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Measurements of Fundamental Parameters
JILA scientists are experts in the field of precision measurement. Using sophisticated instrumentation, they are able to measure small shifts in energy inside atoms and molecules as well as in larger physical systems. For instance, measurements of electronic transitions in OH molecules in the laboratory at JILA are allowing scientists to compare the value of the fine structure constant today with its value 10 billion years ago. A sophisticated experiment under construction at JILA might be able to detect and measure the electron’s electric dipole moment — if it exists. Comparisons of clock transition frequency ratios of optical atomic clocks located at JILA and elsewhere around the world are constraining the values of fundamental constants related to the coupling between quantum physics and gravity predicted by new theories in physics. JILA scientists also investigate precision geophysical measurements. They are engaged in determining the Newtonian gravitational constant (G) and the acceleration of gravity (g) as well as working on mapping Earth’s gravity field.
The Jun Ye group is using cold molecules for precision tests of fundamental laws of nature, including searches for possible time variations of physical constants such as α, the fine structure constant. α is the "coupling constant," or measure of the electromagnetic force than governs how electrons, muons, and light interact. New models for the basic structure of matter predict that α may have changed over vast spans of cosmic time. However, conventional physics wisdom says α has always been the same. Our basic understanding of our world depends on scientists’ ability to determine whether or not α is an "inconstant" constant.
The Ye group is teaming up with astronomers to solve this mystery by taking advantage of the special properties of highly reactive OH molecules. This molecule is abundant in outer space and naturally emits microwave radiation at four specific frequencies when it returns to its ground state after being excited by radiation from nearby stars or collisions with other interstellar molecules. These natural "masers" can be detected and measured on Earth. And, some of them were created billions of years ago in the early Universe.
Because each OH transition frequency has a different dependence on the value of a, it should be possible to compare the value of a billions of years ago with its value today – provided astronomers and laboratory scientists like those in the Ye group can measure the four transition frequencies precisely enough to detect small differences in α. In 2006, the precision-measurement specialists in the Ye group were able to improve the precision of one OH transition frequency measurement by 25-fold, and the other three by tenfold.
To answer the question whether the value of a has changed over eons of times, the Ye group is waiting for astronomers to make their own precise measurements of the same four OH transitions in a 10-billion-year-old maser. Preliminary measurements of these transitions by the Westerbork Synthesis Radio Telescope in 2004–2005 provided tantalizing, but not statistically significant, hint of a possible small change in α.
The Eric Cornell and Jun Ye groups are investigating whether the electron has an electric dipole moment (eEDM). The standard model of elementary particle physics posits that this dipole moment would be many orders of magnitude below what can be measured experimentally. However, various extensions of the standard model predict a much larger eEDM that might be within reach of a cleverly designed experiment.
That tantalizing idea inspired Cornell to collaborate with theorist John Bohn on a multiyear project to try and measure the eEDM in ultracold trapped molecular ions. If the Cornell and Ye groups succeed in detecting an eEDM, it will show that running time forward or backward at the quantum level makes a difference in the behavior of individual particles.
After completing a theoretical analysis of candidate molecules for eEDM experiments, the Bohn group suggested several possibilities to the experimentalists. The Cornell group selected hafnium fluoride ion (HfF+) and got to work building the experiment. By combining a mixture of Hf and Hf+ with sulfur hexafluoride (SF6), the researchers come up with a blend of HfF, HfF+, and lots of other things. A combination of cooling and skimming allows the removal of the HfF molecules. The HfF+ ions can then be brought to a stop in a radio-frequency (rf) trap.
The Cornell group is working on two different strategies to measure the energy levels of the HfF ions and molecules, compare them with the Bohn group’s theoretical calculations, and possibly use them in the actual eEDM experiment. In one effort, Cornell group members are working with the Ye group to perform spectroscopy with a femtosecond comb laser to characterize the energy levels of purified HfF+ ions. In another effort, researchers remove the ions from the original mixture to obtain pure HfF molecules. They photoionize these molecules, and create ions with as few internal rotational states as possible. The researchers investigate the energy states that are populated in the resulting ions. Their goal is to put the ions into the exact state needed for the actual eEDM experiment — which is a long-lived excited state.
Once either strategy proves successful for making the exact HfF+ ions needed for the eEDM experiment, the researchers will load the ions into a trap where they can vary the external electric field. The changing electric field will polarize the ions, giving them a circular motion as they move through the trap. They will then perform precision spectroscopy on the ions as they change them from spin down to spin up and precisely measure any energy difference between the two states. If the electron has an EDM, the electron will interact with the ion’s high internal electric field, resulting in a small, but measurable, energy shift as the ions switch from one spin state to another.
Scientists at JILA and the National Institute of Standards and Technology (NIST) recently used their high-accuracy optical atomic clocks to set new, much lower limits on the strength of any coupling that might exist between quantum and gravitational effects. The tighter limits have placed more stringent constraints on advanced theories that include coupling between gravity and quantum effects that come into play between atomic and subatomic particles in close proximity. Physicists want to find evidence of such coupling to develop a single theory that would explain everything about our world.
The idea for an experiment to look for coupling between quantum physics and gravity came from theoretical physicist Victor Flambaum of Australia’s University of New South Wales. Flambaum suggested the use of the exquisite precision of modern atomic clocks to look for evidence of this thus-far elusive effect. He proposed comparing different atomic clocks to see whether the ratio of their clock transition frequencies depends on Earth’s distance from the Sun. This distance varies because Earth travels around the Sun in an elliptical orbit. During a year, the gravitational potential varies by 3.3%.
If coupling between quantum effects and gravity exists, it might manifest as annual variations in atomic-clock transition-frequency ratios; high-precision clock comparisons would either reveal this coupling or place a limit on its strength. Plus, a long-term record of the ratio of different clock frequencies could provide evidence of the constancy, or inconstancy, of such fundamental constants as the fine-structure constant, α, or the electron-proton mass ratio, µ.
In early 2007, NIST researchers reported that they had been unable to detect any variations in fundamental constants related to Earth’s orbital position during a six-year study. Later during a comparison of the Sr-lattice clock at JILA with the Cs primary frequency standard at NIST, the Jun Ye group looked for coupling of gravity with three fundamental constants: α, µ, and the light quark mass; they also enlisted help for this project from colleagues in France and Japan. At the same time, NIST researchers decided to look for evidence of coupling between α and gravity during a comparison of atomic clocks based on single ions of mercury and aluminum.
The researchers found no evidence for coupling between gravity and α, µ, or the light quark mass. If coupling exists, it is still too small to detect with today’s most accurate and precise optical atomic clocks.
Precision measurements of distance have been carried out in space since the early days of the space program. One recent effort was the Gravity Recovery and Climate Experiment (GRACE) mission. GRACE has been mapping the Earth's gravity field since 2002. Its goal is to identify changes in total water storage in different areas of the world. A planned GRACE-2 mission is expected to make use of precise measurements of distance changes between carefully shielded test masses inside two drag-free satellites (roughly 50 km apart) over about a 10-year period. Peter Bender works with colleagues in CU’s Aerospace Engineering Sciences Department to understand the spatial resolution that can be achieved for determining changes in total water storage in areas such as medium-sized river basins.
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