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Angewandte Chemie International Edition in English

Experiments with an Isolated Subatomic Particle at Rest (Nobel Lecture)

HANS G. DEHMELT

Published Jul 1, 1990• 17 citations
Publisher

Abstract

Individual atomic and subatomic particles may be observed over long periods by confining them in “traps without material walls”. The Nobel Prize in physics was awarded in 1989 to Hans Dehmelt and Wolfgang Paul (together with Norman F. Ramsey) for first introducing this concept. Such traps are made possible by extending the principles of two‐dimensional focusing of particles to three dimensions. This led, in turn, to precision radiofrequency spectroscopy and quadrupole mass spectrometry. For example, side‐band cooling, in which electrons absorb photons with an energy just below the resonance energy and use their kinetic energy to supply the balance, allowed an electron to be held for ten months in the center of a magnetic trap. In this way, ½g was determined to be 1.001159652188(4).

DOI
10.1002/anie.199007341

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Individual atomic and subatomic particles may be observed over long periods by confining them in “traps without material walls”. The Nobel Prize in physics was awarded in 1989 to Hans Dehmelt and Wolfgang Paul (together with Norman F. Ramsey) for first introducing this concept. Such traps are made possible by extending the principles of two‐dimensional focusing of particles to three dimensions. This led, in turn, to precision radiofrequency spectroscopy and quadrupole mass spectrometry. For example, side‐band cooling, in which electrons absorb photons with an energy just below the resonance energy and use their kinetic energy to supply the balance, allowed an electron to be held for ten months in the center of a magnetic trap. In this way, ½g was determined to be 1.001159652188(4).
Individual atomic and subatomic particles may be observed over long periods by confining them in “traps without material walls”. The Nobel Prize in physics was awarded in 1989 to Hans Dehmelt and Wolfgang Paul (together with Nor‐man F. Ramsey) for first introducing this concept. Such traps are made possible by extending the principles of two‐dimensional focusing of particles to three dimensions. This led, in turn, to precision radiofrequency spectroscopy and quadrupole mass spectrometry. For example, side‐band cooling, in which electrons absorb photons with an energy just below the resonance energy and use their kinetic energy to supply the balance, allowed an electron to be held for ten months in the center of a magnetic trap. In this way, ½ g was determined to be 1.001159652188(4).
Of the many application of lasers, the one with possibly the most dramatic impact on fundamental physics is laser cooling and trapping: the Nobel Prize for physics was awarded for laser cooling in 1997 [1]. The staggering progress achieved by laser cooling is reflected by considering a thought experiment that begins ‘take an atom. . . ’. Such an experiment was impossible to realize in practice because room temperature atoms move at supersonic speeds and, as atoms are neutral, there was no easy way to grab hold of them. Laser cooling allows one to obtain samples of atoms essentially at rest, with temperatures in the microKelvin region being routine. At such low temperatures, it becomes easy to trap or move atoms with modest electromagnetic forces [2, 3] and a whole new class of experiments previously confined to the realm of fantasy becomes possible. For example, one can throw atoms upwards and use precision spectroscopy to measure time or gravity with unprecedented accuracy. One can place atoms at a desired location on a surface creating a novel lithographic technique or place an atom just above the surface to create a qubit in a quantum information processor. One can take many atoms and cool them further by evaporation to form a Bose-Einstein condensate [4] or degenerate Fermi gas. This last development has propelled atomic and optical physics right into the heart of condensed matter physics and resulted in the award of the Nobel Prize in 2001.
Zero-point level confinement1 in a trap is the quantum-mechanical equivalent of the classical single particle at rest in free space. Such confinement has been demonstrated—by the continuous Stern-Gerlach effect—only for the 150-GHz cyclotron motion in geonium, a single electron permanently confined in a cold Penning trap. Localizing the electron to ≈ 60 μm in the node of a standing wave in the trap cavity approximated free space, but decreased spontaneous emission tenfold. Driving the 60-MHz axial motion on a sideband higher by the ≈ 12-kHz magnetron frequency forced the magnetron motion to absorb the excess photon energy, shrinking its radius to ~ 15μm. Analogous laser cooling has reduced the oscillation amplitude of a Ba+ ion in a Paul rf trap to < 120 nm. As in nuclear magnetic resonance, confinement is now much smaller than the wavelength and sidebands disappear. Quantum jumps to and from an electronic metastable level of Ba+ have been demonstrated. Using In+ or a similar ion, this may make a monoion oscillator optical frequency standard with a 1000-day reproducibility of 10-18 possible.
Of the many applications of lasers, the one with possibly the most dramatic impact on fundamental physics is laser cooling and trapping: the Nobel Prize for physics was awarded for laser cooling in 1997. Laser cooling allows one to obtain samples of atoms essentially at rest, with temperatures in the micro-Kelvin region being routine. At such low temperatures, it becomes easy to trap or move atoms with modest electromagnetic forces and a whole new class of experiments previously confined to the realm of fantasy becomes possible. The high brightness of masers and lasers makes it possible to access a regime of atomic excitation for which Einstein’s rate equations are inadequate. Three-dimensional optical molasses using counter-propagating laser beams along three mutually orthogonal directions was first observed in 1985 by Chu et al. The Sisyphus mechanism works well for atoms that are already cold and has a smaller capture velocity than conventional Doppler cooling.
Zero‐point level confinement in a trap is the quantum‐mechanical equivalent of the classical single particle at rest in free space. Such confinement has been demonstrated—by the Continuous Stern‐Gerlach Effect—for only the 150 GHz cyclotron motion in geonium, a single electron permanently confined in a cold Penning trap. This confinement made possible a new determination of the radius of the electron, some 10 000 times smaller than the accepted upper limit of ≊10−16 cm. Localizing the electron to ≊60 μm in the node of a standing wave in the trap cavity approximated free space, however decreasing spontaneous emission ten‐fold. Driving the 60 MHz axial motion on a side band higher by the ≊12 kHz magnetron frequency, forced the magnetron motion to absorb the excess photon energy, and shrank its radius to ≊15 μm. Analogous laser cooling has reduced the oscillation amplitude of a Ba+ ion in a Paul rf trap to <120 nm. As in Nuclear Magnetic Resonance, confinement here is much smaller than the wave length and side bands disappear. Quantum jumps to and from an electronic metastable level of Ba+ have been demonstrated. Using In+ or similar, these techniques may make a mono‐ion oscillator optical frequency standard with a 1000‐day reproducibility of 10−18 possible. Further experiments are proposed.