The
atoms assume definite orientations relative to the field, and in
consequence of the nonuniformity of the field, are deflected by
differing amounts. The experiment is considered to provide proof
that there exist only certain permitted orientations: otherwise,
instead of beam splitting, there would merely be a broadening of
the beam due to random atom orientation. Proof of the existence
of only permitted orientations in turn provides proof of the
quantization of the angular momentum of the electron.
The following points are made by B. Friedrich and D. Herschbach
(Physics Today 2003 December):
1) The demonstration of space quantization, carried out in
Frankfurt, Germany, in 1922 by Otto Stern (188-1969) and Walther
Gerlach (1889-1979), ranks among the dozen or so canonical
experiments that ushered in the heroic age of quantum physics.
Perhaps no other experiment is so often cited for elegant
conceptual simplicity. From it emerged both new intellectual
vistas and a host of useful applications of quantum science.
2) The success of Stern and Gerlach in splitting a beam of silver
atoms by means of a magnetic field startled, elated, and
confounded pioneering quantum theorists, including several
eminent physicists who beforehand had regarded an attempt to
observe space quantization as naive and foolish.
3) Descendants of the Stern-Gerlach experiment (SGE) and its key
concept of sorting quantum states via space quantization are
legion. Among them are the prototypes for nuclear magnetic
resonance, optical pumping, the laser, and atomic clocks, as well
as incisive discoveries such as the Lamb shift and the anomalous
increment in the magnetic moment of the electron, which launched
quantum electrodynamics. The means to probe nuclei, proteins, and
galaxies; image bodies and brains; perform eye surgery; read
music or data from compact disks; and scan bar codes on grocery
packages or DNA base pairs in the human genome all stem from
exploiting transitions between space-quantized quantum states.
4) Otto Stern received his doctorate in physical chemistry at the
University of Breslau in 1912. In his dissertation, he presented
theory and experiments on osmotic pressure of concentrated
solutions of carbon dioxide in various solvents -- just
generalized soda water. His proud parents offered to support him
for postdoctoral study anywhere he liked. "Motivated by a spirit
of adventure," Stern became the first pupil of Albert Einstein
(1879-1955), then in Prague; their discussions were held "in a
cafe which was attached to a brothel."(2) Soon Einstein was
recalled to Zurich. Stern accompanied him there and was appointed
privatdozent for physical chemistry.
5) Walther Gerlach received his doctorate in physics at the
University of Tubingen in 1912. His research dealt with
blackbody radiation and the photoelectric effect. While serving
in the military during World War I, Gerlach worked with Wilhelm
Wien (1864-1928) on the development of wireless telegraphy. After
a brief interlude in industry, Gerlach obtained an appointment in
1920 at Frankfurt as assistant in the Institute for Experimental
Physics, adjacent to Born's institute.(1,3-5)
References (abridged):
1. For fuller descriptions of the context and legacy of the
Stern-Gerlach experiment, see J. Mehra, H. Rechenberg, The
Historical Development of Quantum Theory, vol. 1, part 2,
Springer-Verlag, New York (1982), p. 433, and B. Friedrich, D.
Herschbach, Daedalus 127, 165 (1998)
2. K. Mendelssohn, The World of Walther Nernst: The Rise and Fall
of German Science, Macmillan, London (1973), p. 95
3. F. Hund, Geschichte der Quantentheorie [The History of Quantum
Theory], Bibliographisches Institut, Mannheim, Germany (1975)
4. O. Stern, in Les Prix Nobel en 1946, Imprimerie Royale
Norstedt and Soner, Stockholm (1948), p. 123. Available online at
http://www.nobel.se/physics/laureates/1943/stern-lecture.html
5. I. Estermann, Am. J. Phys. 43, 661 (1975) [INSPEC]. See also
I. Estermann in I. Estermann, ed., Recent Research in Molecular
Beams: A Collection of Papers Dedicated to Otto Stern, Academic
Press, New York (1959)
Physics Today
http://www.physicstoday.org
--------------------------------
ON SPIN-BASED ELECTRONICS
The following points are made by S.A. Wolf et al (Science 2001
294:1488):
1) Until recently, the spin of the electron was ignored in
mainstream charge-based electronics. A technology has emerged
called "spintronics" (spin transport electronics or spin-based
electronics), where it is not the electron charge but the
electron spin that carries information, and this offers
opportunities for a new generation of devices combining standard
microelectronics with spin-dependent effects that arise from the
interaction between spin of the carrier and the magnetic
properties of the material.
2) Traditional approaches to using spin are based on the
alignment of a spin (either "up" or "down") relative to a
reference (an applied magnetic field or magnetization orientation
of the ferromagnetic film). Device operations then proceed with
some quantity (electrical current) that depends in a predictable
way on the degree of alignment. Adding the spin degree of freedom
to conventional semiconductor charge-based electronics or using
the spin degree of freedom alone will add substantially more
capability and performance to electronic products. The advantages
of these new devices would be nonvolatility, increased data
processing speed, decreased electric power consumption, and
increased integration densities compared with conventional
semiconductor devices.
3) Major challenges in this field of spintronics that are
addressed by experiment and theory include the optimization of
electron spin lifetimes, the detection of spin coherence in
nanoscale structures, transport of spin-polarized carriers across
relevant length scales and heterointerfaces, and the manipulation
of both electron and nuclear spins on sufficiently fast time
scales. In response, recent experiments suggest that the storage
time of quantum information encoded in electron spins may be
extended through their strong interplay with nuclear spins in the
solid state. Moreover, optical methods for spin injection,
detection, and manipulation have been developed that exploit the
ability to precisely engineer the coupling between electron spin
and optical photons.
4) The author describes a new paradigm of electronics based on
the spin degree of freedom of the electron. Either adding the
spin degree of freedom to conventional charge-based electronic
devices or using the spin alone has the potential advantages of
nonvolatility, increased data processing speed, decreased
electric power consumption, and increased integration densities
compared with conventional semiconductor devices. To successfully
incorporate spins into existing semiconductor technology, one has
to resolve technical issues such as efficient injection,
transport, control and manipulation, and detection of spin
polarization as well as spin-polarized currents. Recent advances
in new materials engineering hold the promise of realizing
spintronic devices in the near future.(1-5)
References (abridged):
1. M. Baibich, et al., Phys. Rev. Lett. 61, 2472 (1988)
2. J. Barnas, A. Fuss, R. Camley. P. Grunberg and W. Zinn, Phys.
Rev. B 42, 8110 (1990)
3. G. Prinz, Science 282, 1660 (1998)
4. B. Dieny, et al., J. Appl. Phys. 69, 4774 (1991)
5. S. Parkin, D. Mauri, Phys. Rev. B 44 7131 (1991)
Science
http://www.sciencemag.org
--------------------------------
ON SPINTRONICS AND QUANTUM COMPUTING
The following points are made by J.H. Smet et al (Nature 2002
415:281):
1) Discussions of future information-processing technologies
often assign a prominent role to the spin degree of freedom in
addition to (or instead of) the charge degree of freedom(1),
which is exploited in today's mainstream electronics. In the
short term, this "spintronics" may deliver products with enhanced
functionality or improved performance, such as high-speed, high-
density non-volatile random access memories: whereas on a much
longer timescale, contributions to the very challenging realm of
quantum computation(2,3) have been anticipated.
2) Quantum computation attempts to benefit from correlations and
dissipationless transformations of coupled quantum-mechanical
systems. The main incentive is a certain degree of parallelism
that computational schemes based on such principles bring with
them. For example, such schemes offer algorithms for prime
factorization(4) and for exhaustive search(5); unlike any
apparatus based on classical physics, a quantum computer should
be able to solve these problems in polynomial time -- provided
that it can be implemented in a real machine, as energy
dissipation is a fundamental source of concern.
3) There has been a wide variety of proposals for practical
implementations of rudimentary logic gates in which quantum
memory registers -- based on any of the abundant two-level
systems in physics, like spin-1/2 electrons and nuclei -- can be
externally manipulated. These proposals range from trap
configurations in atom or ion physics, to techniques of nuclear
magnetic resonance spectroscopy as used in organic chemistry, to
a very bold all-electronic approach for nuclear spin solid-state
devices that would marry the merits of electronics fabrication
technology with the virtues of quantum computation.
4) Many of the ideas produced by workers in the spintronics and
quantum computing communities may be deemed far out of reach. But
they have sparked efforts to develop new ways to accomplish the
more fundamental task of controlling and measuring the nuclear
spin polarization in solid-state devices, in view of the dearth
of existing techniques for locally manipulating nuclear spins.
Particularly appealing is the use of mobile objects, like
conduction electrons in semiconductors, as mediators to both
probe and modify nuclear spins. Gating and optical techniques are
able to tailor precisely the population and energy distribution
of such electrons, especially when they are constrained to move
in two dimensions -- as in quantum wells or field-effect
transistors -- or even fewer dimensions. The creation of non-
equilibrium populations of spin-polarized electrons using
coherent polarized light pulses or gating techniques have, for
example, already enabled dynamical control of nuclear spins or
the electronic generation of net nuclear spin polarization.
Progress in this area will rely on experiments specifically
geared towards expanding limited knowledge of controlled spin
interactions and the microscopic interaction processes that take
place between spin systems in such low-dimensional structures.
5) In summary: The authors report procedures that carry out the
controlled transfer of spin angular momentum between electrons --
confined to two dimensions and subjected to a perpendicular
magnetic field -- and the nuclei of the host semiconductor, using
gate voltages only. The authors demonstrate that the spin
transfer rate can be enhanced near a ferromagnetic ground state
of the electron system, and that the induced nuclear spin
polarization can be subsequently stored and "read out". These
techniques can also be combined into a spectroscopic tool to
detect the low-energy collective excitations in the electron
system that promote the spin transfer. The existence of such
excitations is contingent on appropriate electron-electron
correlations, and these can be tuned by changing, for example,
the electron density via a gate voltage.
References (abridged):
1. Prinz, G. A. Magnetoelectronics. Science 282, 1660-1663 (1998)
2. Bennett, C. H. & DiVincenzo, D. P. Quantum information and
computation. Nature 404, 247-255 (2000)
3. Steane, A. Quantum computing. Rep. Prog. Phys. 61, 117-173
(1998)
4. Ekert, A. & Jozsa, R. Quantum computation and Shor's factoring
algorithm. Rev. Mod. Phys. 68, 733-753 (1996)
5. Grover, L. K. Quantum mechanics helps in searching for a
needle in a haystack. Phys. Rev. Lett. 79, 325-328 (1997)
Nature
http://www.nature.com/nature
ScienceWeek
http://www.scienceweek.com
