# Laser Technology in Photonic Applications for Space
## 3 Lasers in Space
### Science
#### 3.1.3 Quantum Scientific Technologies  
Scientific experiments addressing quantum physics challenges have also been launched into space. Atomic clocks and atom interferometers, which can be used for various space applications, are some of the devices that have been tested. Atomic clocks based on laser-cooled atoms are widely used as primary frequency standards and are a key component of the Global Navigation Satellite System (GNSS). Atom interferometers are used in applications described in the article of [PERSON] et al. (2018), such as measurements of the Earth's gravitational field, gravitational wave detection, navigation, and tests of the weak equivalence principle. In the same paper, authors reported an atom interferometer device built with a laser system based on a hybrid approach using fiber-based telecommunication components and high-power laser diode technologies, designed in the framework of the space mission Space-Time Explorer and QUantum Equivalence Principle Space Test (STE-QUEST). The purpose of this mission is to perform quantum experiments to test the Einstein Equivalence Principle to a high precision level, as well as search for new fundamental constituents and interactions in the Universe ([PERSON], 2018). Similar technologies were demonstrated in the Quantus project, and in rocket mission experiments, such as MAIUS and Kalevas. The first cold atom experiment in space was reported in 2017 with the Matter-Wave Interferometry in the Microgravity (Maius 1) mission ([PERSON], 2018), although it only lasted six minutes. The laser module was based on a MOPA configuration where the oscillator was built of a 1 mm long double quantum well, AlGaAs based ridge-waveguide distributed feedback diode laser emitting at 766.7 nm with a linewidth of approximately 1 MHz ([PERSON], 2018), and where a self-seeded tapered amplifier (TA) was used to generate an output power of more than 1 W ([PERSON], 2018). The output power emitted through the back facet of the Distributed FeedBack (DFB) diode laser was monitored with a photo diode. Nevertheless, there has been some controversy on which experiment was performed first, since China started a mission called Cold Atom Clock Experiment in Space (CACES) in 2011 that resulted in launching an atomic clock in space in 2016, but the results were only reported one year later. In this case, the system used two input distributed Bragg reflector diode laser beams for laser cooling of rubidium ([PERSON], 2018). The Cold Atom Lab experiment, launched to the ISS in May 2018, was designed and built at NASA's Jet Propulsion Laboratory (JPL). It uses a very complex design based on commercial laser equipment and has successfully produced Bose-Einstein Condensates (BEC) of Rubidium atoms in orbit for the first time. Thanks to the microgravity conditions of the ISS, the BEC produced are colder and more stable, which should allow to observe new quantum phenomena and test fundamental laws of physics which could be applied in quantum detectors, optical clocks, and gravity monitoring. The light source consists in external cavity diodes, where a reference laser is used for each species: one at 766.701 nm, the other at 780.240 nm. The laser cooling system uses stabilized New Focus Vortex Plus diode lasers and New Focus TA-7600 tapered amplifiers ([PERSON], 2018). Further atomic clock missions in space will include ESA's Atomic Clock Ensemble in Space (ACES), which has been tested and integrated on the payload and will be ready for launch to the Space Station by 2020. The objective is to place atomic clocks in orbit and compare their performance to ground clock systems, as well as perform fundamental scientific experiments, such as gravitational red-shift and light anisotropy measurements, amongst others ([PERSON], 2018). The cesium atomic clock system called PHARAO (Project d'Horloge Atomique Par Refroidissement d'Atomes en Orbite), to be deployed within the ACES mission, has been developed by the CNES (Figure 3).