ScienceWeek

HISTORY OF PHYSICS: ON FRITZ LONDON (1900-1954)

The following points are made by Philip W. Anderson (Nature 2005 437:625):

1) Fritz London began his career in physics as one of the originators of quantum theory during 1925-27. His training as a philosopher, before taking up physics, no doubt enhanced his contribution to the "Copenhagen interpretation" -- the first general attempt to understand the world of atoms according to quantum mechanics. But London did much more than create the first theory of the chemical bond, and has not had the recognition he deserves. He was among the few pioneers who deliberately chose, once atoms and molecules were understood, not to focus his research on further subdividing the atom into its ultimate constituents, but on exploring how quantum theory could work, and be observed, on the macroscopic scale.

2) For a few years, London worked at trying to found chemistry on quantum theory, but in the end was overwhelmed by Linus Pauling's more heuristic approach; he never published his book on the subject. He then became intrigued by the twin phenomena of superfluidity and superconductivity, which, he was convinced, were macroscopic manifestations of quantum mechanics. In 1935, London was the first to propose that superfluidity was Bose-Einstein condensation, and then in the late 1930s, with his brother Heinz, he developed the first heuristic theory of superconductivity. His pair of books on these subjects appeared around 1950 and admirably framed the questions that were soon to be answered -- in the one case by Oliver Penrose, Lars Onsager and Richard Feynman, and in the other by John Bardeen, Leon Cooper and Robert Schrieffer. But London fell ill in 1950 and died in 1954, so he did not live to see the triumphs of his intuitions.

3) He had paid, however, for his unpopular choice of subject matter -- quantum theory on the macroscopic scale -- by having to settle for a job in the pre-war South. This meant being out of mainstream physics, and may have resulted in him being excluded from the Manhattan bomb project on which all his early associates worked. In 1939, in an obscure paper called "The observation problem in quantum mechanics", London and Edmond Bauer took on the notorious Bohr-Einstein debates. This is the earliest paper known to the author that expresses the most common-sense approach to the uncertainty principle and the philosophy of quantum measurement.

4) The author states that in reading about these debates, he has the sensation of being a small boy who spots not one, but two undressed emperors. Niels Bohr's "complementarity principle" --that there are two incompatible but equally correct ways of looking at things -- was merely a way of using his prestige to promulgate a dubious philosophical view that would keep physicists working with the wonderful apparatus of quantum theory. Albert Einstein comes off a little better because he at least saw that what Bohr had to say was philosophically nonsense. But Einstein's greatest mistake was that he assumed that Bohr was right -- that there is no alternative to complementarity and therefore that quantum mechanics must be wrong. This was a far greater mistake, as we now know, than the cosmological constant.

[Editor's note: The physicist Nobel laureate Philip W. Anderson is another individual. The author in the above report is of the same name but is not the same person.]\

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QUANTUM CHEMISTRY: ON THE CONTROL OF QUANTUM PHENOMENA

The following points are made by H.A. Rabitz et al (Science 2004 303:1998):

1) The control of quantum phenomena is garnering increasing interest for fundamental reasons as well as on account of its possible applications (1). In general, the redirecting of quantum dynamics is sought to meet a posed objective through the introduction of an external control field C(t), often expressed as a function of time and frequently being electromagnetic, arising from laser sources. The field of optimal control theory (OCT) has arisen for the design of controls (2) in simulated systems, and the ultimate interest lies in executing optimal control experiments (OCEs).

2) By employing closed-loop learning control techniques (3), the number of such experiments is rapidly rising (4,5). At this juncture, there are many OCT studies exploring the control of broad varieties of quantum phenomena, and OCEs have similarly addressed several types of physical situations, including the selective breaking of chemical bonds (5), the creation of particular molecular vibrational excitations, the enhancement of radiative high harmonics, the creation of ultrafast semiconductor optical switches, and the manipulation of electron transfer in biological photosynthetic antenna complexes. Applications to other areas can also be envisioned, including quantum information sciences.

3) Typical OCT and OCE studies involve the manipulation of tens or even hundreds of control variables corresponding to the discretization of the control C(t) in either time or the analogous frequency domain representation. Almost all of the OCT design calculations use local search algorithms seeking an optimal control C(t), whereas the current laboratory OCE applications have all used global genetic-type algorithms (3). In the case of OCT, a very striking result is that all of the calculations are generally giving excellent-quality product yields. In the case of OCEs with laser controls, the absolute yields are not known, although a basic finding is the evident ease of discovering control settings that can often dramatically increase the desired final product.

4) In summary: A large number of experimental studies and simulations show that it is surprisingly easy to find excellent quality control over broad classes of quantum systems. The authors prove that for controllable quantum systems with no constraints placed on the controls, the only allowed extrema of the transition probability landscape correspond to perfect control or no control. Under these conditions, no suboptimal local extrema exist as traps that would impede the search for an optimal control. The identified landscape structure is universal for all controllable quantum systems of the same dimension when seeking to maximize the same transition probability, regardless of the detailed nature of the system Hamiltonian. The presence of weak control field noise or environmental decoherence preserves the general structure of the control landscape, but at lower resolution.

References (abridged):

1. H. Rabitz, R. de Vivie-Riedle, M. Motzkus, K. Kompa, Science 288, 824 (2000)

2. S. A. Rice, M. Zhao, Optical Control of Molecular Dynamics (Wiley, New York, 2000)

3. R. S. Judson, H. Rabitz Phys. Rev. Lett. 68, 1500 (1992)

4. C. J. Bardeen et al., Chem. Phys. Lett. 280, 151 (1997)

5. R. J. Levis, G. M. Menkir, H. Rabitz, Science 292, 709 (2001)

Science http://www.sciencemag.org

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QUANTITATIVE MEASUREMENT OF SHORT-RANGE CHEMICAL BONDING FORCES.

M.A. Lantz et al (University of Basel, CH) discuss AFM in the measurement of short-range forces, the authors making the following points:

1) The atomic force microscope (AFM) was originally intended to be a tool capable of measuring the forces acting between a single pair of atoms (1) but has only recently evolved into an instrument capable of producing atomically resolved images of surfaces with characteristic features and defects (2-4). This true atomic-scale contrast is generally interpreted as resulting from the short-range chemical interaction between an atomically sharp AFM tip and the nearest atoms on the surface of the sample. In principle, it should therefore be possible to map the chemical bonding potential between the foremost atom on an AFM tip and a specific atom on the sample.

2) The measurement of short-range bonding forces with the AFM has been difficult to achieve for several reasons. First, at room temperature, thermal drift and piezoelectric scanner creep make it difficult to reliably position the tip above a specific lattice position. Second, most atomic-resolution AFM images have been obtained using a dynamic technique in which the tip-bearing cantilever is driven on its fundamental resonant frequency with a typical amplitude of several nanometers. When the cantilever tip comes close to the sample surface, the force acting on the tip weakly perturbs the cantilever oscillation, giving rise to a small shift f in the resonance frequency. The frequency shift is used as a feedback parameter to control the tip-sample spacing, and images therefore correspond to contours of constant frequency shift. Because of the large tip excursion, the relation between the measured frequency shift and the force acting on the tip is not straightforward. Recently, however, progress has been made in quantitatively understanding and inverting this relation (5). A third difficulty arises because, in general, both short-range forces (such as covalent bonding forces) and long-range forces [such as van der Waals (vdW) and electrostatic forces] act on the tip. Separating these contributions in order to isolate the short-range chemical bonding force is a nontrivial problem. Finally, it is difficult to determine whether the measured chemical force involves more than just a single pair of atoms.

3) In summary: The authors report direct force measurements of the formation of a chemical bond. The experiments were performed using a low-temperature atomic force microscope, a silicon tip, and a silicon (111) 7 x 7 surface. The measured site-dependent attractive short-range force, which attains a maximum value of 2.1 nanonewtons, is in good agreement with first-principles calculations of an incipient covalent bond in an analogous model system. The resolution was sufficient to distinguish differences in the interaction potential between inequivalent adatoms, demonstrating the ability of atomic force microscopy to provide quantitative, atomic-scale information on surface chemical reactivity.

References (abridged):

1. G. Binnig, C. F. Quate, C. Gerber, Phys. Rev. Lett. 56, 930 (1986).

2. F. Ohnesorge and G. Binnig, Science 260, 1451 (1993).

3. F. J. Giessibl, Science 267, 68 (1995).

4. S. Kitamura and M. Iwatsuki, Jpn. J. Appl. Phys. 34, 145 (1995).

5. F. J. Giessibl, Phys. Rev. B 56, 16010 (1997).

Science 2001 291:2580.

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