Recently, several reports with a strong focus on compact, nonstationary optical atomic clocks have been published, including accounts of in-field deployment of these devices for demonstrations of chronometric levelling in different types of environments. We review recent progress in this research area, comprising compact and transportable neutral atom and single-ion optical atomic clocks. The identified transportable optical clocks strive for low volume, weight and power consumption while exceeding standard microwave atomic clocks in fractional frequency instability and systematic uncertainty. Some transportable clock projects additionally address requirements for metrology or serve the joint technology development between industrial and academic stakeholders. Based on the reviewed reports on nonstationary optical atomic clocks, we suggest definitions for transportable, portable and mobile optical atomic clocks. We conclude our article with an overview of possible future directions for developments of optical clock technology.
Funding source: UK Quantum Technology Hub Sensors and Timing
Award Identifier / Grant number: EP/T001046/1
Funding source: European Union
Award Identifier / Grant number: 820404
About the authors
Dr. Markus Gellesch is an experimental physicist. Currently, he is working on transportable lattice clocks as part of the EU quantum flagship project iqClock. Markus also has a strong interest in outreach work and has recently collaborated with a local artist to realise a comic booklet on the theme of optical atomic clocks. (https://quantumclocks.the-comic.org/)
Dr. Jonathan Jones is an early stage researcher with a focus on developing cold atoms and optical clock technology for application in the real world. He has published work on ion trapping, optical clocks, quantum metrology and time variation of fundamental constants. Currently he is leading the group’s work on optical cavities in addition to working on transportable lattice clocks as part of the EU quantum flagship programme “iqClock”.
Richard Barron joined the group during his master’s project in 2016 and has carried on through to a PhD, working mostly on the production of atoms to load into a MOT. Currently on the iqClock project.
Dr. Alok Singh is an experimental Optical and Atomic Physicist and he works at precision hyperfine frequency measurements, coherent population trapping/oscillation, Bose-Fermi mixture, dipole-dipole/quadrupole Forster resonance transitions in Rydberg atoms and cold atoms for atomic clock. He has published on the impact of cold atoms, precision frequency measurements and coherent population trapping/oscillation. His recent research focuses on atomic optical lattice clock using Sr atoms with Miniature Optical Lattice Clock project and iqClock-Integrated Quantum Clock project.
Dr. Qiushuo Sun is a knowledge exchange fellow at the Centre for Innovation in Advanced Measurement in Manufacturing (CIAMM) at the University of Birmingham. She joined the group in September 2019 and has been working on the high-finesse optical cavities and the lasers for the optical clocks. She also helps support local SMEs in Research and Development in state–of–the–art optical technologies and instruments.
Professor Kai Bongs is a Principle Investigator at the UK Quantum Technology Hub Sensors and Timing, where he helps to drive the translation of gravity sensors and ultraprecise clocks into technology and applications across a diverse number of different sectors. Professor Bongs is also College Director of Innovation at the University of Birmingham. He is a Royal Society Wolfson Research Merit Fellow, as well as a Fellow of the Institute of Physics and the Institution of Engineering and Technology.
Dr. Yeshpal Singh is a senior lecturer at the University of Birmingham and leads the cold atoms based Sr group. His main research interests are translation of quantum concepts to quantum technology, quantum metrology and precision measurements, Bose–Einstein condensates in microgravity, ultracold Bosons, degenerate Fermi gases and quantum phase transitions, and ultracold molecules and quantum state engineering.
The authors are grateful for receiving funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 820404 (iqClock project) and acknowledge the fruitful joint work with partners from the iqClock Consortium. We further recognise sponsorship from the UK Quantum Technology Hub Sensors and Timing (grant EP/T001046/1).
Author contribution: MG and YS wrote the manuscript with contributions from JJ, AS, QS, RB, and KB. All authors are involved with the iqClock project for realising a transportable optical lattice clock demonstrator. All authors have reviewed and approved the manuscript.
Research funding: The authors are grateful for receiving funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 820404 (iqClock project) and acknowledge the fruitful joint work with partners from the iqClock Consortium. We further recognise sponsorship from the Quantum Hub for Sensors and Metrology (EPSRC funding within grant EP/M013294/1).
Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
 B. L. Schmittberger and D. R. Scherer, “A review of contemporary atomic frequency standards,” arXiv preprint, arXiv:2004.09987, 2020. Available at: https://arxiv.org/pdf/2004.09987.pdf.Search in Google Scholar
 T. Fortier and E. Baumann, “20 years of developments in optical frequency comb technology and applications,” Commun. Phys., vol. 2, p. 153, 2019, https://doi.org/10.1038/s42005-019-0249-y.Search in Google Scholar
 E. Benkler, B. Lipphardt, T. Puppe, et al., “End-to-end topology for fiber comb based optical frequency transfer at the 10−21 level,” Opt. Exp., vol. 27, p. 36886, 2019, https://doi.org/10.1364/oe.27.036886.Search in Google Scholar
 S. M. Brewer, J.-S. Chen, A. M. Hankin, et al., “27Al+ quantum-logic clock with a systematic uncertainty below 10−18,” Phys. Rev. Lett., vol. 123, p. 033201, 2019, https://doi.org/10.1103/physrevlett.123.033201.Search in Google Scholar
 C. W. Chou, D. B. Hume, J. C. J. Koelemeij, D. J. Wineland, and T. Rosenband, “Frequency comparison of two high-accuracy Al+ optical clocks,” Phys. Rev. Lett., vol. 104, p. 070802, 2010, https://doi.org/10.1103/physrevlett.104.070802.Search in Google Scholar
 P. Zhang, J. Cao, H.-L. Shu, et al., “Evaluation of blackbody radiation shift with temperature-associated fractional uncertainty at 10−18 level for 40Ca+ ion optical clock,” J. Phys. B: Mol. Opt. Phys., vol. 50, p. 015002, 2017, https://doi.org/10.1088/1361-6455/50/1/015002.Search in Google Scholar
 M. Chwalla, J. Benhelm, K. Kim, et al., “Absolute frequency measurement of the 40Ca+ 4s2S1/2 – 3d2 D5/2 clock transition,” Phys. Rev. Lett., vol. 102, p. 023002, 2009, https://doi.org/10.1103/physrevlett.102.023002.Search in Google Scholar
 H. S. Margolis, G. P. Barwood, G. Huang, et al., “Hertz-level measurement of the optical clock frequency in a single 88Sr+ ion,” Science, vol. 306, p. 1355, 2004, https://doi.org/10.1126/science.1105497.Search in Google Scholar
 N. Huntermann, C. Sanner, B. Lipphardt, and E. Peik, “Single-ion atomic clock with 3 × 10−18 systematic uncertainty,” Phys. Rev. Lett., vol. 116, p. 063001, 2016. https://doi.org/10.1103/PhysRevLett.116.063001.Search in Google Scholar
 R. M. Godun, P. B. R. Nisbet-Jones, J. M. Jones, et al., “Frequency ratio of two optical clock transitions in 171Yb+ and constraints on the time variation of fundamental constants,” Phys. Rev. Lett., vol. 113, p. 210801, 2014, https://doi.org/10.1103/physrevlett.113.210801.Search in Google Scholar
 S. Falke, N. Lemke, C. Grebing, et al., A strontium lattice clock with 3 × 10−17inaccuracy and its frequency, N. J. Phys., vol. 16, p. 073023, 2014, https://doi.org/10.1088/1367-2630/16/7/073023.Search in Google Scholar
 P. G. Westergaard, J. Lodewyck, L. Lorini, et al., “Lattice-induced frequency shifts in Sr optical lattice clocks at the 10−17 level,” Phys. Rev. Lett., vol. 106, p. 210801, 2011, https://doi.org/10.1103/physrevlett.106.210801.Search in Google Scholar
 T. L. Nicholson, M. J. Martin, J. R. Williams, et al., “Comparison of two independent Sr optical clocks with 1 × 10−17 stability at 103s,” Phys. Rev. Lett, vol. 109, p. 230801, 2012, https://doi.org/10.1103/physrevlett.109.073003.Search in Google Scholar
 Z. W. Barber, C. W. Hoyt, C. W. Oates, et al., “Direct excitation of the forbidden clock transition in neutral Yb 174 atoms confined to an optical lattice,” Phys. Rev. Lett., vol. 96, p. 083002, 2006, https://doi.org/10.1103/physrevlett.96.083002.Search in Google Scholar
 W. M. Itano, J. C. Bergquist, J. J. Bollinger, et al., “Quantum projection noise: population fluctuations in two-level systems,” Phys. Rev. A, vol. 47, p. 3554, 1993, https://doi.org/10.1103/physreva.47.3554.Search in Google Scholar
 E. Oelke, R. B. Hutson, C. J. Kennedy, et al., “Demonstration of 4.8 × 10−17 stability at 1 s for two independent optical clocks,” Nat. Photon., vol. 13, p. 714, 2019, https://doi.org/10.1038/s41566-019-0493-4.Search in Google Scholar
 A. Quessada, R. P. Kovacich, I. Courtillot, et al., “The Dick effect for an optical frequency standard,” J. Opt. B: Quantum Semiclass. Opt., vol. 5, p. S150, 2003, https://doi.org/10.1088/1464-4266/5/2/373.Search in Google Scholar
 J. Lodewyck, P. G. Westergaard, and P. Lemonde, “Nondestructive measurement of the transition probability in a Sr optical lattice clock,” Phys. Rev. A, vol. 79, p. 061401, 2009, https://doi.org/10.1103/physreva.79.061401.Search in Google Scholar
 M. Vermeer, Chronometric Levelling, Helsinki: Geodeettinen Laitos Geodetiska Institutet, 1983.Search in Google Scholar
 N. K. Pavlis and M. A. Weiss, “The relativistic redshift with 3 × 10−17 uncertainty at NIST, Boulder, Colorado, USA,” Metrologia, vol. 40, p. 66, 2003, https://doi.org/10.1088/0026-1394/40/2/311.Search in Google Scholar
 W. F. McGrew, X. Zhang, R. J. Fasano, et al., “Atomic clock performance enabling geodesy below the centimetre level,” Nature, vol. 564, p. 87, 2018, https://doi.org/10.1038/s41586-018-0738-2.Search in Google Scholar
 J. Grotti, S. Koller, S. Vogt, et al., “Geodesy and metrology with a transportable optical clock,” Nat. Phys., vol. 14, p. 437, 2018, https://doi.org/10.1038/s41567-017-0042-3.Search in Google Scholar
 M. Takamoto, I. Ushijima, N. Ohmae, et al., “Test of general relativity by a pair of transportable optical lattice clocks,” Nat. Photonics, vol. 14, p. 411–415, 2020, https://doi.org/10.1038/s41566-020-0619-8.Search in Google Scholar
 I. Ushijima, M. Takamoto, M. Das, T. Ohkubo, and H. Katori, “Cryogenic optical lattice clocks,” Nat. Photon., vol. 9, p. 185, 2015, https://doi.org/10.1038/nphoton.2015.5.Search in Google Scholar
 T. Nicholson, S. Campbell, R. Hutson, et al., “Systematic evaluation of an atomic clock at 2 x 10-18 total uncertainty,” Nat. Com., vol. 6, p. 1, 2015, https://doi.org/10.1038/ncomms7896.Search in Google Scholar
 S. Vogt, C. Lisdat, T. Legero, et al., “Demonstration of a transportable 1 Hz-linewidth laser,” Appl. Phys. B, vol. 104, p. 741, 2011, https://doi.org/10.1007/s00340-011-4652-7.Search in Google Scholar
 Q. F. Chen, A. Nevsky, M. Cardace, et al., “A compact, robust, and transportable ultra-stable laser with a fractional frequency instability of 1 × 10−15,” Rev. Sci. Instrum., vol. 85, p. 113107, 2014, https://doi.org/10.1063/1.4898334.Search in Google Scholar
 D. Świerad, S. Häfner, S. Vogt, et al., “Ultra-stable clock laser system development towards space applications,” Sci. Rep., vol. 6, p. 33973, 2016, https://doi.org/10.1038/srep33973.Search in Google Scholar
 M. G. Tarallo, N. Poli, M. Schioppo, D. Sutyrin, and G. M. Tino, “A high-stability semiconductor laser system for a 88Sr-based optical lattice clock,” Appl. Phys. B, vol. 103, p. 17, 2011, https://doi.org/10.1007/s00340-010-4232-2.Search in Google Scholar
 A. Nevsky, S. Alighanbari, Q.-F. Chen, et al., “Robust frequency stabilization of multiple spectroscopy lasers with large and tunable offset frequencies,” Opt. Lett., vol. 38, p. 4903, 2013, https://doi.org/10.1364/ol.38.004903.Search in Google Scholar
 S. Mulholland, H. A. Klein, G. P. Barwood, et al., “Compact laser system for a laser-cooled ytterbium ion microwave frequency standard,” Rev. Sci. Instrum., vol. 90, 2019, Art no. 033105, https://doi.org/10.1063/1.5082703.Search in Google Scholar
 R. Schwarz, S. Dörscher, A. Al-Masoudi, et al., “A compact and robust cooling laser system for an optical strontium lattice clock,” Rev. Sci. Instrum., vol. 90, 2019, Art no. 023109, https://doi.org/10.1063/1.5063552.Search in Google Scholar
 C. Vishwakarma, J. Mangaonkar, K. Patel, et al., “A simple atomic beam oven with a metal thermal break,” Rev. Sci. Instrum., vol. 90, 2019, Art no. 053106, https://doi.org/10.1063/1.5067306.Search in Google Scholar
 W. Bowden, R. Hobson, I. R. Hill, et al., “A pyramid MOT with integrated optical cavities as a cold atom platform for an optical lattice clock,” Sci. Rep., vol. 9, p. 11704, 2019, https://doi.org/10.1038/s41598-019-48168-3.Search in Google Scholar
 I. A. Semerikov, I. V. Zalivako, A. S. Borisenko, et al., “Three-dimensional Paul trap with high secular frequency for compact optical clock,” Bull. Lebedev Phys. Inst., vol. 46, p. 297, 2019, https://doi.org/10.3103/s1068335619090070.Search in Google Scholar
 S. Schiller, A. Görlitz, A. Nevsky, et al., “The space optical clocks project: Development of high-performance transportable and breadboard optical clocks and advanced subsystems,” IEEE Europ. Freq. Time Forum (EFTF), p. 412, 2012, https://doi.org/10.1109/EFTF.2012.6502414.Search in Google Scholar
 J. Guéna, M. Abgrall, D. Rovera, et al., “Progress in atomic fountains at LNE-SYRTE,” IEEE Trans. Ultrason. Ferroelectr. Freq. Contr., vol. 59, p. 391, 2012, https://doi.org/10.1109/tuffc.2012.2208.Search in Google Scholar
 S. Weyers, V. Gerginov, M Kazda, et al., “Advances in the accuracy, stability, and reliability of the PTB primary fountain clocks,” Metrologia, vol. 55, p. 789, 2018, https://doi.org/10.1088/1681-7575/aae008.Search in Google Scholar
 J. Cao, P. Zhang, J. Shang, et al., “A compact, transportable single-ion optical clock with 7.8 × 10−17 systematic uncertainty,” Appl. Phys. B, vol. 123, p. 112, 2017, https://doi.org/10.1007/s00340-017-6671-5.Search in Google Scholar
 S. Wang, J. Cao, J. Yuan, et al., “Integrated multiple wavelength stabilization on a multi-channel cavity for a transportable optical clock,” Opt. Exp., vol. 28, p. 11852, 2020, https://doi.org/10.1364/oe.383115.Search in Google Scholar
 X. Xue, Z. Zhang, X. Zhang, et al., “A Compact Design of a Transportable Calcium Optical Frequency Standard,” IEEE Int. Freq. Symp. (IFCS), p. 1, 2018, https://doi.org/10.1109/FCS.2018.8597454.Search in Google Scholar
 T. A. Ely, J. Seubert, J. Prestage, et al., 2019AAS/AIAA Astrodynamics Specialist Conf., AIAA; 2019.Search in Google Scholar
 T. A. Ely, D. Murphy, J. Seubert, J. Bell, and D. Kuang, AAS/AIAA Space Flight Mechanics Meeting, AIAA; 2014.Search in Google Scholar
 R. L. Tjoelker, J. D. Prestage, E. A. Burt, et al., “Mercury ion clock for a NASA technology demonstration mission,” IEEE Trans. Ultrason. Ferroelectr. Freq. Contr., vol. 63, p. 1034, 2016, https://doi.org/10.1109/tuffc.2016.2543738.Search in Google Scholar
 Available at: http://www.exphy.uni-duesseldorf.de/optical_clock/soc2/index.php [accessed: May 30, 2020].Search in Google Scholar
 N. Poli, M. Schioppo, S. Vogt, et al., “A transportable strontium optical lattice clock,” Appl. Phys. B, vol. 117, p. 1107, 2014, https://doi.org/10.1007/s00340-014-5932-9.Search in Google Scholar
 G. Mura, T. Franzen, C. A. Jaoudeh, et al., “A transportable optical lattice clock using 171Yb,” in IEEE Joint European Frequency Time Forum & International Frequency Control Symposium. (EFTF/IFC), Prague, Czech Republic, IEEE, 2013, p. 376. https://doi.org/10.1109/EFTF-IFC.2013.6702292.Search in Google Scholar
 K. Bongs, Y. Singh, L. Smith, et al., “Development of a strontium optical lattice clock for the SOC mission on the ISS,” Compt. Ren. Phys., vol. 16, p. 553, 2015, https://doi.org/10.1016/j.crhy.2015.03.009.Search in Google Scholar
 S. Origlia, M. S. Pramod, S. Schiller, et al., “Towards an optical clock for space: compact, high-performance optical lattice clock based on bosonic atoms,” Phys. Rev. A, vol. 98, p. 053443, 2018, https://doi.org/10.1103/physreva.98.053443.Search in Google Scholar
 L. L. Smith, “A transportable strontium optical lattice clock towards space,” PhD Thesis, University of Birmingham, England, 2016.Search in Google Scholar
 M. Schioppo, G. M. Tino, N. Poli, et al., “Development of a transportable laser cooled strontium source for future applications in space,” IEEE Europ. Freq. Time Forum (EFTF), vol. 1, 2010, https://doi.org/10.1109/EFTF.2010.6533689.Search in Google Scholar
 M. Schioppo, N. Poli, M. Prevedelli, et al., “A compact and efficient strontium oven for laser-cooling experiments,” Rev. Sci. Instrum., vol. 83, p. 103101, 2012, https://doi.org/10.1063/1.4756936.Search in Google Scholar
 S. Vogt, S. Häfner, J. Grotti, et al., “A transportable optical lattice clock,” J. Phys.: Conf. Ser., vol. 723, p. 012020, 2016, https://doi.org/10.1088/1742-6596/723/1/012020.Search in Google Scholar
 S. B. Koller, J. Grotti, S. Vogt, et al., “Transportable optical lattice clock with 7 × 10−17 uncertainty,” Phys. Rev. Lett., vol. 113, 2017, Art no. 073601.10.1103/PhysRevLett.118.073601Search in Google Scholar PubMed
 S. Häfner, “Ultra-stabile Lasersysteme für Weltraum- und Bodenanwendungen,” PhD Thesis, Universität Hannover, Germany, 2015.Search in Google Scholar
 U. Sterr, “Frequency stabilization device,” German patent DE102011015489B3, 2012.Search in Google Scholar
 C. Lacroûte, M. Souidi, P.-Y. Bourgeois, et al., “Compact Yb+ optical atomic clock project: design principle and current status,” J. Phys.: Conf. Ser., vol. 723, p. 012025, 2016, https://doi.org/10.1088/1742-6596/723/1/012025.Search in Google Scholar
 W. Brand, R. Fasano, R. Fox, et al., “Portable Yb Optical Lattice Clock: Towards Precision Measurement Outside the Lab,” APS 2019 E01-046, Available at: http://meetings.aps.org/Meeting/DAMOP19/Session/E01.46, 2019.Search in Google Scholar
 R. Fasano, W. Brand, R. Fox, et al., “NIST Yb portable lattice clock: updates and analysis,” IEEE Joint Conf. IFCS-ISAF 2020, IEEE; 2020.Search in Google Scholar
 K. Beloy, N. Hinkley, N. B. Phillips, et al., “Atomic clock with 1 × 10−18 room-temperature blackbody stark uncertainty,” Phys. Rev. Lett., vol. 113, p. 260801, 2014, https://doi.org/10.1103/physrevlett.113.260801.Search in Google Scholar
 International Conference on Quantum Metrology and Sensing 9.-13. December 2019, Paris, France, Abdel Hafiz Moustafa, Arar Bassem, Bergner Klaus, et al. Opticlock: transportable and easy-to-operate optical single-ion clock. Paris: IQuMS; December, 2019.Search in Google Scholar
 K. W. Martin, G. Phelps, N. D. Lemke, et al., “Compact optical atomic clock based on a two-photon transition in rubidium,” Phys. Rev. Appl., vol. 9, p. 014019, 2018, https://doi.org/10.1103/physrevapplied.9.014019.Search in Google Scholar
 Z. L. Newman, V. Maurice, T. Drake, et al., “Architecture for the photonic integration of an optical atomic clock,” Optica, vol. 6, p. 680, 2019, https://doi.org/10.1364/optica.6.000680.Search in Google Scholar
 Available at: https://www.microsemi.com/product-directory/cesium-frequency-references/4115-5071a-cesium-primary-frequency-standard#resources [accessed: May 30, 2020].Search in Google Scholar
 C. Nshii, M. Vangeleyn, J. Cotter, et al., “A surface-patterned chip as a strong source of ultracold atoms for quantum technologies,” Nat. Nanotechnol., vol. 8, p. 321, 2013, https://doi.org/10.1038/nnano.2013.47.Search in Google Scholar
 P. Elgee, A. Sitaram, D. Barker, et al., “Confinement of an alkaline-earth element in a grating magneto-optical trap,” arXiv preprint arXiv:2006.05369, 2020 Available at: https://arxiv.org/abs/2006.05369.10.1063/5.0019551Search in Google Scholar
 I. S. Madjarov, A. Cooper, A. L. Shaw, et al., “An atomic-array optical clock with single-atom readout,” Phys. Rev. X, vol. 9, Art no. 041052, 2019. https://doi.org/10.1103/PhysRevX.9.041052.Search in Google Scholar
 M. A. Norcia, A. W. Young, W. J. Eckner, et al., “Seconds-scale coherence on an optical clock transition in a tweezer array,” Science, vol. 366, p. 93, 2019, https://doi.org/10.1126/science.aay0644.Search in Google Scholar
 S. Hannig, L. Pelzer, N. Scharnhorst, et al., “Towards a transportable aluminium ion quantum logic optical clock,” Rev. Sci. Instrum., vol. 90, Art no. 053204, 2019, https://doi.org/10.1063/1.5090583.Search in Google Scholar
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