Space Optical Clocks for Technology and Physics Applications - 180D-FW-2023/Knowledge-Base-Wiki GitHub Wiki
Introduction
Much like your quartz wristwatch, an Optical Clock tracks time by measuring the electron relaxation period in certain elements. Research institutions have made significant strides in the development of optical atomic clocks, achieving an extraordinary level of precision and accuracy at 10e-18. This unrivaled degree of meticulousness exceeds any other scientific or engineering space, presenting exciting opportunities for cutting-edge quantum sensing applications. These applications encompass delving into the mysteries surrounding dark matter and scrutinizing potential deviations from general relativity's predictions. However, to fully capitalize on these remarkable advancements both scientifically and technologically, it is imperative to fabricate clocks capable of operating dependably outside controlled laboratory environments without requiring constant human supervision.
Why Optical Clocks?
A quartz crystal wristwatch uses a battery in a simple circuit to excite electrons in the crystal, this allows electrons to jump up in energy states and as they relax or transition back to their base (ground) state, that time is measured and is approximately 1 second (accurate up to 10e-3 decimal places). For day to day uses, this is perfectly fine and accurate, however, on average, your watch becomes inaccurate by 1 second after about a year as the battery weakens and loses voltage, the magnetic or electric fields generated by things around you can offset the watch’s accuracy. An Optical Clock is accurate up to 10e-18 decimal places. Where a quartz watch becomes offset by 1 second after a year, an optical clock becomes offset by 1 second in 10 million years. This shows the accuracy, precision, and robustness of these clocks in a laboratory setting. For many experiments, such accurate measures of time are necessary to acquire usable and precise measurements especially when trying to study quantum effects of matter - such as the study of General Relativity (GM), Dark Energy, Dark Matter, or further study of space and our universe.
Figure 1: Historical accuracy of atomic clocks. Since 2000, the uncertainty of optical atomic clocks has rapidly improved to, and beyond, 1 part in 10e18.
As technology advanced, so did the accuracy of these clock systems. The accuracy of various clocks was tested by comparing the elapsed time or phases between two clocks over successive time intervals. This comparison is essentially an integration of the frequency difference between the clocks to enhance sensitivity and avoid interruptions caused by issues like turbulence or satellite visibility constraints.
Applications
Our goal is to lead the way in developing optical atomic clocks that can effectively function on spacecraft or in outdoor terrestrial environments. This project lies at the intersection of quantum physics and practical engineering, necessitating advancements in vibration-resistant lasers and robust ion traps with integrated photonics capabilities. Our overarching aim is to deploy an optical clock into a high Earth orbit for conducting experiments that test general relativity principles outside of our planet, explore dark matter phenomena, and improve the accuracy of very long-baseline interferometry measurements.
What is Dark Matter?
Dark Matter (DM) is an ambiguous form of matter that constitutes about 85% of the universe, yet remains invisible to current astronomical instruments. This is due to DM’s unique nature of not interacting with electromagnetic radiation.. Unlike dark energy, which is responsible for the universe's accelerated expansion and exerts adverse pressure, DM lacks any form of pressure altogether. Its existence is presumed primarily from its gravitational effects. For example, galaxies rotate at speeds that suggest the presence of much more mass than what we can see or measure, thus indicating the gravitational influence of something of which that cannot be seen, dark matter. These scientific observations and findings suggest that ordinary visible matter merely accounts for a staggering 5% of the total energy density present in the universe; thus underlining the essential role of the existence of DM in facilitating galaxy formation. Our understanding of the cosmos depends on DM, despite its elusive nature. It is believed to be non-baryonic, meaning it's not composed of atoms like matter we encounter daily. Theoretical particles like Weakly Interacting Massive Particles (WIMPs) and axions are among the candidates proposed for DM, but none have been directly observed yet.
Clocks and Dark Matter
In the 1930s DM was proposed to exist after physicists and astronomers observed our galaxy and distant stars. It was originally theorized that this mysterious and elusive mass was found in large clumps or clusters that held the universe together. It wasn’t until the 1980s that scientists became convinced that most of the mass that holds galaxies and clusters of galaxies together, was invisible. This, therefore, begged the question: How do we study something we can’t see or easily measure? The detection of DM presents a formidable challenge due to the feeble nature of gravitational interactions at small laboratory scales. Consequently, it becomes imperative to explore alternative methodologies that can aid in its identification. A promising avenue under investigation involves harnessing the power of atomic clocks, which possess the ability to measure frequencies linked with atomic transitions. By scrutinizing whether dark matter exerts an influence on these precise frequencies during experiments conducted on Earth's surface, there exists a potential for effectively detecting and discerning its effects.
Figure 2: While traversing the enigmatic realm of dark matter, an atomic clock experiences a velocity reminiscent of galactic motion. Within this mysterious domain, it is postulated that dark matter exists in extensive conglomerations or clusters. If these aggregations exhibit deviations in fundamental constants such as the fine structure constant (symbolized as α) when compared to the surrounding space, such disparities may give rise to either deceleration or acceleration in the temporal accuracy of said clock.
Figure 3: The presence of fields with exceedingly minuscule mass can lead to the fluctuation of fundamental parameters at the corresponding frequency known as the Compton frequency associated with said field. By subjecting a series of clock measurements, representing frequencies, to Fourier transformation, it becomes feasible to examine the power spectrum for discernible peaks. These distinctive peaks may serve as potential evidence pointing toward the existence of elusive dark matter.
Atomic clocks in a laboratory setting are quite accurate. Unlike your wristwatch or other electronic devices, they are not easily affected by external interference. Therefore the hope, and ultimate goal of sending these clocks into high orbit terrestrial setting, is to see if we can detect any changes in their fundamental frequency, and therefore any sort of change to something so accurate and resilient must be due to something extreme such as DM. This method is currently one of our most promising methods for studying such an elusive substance.
Challenges
While these clocks may be extremely accurate and promising in a laboratory setting, DM is not something we can harvest and bring into a controlled laboratory environment for study and therefore we must go to it. Although sending one of these clocks into space and taking some measurements seems pretty straightforward, it is incredibly difficult and tedious. These clocks achieve such levels of accuracy due to their controlled environments, thus the new challenge is achieving the same levels of accuracy in a non-controlled environment.
Figure 4: COMSOL model of a spherical resonant cavity which will be utilized in future experiments. This particular design will be used to measure the effects of acceleration and vibrational forces that will be encountered during launch that will affect the initial design of the optical cavity (i.e. changes in length and thermal expansions of the mirror cavity due to vibrations during take-off).
One of the largest challenges will be, that once this clock is launched into space, it becomes incredibly difficult to address and fix any electronic or mechanical issues that arise. Granted that there are exponential possibilities for possible errors, one of the biggest and most important components is this cavity, as there are a series of PID controllers that will adjust the electronic components, they do so from measurements from this cavity, therefore making it imperative to design the cavity to resist thermal and vibrational changes.
Conclusion
The exploration of atomic clocks has opened new opportunities in the quest to understand the elusive nature of dark matter and better understand our universe. The strategic design of atomic clocks to withstand thermal and vibrational disturbances has been pivotal in this pursuit, allowing for unparalleled precision in measurement. Although technology advances, we are experiencing less and less exponential gain, meaning we do not experience the same life-changing benefit from the iPhone 14 to the iPhone 15 compared to going from horse and carriage to the first car. As we seek knowledge of DM and the architecture of our universe, we hope to gain new insights that could lead to exponential changes in our understanding of physics and the world we live in.
References
Atomic clocks and dark-matter signatures - iopscience. (n.d.-a).
Fundamental physics with a state-of-the-art optical clock ... - iopscience. (n.d.-b).