No man is an island, entire of itself, said poet John Donne. And no atom neither. Even in the middle of intergalactic space, atoms feel the electromagnetic field---also known as the cosmic microwave background---left over by the Big Bang.
This shows the lattice of laser beams traps small numbers of ytterbium atoms in pancake-shaped "wells." A yellow laser excites the atoms so that they switch between lower (blue) and higher (yellow) energy levels.
The cosmos is filled with interactions that remind atoms they are not alone. Stray electric fields, say from a nearby electronic device, will also slightly adjust the internal energy levels of atoms, a process called the Stark effect. Even the universal vacuum, presumably empty of any energy or particles, can very briefly muster virtual particles that buffet electrons inside atoms, further shifting their energies; this form of self-interaction is known as the Lamb shift.
A new calculation by scientists at the Joint Quantum Institute (JQI) and the University of Delaware shows how still another influence, the warmth thrown off by nearby objects, can shift energy levels. Uncertainties in this "blackbody radiation shift" will soon impose limits on the accuracy of the best atomic clocks. Theoretical work on this subject will give scientists extra confidence when they come to redefine the second in coming years, a recalibration based on how ultracold atoms behave while sitting in special traps.
Modern timekeeping consists nowadays in reliably counting the cycles of light pouring out of those atoms and, more basic still, knowing what the atoms' intrinsic energy levels should be once all external influences are taken into account. On the experimental side, scientists slow the atoms to a near standstill in traps in order to minimize Doppler effects from the emitted light. This, and the ability to detect and count light oscillations at ever shorter wavelengths ---has led to atomic clocks with uncertainties as small as one part in 1017.
This research is Nobel-rich territory. To say nothing of earlier Nobels for atom cooling, the move from microwaves as the atomic "escapement" for clocks to light in the optical range (harder to measure but offering a precision hundreds of thousands of times better) earned several scientists the 2005 Nobel in Physics. One of 2012's Nobelists, David Wineland, is a pioneer in exploiting the properties of single ion held in a trap to develop clocks of the highest stability.
The precision of the clocks, however, is no better than knowledge of the internal energy levels of the atoms themselves, whether they are single ions or a gas of neutral atoms held in space by a network of laser beams---an arrangement called an optical lattice.
Some of the things that impose unwanted shifts on the atoms in a lattice, such as inter-atom collisions or the Stark effect, can be controlled. According to JQI Fellow Charles Clark, one of the largest irreducible parts in the uncertainty budget of an atomic clock is the blackbody radiation emitted by the very chamber enclosing the atoms. The atoms in the lattice might, by virtue of an elaborate cooling process, be at milli-kelvin or even micro-kelvin temperatures, but the surrounding vacuum chamber is generally at room temperature. One of the basic laws of thermodynamics says that material objects radiate heat---the higher the temperature the higher-energy the radiation. This shift is hard to measure experimentally and hard to calculate theoretically.
Coming to grips with this faint form of influence is the purpose of a new paper in the journal Physical Review Letters (**). Clark and his co-authors Marianna Safronova (a JQI Adjunct Fellow) and Sergey Porsev of the University of Delaware, look specifically at how ytterbium atoms are affected by blackbody radiation.
The rare-earth element ytterbium (Yb) is valued not so much for its mechanical properties but for its complement of internal energy levels. "A particular transition in Yb atoms, at a wavelength of 578 nm, currently provides one of the world's most accurate optical atomic frequency standards," said Safronova.
Although only important at a precision level of a part in 1015, accurate knowledge of the blackbody shift is more pertinent now that clocks are closing in on the part-per-1018 level of precision. That is, the uncertainty in the blackbody shift must be comparable to (and eventually lower than) the desired uncertainty of the clock. The new calculation by Safronova, Clark, and Porsev is the best yet since it includes the most complete treatment of the electron-electron correlations within the Yb atoms.
Clark estimates that the amount of uncertainty achieved in the value of an atomic energy level---about 2 times 10-18 --- corresponds to a clock uncertainty of about one second over the lifetime of the universe so far, 15 billion years.
The authors also studied the long-distance interactions among the Yb atoms and atoms of other species as well. This is critical to understanding the physics of dilute gas mixtures in general. Such mixtures are of interest, for example, in studying such things as quantum dipolar material (molecules which, though neutral, possess an electric dipole moment) and many-body quantum simulation. Besides applications in timekeeping and the study of ultracold chemistry, the results of the present work are important for the measurement of the weak force (through subtle parity effects---the process by which nature can tell left from right) and the search for the new physics beyond the standard model of the electroweak interactions.
(*)The Joint Quantum Institute is operated jointly by the National Institute of Standards and Technology in Gaithersburg, MD and the University of Maryland in College Park.
(**) "Ytterbium in quantum gases and atomic clocks: van der Waals interactions and blackbody shifts," M. S. Safronova, S. G. Porsev, and Charles W. Clark, Physical Review Letters, 7 December 2012.
Press contact at JQI: Phillip F. Schewe, firstname.lastname@example.org, 301-405-0989. http://jqi.umd.edu/
Phillip F. Schewe | EurekAlert!
Sharpening the X-ray view of the nanocosm
23.03.2018 | Changchun Institute of Optics, Fine Mechanics and Physics
Drug or duplicate?
23.03.2018 | Fraunhofer-Institut für Angewandte Festkörperphysik IAF
Satellites in near-Earth orbit are at risk due to the steady increase in space debris. But their mission in the areas of telecommunications, navigation or weather forecasts is essential for society. Fraunhofer FHR therefore develops radar-based systems which allow the detection, tracking and cataloging of even the smallest particles of debris. Satellite operators who have access to our data are in a better position to plan evasive maneuvers and prevent destructive collisions. From April, 25-29 2018, Fraunhofer FHR and its partners will exhibit the complementary radar systems TIRA and GESTRA as well as the latest radar techniques for space observation across three stands at the ILA Berlin.
The "traffic situation" in space is very tense: the Earth is currently being orbited not only by countless satellites but also by a large volume of space...
An international team of researchers has discovered a new anti-cancer protein. The protein, called LHPP, prevents the uncontrolled proliferation of cancer cells in the liver. The researchers led by Prof. Michael N. Hall from the Biozentrum, University of Basel, report in “Nature” that LHPP can also serve as a biomarker for the diagnosis and prognosis of liver cancer.
The incidence of liver cancer, also known as hepatocellular carcinoma, is steadily increasing. In the last twenty years, the number of cases has almost doubled...
In just a few weeks from now, the Chinese space station Tiangong-1 will re-enter the Earth's atmosphere where it will to a large extent burn up. It is possible that some debris will reach the Earth's surface. Tiangong-1 is orbiting the Earth uncontrolled at a speed of approx. 29,000 km/h.Currently the prognosis relating to the time of impact currently lies within a window of several days. The scientists at Fraunhofer FHR have already been monitoring Tiangong-1 for a number of weeks with their TIRA system, one of the most powerful space observation radars in the world, with a view to supporting the German Space Situational Awareness Center and the ESA with their re-entry forecasts.
Following the loss of radio contact with Tiangong-1 in 2016 and due to the low orbital height, it is now inevitable that the Chinese space station will...
Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, provider of research and development services for OLED lighting solutions, announces the founding of the “OLED Licht Forum” and presents latest OLED design and lighting solutions during light+building, from March 18th – 23rd, 2018 in Frankfurt a.M./Germany, at booth no. F91 in Hall 4.0.
They are united in their passion for OLED (organic light emitting diodes) lighting with all of its unique facets and application possibilities. Thus experts in...
A new scenario seeking to explain how Mars' putative oceans came and went over the last 4 billion years implies that the oceans formed several hundred million...
23.03.2018 | Event News
19.03.2018 | Event News
16.03.2018 | Event News
23.03.2018 | Materials Sciences
23.03.2018 | Agricultural and Forestry Science
23.03.2018 | Physics and Astronomy