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Combing the Helium Atom

Date

2014-1-7

Testing the theory of quantum electrodynamics inside a tiny atom using precision optical laser metrology.

In 1947 when late Nobel Laureate Willis Lamb tried to measure the energy levels in atomic hydrogen, the simplest atom for which physicists believed they understood the electronic structure very well based on the formulas of quantum mechanics at the time, he found discrepancies after many repeated precise measurements. The puzzle, now known as the “Lamb shift,” was later solved 1947 when Professor Willis Lamb tried to measure some energy levels of atomic hydrogen, the simplest atom that physicists all believed to understand its structure very well after complete formulation of quantum mechanics at that time, a discrepancy has surprised Lamb after repeated precise measurements. This puzzle, now known as “Lamb Shift”, was later solved by Schwinger, Feynman, and Tomonaga in the theory called quantum electrodynamics (QED), which allows virtual particles emerging from the vacuum to interact with the atom and cause shifts in energy levels. Nowadays, QED​ has been tested to a degree of preciseness better than one part in a billion and is the most precisely tested theory in all fields of physics.

With further advances in numerical methods and computing power, scientists are now able to calculate the Lamb shift of the simplest multi-electron atom, helium. A comparison between theories and experiments therefore provides an excellent testing ground for QED effects in this more complicated system. Recently a research team led by Professor Li-Bang Wang and Professor Jow-Tsong Shy from the Physics Department of National Tsing Hua University - a partner university of the University System of Taiwan - has performed laser spectroscopy on singlet state helium (in which one of two electrons is in the ground state while the second spins in an anti-parallel direction). The absolute frequency of the 21S0 ® 21P1 energy transition has been determined with a fractional uncertainty of 10-9 (one part in a billion), a result precise enough to reveal a disagreement with the most precise atomic calculation. The researchers constructed very stable lasers at 2058 nm (1 nm = one billionth of a meter) and used a technology called an optical frequency comb to precisely measure the transition frequency. A fiber-based optical frequency comb developed by Dr. Jin-Long Peng of the Center for Measurement Standards in the Industrial Technology Research Institute (ITRI) in Hsinchu was used for precision optical metrology. To accurately determine the transition frequency, the team had to control and investigate many experimental parameters, e.g., laser power stabilization, magnetic field shielding, discharge condition and helium gas pressure. Various experimental conditions were tested and closely controlled by PhD student Pei-Ling Luo to eliminate any possible systematic effects that might have limited the final precision of the measurement. A spectrum is shown in the figure below. This represents the first precise measurement of this transition. This result can then be compared with theories, and for some of the energy states, serious discrepancies have been found between this result and the most precise theoretical calculation.

The reason for the discrepancies is still not clear, according to Professor Li-Bang Wang, but this will definitely stimulate new theoretical investigations on the singlet states of helium. Different experiments on other transitions will also be performed in the near future to verify this result. With refined QED calculations and improved experimental precision, a tiny atom may be able to reveal new physics that high-energy particle colliders have so far failed to discover.

Vocabulary

helium [n.] 氦

electrodynamics [n.] 電動力學

measurement [n.] 測量法

metrology [n.]  度量衡學 

QED  [n.] 量子電動力學

Source:Taiwan News


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