FAQ: Quantum Dots and Programmable Matter
Wil McCarthy / Version 2.1 / 25 February 2003

(C) Copyright 2002 by Wil McCarthy. This FAQ may be transmitted or
distributed freely without alterations, appendices, or annotations.
Please do not make changes to this text without the author's
permission. Thank you.

Keywords: "quantum dot", "artificial atom", "programmable atom",
"designer atom", "programmable matter", "wellstone", "quantum dot
fiber", "SET", "single-electron transistor"

Q1: What is programmable matter?
Q2: What is wellstone?
Q3: Is this science fiction?
Q4: Where is programmable matter research being conducted?
Q5: Is programmable matter the same thing as nanotechnology?
Q6: Is programmable matter the same thing as MEMS?
Q7: I've heard the term "programmable matter" used to describe a
cellular automaton computer simulation. Is this accurate?
Q8: Does an LCD screen qualify as programmable matter? Does a
transistor?
Q9: What is doping?
Q10: What are quantum dots and artificial atoms? Are they the same
thing?
Q11: What is a quantum well?
Q12: What is programmable matter made of?
Q13: How is programmable matter made?
Q14: Can programmable matter mimic the substances on the periodic
table?
Q15: Is this alchemy? Can it convert lead into gold?
Q16: Can programmable matter mimic transuranic elements?
Q17: Aren't these transuranic elements highly unstable?
Q18: Can programmable matter be used to create superstrong materials?
Q19: What does "unnatural properties" mean?
Q20: What does matter made of artificial atoms feel like? Is it
solid?
Q21: What is programmable matter good for?
Q22: Who is Wil McCarthy, and why is he posting this FAQ?
Q23: Did Wil McCarthy invent programmable matter?
Q24: Do you know what you're talking about? Have you considered
<<pet issue>>?
Q25: Where can I learn more?

--

Q1: What is programmable matter?

A1: Programmable matter is any bulk substance whose physical
properties can be adjusted in real time through the application of
light, voltage, electric or magnetic fields, etc. Primitive forms
may allow only limited adjustment of one or two traits (e.g., the
"photodarkening" or "photochromic" materials found in light-sensitive
sunglasses), but there are theoretical forms which, using known
principles of electronics, should be capable of emulating a broad
range of naturally occurring materials, or of exhibiting unnatural
properties which cannot be produced by other means.

--

Q2: What is wellstone?

A2: Wellstone is a hypothetical form of programmable matter first
proposed by Wil McCarthy in his novella "Once Upon a Matter Crushed"
(Science Fiction Age, May 1999), consisting of nanoscopic
semiconductor threads covered with quantum dots. These threads can
be woven together to form a bulk solid with real-time programmable
properties. The terms "wellstone" and "programmable matter" are
occasionally used interchangeably, although many other forms of
programmable matter exist.

--

Q3: Is this science fiction?

A3: No. Various forms of programmable matter have appeared in
fiction, but are in many cases based on technologies which exist
today, or on reasonable extrapolations from them.

--

Q4: Where is programmable matter research being conducted?

A4: Various aspects of programmable matter are under investigation in
labs all over the world. Major players include (but are by no means
limited to) IBM, Nippon Telehone and Telegraph, Fujitsu, Delft
University, MIT, Harvard, Stanford, Princeton, Cornell, CalTech, and
The University of California at Santa Barbara.

--

Q5: Is programmable matter the same thing as nanotechnology?

A5: Yes and no. The word "nanotechnology" simply means "technology
on the scale of nanometers," or billionths of a meter, i.e.
technology on the molecular scale. Most forms of programmable matter
rely on nano-circuitry, designer molecules, or both, so in this
literal sense they are nanotechnology. However, as originally coined
by K. Eric Drexler in the 1980s and as commonly used by lay persons
today, the word nanotechnology implies nanoscale _machinery_, more
properly known as molecular nanotechnology or MNT.

While bulk materials incorporating MNT may have programmable
properties, they also have moving parts. The term "programmable
matter" does not rule out such materials, but more typically refers
to substances whose properties can be adjusted in the solid state,
with no moving parts other than photons and electrons.

--

Q6: Is programmable matter the same thing as MEMS?

A6: No. Micro Electromechanical Systems, or MEMS, are microscopic
machines crafted using standard methods for the manufacture of
microchips. MEMS have many useful applications in the real world,
but are far too large to exhibit the quantum effects necessary to
affect the bulk properties of matter. However, the "Utility Fog"
substance proposed by J. Storrs Hall in the early 1990s, consisting
of millions or billions of MEMS micromachines -- each with with 12
retractable, linkable arms -- has numerous adjustable bulk properties
and can thus be considered a crude, mechanical form of programmable
matter.

--

Q7: I've heard the term "programmable matter" used to describe a
cellular automaton computer simulation. Is this accurate?

A7: Yes, although technically speaking, a cellular automaton can only
contain _virtual_ programmable matter, whereas physical examples
which meet the definition are available in the real world.

--

Q8: Does an LCD screen qualify as programmable matter? Does a
transistor?

A8: An LCD screen's optical properties can be dramatically altered by
the application of electrical signals. Thus, it is clearly a form of
programmable matter, albeit a simple one. A transistor can switch
between an electrically conductive state and an electrically
insulative one, but is properly a "device" rather than a substance.
However, a bulk material fashioned from transistors (transistronium?)
would be electrically switchable between these two states, and
possibly numerous intermediate states. This meets (trivially) the
definition for programmable matter stated above.

In general, the more capable forms of programmable matter rely on the
doping effects of "artificial atoms" or "quantum dots" inside a bulk
material.

--

Q9: What is doping?

A9: Doping is the addition of impurities (dopants) to a bulk material
(the substrate) in order to adjust its electrical, thermal, optical,
or magnetic properties. The addition of one dopant atom per million
atoms of substrate is often sufficient to cause major changes in the
material's behavior, and impurities in the parts-per-billion can
disrupt the expected behavior of a pure crystal.

--

Q10: What are quantum dots and artificial atoms? Are they the same
thing?

A10: A quantum dot is any device capable of confining electrons in
three dimensions, in a space small enough that their quantum
(wavelike) behavior dominates over their classical (particle-like)
behavior. Under cryogenic conditions, this typically occurs with
dimensions of 1000 nm (0.001 mm) or less. At room temperature,
confinement spaces of 20-30 nm or smaller are required.

Once the electrons are confined, they repel one another and also obey
the Pauli Exclusion Principle, which forbids any two electrons from
having the same quantum state. Thus, the electrons in a quantum dot
will form shells and orbitals highly reminiscent of (though larger
than) the ones in an atom, and will in fact exhibit many of the
optical, electrical, thermal, and (to some extent) chemical
properties of an atom. This electron cloud is therefore referred to
as an artificial atom.

In their various forms, quantum dots may be referred to as
single-electron transistors, controlled potential barriers, Coulomb
islands, zero dimensional electron gases, colloidal nanoparticles or
semiconductor nanocrystals.

--

Q11: What is a quantum well?

A11: A quantum well is a device for confining electrons in one
dimension, such that their quantum (wavelike) behavior dominates over
their classical (particle-like) behavior along the confined axis,
while classical behavior dominates along the other two axes,
permitting the electrons to flow two-dimensionally through the
material like billiard balls on a table. A typical quantum well
consists of an N-type semiconductor, doped with electron donor atoms,
trapped between two layers of P-type semicondictor, doped with
electron borrower atoms.

A quantum well is the primary component of miniature laser pointers.

--

Q12: What is programmable matter made of?

A12: Programmable matter is composed of manmade objects too small to
perceive directly with the human senses. This may include
microscopic or nanoscopic machines, but more typically refers to
fixed arrangements of conductors, semiconductors, and insulators
designed to trap electrons in artificial atoms.

--

Q13: How is programmable matter made?

A13: Current forms of programmable matter fall into three types:
colloidal films, bulk crystals, and quantum dot chips which confine
electrons electrostatically. Quantum dots can be grown chemically as
nanoparticles of semiconductor surrounded by an insulating layer.
These particles can then be deposited onto a substrate, such as a
semiconductor wafer patterned with metal electrodes, or they can be
crystalized into bulk solids by a variety of methods. Either
substance can be stimulated with electricity or light (e.g., lasers)
in order to change its properties.

Electrostatic quantum dots are patterns of conductor (usually a metal
such as gold) laid down on top of a quantum well, such that varying
the electrical voltage on the conductors can drive electrons into and
out of a confinement region in the well -- the quantum dot. This
method offers numerous advantages over nanoparticle ("colloidal")
films, including a greater control over the artificial atom's size,
composition, and shape. Numerous quantum dots can be placed on the
same chip, forming a semiconductor material with a programmable
dopant layer near its surface.

Rolling such chips into cylindrical fibers produces "wellstone," a
hypothetical woven solid whose bulk properties are broadly
programmable.

--

Q14: Can programmable matter mimic the substances on the periodic
table?

A14: Yes. Artificial atoms can easily be constructed which mimic the
properties of any natural atom, except that they are larger and their
electrons are bound more loosely. However, these artificial atoms
have negligible mass, and can exist only inside the quantum-dot
substrate which generates them, usually a semiconductor. Thus, the
final properties of the material are a blend of the simulated element
and the underlying substrate. Note that the color of an artificial
element made of oversized atoms would be redshifted as compared with
the equivalent natural element.

--

Q15: Is this alchemy? Can it convert lead into gold?

A15: Yes and no. An artificial atom of pseudo-lead, trapped
permanently inside a semiconductor material, can be converted to an
artificial atom of pseudo-gold by the subtraction of three
electrons. Sufficient numbers of these pseudoatoms may overwhelm the
natural behavior of the semiconductor to produce a metal-like
material similar to lead or gold, except for its mass and probably
color. Artificial atoms designed to mimic the _colors_ of lead or
gold might have other properties (e.g., electrical or thermal
conductivity) which do not match the original metal.

--

Q16: Can programmable matter mimic transuranic elements?

A16: Yes. An artificial atom can contain any number of electrons,
from 1 to over 1000. The form and properties of highly transuranic
atoms (atomic number >> 92) are dramatically different from those of
natural atoms.

--

Q17: Aren't these transuranic elements highly unstable?

A17: Electrons in an atom are confined by their attraction to the
nucleus, and the nuclei of highly transuranic elements are unstable.
However, an artificial atom does not have a nucleus of its own,
relying instead on geometry, insulative barriers, and/or
electrostatic repulsion to confine its electrons inside a
semiconductor substrate.

--

Q18: Can programmable matter be used to create superstrong materials?

A18: Probably not. The binding energy of artificial atoms cannot
exceed the binding energy of the semiconductor substrate. However,
using diamond fibers or fullerenes as a substrate should allow for
some very tough programmable materials. Also, changes in the
magnetic behavior of a material can affect its stiffness and
tensile/compressive strength in useful ways.

--

Q19: What does "unnatural properties" mean?

A19: Unlike natural atoms, artificial atoms can be square, pyramidal,
two-dimensional, highly transuranic, composed of charged particles
other than electrons (e.g., "holes"), and can even be asymmetrical.
Their size, energy, and shape are variable quantities. Thus,
artificial atoms exhibit optical, electrical, thermal, magnetic,
mechanical, and (to some extent) chemical behaviors which do not
occur in natural materials. This variety is bounded but infinite, in
sharp contrast to the 92 stable atoms of the periodic table.

--

Q20: What does matter made of artificial atoms feel like? Is it
solid?

A20: Artificial atoms can exist only inside a semiconductor
substrate. They are charge discontinuities rather than physical
objects, so they don't "feel" like anything. However, their doping
effects can dramatically alter the properties of the substrate,
causing it to feel different. For example, a dramatic increase in
thermal and electrical conductivity would make the semiconductor feel
(in terms of thermal response) like a metal.

--

Q21: What is programmable matter good for?

A21: Almost anything. It can improve the efficient collection,
storage, distribution, and use of energy from environmental sources.
It can be used to create novel sensors and computing devices,
probably including quantum computers. It can create materials which
are not available by other means, and which change their apparent
composition on demand. Currently, the design of new materials is a
time- and labor-intensive process; with programmable matter, it
becomes a real-time issue, similar to the design and debugging of
software.

--

Q22: Who is Wil McCarthy, and why is he posting this FAQ?

A22: Wil McCarthy, an aerospace engineer, is a contributing editor
for WIRED magazine, the science columnist for the SciFi channel web
site < http://www.scifi.com (Who's computer is this?) >, and an author of numerous book-length
works of science fact and science fiction. He has written
extensively about quantum dots and programmable matter, and faces a
consistent set of questions, objections, and misconceptions when
presenting this material. The FAQ is intended to promote intelligent
discussion of programmable matter and quantum dots by increasing
awareness of their underlying issues and principles.

--

Q23: Did Wil McCarthy invent programmable matter?

A23: No. Single-electron transistors, a form of quantum dot, were
first proposed by A.A. Likharev in 1984 and constructed by Gerald
Dolan and Theodore Fulton at Bell Laboratories in 1987. The first
semiconductor SET, a type of quantum dot sometimes referred to as a
designer atom, was invented by Marc Kastner and John Scott-Thomas at
MIT in 1989. The term "artificial atom" was coined by Kastner in
1993.

However, Wil McCarthy was the first to use the term "programmable
matter" in connection with quantum dots, and to propose a mechanism
for the precise, 3D control of large numbers of quantum dots inside a
bulk material. The most interesting forms of this device or
substance -- known as "quantum dot fiber" or "wellstone" -- are not
produceable using circa 2003 technology, although related products
may be.

The term "wellstone" was coined by McCarthy's business associate, Dr.
Gary E. Snyder.

--

Q24: Do you know what you're talking about? Have you considered
<<pet issue>>?

A24: Quantum dots are a new field with much basic research still
remaining, so the ultimate properties of bulk quantum-dot materials
cannot be known with precision at this time. However, the
experimental evidence overwhelmingly indicates that programmable
quantum-dot materials are feasible, and will play an important role
in future technology.

--

Q25: Where can I learn more?

A25: The best online reference for lay readers is "Ultimate Alchemy,"
a 7,000-word article from WIRED magazine available (minus the
pictures) at:
< http://www.wired.com/wired/archive/9.10/atoms.html (Who's computer is this?) >.

Offline lay-references include Richard Turton's THE QUANTUM DOT: A
Journey into the Future of Microelectronics (Oxford University Press,
1996, ISBN 0-19-510959-7)
< http://www.amazon.com/exec/obidos/tg/detail/-/0195109597 (Who's computer is this?) >

and Wil McCarthy's HACKING MATTER: Levitating Chairs, Quantum
Mirages, and the Infinite Weirdness of Programmable Atoms (Basic
Books, 2003, ISBN 0-46-504428-X).
< http://www.amazon.com/exec/obidos/tg/detail/-/046504428X (Who's computer is this?) >.

On a lighter note, detailed fictional treatments of programmable
matter can be found in Wil McCarthy's THE COLLAPSIUM (Bantam, Dec
2002, ISBN 0-55-358443-X)
< http://www.amazon.com/exec/obidos/ASIN/055358443X (Who's computer is this?) >
and THE WELLSTONE (Bantam, Mar 2003, ISBN 0-55-358446-4)
< http://www.amazon.com/exec/obidos/ASIN/0553584464 (Who's computer is this?) >.

Closely related technologies appear in John Barnes' THE PRINCESS OF
THE AERIE (Warner, Jan 2003, ISBN 0-44-661082-8)
< http://www.amazon.com/exec/obidos/ASIN/0446610828 (Who's computer is this?) >, and the
upcoming THE KILLING OF WORLDS by Scott Westerfeld (Tor, November
2003), and probably other recent science fiction as well. The idea
appears to be catching.

More technical discussions can be found in the (searchable) annals of
Science News, Science, Nature, Physics Today, and related journals,
as well as web pages at many of the research centers listed above.
Happy Googling!

--
Wil McCarthy < http://www.wilmccarthy.com (Who's computer is this?) >
Engineer, Columnist, Author, etc.
Never hurry / never rest. -- Goethe