously reported theoretical predictions. In particular, we measured a relative frequency stability of σy(τ) - 1.

Realization of an ultrastable 578-nm laser for an Yb lattice clock.

In this paper, we describe the development of an ultrastable laser source at 578 nm, realized using frequency sum generation. This source will be used to excite the clock transition (1)S(0) - (3)P(0) in an ytterbium optical lattice clock experiment. Two independent ultrastable lasers have been realized, and the laser frequency noise and stability have been characterized.

Planar-waveguide external cavity laser stabilization for an optical link with 10(-19) frequency stability.

We stabilized the frequency of a compact planar-waveguide external cavity laser (ECL) on a Fabry-Perot cavity (FPC) through a Pound-Drever-Hall scheme. The residual frequency stability of the ECL is 10(-14), comparable to the stability achievable with a fiber laser (FL) locked to an FPC through the same scheme. We set up an optical link of 100 km, based on fiber spools, that reaches 10(-19) relative stability, and we show that its performances using the ECL or FL are comparable.

Microwave cavities for vapor cell frequency standards.

In this paper, we report an analysis of the design criteria of microwave cavities for vapor cell frequency standards. Two main geometries exploited in those devices are considered: the cylindrical cavity, used, for example, in the coherent population trapping maser and in the pulsed optically pumped (POP) clock, and the spherical cavity used in the isotropically laser cooled clock. The cavity behavior is described through a lumped equivalent circuit in which the input coupling loop, the dielectric cell containing the atoms and the diodes for frequency tuning or Q control are taken into account.

Medium-long term frequency stability of pulsed vapor cell clocks.

In this paper we report an analysis of the physical phenomena that can affect the frequency stability of optically pumped vapor cell clocks working in pulsed regime. It is well known that the pulsed approach allows a strong reduction of the light shift that is one of the main sources of frequency instability. However, other instability sources can degrade clock performance by limiting both short- and medium-term frequency stability.

Cryogenic fountain development at NIST and INRIM: preliminary characterization.

This paper describes the new twin laser-cooled Cs fountain primary frequency standards NIST-F2 and ITCsF2, and presents some of their design features. Most significant is a cryogenic microwave interrogation region which dramatically reduces the blackbody radiation shift. We also present a preliminary accuracy evaluation of IT-CsF2.