Stable and Ultra-Stable Laser Systems for a Mobile Strontium Optical Clock by Dariusz Tadeusz Swierad´ A thesis submitted to The University of Birmingham for the degree of DOCTOR OF PHILOSOPHY Ultracold Atoms Group School of Physics and Astronomy College of Engineering and Physical Sciences The University of Birmingham December 2017 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. Abstract This thesis presents a realisation of the most demanding laser systems for a mobile strontium optical clock. First, stable 689 nm cooling lasers are presented: an amplified semiconductor diode laser and a prototype of the semiconductor disc laser (SDL/VECSEL). The first was stabilised to a novel miniaturised multi-laser frequency stabilisation system that allows to stabilise all the six lasers used in the strontium optical clock. The latter was used for the first time in the second-stage cooling of strontium to obtain a cold cloud of trapped atoms. An atomic optical clock requires a laser oscillator for interrogating the clock transition. This thesis presents the construction and discusses the performance of the cutting-edge ultra-stable interrogation lasers at 698 nm. One of the systems is a stationary system based at the University of Birmingham, with the 15 instability of 5 10− . A mobile version of the interrogation laser is also presented and characterised in ⇥ 16 this work. The laser reaches instability of 8 10− , which is one of the best results for a mobile system. ⇥ The mobile laser is a part of the space optical clock project (SOC2), in which a record-low instability for the 16 bosonic strontium was observed of < 4 10− /p⌧. ⇥ Dla Klaudii Dla moich rodzic´ow ACKNOWLEDGEMENTS This thesis has benefited from a vast number of people who are my friends, mentors, advisers and supervisors. At the beginning I would like to thank Prof. Kai Bongs for giving me the opportunity to join the Cold Atoms group at the University of Birmingham, for useful discussions, support and trust. Special thanks to Dr. Christian Lisdat and Dr. Uwe Sterr for their great supervision during my secondment at the Physikalisch- Technische Bundesanstalt (PTB). I will always remember your immense hospitality and help that you gave me. Thank you for extremely useful and instructive discussions and for always speaking English in my company. It was an honour working with you. I also want to thank Dr. Rodolphe Le Targat and Dr. J´erˆomeLodewyck from the Paris Observatory for their help and advice. I would like to thank everyone working with me in the clock experiment at the University of Birmingham. Thanks to Dr. Yeshpal Singh for his understanding, kindness and keeping our experimental group running. I should especially thank Dr. Ole Kock and Dr. Joshua Hughes for guiding me through the spider web of cables in the laboratory and for being keen on sharing their knowledge and experience with me. Particularly, I thank Dr. Lyndsie Smith for being a great lab companion, for many comforting discussions and advice. It was you Lyndsie, who sacrificed your entire lab snack box so that I could have my first one. Also, I want to thank Stefano Origlia for being a good colleague, being helpful and for sharing his Parmigiano-Reggiano with me. Many thanks to Dr. Wei He for his support, kindness, for telling interesting stories, bringing the gummy bears and good mood to our lab. Special thanks to Sruthi Viswam for her help with the experiment, support and smile. I also thank Dr. Huadong Cheng for introducing me to the experiment and performing the first measurements with me. A big thank you to all the summer and 4th year students: Patrick, Dan, Matt, Huang and Alex for their help and contribution to development of the experiment. During my secondment at PTB I have met many friendly people who I must to thank. First of all, I would like to thank Dr. Sebastian H¨a↵ner for sharing his knowledge and experience on the ultra-stable lasers and providing help for the entire time during my visit. Also, thanks to Dr. Stefan Vogt for his precious time while writing his thesis and Dr. Christian Grebing for his help with the frequency counters. Many thanks to Jacopo, Ali, S¨oren, Andreas and Silvio for being friendly and helpful. Special thanks to Dr. Thomas Legero for teaching me the “dark art” of optical contact bonding. I also thank Dr. Harald Schnatz for his kindness and making me feel welcome in his group. Thank you to the whole Cold Atoms group. You are growing so fast that I am not able to list all your members any more. I should begin with thanking Dr. Anna Kowalczyk for her help and good advise in every topic from the science to booking flights. Special thanks to Clemens, George, Marisa, Alex, Ole, Nadine, Ania and Artur for many good moments together. I also want to thank my peers Harry for his extreme kindness, Andreas for his friendliness and Daniel for his calmness. Many thanks to Dr. Vincent Boyer and Dr. Jon Goldwin for very useful discussions. I would also like to thank Dr. Plamen Petrov for his advise and explanations. Also, thanks to Dr. Jochen Kronj¨agerfor his help with electronics. I must to thank Chris, Plamen, Andy, Harry and Ania for help in exercising brain when playing bridge at the lunch breaks. Huge thank you to the entire team in the mechanical workshop and Stephen Brookes in particular for their fantastic realisation of my mechanical designs. Thank you to Chow and Ian from the stores for always finding the necessary bits and pieces. I also want to thank John Bryant for access to the clean room facilities. I would like to thank all my friends from the Maria Sk lodowska-Curie Initial Training Network: Clemens, George, Saurabh, Thomas, Noaman, Sindhu, Amruta, Marco, Mattia, Nazanin, Niko, Muhammad and Ivan. I am very glad to know you and you made every of our network meetings worthwhile. Thank you to my second supervisor within the network Dr. Christoph Becker for discussions and friendliness. Big thank you to the management and supervisory board of the network and in particular Dr. Thomas Fernholz for very good coordination. Thanks to Romina Davoudi for continuous report reminders and devotion to the network. I need to thank Matthew Gray from ARUP for his help with vibrational measurements in the lab and Mr. Phil Atkins for a discussion on the acoustic isolation. I would also like to thank Dr. David Pabœuf for his help with the VECSEL. I also want to thank Curtis Hentschel and a dear friend, Dr. Chau Meng Huat, for editorial help and proofreading my thesis. I want to thank the entire SOC2 consortium and especially Prof. Stephan Schiller for the coordination and his friendliness. I acknowledge the support from the FP7 Maria Sk lodowska-Curie Actions of the European Commission, via the Initial Training Network QTea (contract-No. MCITN-317485). Part of the research leading to these results has also received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 263500. Finally, I thank all my friends, family, former supervisors and teachers. Foremost, I want to thank my wife Klaudia for her patience, help, support, love and understanding. CONTENTS 1 Introduction 1 1.1 Short history of clocks . 2 1.2 Clock operation . 5 1.3 Optical atomic clocks . 8 1.3.1 Operation . 9 1.4 Importance of atomic clocks . 9 1.5 State-of-the-art . 11 1.5.1 Optical clocks . 11 1.5.2 Ultra-stable lasers . 12 1.6 Thiswork............................................... 13 2 Frequency standards 15 2.1 Frequencies of the clocks . 15 2.2 Precision and accuracy . 17 2.3 Stability................................................ 18 2.3.1 Allan deviation . 18 2.3.2 Types of noise . 19 2.4 Fabry-P´erot interferometer . 24 2.4.1 Sperical resonators . 26 2.4.2 Thermal noise in the optical resonators . 29 2.5 Atomic reference . 31 2.5.1 Microwave atomic reference . 31 2.5.2 Optical atomic reference . 32 2.5.3 Linewidth broadening . 33 2.5.4 Other uncertainty sources . 35 3 Neutral strontium as a frequency standard 37 3.1 Energy structure of strontium . 37 3.2 Clockcycle .............................................. 38 3.3 First stage cooling . 40 3.3.1 Pre-cooling . 41 3.3.2 Dark state repumping . 44 3.3.3 Blue magneto-optical trap . 44 3.4 Second stage cooling . 45 3.4.1 Broadband cooling . 47 3.4.2 Single frequency cooling . 48 3.5 Optical lattice . 49 3.6 Clock interrogation . 49 3.6.1 First ground state detection and clean up . 50 3.6.2 Clock to ground state repumping . 50 3.6.3 Second ground state detection . 51 3.6.4 Background detection . 51 4 Cooling laser systems 52 4.1 Modular amplified tunable diode laser for second-stage cooling . 52 4.1.1 Laser design . 54 4.1.2 Narrowing the 689nm laser . 56 4.1.3 Laser current source . 56 4.2 Semiconductor disk laser for atom cooling . 57 4.2.1 Laser chip . 57 4.2.2 VECSEL construction .