Cold-atom sources for the Matter-wave laser Interferometric Gravitation Antenna (MIGA).

The Matter-wave laser Interferometric Gravitation Antenna (MIGA) is an underground instrument using cold-atom interferometry to perform precision measurements of gravity gradients and strains. Following its installation at the low noise underground laboratory LSBB in the South-East of France, it will serve as a prototype for gravitational wave detectors with a horizontal baseline of 150 meters. Three spatially separated cold-atom interferometers will be driven by two common counter-propagating lasers to perform a measurement of the gravity gradient along this baseline. This article presents the cold-atom sources of MIGA, focusing on the design choices, the realization of the systems, the performances and the integration within the MIGA instrument.

Phase noise characterization of sub-hertz linewidth lasers via digital cross correlation.

Phase noise or frequency noise is a key metric to evaluate the short-term stability of a laser. This property is of great interest for the applications but delicate to characterize, especially for narrow linewidth lasers. In this Letter, we demonstrate a digital cross-correlation scheme to characterize the absolute phase noise of sub-hertz linewidth lasers. Three 1542 nm ultra-stable lasers are used in this approach. For each measurement, two lasers act as references to characterize a third one. Phase noise power spectral density from 0.5 Hz to 0.8 MHz Fourier frequencies can be derived for each laser by a mere change in the configuration of the lasers. To the best of our knowledge, this is the first time showing the phase noise of sub-hertz linewidth lasers with no reference limitation. We also present an analysis of the laser phase noise performance.

High Stability Comparison of Atomic Fountains using two Different Cryogenic Oscillators.

We present a detailed characterization of two atomic clock interrogation systems based on two different cryogenic sapphire oscillators operated simultaneously. We use them as references for two accurate fountain clock frequency standards participating in international atomic time and operating both at the quantum projection noise frequency limit. The two fountain comparison down to a few 10􀀀16 over 28 days demonstrates the potential of a cryocooled oscillator to replace a He refilled cryogenic oscillator.

Progress in atomic fountains at LNE-SYRTE.

We give an overview of the work done with the Laboratoire National de Métrologie et d'Essais-Systèmes de Référence Temps-Espace (LNE-SYRTE) fountain ensemble during the last five years. After a description of the clock ensemble, comprising three fountains, FO1, FO2, and FOM, and the newest developments, we review recent studies of several systematic frequency shifts. This includes the distributed cavity phase shift, which we evaluate for the FO1 and FOM fountains, applying the techniques of our recent work on FO2. We also report calculations of the microwave lensing frequency shift for the three fountains, review the status of the blackbody radiation shift, and summarize recent experimental work to control microwave leakage and spurious phase perturbations. We give current accuracy budgets. We also describe several applications in time and frequency metrology: fountain comparisons, calibrations of the international atomic time, secondary representation of the SI second based on the (87)Rb hyperfine frequency, absolute measurements of optical frequencies, tests of the T2L2 satellite laser link, and review fundamental physics applications of the LNE-SYRTE fountain ensemble. Finally, we give a summary of the tests of the PHARAO cold atom space clock performed using the FOM transportable fountain.

Characterizing a fiber-based frequency comb with electro-optic modulator.

We report on the characterization of a commercial- core fiber-based frequency comb equipped with an intracavity free-space electro-optic modulator (EOM). We investigate the relationship between the noise of the pump diode and the laser relative intensity noise (RIN) and demonstrate the use of a low-noise current supply to substantially reduce the laser RIN. By measuring several critical transfer functions, we evaluate the potential of the EOM for comb repetition rate stabilization. We also evaluate the coupling to other relevant parameters of the comb. From these measurements, we infer the capabilities of the femtosecond laser comb to generate very-low-phase-noise microwave signals when phase-locked to a high-spectral-purity ultra-stable laser.

Advanced noise reduction techniques for ultra-low phase noise optical-to-microwave division with femtosecond fiber combs.

We report what we believe to be the lowest phase noise optical-to-microwave frequency division using fiber-based femtosecond optical frequency combs: a residual phase noise of -120 dBc/Hz at 1 Hz offset from an 11.55 GHz carrier frequency. Furthermore, we report a detailed investigation into the fundamental noise sources which affect the division process itself. Two frequency combs with quasi-identical configurations are referenced to a common ultrastable cavity laser source. To identify each of the limiting effects, we implement an ultra-low noise carrier-suppression measurement system, which avoids the detection and amplification noise of more conventional techniques. This technique suppresses these unwanted sources of noise to very low levels. In the Fourier frequency range of ∼200 Hz to 100 kHz, a feed-forward technique based on a voltage-controlled phase shifter delivers a further noise reduction of 10 dB. For lower Fourier frequencies, optical power stabilization is implemented to reduce the relative intensity noise which causes unwanted phase noise through power-to-phase conversion in the detector. We implement and compare two possible control schemes based on an acousto-optical modulator and comb pump current. We also present wideband measurements of the relative intensity noise of the fiber comb.

Low-phase-noise frequency synthesizer for the trapped atom clock on a chip.

We report on the realization of a 6.834-GHz synthesis chain for the trapped atom clock on a chip (TACC) that is being developed at LNE-SYRTE. The chain is based on the frequency multiplication of a 100-MHz reference signal to obtain a signal at 6.4 GHz. It uses a comb generator based on a monolithic GaAs nonlinear transmission line. This is a novelty in the fabrication of high-stability microwave synthesizers. Measurements give a low flicker phase noise of -85 dBrad(2)/Hz at 1-Hz offset frequency and a white phase noise floor < -115 dBrad(2)/Hz. Based on these results, we estimate that the performance of the synthesizer is at least one order of magnitude better than the stability goal of TACC. This ensures that the synthesizer will not be limiting the clock performance.

THz photomixing synthesizer based on a fiber frequency comb.

A frequency doubled erbium doped modelocked fiber frequency comb is used to implement a THz photomixing synthesizer. The useful THz linewidth is in order of 150 kHz and has been assessed along with the frequency accuracy by spectroscopic measurements demonstrating a relative accuracy of 10(-8) at frequencies around 1 THz. The THz synthesizer is used to implement a THz spectrometer to study the rotational absorption spectrum of carbonyl sulfide (OCS). Measurement of the principal transitions between 813 GHz and 1283 GHz allowed the properties of the THz spectrometer to be compared with competing techniques, and demonstrates the potential of the THz photomixing synthesizer as an alternative means to explore the THz domain.

Switching atomic fountain clock microwave interrogation signal and high-resolution phase measurements.

This paper focuses on the development of tools aiming to solve several problems related to the microwave interrogation signal in atomic fountains. We first consider the problem related to cycle synchronous phase transients caused by the sequential operation of the atomic fountain. To search for such systematic phase variations deeply buried in the microwave synthesizer phase noise, we have developed a novel triggered-phase transient analyzer capable of processing the microwave signal to extract the phase in a synchronous manner even in the presence of frequency modulation. With this device we check in vivo the LNE-SYRTE fountain's interrogation signals with a resolution approaching 1 microradian. In addition, using this device, we investigate an innovative approach to solve a second problem, namely, the shift caused by microwave leakage in the fountain. Our approach consists of switching off the fountain microwave interrogation signal when atoms are outside the microwave cavity. To do that, we have developed a switch that is almost free of phase transients and is thus able to eliminate the frequency shift caused by microwave leakage without inducing significant phase transients on the interrogation signal.

Design and metrological features of microwave synthesizers for atomic fountain frequency standard.

In this paper we describe the improved redesign of the microwave frequency synthesizers for Laboratoire National d'Essais-Systèmes de Référence Temps-Espace (LNE-SYRTE) atomic fountains. The synthesizers use a cryogenic oscillator to generate both Cs and Rb hyperfine frequencies based on a new distribution frequency of 1 GHz. The main metrological features (phase noise, long-term phase stability, and spectral purity) of the synthesizers have been measured in situ connected to an atomic fountain and are compatible with an accuracy goal of 10(-16) for the atomic fountains. The simultaneous test of two different synthesizers on the FO2 atomic fountain at the 10(-16) level also is reported.