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Advances in Atomic and Optical Clocks: Pushing the Boundaries of...
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Advances in Atomic and Optical Clocks: Pushing the Boundaries of Precision Timing

A special issue of Time and Space (ISSN 2813-9526).

Deadline for manuscript submissions: closed (30 April 2026) | Viewed by 2706

Special Issue Editors

Dr. Thejesh N. Bandi

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Guest Editor
Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487-0324, USA
Interests: GNSS; GPS; optical clocks; metrology; precision timing and navigation
Dr. Judah Levine

E-Mail Website1 Website2
Guest Editor
Department of Physics, University of Colorado, Boulder, CO 80309-0390, USA
Interests: atomic clocks; frequency standards; time measurement

Special Issue Information

Dear Colleagues,

This Special Issue invites submissions on optical clocks, frequency combs, time and frequency synchronization, and optical transmission techniques, particularly those that focus on advances in optical clock technologies and scientific explorations for ground and space.

Atomic microwave clocks, including microwave time and frequency transfer techniques, have revolutionized timekeeping and space navigation. Due to their inherently higher transition frequencies, optical clocks are at least an order of magnitude better in terms of performance compared to the microwave standard. Optical clocks have begun to be considered in practical and portable applications, not just in laboratory settings.

Paper submissions are solicited, covering the basics and advances in optical clocks, including optical lattice clocks, trapped ion optical clocks, and nuclear clocks. Studies on the atomic species used for optical and nuclear clocks, such as strontium, ytterbium, aluminum, calcium, rubidium, iodine, methane, thorium, etc., are of relevant interest. Recent progress in the development, evaluation, and application of high-performance clocks, which continue to drive the frontiers of precision measurement in both fundamental science and applied metrology, are of relevance to this Special Issue. Optics-based time and frequency transfer methods are of significant interest to ensure the synchronization between distant optical clocks.

The contribution of optical clocks to UTC has already begun, and it will increase in the future; this is a requirement to progress towards a redefinition of seconds based on one or several species. These optical devices could potentially improve the stability and accuracy of TAI. Furthermore, optical-clock-driven timescales will be implemented in some laboratories, enhancing the metrological quality of local representations of UTC. Improvements in the reliability and frequent availability of optical clock signals is of relevance for the above-mentioned practical applications.

The key aspects of optical clocks, such as clock laser stabilization methods, vacuum and thermal control advances, compact and portable frequency combs, innovative approaches to improving accuracy, quantitative characterizations of perturbation factors, fractional frequency instability (Allan deviation), long-term performance, low-frequency drift, time deviation, reproducibility, compactness, portability, robustness, reliability, accurate time and frequency transfer, etc., are of relevance to this Special Issue.

Emerging applications of optical lattice/ion/nuclear atomic clocks, such as the search for new physics, gravitational wave detection, and relativistic geodesy, are particularly encouraged.

Dr. Thejesh N. Bandi
Dr. Judah Levine
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 250 words) can be sent to the Editorial Office for assessment.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Time and Space is an international peer-reviewed open access quarterly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1000 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • optical clocks
  • UTC
  • redefinition of time
  • optical cavity
  • frequency combs
  • long-term stability
  • optical time and frequency transfer

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Published Papers (2 papers)

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Research

16 pages, 5669 KB  
Article
Space System Time-Error Buildup and the Allan Deviation
by James Camparo
Time Space 2026, 2(2), 3; https://doi.org/10.3390/timespace2020003 - 25 Mar 2026
Viewed by 442
Abstract
Relating the standard deviation of time-error buildup, σt(T), at some time T after synchronization to a clock’s Allan deviation, σy(T), is problematic for several reasons. Notably, the stochastic integrals of various relevant noise types do not exist in closed [...] Read more.
Relating the standard deviation of time-error buildup, σt(T), at some time T after synchronization to a clock’s Allan deviation, σy(T), is problematic for several reasons. Notably, the stochastic integrals of various relevant noise types do not exist in closed form, and the standard deviation does not necessarily converge for the noise types of relevance for atomic clocks and crystal oscillators. Consequently, as an expedient, one often writes σt(T) = kσy(T)T, where k is a constant that depends on the noise type under consideration, as well as the statistical question of interest. Here, we consider the question of Clock Family Time-Error (CFTE) buildup and compute k for noise processes of relevance to atomic timekeeping in space. One of the interesting results of the present work is the k-value that we obtain for flicker frequency noise, which shows a dependence on the time after synchronization. Full article
(This article belongs to the Special Issue Advances in Atomic and Optical Clocks: Pushing the Boundaries of Precision Timing)
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10 pages, 1147 KB  
Article
Optical Measurements of Binary Buffer-Gas Partial Pressures for Vapor-Cell Atomic Clocks
by Andrew Householder and James Camparo
Time Space 2025, 1(1), 4; https://doi.org/10.3390/timespace1010004 - 24 Oct 2025
Cited by 1 | Viewed by 1322
Abstract
In vapor-cell atomic clocks, a buffer gas is employed to slow the collision rate of atoms with the vapor-cell’s walls, which dephases the atomic coherence and thereby contributes to the 0-0 hyperfine transition’s linewidth. However, the buffer gas also gives rise to a [...] Read more.
In vapor-cell atomic clocks, a buffer gas is employed to slow the collision rate of atoms with the vapor-cell’s walls, which dephases the atomic coherence and thereby contributes to the 0-0 hyperfine transition’s linewidth. However, the buffer gas also gives rise to a temperature-dependent pressure shift in the hyperfine transition, Δνhfs. As a consequence, the clock’s frequency develops a temperature dependence, manifesting as a clock environmental sensitivity, which can degrade the clock’s long-term frequency stability. To mitigate this problem, it is routine to employ a buffer-gas mixture in a vapor cell. With an appropriate choice of buffer gases, d[Δνhfs]/dT = 0 at a vapor temperature Tc, “zeroing out” the clock’s buffer-gas temperature sensitivity. Unfortunately, Tc depends on the exact mix of buffer-gas partial pressures, and if not properly achieved, Tc will be far from the vapor temperature that yields useful atomic clock signals, To. Therefore, understanding buffer-gas partial pressures in sealed vapor cells is crucial for optimizing a vapor cell clock’s performance, yet, to date, there have been no easy means for measuring buffer-gas partial pressures non-destructively in sealed glass vapor cells. Here, we demonstrate an optical technique that can accurately assess partial pressures in binary buffer-gas mixtures. Moreover, this technique is relatively simple and can be easily implemented. Full article
(This article belongs to the Special Issue Advances in Atomic and Optical Clocks: Pushing the Boundaries of Precision Timing)
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