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Symposium Web Page: https://www.qdmlab.com/9fsm2023
History of the Symposium: https://www.qdmlab.com/9fsm2023-history
*REGISTRATIONS CLOSED on the 5th of SEPTEMBER*
The symposium is intended to serve as an international discussion forum on precision frequency standards throughout the electromagnetic spectrum, and associated precision and quantum metrology. It focuses on the fundamental scientific aspects of the latest ideas, results, and applications in relation to these frequency standards and measurement techniques. More than Seven years after the last symposium, significant progress has occurred across all associated fields.
Following tradition and by design, the conference will consist of a single series of invited talks (~50) and invited poster presentations (>100).
The book of abstracts is now available on the website.
https://indico.cern.ch/event/1226483/book-of-abstracts.pdf
All participants are requested to supply a paper for inclusion in the conference proceedings.
**Important dates:**
Topics Include
Code of Conduct
The Symposium is dedicated to providing a discrimination- and harassment-free experience for all attendees. All speakers and participants are expected to comply with the EQUS Code of Conduct.
CANCELLATION:
All refund requests must be in writing by mail to the Conference Secretary and Chair as soon as possible. The Conference committee reserves the right to refuse reimbursement of part or all of the fee in the case of late cancellation. However, each case of cancellation would be considered individually.
In this talk, I will talk about the many different manifestations of neutron stars in our galaxy and the greater cosmos that emit everything from gamma rays to radio waves in either regular pulses or once-off "Fast Radio Bursts".
Normal pulsars die after typically just a few Myr, but some are fortunate to be recycled by material fed to them from a stellar companion. These recycled pulsars offer some amazing scientific opportunities, for both tests of General Relativity and the detection of a stochastic background of gravitational waves from supermassive black holes using an array of millisecond pulsars, nature’s most accurate naturally occurring clocks.
The paper highlights the importance of the time unit definition,by means of the atomic Cs frequency standard, in the definition of the base units of the International System of units (SI).
Masses of light nuclei provide a network of essential parameters used for the fundamental nature description. For example, the mass difference of tritium and helium-3 allows for an independent check of the limit on the electron-antineutrino mass.
The most precise mass measurements of the lightest nuclei, including helium-3, revealed considerable inconsistencies between the values reported by different experiments. In order to provide an independent cross-check, we have performed the most precise measurements of the atomic masses of the proton, the deuteron and the HD+ molecular ion using the multi-Penning trap mass spectrometer LIONTRAP.
PENTATRAP allows for ultra-precise mass measurements on highly charged heavy ions with relative uncertainties in the low 1E-12 region. Among others the excitation energies of low-lying metastable electronic states could be measured by their mass differences to the ground states. Thus, possible new clock transitions in the extreme ultraviolett (XUV) regime could be detected.
The most recent intriguing results by LIONTRAP and PENTATRAP as well as possible applications of these ultra-precise mass data will be presented.
Phonons, quanta of Acoustic vibration, have much in common with photons, elementary excitation of Electro-Magnetic fields. Despite the fact that photonic devices have dominated physics and engineering for at least a century, and the acoustical systems have almost been forgotten. One of the main reasons for that is much lower energy losses exhibited by well designed photonic systems, e.g. optical cavities. The situation started to change over the last decade when practical implementation of extremely low loss resonant acoustical systems at low temperatures was demonstrated. This was achieved due to exceptional engineering of phonon trapping Quartz Bulk Acoustic Wave (BAW) devices that have much in common with optical Fabry-Perot cavities. Initially used in frequency control devices, BAW resonators demonstrated that at low temperatures their performance is only limited by fundamental phonon-phonon interaction as well as two level systems. With Quality factors well exceeding $10^9$ in many modes, BAW cavities often outperform many photonic counterparts and open new possibilities in physics and engineering. Started from a systematic measurements of losses in a solid state, this research lead to a discovery of a physical platform that can answer some fundamental questions about our Universe such as validity of fundamental symmetries postulated in all current theories, existence of Dark Matter, Quantum Gravity, variation of fundamental constants and primordial gravitational waves. More over, many research groups started to use such acoustic systems as a building block of Quantum Hybrid systems, a future base for quantum computing, measurement and control.
Over the past 20 years optical frequency combs [1], with atomic clocks [2], have been a powerful and enabling technology in the context of time and frequency measurement [1,2]. Impressively, optical atomic clocks have yielded an 8 order of magnitude improvement in accuracy in the past 30 years. These improvements are fueling a push toward redefinition of the SI second to optical atomic references [3], as well as application of atomic clocks to tests of fundamental physics [4] and as relativistic gravitational sensors [5-6]. Unfortunately, the long measurement times needed to average down clock quantum projection noise and local oscillator noise to reach measurement stabilities at and beyond the 10-18 level, limit the feasibility of next-generation applications.
I will present the improved instability results for an inter-species optical atomic clock comparison us-ing a differential measurement technique, Figure 1. In this technique, the single ion 27Al+ clock near and the 171Yb lattice clock shared a common local oscillator using the phase coherent wavelength con-version with an optical frequency comb. This technique enabled nearly a factor of 10 improvement in 1-s measurement resolution and a 100-time improvement in averaging time to reach a measurement instability of 10-18. Improvements in the measurement stability was achieved via a minimization of laser noise aliasing, and via improvement in the 27Al+ clock quantum projection noise by increasing its probe time by mitigating laser-atomic decoherence [7].
References
[1] Fortier, T.M. and Baumann, E., “20 years of developments in optical frequency comb technology and applica-tions,” Communications Physics 2, Article number 153 (2019).
[2] Ludlow, A. D., Boyd, M. M., Ye, J., Peik, E. & Schmidt, P. O., “Optical atomic clocks,” Rev. Mod. Phys. 87, 637–701 (2015).
[3] Riehle, F., Gill, P., Arias, F. & Robertsson, L. “The CIPM list of recommended frequency standard values: guidelines and procedures. Metrologia,” 55, 188–200 (2018).
[4] BACON collaboration: “Frequency ratio measurements at 18-digit accuracy using an optical clock network,” Na-ture 591 (2021).
[5] Rosenband, T. et al. “Frequency ratio of Al+ and Hg+ single-ion optical clocks; metrology at the 17th decimal place,” Science 319, 1808–1812 (2008).
[6] Mehlstäubler, T. E., Grosche, G., Lisdat, C., Schmidt, P. O. & Denker, H. Atomic clocks for geodesy. Rep. Prog. Phys. 81, 064401 (2018).
[7] Kim, M et al., “Improved interspecies optical clock comparisons through differential spectroscopy,” Nat. Phys. 19 (2023)
Optical atomic clocks are the most precise and accurate measurement devices ever constructed, reaching fractional systematic uncertainties below one part in $10^{-18}$ [1]. Their exceptional performance opens up a wide range of applications in fundamental science and technology. The extreme properties of highly charged ions (HCI) make them highly sensitive probes for tests of fundamental physical theories [2, 3]. Furthermore, these properties make them significantly less sensitive to some of the leading systematic perturbations that affect state-of-the-art optical clocks, making them exciting candidates for next-generation clocks [4, 2]. The technical challenges that hindered the development of such clocks have now all been overcome, starting with their extraction from a hot plasma and sympathetic cooling in a linear Paul trap [5], readout of their internal state via quantum logic spectroscopy [6], and finally the preparation of the HCI in the ground state of motion of the trap [7], which allows levels of measurement accuracy to be reached that were previously limited to singly-charged and neutral atoms. Here, we present the first operation of an atomic clock based on an HCI (Ar$^{13+}$ in our case) and a full evaluation of systematic frequency shifts [8]. The achieved uncertainty is almost eight orders of magnitude lower than any previous frequency measurements using HCI. Measurements of some key atomic parameters confirm the theoretical predictions of the favorable properties of HCIs for use in clocks. The comparison to the $^{171}$Yb$^+$ E3 optical clock [9] places the frequency of this transition among the most accurately measured of all time. Furthermore, by comparing the isotope shift between $^{36}$Ar$^{13+}$ and $^{40}$Ar$^{13+}$ to improved atomic structure calculations, we were able for the first time to resolve the largely unexplored QED nuclear recoil effects. Finally, prospects for 5th force tests based on isotope shift spectroscopy of Ca$^+$/Ca$^{14+}$ isotopes and the high-sensitivity search for a variation of the fine-structure constant using HCI will be presented. This demonstrates the suitability of HCI as references for high-accuracy optical clocks and to probe for physics beyond the standard model.
References
[1] Brewer, S. M. et al., Phys. Rev. Lett. 123, 033201 (2019).
[2] Kozlov, M. G. et al., Rev. Mod. Phys. 90, 045005 (2018).
[3] Safronova, M. S. et al., Rev. Mod. Phys. 90, 025008 (2018).
[4] Schiller, S., Phys. Rev. Lett. 98, 180801 (2007).
[5] Schmöger, L. et al., Science 347, 1233–1236 (2015).
[6] Micke, P. et al., Nature 578, 60–65 (2020).
[7] King, S. A. et al., Phys. Rev. X 11, 041049 (2021).
[8] King, S. A. et al., Nature 611, 43–47 (2022).
[9] Lange, R. et al., Phys. Rev. Lett. 126, 011102 (2021).
We discuss some of the many advantages that lutetium offers as an optical frequency reference. We illustrate the ease at which a comparison at the level of 1e-18 can be achieved and we show how the use of two available clock transitions can be used to verify clock performance between two systems.
In 2012, we proposed multi-ion spectroscopy to improve the stability of optical ion clocks which is fundamentally limited by the quantum projection noise of the single ion. Multi-ion clocks will not only improve the stability by exploiting the higher signal to noise of multiple ions or their uncertainty by allowing for sympathetic cooling of clock ions using a separate ion species, but will be the basis for future entangled clocks and cascaded clocks. For the multi-ion approach we have developed and qualified scalable high-precision ion traps, which are already in use in several experiments. A challenge is the high level of control of systematic shifts when scaling up a single trapped ion to a complex many-body system. I will discuss our results in characterizing the shifts in multiple trapped ions and from lessons learned the potential of multi-ion spectroscopy. The multi-ion clock is operated in a recent dedicated experiment, where 115In+ ions are sympathetically cooled by 172Yb+ ions. Here, I will report on the status of clock operation and international clock comparisons. Last but not least, I will briefly discuss new limits we obtained in our work on an improved test of local Lorentz invariance using 172Yb+ ions and the search for new physics using the even Yb+ isotopes.
We will report on absolute E3 frequency measurements and E3/E2 optical frequency ratio measurements in $^{171}$Yb$^+$, local $^{171}$Yb$^+$/ $^{87}$Sr clock frequency ratios, related uncertainty budgets, and improvements in automation and robust operation of the $^{171}$Yb$^+$ clock system at NPL. We will also show how these measurement results have been used to constrain temporal variation of the fine structure constant and exclude regions of parameter space in theories beyond the Standard Model, such as those which include ultralight scalar dark matter.
Over the past decades, atoms are trapped and laser cooled to near zero temperature, minimizing the motional effects in spectroscopy. Internal states of atoms can be coherently manipulated and pre-pared in pure quantum states, including entangled states that are impactful in quantum information processing, sensing, and metrology. This talk will describe the effort of the Ion Storage group at NIST in bringing molecular ions to equal footings with atoms in terms of state control and spectro-scopic precision. The project builds on laser cooling and trapping techniques, frequency comb tech-nology, and quantum-logic spectroscopy protocols nowadays routinely employed in cold-atom re-search and trapped ion optical clocks. That enables demonstrations, on single molecular ions, of co-herent quantum state manipulation [1], nondestructive state detection [1-3], rotational [4, 5] and vibrational [6] spectroscopy with better than part-per-trillion resolution, and quantum entanglement [7]. The group is exploring new opportunities in physics and chemistry offered by the richer struc-ture and broader species selections in molecules.
*In collaboration with Alejandra Collopy, Yiheng Lin, Christoph Kurz, Michael E. Harding, Philipp N. Plessow, Tara Fortier, Scott Diddams, Yu Liu, Zhimin Liu, Julian Schmidt, Dalton Chaffee, Ba-ruch Margulis, David R. Leibrandt, and Dietrich Leibfried
Here our progress on the Ca+ ion optical clocks for the last few years will be reported, including both the laboratory clocks and the transportable clock.
First of all, the clock stability is greatly improved, with long term stability reaches the E-18 level; recently with a low E-16 level stability clock laser, the clock stability has been improved to ~ 1E-15/√τ, about another factor of 2 improvement and about an order of magnitude smaller than that in 2016. Secondly, a cryogenic Ca+ clock at the liquid nitrogen environment is built, with the blackbody radiation (BBR) shift uncertainty greatly suppressed, and improvements made with other systematic uncertainties, the overall systematic uncertainty of the clock is evaluated as 3.0E-18. Thirdly, the Ca+ clock at room temperature is also improved. The systematic uncertainty of the room temperature clock was at the E-17 level, limited by the BBR shift uncertainty. To lower the BBR shift uncertainty, the precise measurement of the differential scalar polarizability through of the clock transition is taken, and the active liquid-cooling scheme is adopted, combined with the precise temperature measurement with 13 temperature sensors. The BBR field temperature uncertainty is then evaluated as 0.4 K, corresponding to a BBR shift uncertainty of 4.6E-18, then the overall systematic uncertainty of the room temperature clock is evaluated as 4.9E-18. Clock frequency comparison between the room temperature clock and the cryogenic clock is taken for testing the systematic shift uncertainty evaluations, and the two clocks show an agreement at the E-18 level after the systematic shift corrections: With the systematic shift corrections, the frequency difference between the two clocks is measured as 1.8(7.5)E-18, the overall uncertainty includes a statistic uncertainty of 4.9E-18 and a systematic uncertainty of 5.7E-18.
Besides the laboratory clocks mentioned above, a transportable Ca+ ion clock is also built, with an uncertainty of 1.3E-17 and an uptime rate of > 75%. With the comparison between the transportable clock and the laboratory clock, a demonstration of geopotential measurement with clocks has been made. The clock is then transported for > 1200 km to another institute, the absolute frequency measurement is made there with an uncertainty of 5.6E-16, about 5 times smaller than our previous result. Recently, a new round, 35-day-long absolute frequency measurement is taken, with improvements made such as the increase of the uptime rate to 91.3 %, the reduced statistical uncertainty of the comparison between the optical clock and hydrogen maser, and the use of longer measurement times to reduce the uncertainty of the frequency traceability link. The uncertainty of the absolute frequency measurement is further reduced to 3.2E-16, which is another factor of 1.7 improvement.
We describe recent techniques, strategies, and efforts toward realizing next-generation optical clock uncertainty and stability with the NIST Yb optical lattice clock.
This talk will discuss a vibrational molecular lattice clock based on ultracold strontium dimers, its systematic evaluation at the 14th decimal digit, the current limitations, and paths forward.
We will present the progress of our Hg optical lattice clock. This work is motivated, in particular, by the low sensitivity of Hg to blackbody radiation and stray electric fields and by the possibility to use ratio between Hg and other optical transitions for fundamental physics and metrology. We re-port on our work done with the 199Hg fermionic isotope to improve uncertainty, stability and relia-bility [1][2], noting that managing the required deep UV wavelength remains a significant experi-mental challenge. We will also report on a series of frequency ratio measurements with 87Sr and other species, as allowed by the optical fiber network in Europe, including the REFIMEVE infra-structure in France [3]. The excited clock state 3P0 in 199Hg has a rather short spontaneous decay time compared to other species, which can become a limit to exploit the most recent ultra-stable lasers. Bosonic isotopes can circumvent this limit. We will describe our on-going work to develop a Hg clock based on bosonic isotopes and making use of the quenching method [4] and of hyper-Ramsey interrogation [5][6].
We are also developing a non-destructive detection scheme adapted to Sr optical lattice clocks. The scheme is based on a differential heterodyne measurement of the dispersive properties the atomic sample, enhanced by a high finesse cavity. A first implementation demonstrated how to implement the scheme in a technically robust manner and clarified the path to achieve the quantum non-destructive regime [7]. We will report on our new implementation, on its characterization it terms of quantum noise and destructivity, and on its practical potential to improve optical lattice clocks [8].
Finally, we will report on our investigation of laser stabilization using spectral hole burning in rare-earth doped crystals at ultra-low temperature. We have developed agile heterodyne dispersive prob-ing methods based digital signal generation, modulation and demodulation that gives low detection noise, slow fading of the spectral hole and immunity to perturbations present in the cryogenic envi-ronment [9]. We will report on our investigation of properties of spectral holes at 1.4 K [10] and at cryogenic dilution temperature of a few 100 mK, at which favorable conditions to realize laser sta-bilization at 10-17 or better.
These advances, individually or combined for example with spectral purity transfer with combs and composite clock approaches, shall bring significant progress in clock stability and accuracy.
Precise quantum state engineering, many-body physics, and innovative laser technology are revolutionizing the performance of atomic clocks and metrology, providing opportunities to explore emerging phenomena and probe fundamental physics. A Wannier-Stark optical lattice configuration highlights such an example. Atom-light and atom-atom interactions in the shallow optical lattice are precisely controlled and determined to the $10^{-19}$ level, representing key steps toward achieving inaccuracy below $10^{-18}$ for an optical lattice clock. On the front of clock precision, the use of microscopic imaging and cavity-QED-based nondemolition measurement has allowed us to measure gravitation time dilation across a few hundred micrometers, and demonstrate spin squeezing-enabled metrological gain for clock comparison.
The Consultative Committee for Time and Frequency (CCTF) established a Task Force [1] in 2020 to update the roadmap towards the redefinition of the SI second, following a first roadmap agreed in 2016. This paper illustrates the work of the entire task force [2] formed by about 40 people repre-senting the CCTF countries, with some additional experts.
We will present real-time optically steered timescales generated at the same time at OP and NPL. After a detailed description of the experimental chains, we will present the implemented algorithms for outlier filtering and frequency steering estimations. We will then analyse the performance of the experimental timescales based on local comparison against the local UTC(k) and remote comparisons performed via UTC and using the GPS Precise Point Positioning (PPP) technique, before presenting strategies for improvement.
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Background ultralight scalar fields that are considered as a viable candidate for galactic dark matter may manifest themselves in apparent variation of fundamental constants, see, for example, [1,2].
In this talk, we will discuss some of the recent work of our group and collaborators, for example [3- 6], where we search for oscillating dark matter with Compton frequencies from near DC up to 100 MHz.
References
[1] A Arvanitaki, J Huang, and K Van Tilburg, “Searching for dilaton dark matter with atomic clocks,” Phys. Rev. D 91, 015015, 2015
[2] D Antypas, D Budker, VV Flambaum, MG Kozlov, G Perez, and J Ye, “Fast apparent oscillations of fundamental constants,” ANNALEN DER PHYSIK 2020, 1900566; arXiv:1912.01335
[3] A Banerjee, D Budker, M Filzinger, N Huntemann, G Paz, G Perez, S Porsev, and M Safronova, “Oscillating nuclear charge radii as sensors for ultralight dark matter,” arXiv:2301.10784 (2023)
[4] I.M. Bloch, D. Budker, V.V. Flambaum, I.B. Samsonov, A.O. Sushkov, and O. Tretiak, “Scalar dark matter induced oscillation of permanent-magnet field,” Phys. Rev. D 107, 075033 (2023), arXiv:2301.08514
[5] X Zhang, A Banerjee, M Leyser, G Perez, S Schiller, D Budker, and D Antypas, “Search for ultralight dark matter with spectroscopy of radio-frequency atomic transitions,” arXiv:2212.04413 (2022)
[6] O Tretiak, X Zhang, NL Figueroa, D Antypas, A Brogna, A Banerjee, G Perez, and D Budker, “Improved bounds on ultralight scalar dark matter in the radio-frequency range,” Phys. Rev. Lett. 129, 031301 (2022); arXiv:2201.02042
We report on results from long-term operation of the Yb1 ion optical clock of PTB, where we have obtained uptimes exceeding 80% over typical TAI reporting intervals of 30 days. Using these data and the special electronic structure of Yb+ allows us to improve searches for a coupling of ultra-light dark matter (UDM) to photons, temporal drifts of the fine structure constant and its potential dependence on the gravitational field. Interestingly, the same optical clock comparison data can also be used to probe UDM-nuclear couplings and provides competitive sensitivity.
We will also report on a composite system with Yb+ and Sr+ ions and on our efforts to employ a transportable optical clock based on the 171Yb+ E2 transition for contributions to TAI and frequency measurements at other institutes in Europe.
Cadmium is attractive for optical lattice clocks and for searches for Dark Matter and beyond-Standard-Model physics via isotope shift measurements. The cadmium clock transition has a small sensitivity to blackbody radiation and it has 8 stable isotopes, 6 spin 0 bosonic isotopes, and 2 spin ½ fermionic isotopes. Without using 229 nm light to drive the singlet transition, we capture thermal Cd atoms directly into a 326 nm narrow-line MOT. We then increase the loading rate by capturing atoms using the 361 nm $^{3}\textrm{P}_{2}\rightarrow\,^{3}\textrm{D}_{3}$ transition. We measure the isotope shifts of the 326 nm intercombination transition, and the 480 nm $^{3}\textrm{P}_{1}\rightarrow\,^{3}\textrm{S}_{1}$ and $^{3}\textrm{P}_{2}\rightarrow\,^{3}\textrm{D}_{3}$ transitions. These clarify a discrepancy of the nuclear charge radius and suggest that cadmium isotope shifts can sensitively test beyond standard model physics.
The paper highlights the importance of the time unit definition,by means of the atomic Cs frequency standard, in the definition of the base units of the International System of units (SI).
Since August 2021, NICT has been generating UTC(NICT) with its scale interval calibrated by the intermittent operation of a Sr lattice clock . The improvement in the deviation of UTC-UTC(NICT) as well as the current limitation will be discussed.
In this paper, we discuss the possibility to redefine the SI second using the geometric mean of several optical clock transitions. This definition would allow to take advantage of the many high performance optical frequency standards currently available.
Here, we describe the fundamental properties of this definition and its practical implementation. Finally, we discuss its strengths and weakness, as compared to a definition involving a single transition.
The recently concluded collaborative European project "Robust optical clocks for international timescales" (ROCIT) tackled some of the key challenges on the roadmap towards a redefinition of the second. The overall aim was to bring European optical clocks to the stage where they can contribute regularly to International Atomic Time as secondary representations of the second.
We describe our experiments towards the resonant laser excitation of the 8.3 eV nuclear transition in 229Th with the motivation to develop a highly accurate nuclear optical clock for metrology and tests of fundamental physics.
Toward a $^{229}$Th nuclear clock, we performed laser spectroscopy of the triply charged $^{229}$Th isomer ($^{229\rm{m}}$Th$^{3+}$) in an ion trap. The $^{229\rm{m}}$Th$^{3+}$ ions were obtained as a decay product of $^{233}$U. We determined the hyperfine constants of the electronic state of $^{229\rm{m}}$Th$^{3+}$ and derived the magnetic dipole and electric quadrupole moments of $^{229\rm{m}}$Th. We also investigated the nuclear decay lifetime of $^{229\rm{m}}$Th$^{3+}$ which was a key parameter to estimate the performance of a $^{229}$Th$^{3+}$ nuclear clock.
We show that low-phase noise and high-frequency stability can be simultaneously achieved in microwave sapphire oscillators. We describe the 9 GHz sapphire oscillator with interferometric signal processing, which was phase-locked to a stable RF reference by controlling microwave power dissipated in the sapphire resonator. The SSB phase noise of the oscillator was measured to be close to -170 dBc/Hz at Fourier frequency F = 10 kHz [1]. The fractional instability of the oscillator frequency was approximately 2x10^{-13} for integration times from 5 to 50 s.
The use of cryogenic sapphire resonators promises significant improvements in the phase noise performance of microwave oscillators [2]. Yet, serious attention must be paid to the noise mechanisms affecting the cryogenic resonators. The vibrations induced by cryocoolers and power-to-frequency conversion in the sapphire resonator are expected to be the leading causes of the oscillator's excess phase noise. In our recent experiments, we measured the power-to-frequency conversion of the cryogenic sapphire resonator as a function of Fourier frequency. We found that the resonator response to the fast variations of dissipated microwave power is similar to the transfer function of the 1st-order low-pass filter with corner frequency close to the resonator's loaded bandwidth [3]. The measurements were performed with three almost identical resonators cooled to 6 K and excited in the same whispering gallery mode with a resonant frequency near 11.2 GHz. Having measured the cryogenic sapphire resonator's power-to-frequency conversion, we predicted the phase noise spectrum of the cryogenic sapphire oscillator.
References
1. E. N. Ivanov and M. Tobar, “Low Phase Noise Sapphire Crystal Microwave Oscillators: Current Status”, IEEE Trans. on UFFC, v. 56, no.2, pp.263-269, 2009.
2. E. Ivanov and M. Tobar, “Noise Suppression with Cryogenic Resonators,” Microwave and Wireless Components Letters, vol. 31, Issue 4, pp. 405-408, April 2021, DOI: 10.1109/LMWC.2021.3059291, Print ISSN: 1531-1309, Online ISSN: 1558-1764
3. E. Ivanov and M. Tobar, “Power-to-Frequency Conversion in Cryogenic Sapphire Resonators,” Microwave and Wireless Components Letters, page(s): 1-4, Print ISSN: 2771-957X. Online ISSN: 2771-9588, Digital Object Identifier: 10.1109/LMWT.2023.3264975
Atomic fountain clocks have become ubiquitous in maintaining the most stable national time scales within nanoseconds of UTC. After three decades of development, they offer accuracy of about 1 part in 10^16. Recent efforts have been aiming at increased reliability and robust commercial design. We present the distinctive design approach and performance of the NPL built fountains, as well as current work to miniaturise the device and prospects for new applications.
The Atomic Clock Ensemble in Space (ACES) mission is developing high performance clocks and links for space to test Einstein's theory of general relativity. From the International Space Station, the ACES payload will distribute a clock signal with fractional frequency instability and inaccuracy of $1\times10^{-16}$ establishing a worldwide network to compare clocks in space and on ground. ACES will provide an absolute measurement of Einstein's gravitational redshift, it will search for time variations of fundamental constants, contribute to tests of topological dark matter models, and perform Standard Model Extension tests. The network of ground clocks participating to the ACES mission will additionally be used to compare clocks over different continents and measure geopotential dif-ferences at the clock locations.
After some technical delays, the ACES flight model is now approaching its completion. System tests involving the laser-cooled Cs clock PHARAO, the active H-maser SHM and the on-board fre-quency comparator (FCDP) have measured the performance of the clock signal delivered by ACES. The ACES microwave link MWL is currently under test. The single-photon avalanche detector of the ACES optical link ELT has been tested and will now be integrated in the ACES payload.
The ACES mission concept, its scientific objectives, and the recent test results will be presented together with the major milestones that will lead us to the ACES launch.
An “optical lattice clock” benefits from a low quantum-projection noise (QPN) by simultaneously interrogating many atoms trapped in an optical lattice [1]. The essence of the scheme is an engineered perturbation based on the “magic frequency” protocol, which has been proven successful up to 10-18 uncertainty [2-4]. About a thousand atoms enable such clocks to achieve 10-18 stability in a few hours. This superb stability is especially beneficial for chronometric leveling [5-7], which determines a centimeter-level height difference of the clocks located at remote sites by the gravitational redshift [8].
In transportable clocks [9], the potential stability of the optical lattice clocks is severely limited by the Dick effect [10] caused by the frequency noise of a compact clock laser. We proposed a “longitudinal Ramsey spectroscopy” [11] to improve the clock stability by continuously interrogating the clock transition. Two key ingredients for the continuous clock, continuous loading of atoms into a moving lattice [12] and longitudinal excitation of the clock transition, are reported. In addition, we report our recent development of compact and accurate optical lattice clocks in collaboration with industry partners.
We describe work at NIST to develop next-generation chip-scale atomic clocks based on optical transitions in vapor cells and thermal beams.
We report on development of a strontium optical lattice clock built with integrated photonics. We implement free-space laser beam control of positioning, pointing, shaping, polarization, and integration with metasurface optics, and absolute laser-frequency stabilization with waveguide supercontinuum generators. Such use of integrated photonics can simplify the system integration of Sr clocks. We demonstrate laser cooling to microKelvin temperature with narrow-line cooling, and we will describe ongoing work to probe the clock transition with lattice-trapped atoms.
abstract attached below.
The precision of a quantum clock near zero temperature, depends on how it is driven and how it is measured. We investigate both limits to precision using quantum stochastic thermodynamics, and illustrate the results with examples (superconducting and nano mechanical). Of particular relevance is the nature of the measurement as the clock signal ultimately depends on estimating the fluctuations in the period extracted from the measurement signal. We describe precision in terms of a kinetic uncertainty relation, a recently developed method to bound parameter estimation in continuously measured quantum systems.
A nuclear-spin-based rotation sensor is implemented based on simultaneous measurements with two nitrogen isotopes intrinsic to nitrogen-vacancy centers in diamond, employing a microwave-free technique with optical addressing of nuclear spins. Differential measurements suppress systematics related to magnetic-field and temperature variations.
Networks of optical clocks find applications in precise navigation, in efforts to redefine the fundamental unit of the ‘second’ and in gravitational tests. As the frequency uncertainty and instability for state-of-the-art optical clocks has reached the 10^−19 level, the vision of a global-scale optical network that achieves comparable performances requires the dissemination of time and frequency over a long-distance free-space link with a similar instability of 10^−19.
Here, we report time–frequency dissemination with an offset of 6.3 × 10−20 ± 3.4 × 10−19 and an in-stability of less than 4 × 10−19 at 10,000 s through a free-space link of 113 km [1]. Key technologies essential to this achievement include the deployment of high-power frequency combs, high-stability and high-efficiency optical transceiver systems and efficient linear optical sampling. We observe that the stability we have reached is retained for channel losses up to 89 dB.
The experiment was performed in Urumqi, Xinjiang Province. Two terminals (A and B) are located at Nanshan and Gaoyazi with a distance of 113 km. Each terminal is equipped with an ultra-stable laser (USL), two 1-W optical frequency combs with different wavelengths centered at 1,545 nm and 1,563 nm, two LOS modules and an optical transceiver telescope. The OFC optical phase locked to the USL is used as the carrier and reference signals of the local sampling [2]. By frequency multi-plexing the common free-space channel, we establish two independent two-way time–frequency transfer links, enabling precise evaluation of the link performance without limitation from the USL. As the two multiplexing channels share the same free-space link, common-mode noise occurs. To better evaluate our system, we also established an independent fibre link connecting the two termi-nals with a distance of 209 km. All links share the same USL at each terminal. Experimental results are shown in Fig. 1.
The work successfully evaluates the possibility of a satellite-based time–frequency dissemination on loss and noise. Next step, we will try to overcome other difficulties such as Doppler effects, link back-forward asymmetry and so on. Hopefully, we can have global optical clock networks in near future.
References
[1] Q. Shen et al., “Free-space dissemination of time and frequency with 10−19 instability over 113 km,” Nature, vol. 610, no. 7933, 661–666, 2022.
[2] FR Giorgetta, WC Swann, LC Sinclair, E Baumann, I Coddington, and NR Newbury, “Optical two-way time and frequency transfer over free space,” Nature Photonics, vol. 7(6), 434–438, 2013.
REFIMEVE is a national metrological network for time and frequency dissemination using the academic fiber network. It enables the coherent dissemination of time and/or frequency reference signals from LNE-SYRTE to around 15 labs and more than 30 in the future. We will show the latest extensions of the network in the Paris urban area and all over France, the architecture required for such a network, and the progress in terms of robustness and uptime. We will also present some applications to precision measurements, in particular ultra-high-resolution molecular spectroscopy in the mid-infrared spectral range.
Frequency comparison using ultra-stable, free-space laser links between transportable optical atomic clocks will result in globally significant advancements in applications spanning from fundamental physics, to outputs with immediate societal impact. Here, we report on our work to demonstrate a low size, weight, and power, continuous-wave laser technology that is capable of free-space frequency comparison between fast-moving optical clocks.
Applications of time and frequency signals on the fiber.
Research on room-temperature trapped ion atomic clocks at the Jet Propulsion Laboratory recently culminated in the launch of NASA's Deep Space Atomic Clock (DSAC) mission in 2019. Operating in space for 2 years, DSAC achieved a new level of performance among the most stable space clocks now in use and is expected to enable new space clock applications that require both high stability and autonomy. In this paper we will describe the DSAC mission and results, applications, and future directions.
As communication and navigation systems increasingly rely on precise timing signals from atomic clocks, the interest in smaller and more power-efficient clocks has grown in recent years. However, achieving high performance in clock frequency stabilities while reducing size, weight, and power (SWaP) has proven to be a challenge. Surveying existing atomic clocks in use clearly shows a stubborn trade-off [1]. Improving clock stability means a higher number of atoms, better vacuum conditions, higher laser powers, and more complex system control, all of which inevitably result in larger sizes and higher power consumption. In this paper, we will present the development of micro mercury trapped ion clocks (M2TIC) that clearly broke away from the typical trend.
[1] Marlow, B. L. S. & Scherer, D. R. A review of commercial and emerging atomic frequency standards. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 68, 2007–2022 (2021).
We have developed several sub-10 mL vacuum-gap Fabry-Perot cavities that provide ~$10^{-14}$ factional frequency stability and ultralow phase noise, using scalable lithographic techniques to fabricate million-finesse mirrors.
We present an overview of the latest activities relating to optical lattice clocks at NPL inlcuding: contributions to the BIPM for steering TAI, a full systematic evaluation of NPL-Sr1 with a target uncertainty below 5e-18, composite clock schemes making use of quantum non-demolition readout to improve the precision of clock comparisons, and the outlook for a new Yb optical lattice clock with emphasis on low measurement deadtime and high autonomy.
Here we report an optical frequency divider (OFD), which can realize optical frequency ratio measurement as well as optical frequency division to other desired frequencies. Using the OFD, we measure the frequency ratio between the fundamental and its second harmonic with an uncertainty of 3E-22. Meanwhile, we also demonstrate an optical frequency synthesizer referenced to a Yb optical clock. Moreover, we demonstrate the transportability, long-term operation and multi-channel division of the OFD.
Lasers with long coherence time and narrow linewidth are an essential tool for quantum sensors and clocks. We will report on the progress using low thermal noise crystalline mirror coatings with cryogenic silicon cavities, discuss alternatives for improving the stability, and give an outlook for more reliable, maintenance free and robust cryogenic silicon cavity setups that will enable also transportable optical clocks to benefit from their performance.
Mode-locked Kerr frequency combs generated in nonlinear resonators pumped with coherent monochromatic light have attracted significant attention because of their practical importance associated with their applications in optical and microwave frequency generation, signal synthesis, clocks and others. Dichromatic resonant continuous wave pumping of a nonlinear optical resonator can result in generation of broad microcombs at low power levels as well as other comb structures different from the usual Kerr combs. These frequency combs can be fully stabilized by means of pump harmonics and the repetition rate of the microcombs can be significantly smaller than the frequency difference between the pump frequencies. These combs can be considered as realizations of large order discrete time crystals and can be used as regenerative photonic frequency dividers. In this presentation we will discuss properties and applications of the optical frequency combs generated in cavities by means of dichromatic light.