Pick up a phone and you are holding one of the most successful applications of quantum mechanics ever made. Its processor contains billions of switches, its screen glows with light from semiconductor crystals, and its map knows where you are because satellites carry clocks that count the vibrations of atoms. Historians and engineers now call this the first quantum revolution. QUAN 102 tells how those rules were found; this course is about what people built with them.
Why solids need quantum theory
Classical physics could not explain a basic fact: why copper carries electric current easily while glass does not. Quantum mechanics answered it. In a crystal, electrons cannot have just any energy. Quantum theory explains why some materials conduct and others insulate: electrons in a solid may only occupy allowed energy bands, separated by forbidden gaps. In a metal, a band is partly empty, so electrons move freely. In an insulator, a wide gap blocks them. In a semiconductor such as silicon the gap is small, and adding tiny traces of other elements, called doping, lets engineers decide exactly how the material behaves.
The transistor and the chip
In December 1947, John Bardeen and Walter Brattain at Bell Labs in New Jersey built the first working transistor, and their colleague William Shockley soon designed an improved version. The three shared the 1956 Nobel Prize in Physics. A transistor is a switch with no moving parts: a small voltage controls a larger current flowing through a semiconductor. Jack Kilby and Robert Noyce, working separately in 1958 and 1959, put many transistors on one piece of material, the integrated circuit. A modern processor holds tens of billions of them. Quantum effects show up in another everyday device too. Flash memory, in phones and USB sticks, stores data by trapping electrons that pass through a thin insulating layer by quantum tunneling, the ability of a particle to cross a barrier it could not climb over in classical physics.
Lasers and LEDs
In 1917 Albert Einstein worked out how light and atoms trade energy. He described stimulated emission: an atom in an excited state can be triggered by a passing photon to release a second photon identical to the first. Arrange many excited atoms between mirrors and each photon triggers more, building a beam of light that is all one color and marches in step. Theodore Maiman at Hughes Research Laboratories made the first laser, using a ruby crystal, in 1960. Lasers now carry internet traffic through glass fibers, read barcodes, cut steel and reshape corneas.
The light emitting diode works by the band picture. When an electron drops across the gap in a semiconductor, it gives off a photon. The color of an LED's light is set by the size of the band gap in its semiconductor: a bigger gap gives bluer light. Red and green LEDs came early, but a bright blue one took decades. Isamu Akasaki, Hiroshi Amano and Shuji Nakamura solved it in the early 1990s and shared the 2014 Nobel Prize in Physics, because blue light made efficient white LED lighting possible.
MRI and atomic clocks
The nuclei of hydrogen atoms have a quantum property called spin that makes each act like a tiny magnet. An MRI scanner builds images of the body by detecting radio signals from the spins of hydrogen nuclei, mostly in water and fat, inside a strong magnet. Unlike X-rays, it uses no ionizing radiation. Paul Lauterbur and Peter Mansfield shared the 2003 Nobel Prize in Physiology or Medicine for turning the effect into imaging, and the strong magnets themselves are usually superconducting, another quantum effect.
Identical atoms absorb and emit radiation at exactly the same frequencies, which makes them ideal timekeepers. Since 1967 the second has been defined as 9,192,631,770 cycles of a particular radiation from cesium-133 atoms. Every GPS satellite carries atomic clocks, and your receiver works out its distance from each satellite by timing the signals. Light travels about 30 centimeters in a billionth of a second, so GPS needs clocks accurate to billionths of a second to place you within a few meters.
From crowds to individuals
All of these devices rely on enormous numbers of electrons, atoms or photons behaving together. The first quantum revolution used quantum rules to build devices out of huge crowds of particles; the second aims to control individual quantum systems, such as single atoms, photons or electrons, and to exploit their superposition and entanglement. The phrase "second quantum revolution" was popularized by the physicists Jonathan Dowling and Gerard Milburn in a 2003 paper. The rest of this course follows that second revolution into computing, cryptography and sensing.
Worked example: auditing a phone
Take one ordinary smartphone and list its quantum parts. The processor and memory chips are built from doped silicon whose behavior band theory explains, and the photos stored in flash memory sit as electrons that tunneled into place. The screen glows because electrons fall across band gaps in LEDs. The location dot depends on cesium and rubidium clocks orbiting overhead, and the video call travels part of its journey as laser light in undersea cables. The point: quantum technology is not science fiction waiting to arrive; it already underpins modern life. The newer devices try to go one step further, from using quantum crowds to steering single quantum objects.