Letters from the Past - A PRL Retrospective

2001

These two Letters, together with the 1998 Milestone Letter by Y. Fukuda et al. (Super-Kamiokande Collaboration), Phys. Rev. Lett. 81, 1562 are the landmark papers on the discovery of neutrino oscillations by the Super-Kamiokande and Sudbury Neutrino Observatory collaborations. The discovery helped resolve the apparent paradox of missing solar neutrinos, but also proved that neutrinos must have a small rest mass. The source of that mass is an area of active study. The Nobel Prize in Physics for 2015 went to Takaaki Kajita of the University of Tokyo (Super-Kamiokande) and Arthur McDonald of Queen’s University in Kingston, Canada (SNO).

For more information, see the 1998 Milestone, the Focus story at Physics 8, 97 (2015), and the information at the Nobel Prize web site.

2000

This research represented the culmination of a decades-long effort by many individuals on short highly stable optical pulses. The resulting device, reported in these Letters, produces laser light with a spectrum consisting of sharp peaks separated by precisely determined frequencies (like the teeth of a comb). This has made possible many measurements with extraordinary accuracy.

The two groups, led by J. L. Hall and T. W. Hänsch, combined in the first of the Letters listed above; a sign of collaboration and shared credit for the research. The second Letter was an earlier independent publication by the Hänsch group. It followed a separate publication by the Hall group in Science.

The 2005 Nobel Prize in Physics was awarded to Roy J. Glauber “for his contribution to the quantum theory of optical coherence” (see the 1963 Milestones), and to John L. Hall and Theodor W. Hänsch “for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique”.

For further information, see the Focus story Phys. Rev. Focus 16, story 13, the Nobel Lectures of Hall (Rev. Mod. Phys. 78 1279 (2006) [Free PDF]) and Hänsch (Rev. Mod. Phys. 78 1297 (2006) [Free PDF]), and the information on the Nobel Prize site.

In 1967 V. G. Veselago published, in Russian, an article that showed that materials with simultaneously negative dielectric constant ε and magnetic permeability μ had very interesting and unusual properties. An English translation was published in Soviet Physics Uspekhi 10, 509 (1968). Such materials were termed “left-handed”, since the electric field E, the magnetic field H, and the propagation vector k form a left-handed triad.

There were no examples of naturally occurring left-handed materials, although it was generally granted that they could exist. Some efforts were made in recent years to develop artificially constructed materials. The Letter by Smith et al. described the first working material with these properties. Their device, operating in the microwave region of the electromagnetic spectrum, was based on previous developments by Pendry and coworkers, as acknowledged by Smith et al. Among the unusual properties predicted for the left-handed materials were a negative index of refraction and a negative Doppler effect.

In the second Letter it was pointed out by Pendry that a “perfect lens” could be made from materials with a negative index of refraction. This inspired many objections (see the list of Comments on this Letter and Pendry’s Replies, as well as the Focus story Phys. Rev. Focus 9, story 23 on the controversy). In the end Pendry’s views have prevailed. The possibility of a perfect lens appears to be real, but has not yet been realized because it requires a material with very low absorption, which has not yet been produced.

The subject of left-handed materials, their properties, and their potential applications has been clarified and continues to be developed.

For further information see a Physics Today article by Pendry and Smith (Physics Today, June 2004, p. 37 [Free PDF]), a review by Bliokh et al. (Rev Mod Phys 80 1201 (2008) [Free PDF]), and also (where available) the references in that review to articles by Veselago and Narimanov, and by Smith, Pendry, and Wiltshire.

1999

String theory, which is attractive to particle theorists because it provides a mechanism for unifying quantum mechanics and general relativity, requires that space have nine dimensions, which prompts the question of how the additional dimensions could have escaped detection so far. For years, the approach followed to this question was to develop theories in which the extra dimensions were “compactified” – limited to a very small size and thus not detectable with even the most powerful particle accelerator.

In these two Letters, Randall and Sundrum introduced an alternative approach, in which one of the additional dimensions is “warped”. In such models, the strong, weak, and electromagnetic forces are confined to three dimensions (on a 3-brane, a three-dimensional “membrane”, at a specific location in the other dimensions), but gravity propagates in the warped space as well. In such models, quantum effects due to gravity are not out of experimental reach.

These papers stimulated a whole body of work developing string-inspired models using branes in a warped space, and predicting collider signals for such theories. Looking for such signatures of brane theories will be one of the major activities at the CERN Large Hadron Collider.

For further information, see Phys. Rev. Focus 4, story 28.

1998

This Letter provided important evidence about fundamental neutrino properties. Since there are three “flavors” of neutrinos, neutrinos of one type can change into a different type, a phenomenon known as oscillation. This can only occur if neutrinos have mass (and not all the same mass).

The observation reported here was a result of a line of research that was not originally directed at neutrino physics. The Kamiokande and IMB detectors were constructed to search for proton decay (see the 1987 Milestone) and collected data about “atmospheric neutrinos” – neutrinos created by the interaction of cosmic rays with the atmosphere –- only because such events could be a significant background to any proton-decay detection. However, the first measurements by IMB and Kamiokande detected fewer muon neutrinos than expected, by a factor of about 2, and it seemed that the atmospheric neutrinos were worth studying in their own right. The measurements reported in this Letter, done at the Super-Kamiokande detector (the successor to Kamiokande), confirmed the earlier results and also clearly showed a dependence on direction which indicated that the deficit in muon neutrinos resulted from oscillation. This experiment firmly established the existence of neutrino oscillations and hence of neutrino mass.

Masatoshi Koshiba shared the 2002 Nobel Prize with Raymond Davis and Riccardo Giacconi “for pioneering contributions to astrophysics” (see the 1962 and 1968 Milestones). The award to Koshiba was in recognition of his role in the Kamiokande and Super-Kamiokande collaborations, whose important results included the observation of supernova neutrinos (see the 1987 Milestone) and this work.

For further information, see the links in the 1987 Milestone, the Focus story Phys. Rev. Focus 2, story 10, and the review article by M.C. Gonzalez-Garcia and Y. Nir, Rev. Mod. Phys. 75, 345 (2003) [Free PDF]. See the 2001 Milestone for more information on the 2015 Nobel Prize in Physics.

1997

This Letter reports an advance in the quantitative understanding of nonequilibrium processes without the stringent requirement that the processes be quasistatic – i.e., infinitely slow. The central result shows how to determine free energy changes by measuring in a controlled way only the work performed on the system, without the need to know explicitly the accompanying entropy changes. This requires a series of measurements on the system, i.e., making an ensemble of measurements and averaging the results over the distribution of the work performed. As the author has pointed out, the practical applicability of the derived equality requires that the work done be comparable to the average molecular thermal energy. This in practice makes it difficult to apply to macroscopic systems, but Jarzynski’s equality (Eqs. 2 of this Letter) has been instrumental in the interpretation of experiments on nanoscale systems, such as biomolecules, of much current interest. It has led to a number of surprising results in the application of thermodynamic concepts to small systems, with many contributions by workers in statistical mechanics.

For further discussion of these results and of additional developments in the field see a review article by Jarzynski, European Physical Journal B 64, 331 (2008), which is available on Jarzynski’s web site [free PDF]. Also, see the Physics Today article by Bustamante, Liphardt, and Ritort, “The Nonequilibrium Thermodynamics of Small Systems” Physics Today 58(7), 48 (July 2005). We thank Physics Today and the American Institute of Physics for making this article freely available for our readers.

1996

As mentioned in the description of the 1995 Milestone, the 2001 Nobel Prize in physics was awarded not only for the achievement of Bose-Einstein condensation but also “for early fundamental studies of the properties of the condensates”. This Letter is among the earliest of those studies.

For further information see the Nobel committee Information for the Public and press release, as well as Phys. Rev. Focus 8, story 20.

1995

The 2001 Nobel Prize in Physics was awarded to Eric Cornell, Wolfgang Ketterle, and Carl Wieman “for the achievement of Bose-Einstein condensation in dilute gases of alkali atoms, and for early fundamental studies of the properties of the condensates”. Their work followed many important contributions by others over a period of years in which the achievement of a Bose Einstein condensate was a goal. The first observation of the condensate was by the group of Wieman and Cornell, published in Science in 1995. See also the 1996 Milestone for further BEC research by Cornell and Wieman published in Physical Review Letters.

The observation of the condensates required trapping the atoms in a small volume and cooling them to the required temperature for condensation. Ketterle was able to do this for a collection of sodium atoms using a novel trap that produced a very high-density condensate – higher than the density of rubidium atoms in the condensate of Wieman and Cornell. Together their research and the many following developments have produced a greater understanding of quantum mechanics and of statistical physics.

For further information see the 1996 Milestone.

1994

These Letters reported the culmination of the search for the top quark, begun soon after the discovery of the bottom quark [S.W. Herb et al., Phys. Rev. Lett. 39, 252 (1977)]. The search took nearly twenty years because the mass of the top quark turned out to be unexpectedly large, around 40 times the mass of the next heaviest quark (the bottom quark) and 180 times the mass of the proton. As a result, the discovery could only happen after the development of the Fermilab Tevatron Collider, which provided proton-antiproton collisions at high enough energy and in sufficient quantity that the very rare events of creation of pairs of top quarks and antiquarks could be reliably detected at a statistically significant level.

The first Letter listed above, from the CDF (Collider Detector at Fermilab) Collaboration, reported their results from the 1992-1993 run, which provided evidence for the top quark, but with insufficient statistical significance to definitively claim its observation. This had to wait for the 1994-1995 run, which resulted in the other two Letters, simultaneous reports from CDF and the other large Fermilab group, the D0 Collaboration (named after the location of their detector on the Fermilab ring).

With the discovery of the top quark, all of the fundamental fermions predicted by the standard model of particle physics had been observed. Of the particles expected in the standard model, only the Higgs boson has not yet been detected – searching for it is a major goal of the CERN Large Hadron Collider. See the Milestone for 1964.

For more information about the discovery of the top quark, see the review article by C. Campagnari and M. Franklin, Rev. Mod. Phys. 69, 137 (1997) [free PDF].

1993

Classically, copying an object involves making very careful measurements and then producing an object with corresponding properties. In quantum mechanics, an obstacle to such copying is the uncertainty principle, which in general prevents a complete set of precise measurements. In this Letter, the authors present a protocol by which sufficient information can be obtained to create a replica of an initially unknown quantum state. This involves the use of an entangled pair of quantum states (see Milestones for 1981 and 1991). After interaction of the unknown state with one part of the entangled pair, enough information can be obtained to put the other part of the pair into the initially unknown state. The price paid because of quantum mechanics is that the initial object is no longer in its original state. In a sense, this process realizes the science-fiction process of teleportation, in which an object is dissolved at its original location and reproduced elsewhere (“beam me up”) – but in the case of quantum teleportation, this is of course not at the level of a human or other macroscopic object!

Quantum teleportation has since been demonstrated experimentally with photons, atoms, and ions, and is playing an important role in the development of quantum computation.

1992

This Letter laid the foundations of the density matrix renormalization group approach to strongly-correlated many-body quantum systems. The method emerged as a generalization of Wilson’s (Rev. Mod. Phys. 47, 773 (1975) [free PDF]) analysis of the Kondo problem. The Letter reported on an efficient procedure for the accurate determination of the ground state and the lowest-lying excited states of one-dimensional lattice systems, which can be generalized to higher dimensions.

The novel idea in White’s formulation was that, rather than keeping at each iteration the lowest-lying eigenstates of the Hamiltonian, the focus should be on the identification of the most significant eigenstates of the density matrix. The idea was tested on antiferromagnetic Heisenberg spin chains with S = 1/2 and S = 1 with great success, especially with the accurate determination of the gap between the ground state and the first excited state in the latter case. An indispensable addition to the Letter was the much more detailed paper by White in Phys. Rev. B 48, 10345 (1993), in which the author presented the density matrix renormalization group algorithms for both finite- and infinite-lattice calculations . The idea has since been extended to the calculation of a number of static quantities for a variety of systems with strongly interacting particles, such as Hubbard and Kondo chains. Various extensions have also been worked out in order to handle finite-temperature behavior, dynamics, and, notably, higher dimensional cases. The interest in the method has also been extended by its reformulation from the standpoint of quantum information.

There are few other topics of current interest that have generated as many extensive reviews as this Letter and the accompanying Physical Review B article. A recent very comprehensive discussion by U. Schollwöck can be found in Rev. Mod. Phys. 77, 259 (2005) [free PDF]. A perusal of the abstract, table of contents, and the first few pages of this review will give the non-expert reader a taste of the far-reaching applications of this Milestone.

1991

These two Letters are important early contributions to the development of quantum cryptography, the use of the properties of quantum mechanics to provide secure keys for encrypting communications. The first protocol for quantum encryption was developed by C.H. Bennett and G. Brassard in 1984 (and later dubbed BB84: Proceedings of the IEEE International Conference on Computers, Systems and Signal Processing, Bangalore, India (IEEE, New York, 1984), p. 175), but initially attracted little attention among physicists because it was reported only at a computer-science conference, not published in a journal. (One of the potential problems in interdisciplinary research is attracting the attention of all the relevant communities!) The BB84 protocol was based on one party’s sending polarized particles to the other and relied on the Heisenberg uncertainty principle for security, since any eavesdropping will alter the results and thus be detectable.

In the first Letter a scheme was reported which was based on a source of pairs of entangled particles, as first discussed by Einstein, Podolsky, and Rosen, with the particles of each pair being sent respectively to the two parties wishing to communicate. In this case, security is provided by applying the generalized Bell inequalities of Clauser et al. (see Milestone for 1981); the activity of an eavesdropper would give rise to results differing from those expected from quantum mechanics in the absence of such activity. The second Letter showed that this scheme was essentially equivalent to that of BB84, and it was therefore not necessary to specifically use either Bell’s theorem or Einstein-Podolsky-Rosen states; the fundamental properties of quantum mechanics were sufficient.

For more information about the development of quantum cryptography, see the review article by N. Gisin et al. [Rev. Mod. Phys. 74, 145 (2002) (free PDF)].

1990

At the time these Letters were published classical chaos (as found in nonlinear dynamical systems with extreme sensitivity to initial conditions) had already been studied extensively. Great advances had been made, including the understanding that there were only a few universal categories of chaos. The most familiar example of chaos, which attracted much public attention, was the observation by Lorenz that the beating of a butterfly’s wings could, over a period of time, change weather patterns throughout the world, thus making long-term predictions impossible.

Independently, just two days apart in their dates of receipt, the two Milestone Letters initiated a significant shift in the focus of research on chaotic systems. Going beyond characterization, they provided practical, yet mathematically rigorous, schemes to overcome and control chaos in naturally occurring or artificially designed systems. The Letter by Pecora and Carroll describes how two subsystems could be synchronized (and thus controlled) by linking them with a common signal. The authors clearly indicated the numerous experimentally and theoretically significant questions that their study raised; some of the most intriguing prospects related to neural processes and networks. The Letter by Ott et al. develops an approach to controlling chaos by making small perturbations of a system parameter that serves as a handle to drive the chaotic behavior of a nonlinear system into any one of a large number of possible regular (periodic) motions. No prior knowledge of the system’s dynamics is needed as the implementation involves direct experimental monitoring of the control parameters with subsequent feedback. The observation was made that chaos need not be a burdensome complication. Controlled chaos could, as the authors note, be an important ingredient in the regulation of higher life forms by the brain.

The two Letters have been widely recognized and cited, because of their implications and comprehensibility, beyond the immediate communities to which they were addressed. They opened up new routes of research in many seemingly unrelated disciplines.

A relatively recent Resource Letter by Daniel Gauthier in Am. J. Phys.71, 750 (2003) gives in its introductory pages a good idea of the ramifications of chaos control. We thank the American Association of Physics Teachers for allowing us to make this article freely accessible.

1989

Periodic structures that do not allow the propagation of a range of frequencies, directions, and polarizations of electromagnetic radiation have been known and studied since the nineteenth century. Rayleigh and later Darwin (for x rays) showed that one-dimensional structures can be produced in which electromagnetic radiation cannot propagate. Interest in the production of three-dimensional structures with such properties arose again in work by E. Yablonovitch PRL 58, 2059 (1987) and S. John PRL 58, 2486 (1987), who particularly wanted to show that such structures could suppress the decay of excited atoms or other excitations in solids. But the key question remained – how to fabricate the requisite materials. In the case of a crystal it is necessary to produce energy bands with a gap at the zone boundaries, which themselves should have symmetry as close to spherical as is possible.

In the first of the Milestone Letters chosen here the authors describe the fabrication of a lattice of spherical holes – “air atoms” – in a dielectric background. This paper is also notable for the introduction of the term “photonic crystal”, for which there is a gap of photon frequencies in which the crystal is completely reflective, irrespective of the direction of the incident photon. While the authors claimed that they had in fact produced such a crystal, they had overlooked a degeneracy of bands at one of the points on the Brillouin zone boundary, so that there was no gap, and hence no reflectivity, in the directions around that point (i. e. a pseudogap). This was pointed out to the authors by K. M. Leung and Y. F. Liu PRL 65, 2646 (1990). Subsequently, in the second Milestone Letter above, this error was corrected by making a crystal with non-spherical “atoms”, so that the degeneracy at the zone boundary was lifted and a true photonic crystal was produced.

The field of photonics has developed rapidly, and many applications have evolved after these first fabrication efforts. For a clear discussion of photonic crystals and some of their applications go to http://optoelectronics.eecs.berkeley.edu/ and download Yablonovitch’s 2001 Scientific American article.

1988

After the demonstration of “optical molasses” by Steven Chu et al. (see Milestone for 1985), several groups made attempts to overcome the limits that were apparently set by those authors on the lowest temperatures reachable by their techniques. The Letters above reported that laser cooling could in fact lower the temperature of atoms below those limits.

In the first Letter, sodium atoms were measured to have reached a temperature of around 40 μK, a factor of 6 below the “Doppler limit”, which had been achieved by Chu et al.; this result was a great surprise. It was soon realized that while the theory leading to the Doppler limit assumed a two-level atom, real atoms have several Zeeman sublevels. These levels are differently excited depending on the polarization of the laser field, which varies rapidly with space in the optical molasses created by multiple opposing laser beams; a full analysis of this situation led to an understanding of the ability to achieve lower temperatures.

The second Letter reports achieving a temperature below the “recoil limit”. This limit arose from the fact that (in prior laser-cooling experiments) all the atoms were continuously absorbing and emitting photons; these processes inevitably gave the atom a small recoil energy. For the helium atoms involved in this experiment, the recoil limit was 4 μK, but the atoms reached a temperature of 2 μK. The experimenters accomplished this by pumping those atoms which were already moving slowly into a nonabsorbing coherent superposition of states; since they were no longer absorbing they did not recoil, and the recoil limit did not apply. Later experiments eventually reached a temperature which was a factor of 22 below the recoil limit. The net result of both these experiments was that it is possible to make a “thicker optical molasses” than was initially accomplished.

As stated in the Chu et al. Milestone, the 1997 Nobel Prize in physics was awarded jointly to Steven Chu, William D. Phillips, and Claude Cohen-Tannoudji “for development of methods to cool and trap atoms with laser light.”

For further information, see the links in the 1985 Milestone, the Nobel lectures of Phillips [Rev. Mod. Phys. 70, 721 (1998) (free PDF)] and Cohen-Tannoudji [Rev. Mod. Phys. 70, 707 (1998) (free PDF)], and Phys. Rev. Focus 21, story 11.

Magnetoresistance is the change in resistance of a conductor in the presence of a magnetic field. In the mid 19th century Lord Kelvin observed that in iron there is a difference in the magnetoresistance when the electric current is along or perpendicular to the direction of the spontaneous magnetization. This anisotropy, which is a relatively weak effect, has been used to read the direction of magnetic fields in the magnetic memories of computers.

The work highlighted here involves the discovery of a much more pronounced effect – dubbed giant magnetoresistance– in artificially produced nanoscale materials. In the Letter of Baibich et al., the group of Albert Fert described their production of a stack of alternating thin layers of iron and chromium, which displayed giant magnetoresistance. We have included also the work of the group of Peter Grünberg which, while published later in Physical Review B, was actually submitted before the Letter of the Fert group. They reported on the large magnetoresistance observed, but with a single layer of chromium. The magnetoresistance that they observed was smaller than that seen by the Fert group, for two reasons. First, the Fert observations were on multiple layers, so that the effect was considerably enhanced, and second, they carried out their observations at liquid helium temperatures. Grünberg’s group worked initially at room temperature (of great importance for applications), except for a single measurement at low temperature. Credit clearly belongs to both groups.

This discovery has had a significant practical impact in greatly shrinking the size and increasing the reliability of computer memories and of other magnetic measuring devices. It is a prime example of the fast pace of transforming a fundamental discovery into a large-scale technological application. The 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter Grünberg “for the discovery of Giant Magnetoresistance.”

For further information see the Nobel Committee’s Information for the Public and other links on that site, and Phys. Rev. Focus 20, story 13.

1987

These two Letters reported the observation in two large underground water Cherenkov detectors of neutrinos associated with the supernova 1987A. This was the first supernova observable with the naked eye since the one studied by Kepler in 1604, and the first astronomical event studied via neutrinos as well as by observation of light and other electromagnetic radiation.

The two detectors – the Kamiokande detector located in the Kamioka mine in Japan and the Irvine-Michigan-Brookhaven (IMB) detector located in the Morton-Thiokol mine in Ohio – were originally constructed to search for proton decay, which was predicted by certain unified theories of the strong, weak, and electromagnetic interactions. No proton decay was detected (thus eliminating some theories), but the large size and underground location of the detectors (designed to make the presumably rare signal of proton decay detectable by minimizing background from cosmic rays) also made them good detectors for rare neutrino events. (The last three letters of “Kamiokande” – NDE - originally stood for “Nucleon Decay Experiment”, but later were reinterpreted as “Neutrino Detection Experiment”.)

The observation of the neutrinos helped confirm the understanding of the physics of supernovae, provided limits on the neutrino rest mass and other properties and ushered in a new era of neutrino astronomy.

Masatoshi Koshiba, of the Kamiokande collaboration, shared the 2002 Nobel Prize with Raymond Davis and Riccardo Giacconi “for pioneering contributions to astrophysics” (see the 1962 and 1968 Milestones).

For further information, see Koshiba’s Nobel lecture [Rev. Mod. Phys. 75, 1011 (2003) (free PDF)], the Nobel committee’s Information for the Public and the Nobel Focus story [Phys. Rev. Focus 10, story 18].

A 1986 article by J. G. Bednorz and K. A. Müller (Zeitschrift für Physik B 64, 189 (1986)) with the rather tentative title “Possible High T_c Superconductivity in the Ba-La-Cu-O System” suggested a 30K transition temperature, higher than the previous 23K record, which had held for more than a decade. This started the excitement that came to full flower in 1987. During that year many other researchers around the world verified Bednorz and Müller’s results, joined in the chase, and found other related compounds with higher transition temperatures. The principal means of rapid communication at that time was the FAX machine, and these were worked overtime with preprints and graphs of experimental results.

The Letter by Wu et al. described the first instance of a transition temperature higher than the 77K of liquid nitrogen, which had significant potential technological implications, and which therefore attracted great attention. It seemed a major step towards a room temperature superconductor.

Three other events during the year were of interest. At a meeting of the American Physical Society in New York a packed March 18th special session on high temperature superconductors continued into the small hours of the morning, and was dubbed, because of the passion and enthusiasm of the participants, the “Woodstock” of physics. The excitement continued with President Reagan’s formation of the “Council on Superconductivity for American Competitiveness” at a meeting in Washington where he was photographed watching a disk of the high T_c material being magnetically levitated as liquid nitrogen was poured over it. But, while there have been numerous applications of these materials, it is fair to say that more was expected in 1987 than has so far been accomplished. Research on these superconductors continues to this day, using all conceivable experimental and theoretical methods.

The year ended with the well-deserved award of the Nobel Prize in physics to J. G. Bednorz and K. A. Müller “for their important breakthrough in the discovery of superconductivity in ceramic materials”. See their joint Nobel lecture [Rev. Mod. Phys. 60, 585 (1988) (free PDF)] and the Nobel press release for further information.

1986

The scanning tunneling microscope (see the 1982 Milestones) made possible an unprecedented resolution for the surface structure of condensed matter. It was dependent on the measurement of tunneling currents into electronic states, which was doable for metals and some semiconductors, but not for insulators. In this Letter the authors describe a method for measuring not the current, but the varying force on the probe as it is scanned over the surface. This extended the range of surfaces that could be studied in very important ways, not only to insulators, but also to molecules and other structures resting on surfaces. Of particular significance is the imaging of biological samples like DNA which should be kept wet. An excellent early summary of atomic force microscopy and biological applications is in a 1995 Physics Today article by C. Bustamante and D. Keller, “Scanning Force Microscopy In Biology”. We thank Physics Today for making this article freely available for our readers.

1985

This Letter reported the demonstration of a powerful method for slowing down (cooling) atoms with the use of an array of six laser beams – two opposed beams in each of three orthogonal directions. In the experiment, sodium atoms were used and the laser beams were all slightly redshifted with respect to the characteristic wavelength absorbed by stationary sodium atoms. This meant that due to the Doppler shift for the moving atoms, an atom could absorb only photons from laser beams opposed to that atom’s motion; this absorption would give the atom momentum in the direction of the laser beam, opposite to the atom’s motion, thus slowing it. Though the atom would later emit a photon, this would be in a different direction, so that after repeated absorption and emission its forward motion would be slowed. The environment of the laser-beam array which thus slowed motion of the atoms in any direction was dubbed “optical molasses.” With this technique, the atoms were cooled to a temperature of 240 μK, thought at the time to be the limiting temperature for cooling these atoms.

The development of laser-cooling techniques made possible important progress in atomic physics, including substantially improved atomic clocks and the discovery of Bose-Einstein condensation in atomic gases.

The 1997 Nobel Prize in physics was awarded jointly to Steven Chu, William D. Phillips, and Claude Cohen-Tannoudji “for development of methods to cool and trap atoms with laser light.”

For further information, see the Nobel press release and Chu’s Nobel lecture [Rev. Mod. Phys. 70, 685 (1998) (free PDF)]. For further information on the work of Phillips and Cohen-Tannoudji see the 1988 Milestones.

1984

This Letter reports the fabrication and observation of what is now known as a quasicrystal – a solid with symmetry between that of a glass, with no long-range order, and a crystal, with long-range translational symmetry. A quasicrystal can have, for example, local fivefold rotational symmetry, which is forbidden for a crystal. This discovery has caused a revolutionary change in the science and mathematics of crystallography, since the diffraction pattern for a quasicrystal produces Bragg peaks of the same sharpness as does a crystal.

Mathematicians knew of a two-dimensional analogue of these patterns, called Penrose tilings after their discoverer, since the 1970s. The first materials found to have these properties, like those of the alloys discussed in this Letter, are metastable, and are produced by rapid cooling from the molten alloy. Later, stable quasicrystals were discovered. On the theoretical front the first treatment was in a Letter by Levine and Steinhardt, who had been researching generalizations of the Penrose tilings to three dimensions prior to the discovery of Shechtman et al. They showed that quasiperiodicity enables rotational symmetries forbidden to periodic crystals, first used the term quasicrystal in their Letter, and showed that their concepts accounted for the diffraction pattern observed by Shechtman et al. Subsequent theoretical developments by many researchers treated a quasicrystal as a projection of a higher dimensional crystal onto a three-dimensional space, or as an extension of the definition of a crystal in three-dimensional space.

The influence of the Shechtman et al. discovery has been enormous, with publications, conferences and books on the subject still being produced, and showing no signs of abatement.

See also Physical Review Focus 28, story 14 and the Nobel Prize press release for further information.

1983

When the Nobel Prize for the fractional quantum Hall effect experiment and the theory – this Letter – were announced, Robert Laughlin was interviewed by the New York Times. After seeing the experimental results he said that he formulated an explanation for the effect that was “completely wrong, and which, fortunately, was rejected by the referees at the journal [PRL] to which I submitted our paper”. (Of course, papers are accepted or rejected by editors, not referees, but the reports are taken very seriously.)

Soon he found a full explanation for the experimental results, which was developed and published in this remarkable Letter. The quantized plateau with Hall conductance (1/3)(e²/h) is observed when the magnetic field strength corresponds to three quanta of magnetic flux for each electron in the two-dimensional system. Under these conditions, Laughlin showed that the electrons could form a stable, highly correlated ground state, with an energy gap separating it from excited states. He also showed that if an electron is added to this ground state, it would split into three stable “quasiparticles” of charge e/3.

Since the experimental discovery of the 1/3 effect other fractional Hall plateaus have been found, including 2/5, 3/7, 4/9, 5/2,…, etc., which could be explained by generalizations of Laughlin’s quantum fluid. This extraordinary theory has not, as Laughlin has stated, had practical application, but has shown how many-body quantum effects can lead to very unusual consequences.

See, as for the 1982 experimental milestone, Physical Review Focus 2, story 18 and the Nobel press release for further information.

1982

Following on the discovery of the integer Quantized Hall Effect by von Klitzing and coworkers, Tsui, Stormer, and Gossard undertook studies of a two-dimensional electron fluid at higher magnetic fields and lower temperatures than had previously been done. In this Letter they presented surprising results showing a plateau of the Hall effect at 1/3 the von Klitzing conductance value, (1/3)(e²/h). Several explanations for the results were discussed in the Letter, but the authors finally concluded “At the present there is no satisfactory explanation for all of our observations”. Many possible explanations were put forward by others in a flurry of papers, but the conclusive explanation was given by Laughlin in 1983 (selected as a Milestone for that year). He showed that a new state of matter with many-particle interactions accounted for the experimental results.

The 1998 Nobel Prize in Physics was awarded to R. B. Laughlin, Horst Stormer, and Daniel Tsui “for their discovery of a new form of quantum fluid with fractionally charged excitations”. See pPhysical Review Focus 2, story 18](http://physics.aps.org/story/v2/st18) and the Nobel Prize press release for readable descriptions of this work.

This Letter reports the first scientific results on surface topography obtained by scanning tunneling microscopy (STM) in ultrahigh vacuum. The authors explain the straightforward principle of the STM that consists in scanning a metal tip over the surface at constant tunneling current, maintained by piezoelectric feedback drives. The idea of an STM was old, but the practical realization eluded experimenters until the work of these authors. The demonstration of feasibility was published in references 1 and 2 of this Letter, and the practicality was clearly shown here by the measurement of the topography of the surfaces of crystals of gold and of CaIrSn4. An unprecedented resolution, about the size of a hydrogen atom, was ultimately made possible by this relatively simple mechanism.

Subsequent developments have followed. The constant height mode of operation came into use in addition to the original constant current mode. Then, the tip was fixed at a specific location, the voltage was swept, and the tunneling current measured. This scanning tunneling spectroscopy mode gave access to the energy dimension and to the unraveling of local electronic structure, including peculiar surface states. The initial device worked between a metal tip and a metal or semiconductor surface, but later an insulator surface could also be studied, following the development of Atomic Force Microscopy – see the Milestone for 1986 for details.

The 1986 Nobel Prize in Physics was awarded to Ernst Ruska “for his fundamental work in electron optics, and for the design of the first electron microscope” and to Gerd Binnig and Heinrich Rohrer “for their design of the scanning tunneling microscope”.

See the Nobel press release and a related Nobel description for further information.

1981

In these Letters, Aspect and collaborators experimentally tested the spacetime behavior of an entangled system. (While two of the Letters were published in 1982, they are included here as part of the 1981 Milestone selection.) According to quantum mechanics, strong correlations are to be expected between measurements performed on systems that have interacted, even though they are separated at the time of measurement. Consideration of these predictions had led A. Einstein, B. Podolsky, and N. Rosen [Phys. Rev. 47, 777 (1935); see also Physical Review Focus 16, story 10] to argue that quantum mechanics cannot be a complete description of reality. Nearly 30 years later, John S. Bell proved that certain inequalities must hold among polarization measurements performed on two separated particles which had previously interacted if, as Einstein and collaborators felt, quantum mechanics is underlain by local “hidden variables”; these inequalities can be violated in a purely quantum-mechanical system.

Aspect and collaborators tested Bell’s inequalities as generalized by J.F. Clauser, M.A. Horne, A.Shimony, and R.A. Holt [Phys. Rev. Lett. 23, 880 (1969)], in a series of experiments which approached ever closer to the ideal experiment first envisioned by David Bohm, based on the concerns of Einstein, Podolsky, and Rosen. These experiments used pairs of correlated photons produced by laser excitations of an atomic radiative cascade. In the first Letter, the use of single-channel analyzers did not allow the direct measurement of all polarization states; two-channel analyzers were used in the second Letter to overcome this limitation. In the third Letter, variable polarizers were used to eliminate the possibility that unknown interactions among the measuring instruments could evade the inequalities. In these Letters, and in subsequent work by a number of groups, the results showed violations of the generalized Bell inequalities, in accordance with quantum mechanics and in disagreement with local hidden-variable theories. Thus, more than 45 years after the paper of Einstein, Podolsky, and Rosen, the gedankenexperiment suggested by their work was performed, with results that would have disappointed them. Further research on entanglement has led to the new field of quantum information.

1980

This Letter reports the experimental discovery of what became known as the quantum Hall effect (QHE). The classic effect was named after Edwin Hall who, as a student in 1879 of Professor Rowland at Johns Hopkins University, found that a sideways (Hall) voltage was created when a magnetic field was applied transversely to a current-carrying conductor. The discovery earned him a position at Harvard. The discovery of the QHE earned von Klitzing the Nobel Prize in Physics for 1985. von Klitzing studied the two dimensional electron gas confined in the inversion layer of a metal-oxide-semiconductor field-effect transistor. At low temperature the Hall voltage exhibited a series of plateaus at extremely well-defined and reproducible values as the strong transverse field was varied. The experimental results were explained by von Klitzing as arising from the interplay of spatial confinement and orbital quantization of the electron motion in a magnetic field. The remarkable plateaus indicated quantization of the Hall conductance with values at integer multiples of e²/h. The quantum of conductance is entirely determined by fundamental constants (the elementary electric charge e and Planck’s constant h). Its reciprocal provides an associated standard of resistance whose value was named, in 1990, after von Klitzing: R_K = h/e² = 25,812.8 Ohm. The implications for metrology are immediately obvious as the integer Hall effect makes possible a highly accurate determination of another fundamental quantity, the fine-structure constant. The remarkable insensitivity of the position of the quantized plateaus to ubiquitous experimental imperfection was suggestive of some deeper explanation. Later consideration led to theories invoking concepts such as gauge invariance, Berry phases, and a topological invariant known as the Chern number. This is clearly described in a Physics Today article by Avron et al., and we thank Physics Today for making it freely available.

A fuller description of the physics of the quantum Hall effect can be found in the Nobel Prize press release.

1979

In 1977 the Nobel Prize in physics was awarded jointly to P. W. Anderson, Sir Neville Mott, and J. H. Van Vleck “for their fundamental theoretical investigations of the electronic structure of magnetic and disordered systems”. Anderson was cited particularly for a pioneering 1958 Physical Review article titled “Absence of Diffusion in Certain Random Lattices”. In it he showed that, for a simple but reasonably realistic three-dimensional model, transport of electrons does not take place; rather the exact wave functions are localized in space. He also gave an estimate of the critical density at which the theorem fails and extended waves can propagate. Subsequently, Mott, and Thouless, showed that one-dimensional random systems are always localized. But the existence of extended states in two dimensions was a matter of controversy until the publication of this Letter. The authors showed that the waves are weakly localized, i.e. they fall off algebraically, rather than exponentially, to zero. This result was subsequently verified in many experiments, and stimulated the use of size scaling in other theoretical problems.

1978

Appreciable interest in phase transitions in two dimensions arose in the nineteen sixties and seventies. The exact solutions of the two dimensional Ising model and the solutions of Lieb on two dimensional ice and ferroelectrics and of Baxter on the eight vertex model showed that phase transitions to an ordered phase could occur in two dimensions. On the other hand the work of Mermin and Wagner and of Hohenberg proved that the two-dimensional Heisenberg model of magnetism did not have a transition to an ordered phase. But intermediate phase transitions were proposed, for other Hamiltonians, beginning with the work of Stanley and Kaplan, and Kosterlitz and Thouless, in which phase transitions in two dimensions to states without long-range order occur, but with a power-law decay of the correlation functions.

In this Letter Halperin and Nelson outlined a detailed theory of the melting of a two-dimensional solid, based in part on the research of Kosterlitz and Thouless, as produced by dislocations in the solid. A series of transitions results, leading first to melting of the solid, then to a liquid crystal type phase with power law decay of orientational order, and finally to an isotropic fluid. This work has found applications in many examples of thin-film physics and in other theoretical problems.

In the erratum an error in sign, pointed out by A. P. Young [Phys. Rev. B 19, 1855 (1979)] (who was also working on the same problem) showed that, while the qualitative results remained correct, the critical exponent for the solid melting was wrong. This was corrected in a detailed article by Halperin and Nelson [Phys. Rev. B 19, 2457 (1979)]. The theory is often referred to by the initials “KTHNY” (Kosterlitz Thouless, Halperin, Nelson, Young) to indicate the contributions of each of these authors.

See also Erratum, Phys. Rev. Lett. 41, 519 (1978).

1977

The story of this Letter and the chemistry Nobel Prize is interesting, amusing, and illustrative of the importance of scientific conferences and of interdisciplinary research.

Polymers were generally considered to be insulating materials, but in an accidental discovery Shirakawa, in Japan, found that polyacetylene, when prepared with too much catalyst, was not the usual color of an insulating material, but had a silvery tint. Chemists Shirakawa and MacDiarmid met at a conference in Japan, where both presented aspects of their research. (MacDiarmid had been working with Heeger in the US on another polymer that had a metallic appearance.) When they encountered one another during a coffee break MacDiarmid invited Shirakawa to come to the University of Pennsylvania, where the three continued their research on doped polyacetylene. Measurements by physicist Heeger and his students showed that, depending on the mode of preparation of the polymers the electrical conductivity could be varied over eleven orders of magnitude! Theoretical explanations for this variation showed great complexity in the mechanisms for the conductivity. Possible practical applications also became apparent, and several have already been realized.

The 2000 Nobel Prize in Chemistry was awarded to Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa “for the discovery and development of conductive polymers”. Heeger stated in the concluding remarks of his Nobel Lecture [Rev. Mod. Phys. 73, 681 (2001) (free PDF)] that he had become an interdisciplinary scientist, between chemistry and physics. Both aspects were essential for this discovery. The opening of the Nobel Committee’s “Information for the Public” describes the accidents that led to this collaboration. Another excellent description of the work is contained in Phys. Rev. Focus, 6, story 18.

This Letter reports the measurement, by apparatus carried on a NASA U-2 aircraft, of the dipole moment of the anisotropy of the cosmic microwave background (CMB) radiation. Similar results were obtained by D.T. Wilkinson and collaborators in balloon-borne experiments done before and after the U-2 experiments (published in astronomy journals). The dipole can arise from the Doppler shift due to the motion of the measuring instrument with respect to the rest frame of the CMB. The simplest interpretation was that the Milky Way was moving at around 600 km/s; at the time, such a motion was inconsistent with that inferred from measurements of the velocities of nearby galaxies. Today, this motion is understood to arise from the gravitational acceleration of the Milky Way due to the inhomogeneous distribution of galaxies. The U-2 experiment did not have sufficient sensitivity to measure the intrinsic anisotropy of the CMB, whose amplitude is now known to be almost two orders of magnitude smaller.

At the time of this experiment NASA had issued a call for proposals for satellite experiments. One of the resulting proposals, from authors George Smoot and Richard Muller, was for detailed measurements of the CMB, and made use of concepts similar to those used for the U-2 experiment. This proposal and two proposals from other groups led to the development of the Cosmic Background Explorer (COBE) satellite. COBE made the first detection of the intrinsic anisotropy of the CMB, providing an important confirmation for a key idea in big bang cosmology - namely that structure in the distribution of matter arises from the gravitational amplification of tiny primeval fluctuations.

George Smoot and John Mather shared the 2006 Nobel Prize in physics “for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation,” as a result of the COBE work, which was published in astronomy journals. Smoot’s Nobel lecture [Rev. Mod. Phys. 79, 1349 (2007) (free PDF)] discusses the U-2 experiment as well as the COBE results. Mather’s Nobel lecture [Rev. Mod. Phys. 79, 1331 (2007) (free PDF)] provides more information about the history of the COBE satellite. See also the Nobel press release for still more detailed information.

1976

The theory of the free electron laser (FEL), in which a beam of electrons from an accelerator is passed through a periodic transverse magnetic field, was given by Madey in a 1971 paper [J. M. J. Madey, [J. Appl. Phys. 42, 1906 (1971)[(http://dx.doi.org/10.1063/1.1660466)]. The experimental demonstration of the laser was done in two stages and was presented in the above Letters. (While the second Letter appeared in 1977 it is combined here as part of the 1976 Milestones selection.) The first of these Letters shows the gain in the intensity of radiation from a CO₂ laser as it passed along the electron beam in the periodic magnetic field. In work reported in the second Letter the incident radiation was replaced by a mirrored cavity, so that the electromagnetic radiation made multiple passes through the magnetic field and through the electron bunches. The lasing action produced radiation with a narrow spectral line and high peak power.

Many free electron lasers have been built around the world, with operation at shorter and shorter wavelengths. At present there are projects under construction to make free electron lasers operating at X-ray wavelengths. Many applications of FELs have been both demonstrated and proposed.

See a US National Academies 1994 report, and a world wide web virtual library report for further information.

1975

When Martin Perl began his studies in the 1960’s of the elementary particles called leptons there were two known members of that class: the electron and the muon, each with their associated neutrinos (see the 1962 Milestones). He was looking for the possible existence of another heavier lepton. It was not until the availability of electron-positron colliding beam storage rings that the ideal instrument for those studies became available. The SPEAR instrument at the Stanford Linear Accelerator Center, which was involved in the discovery of the ψ particle featured in the 1974 Milestones, was also used by Perl and his collaborators for the search. In the last paragraph of this Letter a claim was made by the authors for the discovery of something new, but they held back from stating that it was a new lepton. It was a long path involving many additional studies by Perl’s group and by many others until other explanations of the results could be eliminated and a clear case made for the discovery of what was named the tau lepton and its associated neutrino. This “third generation” of leptons was followed also by a third generation of quarks, a parallel development for hadrons.

The 1995 Nobel Prize in Physics was awarded “for pioneering experimental contributions to lepton physics” to Martin Perl (“for the discovery of the tau lepton”) and to Frederick Reines (“for the detection of the Neutrino”). The latter’s research was first published in Physical Review in 1953 [Phys. Rev. 92, 830 (1953)], prior to the establishment of Physical Review Letters. See the Focus story Phys. Rev. Focus 19, story 13 for a description of this work.

For further information see the Nobel press release and a recent essay by Martin Perl in Physical Review Letters.

1974

The research reported in these Letters described the observation of a new long-lived particle that engendered considerable excitement among particle physicists. The observation was accomplished by two different routes. The J particle was observed in an experiment using high energy protons which produced e⁺e^- pairs as a product of the decay of the new particle. The same particle (named ψ by the authors of the second Letter) was produced in the collisions of electrons and positrons in a storage ring and studied in great detail by the authors of that Letter. The proton experiment provided, by measurement of the number of e⁺e^- pairs as a function of their energy, a clear indication of the existence of a peak, while the colliding beams could do both the search (although with greater difficulty) and then look in great detail at the properties of the particle. A number of explanations were immediately put forward for the observations. At the time of the experiments there was evidence for three quarks (called “up”, “down”, and “strange”) and an earlier proposal for a fourth (called “charm” by Bjorken and Glashow, the proposers). The ultimate explanation was that the particle was a bound state of a charm—anti-charm quark pair, dubbed “charmonium”, analogous to the bound state positronium of an e⁺e^- pair.

The 1976 Nobel Prize in Physics was awarded to Burton Richter and Samuel C. C. Ting “for their pioneering work in the discovery of a heavy elementary particle of a new kind”. See the Nobel press release for a readable explanation of the discovery. An indication of the excitement and the revolutionary nature of this discovery is that less than two years elapsed between the publications and the award of the prize. This can be contrasted with some of the awards that followed the initial publication by forty years!

1973

In these Letters the authors showed that in certain theories of the strong interactions — the forces that hold nuclei together — it was possible for forces to become weaker as the energy increased. This “asymptotic freedom” meant that calculations using perturbation theory could be carried out to obtain results that might be compared with experiment. Wilczek (at Princeton) and Politzer (at Harvard) did their research as graduate students, and Gross was Wilczek’s thesis adviser. Their work withstood the test of time, and the 2004 Nobel Prize in Physics was awarded to David Gross, H. David Politzer, and Frank Wilczek “for the discovery of asymptotic freedom in the theory of the strong interaction”.

See also the Nobel committee’s information for the public and the Nobel Focus story Phys. Rev. Focus 14, story 15. These give a detailed yet understandable description of this work and its significance.

1972

Kenneth Wilson received the 1982 Nobel Prize in physics for his theory of phase transitions based on what is called the renormalization group. He built on previous work by Leo Kadanoff and Michael Fisher, with whom he shared the 1980 Wolf Prize. Wilson’s Nobel Prize winning work was published in 1971 in Physical Review B [PRB 4, 3174, and PRB 4, 3184]. In the Letters quoted here several consequences of the work in the PRB publications were developed and were used in much future research. An excellent, accessible summary of the background and content of Wilson’s research and of its impact is available in the Nobel committee’s 1982 press release.

Osheroff, Richardson, and Lee were looking for a phase transition to a kind of magnetic order in solid helium-3 at very low temperatures. They were achieving these temperatures by using what was called “Pomeranchuk cooling” – putting pressure on a mixture of solid and liquid helium-3. In the first Letter two anomalies in the measurement were interpreted as phase transitions in solid helium-3. But because of uncertainties in the interpretation the authors used a crude version of what later was named “magnetic resonance imaging” to determine that the anomalies were in the liquid. Other measurements led to the conclusion that they had in fact discovered superfluid helium-3. An important element in this interpretation was the theoretical work of Leggett (see following Milestone). For this research Osheroff, Richardson, and Lee received the 1996 Nobel Prize in Physics “for the discovery of superfluid helium-3”.

For further information see David Lee’s Nobel lecture [Rev. Mod. Phys. 69, 645 (1997) (free PDF)] and the Nobel Committee’s press release.

Leggett describes in his Nobel lecture how he interrupted a holiday (because it rained on that day!) to meet with Robert Richardson at Leggett’s office in Sussex. Richardson described the experimental results on phase transitions in liquid He³, which set Leggett into deep thought on their theoretical explanation. This Letter presents the foundation for understanding the properties of these complex superfluid phases. Many others contributed to that understanding as well, but Leggett was credited with the complete theoretical underpinning. A. A. Abrikosov, V. L. Ginzburg, and A. J. Leggett were awarded the 2003 Nobel Prize in Physics “for pioneering contributions to the theory of superconductors and superfluids”.

[See also an important erratum: PRL 30, 411 (1973)]. Also, see Leggett’s Nobel Lecture [Rev. Mod. Phys. 76, 999 (2004) (free PDF)], and the Nobel Focus story Phys. Rev. Focus 12, story 16.

1971

Exact mathematical solutions of model systems that undergo phase transitions are of considerable interest. The earliest and most famous of these was Lars Onsager’s solution (Phys. Rev. 65, 117 (1944)) of the two dimensional Ising model, which illustrates the temperature dependent behavior of a ferromagnet above and below the transition temperature. Subsequently a number of other two dimensional models (called the “ice”, “F” and “KDP” models) were solved by Elliott Lieb (Phys. Rev. 162,162; PRL 18, 1046, and 19, 108; all 1967) using novel mathematical methods. The “F” and “KDP” models also contained an electric field. In this Letter Baxter, motivated by Lieb’s results, solved a problem that contains Lieb’s models (but without the presence of the electric field) as special cases. The properties of the solution are unusual; they have generated continued interest and research, including further developments by Baxter, to this day.

1970

This Letter is regarded as the beginning of the use of light to trap particles, which has led to many famous applications. That light exerts pressure on matter has been known since Maxwell’s development of the theory of electromagnetism. In this paper Ashkin reports his experiments using the pressure from laser light to move and trap small transparent spheres in a transparent liquid. In addition he foresees possible developments and their uses of this technique, among them the trapping of individual atoms and viruses. He carried out and participated in many of these important experiments, which have gained wide recognition.

1969

Scattering of particles by molecules, atoms and nuclei has long been used to elicit information about the internal structure of those entities. A beam of particles is directed at a sample of the objects under study; measurement of the angular distribution of the particles and their energy after the collisions, as well as other particles produced by the collisions, gives information about the internal structure of the objects. The first, and most famous, example was the experiment in Ernest Rutherford’s laboratory in which alpha particles were directed at a foil. The surprising result that occasionally one of the alpha particles was scattered directly back at the source led to the planetary model of the atom, with electrons surrounding a small nucleus. In more recent times experiments by Robert Hofstadter using electrons of up to 1 GeV from an accelerator gave information about the size of the proton and of other atomic nuclei. (Hofstadter shared the 1961 Nobel Prize in physics for his experiments). With the completion of the two-mile-long 20 GeV Stanford Linear Accelerator various energy electron beams were turned on a hydrogen target, as described in these Letters, with the object of determining the internal structure of the proton, using deep inelastic scattering. The results inspired a number of different explanations, but eventually the combination of inelastic electron and neutrino scattering demonstrated the reality of quarks as part of the proton structure. For their role in this research Jerome Friedman, Henry Kendall, and Richard Taylor shared the 1990 Nobel Prize in Physics (see also the Nobel press release for this award).

1968

The path for Raymond Davis from the beginnings in the 1950s of his search for neutrinos from the sun to the 2002 Nobel Prize was long and uncertain - see his Nobel lecture (presented by Davis’ son Andrew), reprinted in Rev. Mod. Phys. 75, 985 (2003) [free PDF]. It required the establishment of an observatory in a mine 1500 meters below ground level (to shield out other radiation that might give false signals) with a 380,000 liter tank of perchloroethylene, a cleaning fluid. The chlorine 37 nucleus captures a neutrino and emits an electron, producing argon 37, which is then detected in a complex process. This requires a precise calculation of the theoretical production rate of solar neutrinos according to models of the solar interior. The vital theory was carried out by John Bahcall and coworkers over many years, in a long collaboration with Davis. The Letters presented here give initial results from the experiment and their interpretation. The solar neutrino puzzle, that only about one third the expected number of neutrinos was found by Davis, gathered over the years a greater measure of precision. For the resolution, which contributed not only to the understanding of the sun’s interior but to the nature of neutrinos themselves, see Davis’ Nobel lecture and Phys. Rev. Focus 10, story 18.

The 2002 Nobel Prize was awarded to Raymond Davis and Masatoshi Koshiba for the development of neutrino astronomy, and to Riccardo Giacconi for the discovery of cosmic x-ray sources (see the 1962 Milestones).

*The published title erroneously refers to chlorine 36—the abstract and article properly refer to chlorine 37.

1967

Weinberg showed in this famous Letter how the electromagnetic force, carried by the massless photon, and weak force (which is responsible for beta decay), carried by massive W and Z bosons, are unified into a single electroweak force. We observe two different forces because the gauge symmetry of the electroweak force is spontaneously broken (see the Milestone Letters of 1964), giving mass to the W and Z and thus limiting the weak force range. This Letter is the most highly cited research paper in particle physics, yet it was not cited in a journal at all in the first year after its publication, and was referred to only twice in the following two years.

Sheldon Glashow, Abdus Salam, and Steven Weinberg shared the 1979 Nobel Prize for their contributions to this theory.

1966

In this Letter the authors prove that the isotropic Heisenberg model exhibits no long-range magnetic order in either one or two spatial dimensions. As the authors acknowledge, their research was stimulated by P. C. Hohenberg’s similar conclusions for superfluids (Phys Rev 158, 383 (1967)). The results of their theoretical work, while long suspected from approximate calculations, represent a first rigorous proof of the absence of long-range order in these systems, and the process has been widely used, up to the present, in many other theoretical applications. Note that there are many typographical errors in the paper, which are corrected in an erratum (PRL 17, 1307 (1966)).

1965

In this highly cited and frequently downloaded Letter, Zabusky and Kruskal examine the Fermi-Pasta-Ulam-Tsingou paradox, which was found in what arguably was the first computer experiment. Instead of dividing the energy of a nonlinear system among the different modes, they found that, when the computer was run for a sufficient length of time, the initial states recurred. For a fascinating explanation of the inclusion of the unfamiliar name of Mary Tsingou, see T. Dauxois, Physics Today 61, 55 (2008) [PDF].

The following quote from that article, which was discovered while researching the Zabusky-Kruskal Letter, gives an excellent summary of the importance of the Letter:

“In the 1960s, pursuing the solution of the FPU paradox, Norman Zabusky and Martin Kruskal looked at the problem in real space rather than in Fourier space. They were able to explain the periodic behavior in terms of the dynamics of localized excitations now known as solitons. Those localized, or solitary, waves with the properties of particles (hence the suffix “-on”) have many physical applications and are today the subject of a field of study in their own right.”

We thank Professor Thierry Dauxois and Physics Today for permission to quote from that most interesting bit of history, and for making the article freely available on line.

1964

K mesons were discovered and studied through the 1950s and, because of the odd way they decayed, came to be called “strange” particles and assigned a strangeness quantum number. The neutral Κ meson (Κ⁰), with strangeness +1, can mix through the weak interaction with its antiparticle the K⁰, with strangeness -1, to form the mass eigenstates Κ_S⁰ and Κ_L⁰. Both these eigenstates decay through the weak interaction into pions. It was known at the time of this Letter that the weak interaction violated parity P and charge conjugation C, but it was believed that it conserved charge parity CP. Conservation of CP requires that the Κ_S⁰ meson can only decay into an even-pion state (which has CP = +1), and the Κ_L⁰ only into an odd-pion state (CP = -1). This Letter presented experimental evidence that the Κ_L⁰ meson (called Κ₂⁰ at the time) also has a small probability of decaying into 2 pions, so that the weak interaction violates charge parity as well.

For the discovery of CP violation in the decay of neutral K mesons, Cronin and Fitch were awarded the 1980 Nobel Prize in Physics.

 

These Letters contain an explanation that shows how mass could arise in local gauge theories. Gauge symmetries explain how the strong and electroweak forces arise, but such symmetries forbid vector boson mass terms. The authors showed how gauge symmetries could be spontaneously broken in such a way that the vector bosons of the theory acquire mass. A number of earlier Physical Review papers foresaw different aspects of this mechanism. The mechanism predicts the existence of a physical particle, known as the Higgs boson. In 2012, two experiments at the Large Hadron Collider at CERN observed the Higgs boson, thus validating the prediction.

The 2004 Wolf Prize was awarded to Englert, Brout, and Higgs for their contributions to the theories, and the 2010 APS Sakurai Prize was awarded to Englert, Brout, Higgs, Guralnik, Hagen, and Kibble. Brout passed away in 2011. The Nobel Prize in Physics for 2013 was awarded to Englert and Higgs.

1963

This Letter first introduced Glauber’s treatment of the quantum properties of light and its interactions with atoms. He developed this extensively in articles in Physical Review, and presented seventeen(!) lectures on the subject at the 1964 Les Houches summer school (published in the book Quantum Optics and Electronics, Gordon and Breach, 1965). There was much disagreement over the need for a quantum treatment. Glauber argued that the then recent development of the laser and of detectors that could measure a single optical photon made more than a classical treatment essential. Ultimately his view prevailed, and was recognized with the 2005 Nobel Prize.

See also Phys. Rev. Focus 16, story 13.

In this Letter Kerr presented a new exact solution of the Einstein gravitational field equations, only the third such solution up to that time. The expression for the metric tensor, now known as the Kerr metric, describes the space-time geometry, called frame dragging, in the vicinity of an uncharged rotating point mass (or a black hole). It also describes, approximately, the geometry outside an extended rotating body such as the Earth. In spite of its seemingly impenetrable mathematics and terminology the paper was understood, appreciated, and built upon by physicists working in the field. An experimental test of predictions based on the metric has been a major goal of Gravity Probe B, a NASA/Stanford satellite mission launched in 2004, which collected data until 2005.

See also Physics News Update 820 #2.

1962

This Letter reported an experiment that demonstrated a difference in neutrinos produced in the decay of π-mesons into muons, and neutrinos from those produced in β-decay, the decay of nuclei into electrons and neutrinos. The π-mesons were produced by a beam of protons striking a target; the π-mesons then decayed in flight into muons and neutrinos. After passing through shielding to remove all other particles the neutrinos interacted with the matter in the plates of an aluminum spark chamber, where they produced muons but not electrons. For this discovery of the muon neutrino and the development of the neutrino-beam method that made it possible the 1988 Nobel Prize was given to three of the authors of this Letter: Leon Lederman, Melvin Schwartz, and Jack Steinberger.

Riccardo Giacconi shared the 2002 Nobel Prize with Raymond Davis and Masatoshi Koshiba; Davis and Koshiba for the development of neutrino astronomy, and Giacconi for the discovery of cosmic x-ray sources. This Letter reports the first observation of an x-ray source outside the Solar System. The x-ray detector was launched on a rocket to look for x-ray emissions from the Moon; instead they found a bright source of soft x-rays in the constellation Scorpius. This source, now known as Scorpius X-1, is the brightest x-ray source in the sky after the Sun. It has since been identified as a neutron star in a binary-star system some 9,000 light years away.

See also Phys. Rev. Focus 10, story 18.

This Letter reports the first observation of laser action in a solid-state device; it was followed (very) closely by groups at IBM, Lincoln Laboratory of MIT, and by others at General Electric. All of the latter publications appeared in the first volume of Applied Physics Letters. Further development of diode lasers, producing high collimation and intensity and with ease of fabrication, ultimately made possible the optical storage of data, music, and video.

1961

After the successful demonstration of the ruby laser by Maiman it was realized that focusing of the laser beam could produce a very strong electric field in a dielectric. This clearly written Letter is widely recognized as a pioneering demonstration of a nonlinear optical effect, where the intense laser beam produces a second beam at twice the laser light frequency. The paper contains a theoretical outline and experimental example of the second harmonic generation of light in quartz, and it is still heavily read and cited.

In the first of these two Letters, Deaver and Fairbank reported their observation that the magnetic flux trapped in hollow superconducting tin cylinders was quantized in units of hc/2e. Earlier, in 1950, Fritz London had predicted this quantization effect but in units of hc/e, twice the result of Deaver and Fairbank. In the second Letter Byers and Yang interpreted Deaver and Fairbank’s results. They showed that the quantization of magnetic flux through a superconducting ring is closely related to the Meissner effect, in which a superconductor expels magnetic flux from its interior, and that the factor of 1/2 is an indication of the Bardeen-Cooper-Schrieffer pairing of the electrons in the superconducting state.

1960

After the discovery in 1958 of recoil free absorption of gamma rays in iridium by Rudolf Mössbauer there was much interest in performing experiments with different isomers. Attention focused on Fe⁵⁷, with the expectation that an experiment to detect the effect of gravity on gamma radiation, as predicted by Einstein in 1911, could be carried out. Several groups made efforts to observe the resonance in Fe⁵⁷, and once this was accomplished there was an explosion of experiments that led to the observation and application of nuclear hyperfine magnetic and quadrupole splittings and the chemical isomer shift. In this Letter Pound and Rebka describe the results of their experiment, which was the most definitive of the efforts to observe gravitational effects. They had to account for many possible differences between the source and the absorber that could mask the gravitational shift of the frequency, and the results were in agreement with Einstein’s prediction.

See also Phys. Rev. Focus 16, story 1.

Ivar Giaever shared the 1973 Nobel Prize with Leo Esaki and Brian Josephson; Esaki for the junction diode based on tunneling in semiconductors, and Josephson for the tunneling of BCS electron pairs between superconductors. The two Letters shown here were the basis for the award to Giaever. In these experiments he demonstrated the change in the electron density of states (the appearance of a gap) on going from the normal to the superconducting state, in agreement with the BCS theory.

Forty-eight years after the publication of this Letter the Nobel Prize in Physics was awarded to Yoichiro Nambu “for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics”. Also mentioned in the award were two articles in Physical Review.

The award was shared by Makoto Kobayashi and Toshihide Maskawa, who published their work in the Japanese journal Progress of Theoretical Physics.

For further information see Phys. Rev. Focus 22, story 13, the Nobel Press Release, and the Nobel Information for the Public.

1959

Bertram Brockhouse and Clifford Shull shared the physics Nobel Prize in 1994 for their researches using neutron scattering. Brockhouse was cited for his experiments using inelastic scattering, which enabled the study of excitations in solids, while Shull received the award for his elastic scattering studies, which gave information on structure, both magnetic and physical, of condensed matter. It is difficult to cite a single paper that led to this prize, but we give here an example of the work of Brockhouse that uses the techniques (particularly the “triple axis spectrometer”) for which he was cited. The phonon spectra of silicon and germanium are of particular interest because of their considerable practical as well as theoretical importance.

This very mathematical Letter has had a significant impact in all areas of physics and chemistry. The formalism enables the calculation of the thermodynamic properties of quantum-mechanical systems in condensed matter and in quark matter. Hubbard outlines a rederivation of results for the correlation energy of an electron gas, of the Bardeen-Cooper-Schrieffer theory of superconductivity, and of correlations in nuclear matter. The mathematical techniques have also made possible numerical Monte Carlo calculations of the properties of quantum-mechanical systems. After nearly fifty years it is still very frequently downloaded from the APS PROLA archive.

1958

This Letter announced the first synthesis of an isotope of the transuranic element 102 and described a then-new technique to identify high-Z elements. A year earlier researchers at the Nobel Institute in Sweden had claimed to have discovered an isotope of this element with a 10-minute half-life and proposed the name “nobelium” for it [P. R. Fields et al., Phys. Rev. 107, 1460 (1957). This claim could not be confirmed, either by these authors at UC Berkeley [PRL 1, 17 (1958)] or by a group at the Joint Institute for Nuclear Research in Dubna, Russia. The Dubna group did confirm the Berkeley group’s work and further confirming experiments were done at Berkeley in 1966. The Berkeley group eventually got credit for the discovery and the right to name it, but recommended that the name nobelium be retained.

The famous Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, for which the Nobel prize was awarded in 1972, was announced in 1957, before the creation of Physical Review Letters. Many papers following up on that theory subsequently appeared in PRL. In this paper Bardeen raises the question of the oft-made comparison between superconductivity and the superfluidity of liquid helium 4. The phenomenological two fluid (normal and superfluid) model of helium 4 introduced by Fritz London was applied to superconducting electrons as well, but the BCS theory justification for this was not obvious. Bardeen here shows that there is a basis for that comparison, based on reasonable approximations.