Quantum Electronics

Type of physical science: Condensed matterphysics

Field of study: Solids

Quantum electronics is a loosely defined field encompassing interactions between electromagnetic radiation and matter at the atomic level involving quantum levels and resonance phenomena. The field of quantum electronics includes lasers, masers, and their scientific applications. Although the mechanism by which solid-state electronic devices, such as transistors, operate is also quantum-mechanical in nature, these devices are not usually included in the field of quantum electronics.

Overview

Quantum electronics is the branch of physics and engineering that describes and employs interactions between electromagnetic radiation and matter at the molecular and atomic level. The theory and applications of lasers (light amplification by the stimulated emission of radiation) and masers (microwave amplification by the stimulated emission of radiation) form the core of quantum electronics, which also touches on the related topics of nonlinear optics, laser spectroscopy, picosecond spectroscopy, and laser-induced optical breakdown.

The quantum character of the interactions between light and matter results from the discrete nature of energy transitions within individual atoms, molecules, and crystals. The orbital levels of outer-shell electrons in atoms, vibrational and rotational states of bonds within molecules, and spin orientation of paired electrons in molecules are examples of discrete energy states of matter.

At the molecular level, the interaction of a photon of light with an electron or molecular bond depends on the wavelength of the radiation, the energy of the transition within the molecule, and whether the molecule is in the ground state or in some excited (higher-energy) state. Three types of interaction can occur if the energy of the transition matches the energy of the photon. The energy of a photon is a function of its wavelength according to the expression hv = E2 - E1, where v is the wavelength, h is Planck's constant, and E2 - E1 is the difference in energy between excited and ground states.

If the molecule is in the ground state, a photon is absorbed and the molecule is raised to an excited state. A molecule in the excited state rapidly emits a photon by the process of spontaneous emission. If a photon interacts with a molecule that is already in the excited state, it causes a second photon, identical in wavelength and phase, to be emitted in a process known as stimulated emission. Stimulated emission is the basis for the functioning of lasers and masers.

Another, related process, important in laser spectroscopy, is the scattering that occurs when electromagnetic radiation passes through a transparent substance (that is, one with no energy transitions corresponding to the incident wavelength of radiation). Scattering, however, has lower-level energy transitions that absorb a portion of the photon's energy as it passes through the substance and cause a discrete lowering of the photon's energy and lengthening of its wavelength.

Under normal conditions of thermal equilibrium, only a small proportion of molecules exist in the excited state at any given time, and the processes of absorption and spontaneous emission overwhelm any stimulated emission that occurs. The higher the energy of the transition (and thus the shorter the wavelength of the photon emitted), the smaller the population of excited molecules at equilibrium and the less likely stimulated emission is to occur. The phenomenon of stimulated emission was first observed directly in the microwave region of the spectrum, where the populations of excited molecules under conditions of thermal equilibrium were sufficiently high that stimulated emission caused detectable interference in sensitive measurements of microwave-absorption spectra. The corresponding effect at optical wavelengths is negligible.

The main design hurdle to overcome in producing a device that operates by the principle of stimulated emission is that of creating an inverted population, in which there are more molecules in the excited state than in the ground state. Once an inverted population has been achieved, additional radiation of the appropriate wavelength will trigger a cascade, or chain reaction, in which each photon colliding with an excited molecule produces two photons that coincide in phase and wavelength, which in turn produce four, and so on until the population is no longer inverted and the probability of absorption is higher than the probability of stimulated emission.

A variety of methods are used to produce inverted populations. The original ammonia gas maser relied on physical separation of two inversion states of the ammonia molecule in a strong asymmetrical electric field. As a beam of ammonia molecules passed through the field, molecules in one configuration were deflected, while those in the other configuration collected in a resonating cavity. With sufficient input, such a system will sustain oscillations at a specific frequency of 23,870 megacycles and can be used as a time standard.

Pumping—that is, subjecting the material responsible for laser or maser action to electromagnetic radiation in order to raise it to a higher energy level—is the method of inversion used in the first ruby laser and in solid-state masers, which rely on differences in the orientation of electrons with respect to a magnetic field. Devices that rely on pumping employ energy transitions with two or more possible steps. It is not practical to achieve an inverted population in a one-step energy transition by means of pumping alone, because as the proportion of excited molecules increases, so does the probability of stimulated emission due to the pumping radiation. However, if there is an energy level 2 between the ground state 1 and the level 3 produced by pumping, then pumping can produce an inversion with respect to the 3-2 or 2-1 transition.

Other types of lasers use different methods to create a population inversion. In gas lasers, such as the helium-neon laser used in scanning devices, an inverted population is produced by means of electric discharge in the gas. Chemical reactions often produce end products that are in excited energy states, allowing chemical lasers to employ rapid reactions between gases to create an inverted population of molecules. Semiconductor lasers use the energy difference between electrons in the valence band and conducting bands within a semiconductor crystal. Optical pumping and beams of fast electrons can be used to create an inverted population of electrons in the valence band; alternatively, the properties of a p/n junction may be exploited.

The free-electron laser is a laser in the sense that it produces a powerful beam of coherent monochromatic light, but the underlying mechanism is somewhat different from any of the previous types of lasers and masers. A free-electron laser employs a beam of high-energy electrons passing through an array of magnets; this array is termed a "wiggler" because it deflects the beam first in one direction and then in another. An electron deflected by a magnetic field emits a photon whose wavelength varies according to the energy of the electron and the strength of the magnetic field. While gas and dye lasers are tunable to some extent, the free-electron laser surpasses them in this regard, being capable of supplying monochromatic radiation at any wavelength from infrared to near-ultraviolet. Its ability to deliver immensely powerful beams caused it to be dubbed the "Star Wars" laser; its precision and tunability have allowed it to be successfully used in several medical procedures, including neurosurgery and opthalmic surgery. A room-sized, high-energy machine, the free-electron laser remains part of the instrumentation of major laboratories and medical centers, in contrast with portable, inexpensive gas and solid-state lasers, which have become an integral part of consumer electronics.

One property of an inverted population is the ability to detect and magnify an extremely weak incoming signal of an appropriate wavelength. This property is exploited in masers used in conjunction with radio telescopes and remote-sensing devices. Objects in space emit microwave radiation at characteristic frequencies dependent on their temperature and the chemical compounds present in them. A maser gives a radio astronomer a sensitive device for looking at a narrow band in the microwave spectrum for the characteristic spectral fingerprint of a particular molecule.

Lasers, which are typically used as sources of high-energy coherent light rather than as amplifiers per se, incorporate a resonator as well as a pump and lasing medium in their design. A resonator confines radiation and forces it to traverse a defined pathway. The parallel mirrors at the ends of a ruby laser are an example of a resonator.

The most important applications of quantum electronic devices, principally lasers, derive not so much from their effectiveness as amplifiers as from the nature of the radiation that they produce. Laser output is coherent—that is, the crests and troughs of the electromagnetic waves are precisely in phase—and highly monochromatic. In contrast, sunlight or light from an incandescent light bulb consists of a mixture of frequencies that interfere with one another, weakening the signal. Signals of mixed frequencies cannot be focused as precisely as signals from monochromatic sources because the refraction of light is dependent on its wavelength. These two properties give laser-produced radiation its characteristic ability to deliver high levels of electromagnetic energy with great precision.

Applications

Quantum electronic devices have become ubiquitous in science, technology, and industry since their discovery in the 1950s. The applications of masers belong almost entirely to the realm of science and are more easily enumerated. The ammonia gas maser, the first quantum electronic device to be demonstrated, was quickly supplanted by solid-state masers as an electromagnetic detection device but has been proposed as a time and frequency standard, since the frequency of oscillations of a continuously pumped ammonia maser is constant. The quartz crystal in a quartz watch is an analogous time standard, as it is also based on an oscillation frequency inherent in the molecular structure.

Solid-state masers that rely on reorientation of electron spins with respect to a magnetic field are simpler in design and have the added advantage that the energy distance between spin orientations of electrons is dependent on the magnetic field. Therefore, a solid-state maser based on this principle can be tuned over a restricted range of the electromagnetic spectrum by varying the externally applied magnetic field. Solid-state masers are used as detection devices in radio astronomy and remote sensing. Since masers operate best at low temperatures, those used in remote sensing are cooled to the temperature of liquid helium. Such a device is bulky and expensive to operate, a circumstance that has favored the restriction of maser use to major scientific projects.

The development of quantum-well devices is prompting reinstrumentation of remote-sensing and radio-astronomy facilities for the detection of weak electromagnetic radiation. Quantum-well devices are distant cousins of solid-state semiconductor devices, which in turn have multiple theoretical and practical connections with lasers and masers. Semiconductor transistors and diodes rely on quantum transitions between the valence and conducting band within a crystal, which permit the crystal to act as an on-off switch in response to changes in applied voltage across a junction (p/n junction) between areas of differing electron density resulting from the composition of the crystal. A semiconductor crystal containing a p/n junction can operate as a laser if voltage is applied in a forward direction so as to create an inverted population in the conducting band. If spontaneous emission rather than stimulated emission predominates, the device becomes a light-emitting diode. Finally, a photoelectric cell is a device that relies on absorption of electromagnetic radiation to raise electrons to the conducting band, allowing current to flow through the device.

Individual semiconductor p/n junctions, the smallest functional unit on a silicon chip, are virtually microscopic. The individual functional unit of a quantum-well laser or transistor is two orders of magnitude smaller, an atom-sized unit capable of serving as a well for a few electrons. In quantum lasers, electrons combine with positively charged holes as they do in semiconductor lasers. As remote-sensing devices, quantum wells can be arranged in a grid and stacked such that a photon reaching the array has a high probability of hitting a quantum well, increasing the energy of the electron so that it can tunnel through the wall of the well and generate a signal. Quantum-well transistors are a hundred times smaller and several thousand times faster than p/n semiconductor transistors. As a detector of electromagnetic radiation, a quantum-well sensor combines extreme sensitivity with the ease of operation characteristic of solid-state semiconductor devices.

It is possible to enumerate only a few of the applications of lasers in scientific research, industry, medicine, and consumer technology. Of all the technological discoveries in the twentieth century, only transistors are more ubiquitous.

Lasers are invaluable laboratory tools in optics and spectroscopy and for studying the structure of matter. The use of lasers as a source of light for Raman spectroscopy has greatly increased the sensitivity of this technique for studying the energy levels of molecular bonds. A related technique, stimulated Raman spectroscopy, has been used to measure the decay time of excited vibrational states of molecular bonds. First, the target substance is excited with a burst of laser light of comparatively long wavelength corresponding to an excited vibrational state; then the target substance is bombarded with picosecond-length bursts of laser light at half the wavelength (higher energy). Excited bonds add their energy to this series of bursts, creating a scattering of the originally monochromatic light, which declines with time as the excited bonds decay.

Lasers are promising tools in both experimental and practical photochemistry. A wide variety of chemical reactions can be catalyzed by using defined wavelengths of light to activate certain bonds selectively. Lasers find practical use in increasing the speed and yield of industrial chemical reactions. In the research laboratory, lasers give chemists detailed information on the dynamics of collisions. Lasers are important in the study of the mechanisms of photosynthesis and the chemistry of vision. They can also be used for separating isotopes; because different isotopes of the same atom have slightly different absorption spectra, it is possible to activate a bond selectively in one isotope.

In medicine, lasers are used in delicate surgery, where they have the additional desirable property of cauterizing small blood vessels. Lasers attached to fiber-optic cables are used to treat lesions in the digestive tract without invasive surgery. With a laser, it is possible to pinpoint a small area of a chromosome and to induce a mutation in it. Various medical uses have been suggested for the tunable free-electron laser in addition to surgery, including melting fat beneath the skin and directing specific chemical reactions of drugs within a patient's body.

Industrially, lasers are used for welding and cutting and in precision photographic processes, such as directing the layout of a silicon chip. Laser light is used to anneal semiconductors to fix in place the impurities that govern the properties of the silicon chip. It forms the basis for optical scanners, which convert text and graphics into computer files, sort mail, and scan bar codes in the grocery store. The laser printer is common in both the modern office and the home, and the laser compact-disc player was a fixture in most homes in the 1990s and early 2000s. Fiber-optic cables containing semiconductor laser boosters relay telephone messages over long distances more efficiently than metal cables transmitting electrical signals.

In thermonuclear research, the combined output of chains of powerful lasers is focused on pellets of hydrogen in an attempt to create fusion in the laboratory. In holography, output from a laser is used to create changes in the refractive index of the recording material, creating the illusion of a three-dimensional form. Lasers are also important in modern warfare, not so much as weapons themselves but as adjuncts to the guiding systems of sophisticated weaponry.

A more recent application of quantum electronics is in the field of nanotechnology. Crystalline nanoparticles made of semiconductor materials, known as quantum dots, have been used to create lasers that consume less energy and produce a better-quality beam than traditional semiconductor lasers. Nanotechnology in general holds the potential to perform the work of existing technology, such as transistors and display screens, with greater efficiency and using a fraction of the energy, due to the altered properties exhibited by matter at the nanoscopic scale. Significant progress has also been made in the development of quantum computers, with a research team at Yale University constructing the first rudimentary solid-state quantum processor in 2009.

Context

Although the principle of stimulated emission was proposed by Albert Einstein in 1916 as part of his theory of general relativity, practical demonstration of the effect and construction of devices employing stimulated emission did not occur until decades later. After World War II, a comparatively large number of scientists, working independently in several laboratories in the United States and the Soviet Union, contributed to key discoveries in the early development of lasers and masers. When the Nobel Committee decided to recognize research in this area, it awarded the 1964 Nobel Prize in Physics jointly to Charles Hard Townes of Columbia University, who demonstrated the first working maser in 1954, and to Nikolay Gennadiyevich Basov and Aleksandr Mikhailovich Prokhorov of the Lebedev Physics Institute for their work on masers.

Around the same time, in 1958, Theodore H. Maiman demonstrated the first working laser, an optically pumped ruby-crystal laser. In 1964, lasers were still primarily experimental laboratory tools, and the potential range of useful applications was unknown. Within ten years, development of practical lasers—notably the helium-neon laser for low-energy scanning, the carbon-dioxide laser for industrial welding and cutting, and semiconductor lasers for telecommunications—had created a technology that permeated nearly every facet of modern industrial life.

Principal terms

COHERENT RADIATION: radiation in which components of the signal are in phase with one another

PHOTON: a single quantum of light

QUANTUM MECHANICS: a branch of physics that describes the fundamental wave-like and particle-like properties of matter and energy

RAMAN EFFECT: an increase or decrease in the wavelength of light related to quantum interactions of photons with molecules of a transparent substance

Bibliography

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Bertolotti, Mario. Masers and Lasers: An Historical Approach. Bristol: Hilger, 1983. Print. In this technical account intended for a reader with considerable background in the physical sciences, Bertolotti traces the many threads of research leading to the development of masers and lasers. He places in perspective the individuals involved, their unique contributions, and the continuity between the various historical aspects of quantum electronics.

Chang, William S. Principles of Quantum Electronics. Reading.: Addison, 1969. Print. A textbook for graduate-level students in physics and engineering. Four sections cover a review of quantum mechanics, energy levels of materials in quantum-electronic devices, interaction of radiation with matter, and lasers.

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Klyshko, David. Physical Foundations of Quantum Electronics. Ed. Maria Chekhova and Sergey Kulik. Singapore: World Scientific, 2011. Print.

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Rabin, Herbert, ed. Quantum Electronics: A Treatise. New York: Academic, 1975. Print. A collection of review articles presupposing that the reader has a graduate-level education in quantum electronics and optics. The introduction by N. Bloembergen contains a good overview of the field.

Ramirez, Anthony. "A Star Wars Laser Comes to Earth." Fortune 15 Aug. 1988: 65–69. Print. Well-illustrated, nontechnical account of free-electron lasers, their underlying mechanism, and their promise as biomedical devices.

Tartakovskii, Alexander, ed. Quantum Dots: Optics, Electron Transport and Future Applications. New York: Cambridge UP, 2012. Print.

Weber, Marvin J., ed. CRC Handbook of Laser Science and Technology. Boca Raton: CRC, 1982. Print. A reference book intended for professionals in the physical sciences and engineering. Provides reviews and references for many aspects of laser and maser science, particularly applications and devices currently used by laboratories and industry.

Zwanenburg, F. A., et al. "Silicon Quantum Electronics." Reviews of Modern Physics 85.3 (2013): 961–1019. Print.

Generating and Detecting Electromagnetic Waves

Photon Interactions with Molecules

The Effect of Electric and Magnetic Fields on Quantum Systems