# PHARAO/ACES

**LKB Permanent Members**

Christophe Salomon

Christine Guerlin

**SYRTE Permanent Members**

Philippe Laurent

Peter Wolf

Frederic Meynadier

Pacôme Delva

Christophe Leponcin-Lafitte

**SYRTE Students**

Hélène Pihan Le Bars (PhD)

Etienne Savalle (M1 student)

**Selected Publications**

Microwave lensing frequency shift of the PHARAO laser-cooled microgravity atomic clock

P. Peterman, K. Gibble, P. Laurent, C. Salomon

Metrologia 53, 899, (2016)

The ACES /PHARAO space mission

P. Laurent, D. Massonnet, L. Cacciapuoti, C. Salomon

Comptes-Rendus Acad. Sciences, Paris, 540 (2015)

Testing Lorentz symmetry with planetary orbital dynamics

A. Hees, Q. G. Bailey, C. Le Poncin-Lafitte,A. Bourgoin, A. Rivoldini, B. Lamine, F. Meynadier, C. Guerlin, and P. Wolf

PRD **92**, 064049 (2015)

Some fundamental physics experiments using atomic clocks and sensors

C. Guerlin, and P. Delva, P. Wolf

C.R. Physique **16**, 565, (2015)

PHARAO : le premier étalon primaire de fréquence à atomes froids spatial

P. Laurent* et al. *

REVUE FRANÇAISE DE MÉTROLOGIE, 34, Volume 2, (2014)

Reply to comment on: ‘Does an interfrometer test the gravitational redshift at the Compton frequency?’

P. Wolf, L. Blanchet, C.J. Bordé, S. Reynaud, C. Salomon, and C. Cohen-Tannoudji

Class. Quantum Grav. 29, 048002 (2012)

Space Clocks and Fundamental Tests: the ACES experiment

Luigi Cacciapuoti and Christophe Salomon

EPJ Special topics, 172, 57 (2009)

##### Overview

Proposed in 1997, the PHARAO/ACES experiment is a space mission in fundamental physics with two atomic clocks on the International Space Station, a network of ultra-stable clocks on the ground in national metrology institutes, and space-to- ground time transfer systems. The core of the flight instruments is a cold atom cesium clock designed to operate in micro-gravity conditions, the PHARAO clock. This clock has been designed by LKB, SYRTE, and CNES, and constructed by French companies under CNES funding. The other elements of the flight payload include a hydrogen maser developed by SpectraTime (CH), a high precision time transfer system in the microwave domain developed by TimeTech (DE) and Airbus (DE), and a laser time transfer developed by the University of Prague (CZ) and Technical University Munich (DE). The satellite payload is assembled by Airbus (DE) under ESA funding and ESA is responsible for launch and operations of the ACES mission. The ACES flight instruments are completed and launch in space is planned for early 2018 for a mission duration of three years.

The ACES scientific objectives have four main components, the operation of a laser-cooled cesium primary standard in space, a precision measurement of the Einstein effect, the gravitational shift of the clock frequency predicted by General Relativity, tests of Lorentz invariance, and a search for time or spatial variations of fundamental physical constants by long-distance ground clock comparisons.

According to the geometric nature of Einstein’s description of gravity, rates of clocks located at different gravitational potentials will differ by alpha Delta U/c^2 where c is the speed of light in vacuum and alpha =1 in Einstein general relativity. The ACES two-way microwave time transfer system between PHARAO and laboratories equipped with ground terminals will allow to retrieve the desynchronization between PHARAO and ground clocks with picosecond precision. Comparing the measured desynchronization to Einstein’s model will allow to constrain Einstein’s prediction at the level of 2.10^-6 .