36.0 - Introduction
Quantum physics is the branch of science required to fully explain the behavior of
light, its interaction with matter, and the behavior of exceedingly small particles such
as atoms and electrons. As scientists began to discover in the late 19th and early
20 th centuries, certain principles and techniques of classical physics fail utterly when
applied to light and to atomic-scale systems.
Although “quantum physics” often connotes mystery and difficulty, its applications
are very real. You may be pleasantly surprised at how much you can understand
when you are equipped with just a few fundamental concepts from this science.
In particular, you can learn the principles governing the functioning of two of the
most pivotal technologies of the last half-century: semiconductors and lasers.
Why are these two technologies so important? Without semiconductors, there would
be neither transistors, nor the microprocessors built from them. Semiconductor-
based microprocessors serve as the “brains” of computers and are found in digital
cameras, cell phones, and automobiles: wherever engineers want “smart” behavior.
Semiconductors are also used in various types of computer memory, such as
random access memory (RAM). Semiconductor chips not only “think,” they also “remember.”
In recent years, connecting all these semiconductor devices has become the central thrust of the information processing industry. The
Internet, cell-phone “fixed-rate calling plans”, video on demand, downloadable music, and even the Web-based version of the textbook you are
now reading all rely on the cheap and rapid transmission of information over wired or wireless networks. The two technologies most
responsible for creating this networking revolution of rapidly decreasing costs and dramatically increasing bandwidth have been the
microprocessor and the laser.
How do these two technologies enable networking? Communication networks use devices like routers and high-speed switches to transmit
data. These devices rely on microprocessors to determine where to send their information and they form parts of extended physical systems
that use lasers to move the information at light speed over fiber-optic cables.
Lasers give you access to data from sources both distant and nearby. In addition to sending data around the world, they are used on your
desktop or in your home to read the data stored on CDs and DVDs (not to mention their use in stores to read data codes on your purchases).
Without lasers, vinyl records and “floppy disks” might still be the primary means of storing audio and digital data. Believe us: If you have never
used a floppy disk, you haven’t missed much.
How does quantum physics relate to the working of these devices?
Explaining what is meant by “quantum” is the place to start. A key tenet of quantum physics is that particles in some systems, like the electrons
in hydrogen atoms, exist only at certain energy levels. Physicists say the energy levels of the electrons in an atom are quantized.
It may be easiest to explain a quantum property by first considering its opposite, a property that is continuous. Consider the potential energy of
a bucket that is raised or lowered by a rope. You can raise it 1.000 meters off the ground, or 1.001 m, or 1.002 m, or however much you like.
By controlling its height, you can make its potential energy whatever you like. The range of possible energy values is continuous: say 10.00
joules, 10.000017 J, 10.027 J and so forth.
Electrons prove not to be as flexible. The electrons around a hydrogen (or other) atom exist only in states with certain discrete energy values;
for instance, two possible energy levels for the hydrogen electron are í1.51 eV (electron volts) or í3.40 eV (í2.42×10í^19 J or í5.44×10í^19 J
respectively). Between these two values lies a forbidden gap, and a hydrogen atom’s electron is never observed with energies in that range.
Physicists say an electron’s energy is quantized, that it only exists at certain levels. In the example of the hydrogen electron mentioned above,
you will never observe an energy of í1.6 eV or í2.9 eV, since those are forbidden.
You may ask: How does an electron change between energy levels? How can it “move” across a “forbidden gap” to a higher or lower energy
state, if intermediate energy values are forbidden? You may not be satisfied with the answer, but it is most straightforward to say: Those are
simply the only values that have ever been measured. Any time a scientist measures a property of an atomic electron (such as its energy, or its
angular momentum), she only observes results from a particular set of values that can be predicted with extreme accuracy by quantum
physics. It is impossible to “catch” an electron in any in-between state.
The simulation at the right reproduces one of the key experiments that led to the widespread acceptance of the ideas of quantum physics.
Einstein explained data from a more sophisticated version of the same experiment, known as the photoelectric effect, to earn his Nobel Prize.
Describing the experiment is simple. Scientists had noticed that when they shined a beam of light on a metal, electrons were released from the
illuminated surface. From their perspective, this was not particularly surprising: The energy of the light was transferred to the electrons,
allowing some of them to escape their bonds to the atoms of the metal.
The emission of electrons could be explained by classical physics. Light was a wave with energy, and that energy could provide a “kick” to
electrons as the metal absorbed the light.
Although the experiment and the expected outcome are simple in concept, the detailed results were quite surprising and could not be
explained by classical physics. Some colors of light, such as red, could not eject electrons from the surface, no matter how bright the beam
was. On the other hand, other colors, such as violet, were effective at ejecting electrons from the metal surface, even when the beam intensity
was very low. It was the frequency of the light, and not its intensity, that determined whether or not electrons were ejected.