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Topic Name: Physics Successfully Reduce the Casimir Force by Altering the Surface of the Plates
Category: Quantum Computing
Research persons: Ho Bun Chan, Hendrik Casimir
Location: University of Florida, United States
Details
Cymbals don’t clash of their own accord – in our world,
anyway. But the quantum world is bizarrely different. Two metal plates, placed
almost infinitesimally close together, spontaneously attract each other.
What seems like magic is known as the Casimir force, and it has been
well-documented in experiments. The cause goes to the heart of quantum physics:
Seemingly empty space is not actually empty but contains virtual particles
associated with fluctuating electromagnetic fields. These particles push the
plates from both the inside and the outside. However, only virtual particles of
shorter wavelengths — in the quantum world, particles exist simultaneously as
waves — can fit into the space between the plates, so that the outward pressure
is slightly smaller than the inward pressure. The result is the plates are
forced together.
Now, University of Florida
physicists have found they can reduce the Casimir force by altering the surface
of the plates. The discovery could prove useful as tiny “microelectromechanical”
systems — so-called MEMS devices that are already used in a wide array of
consumer products — become so small they are affected by quantum forces.
“We are not talking about an immediate application,” says Ho Bun Chan, an
assistant professor of physics and the first author of a paper. “We are talking
about, if the devices continue to be smaller and smaller, as the trend of
miniaturization occurs, then the quantum effects could come into play.”
More specifically, the finding could one day help reduce what MEMS engineers
call “stiction” — when two very small, very close objects tend to stick
together.
Although stiction has many causes — including, for example, the presence of
water molecules that tend to clump together — the Casimir force can contribute.
Such quantum effects could prove important as the separations between components
in tiny machinery shrink from micrometer, or millionths of a meter, toward
nanometer size, Chan said.
“A lot of people are thinking of ways to reduce stiction, and this research
opens up one possibility,” he said.
Dutch physicist Hendrik Casimir first predicted that two closely spaced metal
plates would be mutually attracted in 1948. It took several decades, but in
1996, physicist Steve Lamoreaux, then at the
University of Washington,
performed the first accurate measurement of the Casimir force using a torsional
pendulum, an instrument for measuring very weak forces.
Subsequently, in a paper published in Science in 2001, Chan and other members of
a Bell Labs team reported tapping the Casimir force to move a tiny metal
see-saw. The researchers suspended a metal sphere an extremely tiny but
well-controlled distance above the see-saw to “push” it up and down. It was the
first demonstration of the Casimir force affecting a micromechanical device.
In the latest research, the physicists radically altered the shape of the metal
plates, corrugating them into evenly spaced trenches so that they resembled a
kind of three-dimensional comb. They then compared the Casimir forces generated
by these corrugated objects with those generated by standard plates, all also
against a metal sphere.
The result? “The force is smaller for the corrugated object but not as small as
we anticipated,” Chan said, adding that if corrugating the metal reduced its
total area by half, the Casimir force was reduced by only 30 to 40 percent.
Chan said the experiment shows that it is not possible to simply add the force
on the constituent solid parts of the plate — in this case, the tines — to
arrive at the total force. Rather, he said, “the force actually depends on the
geometry of the object.”
“Until now, no significant or nontrivial corrections to the Casimir force due to
boundary conditions have been observed experimentally,” wrote Lamoreaux, now at
Yale University, in a commentary accompanying publication of the paper.
Besides Chan, the other authors of the paper are UF doctoral students Yiliang
Bao and Jie Zou, and Bell Labs scientists Raymond Cirelli, Fred Klemens, William
Mansfield and Chien-Shing Pai. The research was funded by the
U.S. Department of Energy.
Note for Casimir Force
In physics, the Casimir effect and the Casimir-Polder force are physical forces
arising from a quantized field. The typical example is of two uncharged metallic
plates in a vacuum, placed a few micrometers apart, without any external
electromagnetic field. In a classical description, the lack of an external field
also means that there is no field between the plates, and no force would be
measured between them. When this field is instead studied using quantum
mechanics, it is seen that the plates do affect the virtual photons which
constitute the field, and generate a net force—either an attraction or a
repulsion depending on the specific arrangement of the two plates. This force
has been measured, and is a striking example of an effect purely due to second
quantization.
Dutch physicists Hendrik B. G. Casimir and Dirk Polder first proposed the
existence of the force and formulated an experiment to detect it in 1948 while
participating in research at Philips Research Labs. The classic form of the
experiment, described above, successfully demonstrated the force to within 15%
of the value predicted by the theory.
Because the strength of the force falls off rapidly with distance, it is only
measurable when the distance between the objects is extremely small. On a
submicrometre scale, this force becomes so strong that it becomes the dominant
force between uncharged conductors. In fact, at separations of 10 nm—about 100
times the typical size of an atom—the Casimir effect produces the equivalent of
1 atmosphere of pressure (101.3 kPa).
Although the Casimir effect can be expressed in terms of virtual particles
interacting with the objects, it is best described and more easily calculated in
terms of the zero-point energy of a quantized field in the intervening space
between the objects. In modern theoretical physics, the Casimir effect plays an
important role in the chiral bag model of the nucleon; and in applied physics,
it is becoming increasingly important in the development of the ever-smaller,
miniaturised components of emerging microtechnologies and nanotechnologies.
The Casimir effect can be understood by the idea that the presence of conducting
metals and dielectrics alter the vacuum expectation value of the energy of the
second quantized electromagnetic field. Since the value of this energy depends
on the shapes and positions of the conductors and dielectrics, the Casimir
effect manifests itself as a force between such objects.
The Casimir effect is an outcome of quantum field theory, which states that all
of the various fundamental fields, such as the electromagnetic field, must be
quantized at each and every point in space. In a simplified view, a "field" in
physics may be envisioned as if space were filled with interconnected vibrating
balls and springs, and the strength of the field can be visualized as the
displacement of a ball from its rest position. Vibrations in this field
propagate and are governed by the appropriate wave equation for the particular
field in question. The second quantization of quantum field theory requires that
each such ball-spring combination be quantized, that is, that the strength of
the field be quantized at each point in space. Canonically, the field at each
point in space is a simple harmonic oscillator, and its quantization places a
quantum harmonic oscillator at each point. Excitations of the field correspond
to the elementary particles of particle physics. However, even the vacuum has a
vastly complex structure. All calculations of quantum field theory must be made
in relation to this model of the vacuum.
Note for Quantum Mechanics
Quantum mechanics is the study of mechanical systems whose dimensions are close
to or below the atomic scale, such as molecules, atoms, electrons, protons and
other subatomic particles. Quantum mechanics is a fundamental branch of physics
with wide applications. Quantum theory generalizes classical mechanics and
provides accurate descriptions for many previously unexplained phenomena such as
black body radiation and stable electron orbits. The effects of quantum
mechanics are typically not observable on macroscopic scales, but become evident
at the atomic and subatomic level. There are however exceptions to this rule
such as superfluidity.
Quantum mechanics is essential to understand the behavior of systems at atomic
length scales and smaller. For example, if Newtonian mechanics governed the
workings of an atom, electrons would rapidly travel towards and collide with the
nucleus, making stable atoms impossible. However, in the natural world the
electrons normally remain in an unknown orbital path around the nucleus, defying
classical electromagnetism.
Quantum mechanics was initially developed to provide a better explanation of the
atom, especially the spectra of light emitted by different atomic species. The
quantum theory of the atom was developed as an explanation for the electron's
staying in its orbital, which could not be explained by Newton's laws of motion
and by Maxwell's laws of classical electromagnetism.
In the formalism of quantum mechanics, the state of a system at a given time is
described by a complex wave function (sometimes referred to as orbitals in the
case of atomic electrons), and more generally, elements of a complex vector
space. This abstract mathematical object allows for the calculation of
probabilities of outcomes of concrete experiments. For example, it allows one to
compute the probability of finding an electron in a particular region around the
nucleus at a particular time. Contrary to classical mechanics, one can never
make simultaneous predictions of conjugate variables, such as position and
momentum, with arbitrary accuracy. For instance, electrons may be considered to
be located somewhere within a region of space, but with their exact positions
being unknown. Contours of constant probability, often referred to as “clouds”
may be drawn around the nucleus of an atom to conceptualize where the electron
might be located with the most probability. It should be stressed that the
electron itself is not spread out over such cloud regions. It is either in a
particular region of space, or it is not. Heisenberg's uncertainty principle
quantifies the inability to precisely locate the particle.
The other exemplar that led to quantum mechanics was the study of
electromagnetic waves such as light. When it was found in 1900 by Max Planck
that the energy of waves could be described as consisting of small packets or
quanta, Albert Einstein exploited this idea to show that an electromagnetic wave
such as light could be described by a particle called the photon with a discrete
energy dependent on its frequency. This led to a theory of unity between
subatomic particles and electromagnetic waves called wave–particle duality in
which particles and waves were neither one nor the other, but had certain
properties of both. While quantum mechanics describes the world of the very
small, it also is needed to explain certain “macroscopic quantum systems” such
as superconductors and superfluids.
Broadly speaking, quantum mechanics incorporates four classes of phenomena that
classical physics cannot account for: (i) the quantization (discretization) of
certain physical quantities, (ii) wave-particle duality, (iii) the uncertainty
principle, and (iv) quantum entanglement. Each of these phenomena is described
in detail in subsequent sections.
Note for Microelectromechanical System
Microelectromechanical systems (MEMS) is the technology of the very small, and
merges at the nano-scale into nanoelectromechanical systems (NEMS) and
nanotechnology. MEMS are also referred to as micromachines (in Japan), or Micro
Systems Technology - MST (in Europe). MEMS are separate and distinct from the
hypothetical vision of Molecular nanotechnology or Molecular Electronics. MEMS
are made up of components between 1 to 100 micrometers in size (i.e. 0.001 to
0.1 mm) and MEMS devices generally range in size from a 20 micrometers (20
millionth of a meter) to a millimeter. They usually consist of a central unit
that processes data, the microprocessor and several components that interact
with the outside such as microsensors. At these size scales, the standard
constructs of classical physics do not always hold true. Due to MEMS' large
surface area to volume ratio, surface effects such as electrostatics and wetting
dominate volume effects such as inertia or thermal mass.
The potential of very small machines was appreciated long before the technology
existed that could make them, for example, Feynman's famous 1959 lecture There's
Plenty of Room at the Bottom. MEMS became practical once they could be
fabricated using modified semiconductor fabrication technologies, normally used
to make electronics. These include molding and plating, wet etching (KOH, TMAH)
and dry etching (RIE and DRIE), electro discharge machining (EDM), and other
technologies capable of manufacturing very small devices.
Researchers in MEMS use various engineering software tools to take a design from
concept to simulation, prototyping and testing. Finite element analysis is often
used in MEMS design. Simulation of dynamics, heat, and electrical domains, among
others, can be performed by ANSYS and COMSOL. Other software, such as MEMS-PRO,
is used to produce a design layout suitable for delivery to a fabrication firm.
Once prototypes are on-hand, researchers can test the specimens using various
instruments, including laser doppler scanning vibrometers, microscopes, and
stroboscopes.
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