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Elements of an optical atomic clock, and the relationship between them. (This illustration is for the case of a trapped ion optical clock, but a similar relation exists between the subcomponents of a clock based on cold neutral atoms.)
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... In addition to applications in ground-based optical frequency metrology, a compact robust optical clock in space opens new opportunities and increased precision across a range of physics and technology applications, including space-based detection of dark matter [7,8], relativistic geodesy [9,10] and enhanced satellite navigation accuracy [11]. As discussed in [12], space-borne optical atomic clocks offer significant opportunities to explore different topics in fundamental physics. They could be used to perform measurements of the gravitational redshift with unprecedented precision, searching for violations of the Einstein Equivalence Principle and detection of gravitational waves from a variety of astronomical entities. ...
Optical atomic clocks demonstrate a better stability and lower systematic uncertainty than the highest performance microwave atomic clocks. However, the best performing optical clocks have a large footprint in a laboratory environment and require specialist skills to maintain continuous operation. Growing and evolving needs across several sectors are increasing the demand for compact robust and portable devices at this capability level. In this paper we discuss the design of a physics package for a compact laser-cooled ⁸⁸Sr+ optical clock that would, with further development, be suitable for space deployment. We review the design parameters to target a relative frequency uncertainty at the low parts in 10¹⁸ with this system. We then explain the results of finite-element modelling to simulate the response of the ion trap and vacuum chamber to vibration, shock and thermal conditions expected during launch and space deployment. Additionally, an electrostatic model has been developed to investigate the relationship between the ion trap geometrical tolerances and the trapping efficiency. We present the results from these analyses that have led to the design of a more robust prototype ready for experimental testing.
... Ultrastable optical atomic clocks, combined with high-performance time/frequency links, open the way to accurate tests of LPI and the panorama of proposals employing them is vast [94]. The proposals for the Space Optical Clock SOC and I-SOC were submitted in 2005 in response to a call for scientific experiments on the ISS. ...
... Also for tests of LLI space offers interesting possibilities. The data obtained by the MICROSCOPE experiment have been already used to cast improved constraints on some SME parameters [120] and atomic sensors have been proposed as an effective way to test LLI in the framework of the SME [94,[121][122][123], thus offering the chance to improve current bounds. Different type of LLI tests can be performed in this context. ...
Advances in quantum technologies are giving rise to a revolution in the way fundamental physics questions are explored at the empirical level. At the same time, they are the seeds for future disruptive technological applications of quantum physics. Remarkably, a space-based environment may open many new avenues for exploring and employing quantum physics and technologies. Recently, space missions employing quantum technologies for fundamental or applied studies have been proposed and implemented with stunning results. The combination of quantum physics and its space application is the focus of this review: we cover both the fundamental scientific questions that can be tackled with quantum technologies in space and the possible implementation of these technologies for a variety of academic and commercial purposes.
... Ultrastable optical atomic clocks, combined with high-performance time/frequency links, open the way to accurate tests of LPI and the panorama of proposals employing them is vast [91]. in the framework of ESA's Cosmic Vision program is the SAGAS [75], which aims at flying highly sensitive atomic sensors on a Solar System escape trajectory. SAGAS envisages employing a Strontium + ion atomic clock with a fractional frequency stability of 10 −17 and aims at measuring solar gravitational redshift at the |α LPI | ∼ 10 −9 level. ...
... Also for tests of LLI space offers interesting possibilities. The data obtained by the MICROSCOPE experiment have been already used to cast improved constraints on some SME parameters [117] and atomic sensors have been proposed as an effective way to test LLI in the framework of the SME [91,[118][119][120], thus offering the chance to improve current bounds. Different type of LLI tests can be performed in this context. ...
Advances in quantum technologies are giving rise to a revolution in the way fundamental physics questions are explored at the empirical level. At the same time, they are the seeds for future disruptive technological applications of quantum physics. Remarkably, a space-based environment may open many new avenues for exploring and employing quantum physics and technologies. Recently, space missions employing quantum technologies for fundamental or applied studies have been proposed and implemented with stunning results. The combination of quantum physics and its space application is the focus of this review: we cover both the fundamental scientific questions that can be tackled with quantum technologies in space and the possible implementation of these technologies for a variety of academic and commercial purposes.
... Important use of atomic clocks is made in satellite-based navigation, where a position is determined via the time it takes for electromagnetic pulses to travel certain distances. With a future generation of optical atomic clocks the precision of navigation may improve up to a point where, for example, small strains of the earth's crust can be measured and applied for earthquake prediction [101,209]. Importantly, however, optical atomic clocks of the highest accuracies have even led to the creation of the novel field of chronometric geodesy [76]. At the achieved accuracies, general relativistic effects enter the game: time dilation caused by the local gravitational field becomes observable. ...
... One famous historic example is the development of the marine chronometer by John Harrison around 1735, which solved the longitude problem and allowed for reliable navigation at sea [108]. Today's cesium atomic clocks are used for satellite-based navigation [136] and the development of optical atomic clocks with their corresponding improvement in accuracy by a factor of 100 will certainly lead to a new generation of satellite-based clocks [101]. Tests along this line have already been performed [204]. ...
The proposal for the development of a nuclear optical clock has triggered a multitude of experimental and theoretical studies. In particular the prediction of an unprecedented systematic frequency uncertainty of about 10-19 has rendered a nuclear clock an interesting tool for many applications, potentially even for a re-definition of the second. The focus of the corresponding research is a nuclear transition of the 229Th nucleus, which possesses a uniquely low nuclear excitation energy of only 8.12±0.11 eV (152.7±2.1 nm). This energy is sufficiently low to allow for nuclear laser spectroscopy, an inherent requirement for a nuclear clock. Recently, some significant progress toward the development of a nuclear frequency standard has been made and by today there is no doubt that a nuclear clock will become reality, most likely not even in the too far future. Here we present a comprehensive review of the current status of nuclear clock development with the objective of providing a rather complete list of literature related to the topic, which could serve as a reference for future investigations.
... Using clocks for determining a local geopotential model with a high spatial resolution was quantitatively investigated by Lion et al. (2017). Moreover, space-borne optical clocks were proposed for physical and geodetic applications (Gill et al. 2008). Efforts toward a compact, high-performance and ultra-stable clock that is aimed to be employed in space were made by Origlia et al. (2018). ...
Quantum optical technology provides an opportunity to develop new kinds of gravity sensors and to enable novel measurement concepts for gravimetry. Two candidates are considered in this study: the cold atom interferometry (CAI) gradiometer and optical clocks. Both sensors show a high sensitivity and long-term stability. They are assumed on board of a low-orbit satellite like gravity field and steady-state ocean circulation explorer (GOCE) and gravity recovery and climate experiment (GRACE) to determine the Earth’s gravity field. Their individual contributions were assessed through closed-loop simulations which rigorously mapped the sensors’ sensitivities to the gravity field coefficients. Clocks, which can directly obtain the gravity potential (differences) through frequency comparison, show a high sensitivity to the very long-wavelength gravity field. In the GRACE orbit, clocks with an uncertainty level of \(1.0\times 10^{-18}\) are capable to retrieve temporal gravity signals below degree 12, while \(1.0\times 10^{-17}\) clocks are useful for detecting the signals of degree 2 only. However, it poses challenges for clocks to achieve such uncertainties in a short time. In space, the CAI gradiometer is expected to have its ultimate sensitivity and a remarkable stability over a long time (measurements are precise down to very low frequencies). The three diagonal gravity gradients can properly be measured by CAI gradiometry with a same noise level of 5.0 \({\mathrm{mE}/\sqrt{\mathrm{Hz}}}\). They can potentially lead to a 2–5 times better solution of the static gravity field than that of GOCE above degree and order 50, where the GOCE solution is mainly dominated by the gradient measurements. In the lower degree part, benefits from CAI gradiometry are still visible, but there, solutions from GRACE-like missions are superior.
... Also under investigation is the installation of optical master clocks in space, where spatial and temporal variations of Earth's gravitational field are smoothed out, such that the space clocks could serve as a reference for groundbased optical clock networks (e.g. Gill et al. 2008, Bongs et al. 2015, Schuldt et al. 2016. Such clock networks could also help to establish a new global height reference system. ...
We review experimental progress on optical atomic clocks and frequency transfer, and consider the prospects of using these technologies for geodetic measurements. Today, optical atomic frequency standards have reached relative frequency inaccuracies below 10-17, opening new fields of fundamental and applied research. The dependence of atomic frequencies on the gravitational potential makes atomic clocks ideal candidates for the search for deviations in the predictions of Einstein's general relativity, tests of modern unifying theories and the development of new gravity field sensors. In this review, we introduce the concepts of optical atomic clocks and present the status of international clock development and comparison. Besides further improvement in stability and accuracy of today's best clocks, a large effort is put into increasing the reliability and technological readiness for applications outside of specialized laboratories with compact, portable devices. With relative frequency uncertainties of 10-18, comparisons of optical frequency standards are foreseen to contribute together with satellite and terrestrial data to the precise determination of fundamental height reference systems in geodesy with a resolution at the cm-level. The long-term stability of atomic standards will deliver excellent long-term height references for geodetic measurements and for the modelling and understanding of our Earth.
... A review of current research in the field of relativistic geodesy and the basic perspectives for application of quantum oscillators in various applications is presented in [5][6][7]. It should be noticed that the relativistic effects in the clock's rate are more apparent in the case of quantum oscillators placed on board of satellites because they move with high velocity at high altitude, and are less sensitive to the influence of local inhomogeneities of the gravitational field of the Earth [8,9]. The article [11] has considered the principles of global monitoring of changes in the gravitational field of the Earth with the help of artificial satellites (including GNSS). ...
... The zenith tropospheric delays were put into the category of estimated parameters. 8 https://igscb.jpl.nasa.gov For evaluation of time scale drift of the mobile standard CH1-1006, we also used the ionosphericfree phase combination P 3 and Φ 3 . ...
We report on the experimental results of testing a new physical method of determination of the gravitational potential differences and orthometric heights by measuring the relativistic effect of gravitational redshift of frequency by means of atomic clocks. The experiment was performed in the Altai Mountains between two geodetic stations, Shebalino and Sieminski Pass, separated by about 850 meters in altitude. The measured mean value of the frequency shift caused by the change in the gravitational potential between the two stations is (δf/f0)grav = 7.980 × 10−14, with the dispersion σ f = 7.27 × 10−15 referred to the time interval of the experiment.
... Also under investigation is the installation of optical master clocks in space, where spatial and temporal variations of the Earth's gravitational field are smoothed out, such that the space clocks could serve as a reference for ground-based optical clock networks (e.g. Gill et al. 2008;Bongs et al. 2015;Schuldt et al. 2016). However, clock networks as well as their possible impact on science, including the establishment of a new global height reference system, are beyond the scope of this work. ...
The frequency stability and uncertainty of the latest generation of optical atomic clocks is now approaching the one part in \(10^{18}\) level. Comparisons between earthbound clocks at rest must account for the relativistic redshift of the clock frequencies, which is proportional to the corresponding gravity (gravitational plus centrifugal) potential difference. For contributions to international timescales, the relativistic redshift correction must be computed with respect to a conventional zero potential value in order to be consistent with the definition of Terrestrial Time. To benefit fully from the uncertainty of the optical clocks, the gravity potential must be determined with an accuracy of about \(0.1\,\hbox {m}^{2}\,\hbox {s}^{-2}\), equivalent to about 0.01 m in height. This contribution focuses on the static part of the gravity field, assuming that temporal variations are accounted for separately by appropriate reductions. Two geodetic approaches are investigated for the derivation of gravity potential values: geometric levelling and the Global Navigation Satellite Systems (GNSS)/geoid approach. Geometric levelling gives potential differences with millimetre uncertainty over shorter distances (several kilometres), but is susceptible to systematic errors at the decimetre level over large distances. The GNSS/geoid approach gives absolute gravity potential values, but with an uncertainty corresponding to about 2 cm in height. For large distances, the GNSS/geoid approach should therefore be better than geometric levelling. This is demonstrated by the results from practical investigations related to three clock sites in Germany and one in France. The estimated uncertainty for the relativistic redshift correction at each site is about \(2 \times 10^{-18}\).
... The Standard Model unifies electromagnetism, the weak, and the strong force [1,2], while Einstein's general relativity is the theory of gravity [3,4]. To this moment general relativity passed all tests with flying colours [5,6] and one of its spectacular predictions, the existence of gravitational waves, was recently confirmed [7]. However, general relativity does not include quantum effects and is therefore incomplete [4,8]. ...
... The Einstein Equivalence Principle (EEP) is the foundation of general relativity and contains three principles: (from [5]) 1. "The trajectory of a freely falling test body (one not acted upon by forces such as electromagnetism and too small to be affected by tidal gravitational forces) is independent of its internal structure and composition. This is known as the weak equivalence principle (WEP). ...
... These principles are pictured in Figure 1.2. The first principle is also called the Universality of Free Fall (UFF), the second is called the Local Lorentz Invariance (LLI) and the third is known as the Local Position Invariance (LPI) [5]. Due to these three principles, a theory of gravity has to be a metric theory which is a theory where the metric tensor alone determines the properties of space-time and its influence on particle trajectories [13]. ...
Quantum sensors, such as atom interferometers and atomic clocks are used for high precision and accurate measurements of inertial forces and time and are therefore ideally suited to address fundamental questions in physics and to test the predictions of general relativity.
The sensitivity of atom interferometers scales quadratically with the free evolution time and the use of quantum sensors in space is predestined to improve the accuracy of such tests by several orders of magnitude. Additionally, precise and accurate sensors for inertial forces are required in the field of navigation or geodesy where mobile devices based on atom interferometry are still rare. This work contributes to the development of highly sensitive and stable mobile quantum sensors. In the course of this thesis, three measurement comparisons of the gravitational acceleration with the mobile atom interferometer GAIN were performed at different geographic locations. The demonstrated stability of 5*10^-11 g after 10^5 s surpasses the one reached by classical gravimeters.
With the goal of space-born atom interferometry, a compact laser system for operation of atom interferometry with Bose-Einstein condensates of rubidium on a sounding rocket was designed, qualified and put in operation. Additionally, three sounding rocket payloads were realized to show the technological maturity of the necessary subsystems. Doppler-free laser spectroscopy of rubidium and potassium was used to realize an optical frequency reference that was compared during the flights to an atomic microwave standard via a frequency comb. This measurement represents the first test of the Local Position Invariance in space.
These activities pave the way for future deployment of quantum sensors in space enabling unprecedented tests of fundamental physics, space geodesy or even gravitational wave detection.
... The Standard Model unifies electromagnetism, the weak, and the strong force [1,2], while Einstein's general relativity is the theory of gravity [3,4]. To this moment general relativity passed all tests with flying colours [5,6] and one of its spectacular predictions, the existence of gravitational waves, was recently confirmed [7]. However, general relativity does not include quantum effects and is therefore incomplete [4,8]. ...
... The Einstein Equivalence Principle (EEP) is the foundation of general relativity and contains three principles: (from [5]) 1. "The trajectory of a freely falling test body (one not acted upon by forces such as electromagnetism and too small to be affected by tidal gravitational forces) is independent of its internal structure and composition. This is known as the weak equivalence principle (WEP). ...
... These principles are pictured in Figure 1.2. The first principle is also called the Universality of Free Fall (UFF), the second is called the Local Lorentz Invariance (LLI) and the third is known as the Local Position Invariance (LPI) [5]. Due to these three principles, a theory of gravity has to be a metric theory which is a theory where the metric tensor alone determines the properties of space-time and its influence on particle trajectories [13]. ...
Quantensensoren, wie Atominterferometer und Atomuhren werden zu hochpräzisen und akkuraten Messungen von Inertialkräften und der Zeit benutzt und sind hervorragend dazu geeignet fundamentale Fragestellungen der Physik anzugehen und die Aussagen der allgemeinen Relativitätstheorie zu testen. Die Empfindlichkeit von Atominterferometern skaliert quadratisch mit der freien Entwicklungszeit und die Verwendung von Quantensensoren im Weltraum ist prädestiniert die Genauigkeit von Tests des Äquivalenzprinzips um mehrere Größenordnungen zu verbessern. Zusätzlich, werden präzise und akkurate Sensoren für Inertialkräfte, im Bereich der Navigation oder Geodäsie benutzt wo mobile auf Atominterferometrie basierende Geräte noch selten sind. Diese Arbeit trägt zur Entwicklung von hochempfindlichen und stabilen mobilen Quantensensoren bei. Im Rahmen dieser Doktorarbeit wurden drei mobile Vergleichsmessungen der Erdbeschleunigung mit dem Atominterferometer GAIN an verschiedenen geographischen Orten durchgeführt. Die demonstrierte Stabilität von 5*10^-11 g nach 10^5 s übertrifft die Stabilität von klassischen Gravimetern. Mit dem Ziel von Weltraumgestützten Atominterferometern wurde ein kompaktes Lasersystem für den Betrieb von Atominterferometrie mit Rubidium Bose-Einstein Kondensaten auf Höhenforschungsraketen entworfen, qualifiziert und in Betrieb genommen. Zusätzlich wurden drei Nutzlasten für dein Einsatz auf Höhenforschungsraketen realisiert um die Reife der notwenigen Subsysteme zu zeigen. Dopplerfreie Laserspektroskopie an Rubidium und Kalium wurde verwendet um eine optische Frequenzreferenz zu realisieren und während der Flüge wurde mit einem Frequenzkamm zu vergleichen. Diese Messung stellt einen ersten Test der Lokalen Lorenz Invarianz im Weltraum dar. Diese Aktivitäten ebnen den Weg für den zukünftigen Einsatz von Quantensensoren im Weltraum die noch nie dagewesene Tests der fundamentalen Physik, Weltraumgeodäsie oder sogar Gravitationswellen ermöglichen.
