Quantum Mechanics

Quantum mechanics (QM; also known as quantum physics, or quantum theory) is a fundamental branch of physics which deals with physical phenomena at nanoscopic scales, where the action is on the order of the Planck constant. The name derives from the observation that some physical quantities can change only in discrete amounts (Latin quanta), and not in a continuous (cf. analog) way. It departs from classical mechanics primarily at the quantum realm of atomic and subatomic length scales. Quantum mechanics provides a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. Quantum mechanics provides a substantially useful framework for many features of the modern periodic table of elements, including the behavior of atoms during chemical bonding, and has played a significant role in the development of many modern technologies.

In advanced topics of quantum mechanics, some of these behaviors are macroscopic and emerge at only extreme (i.e., very low or very high) energies or temperatures (such as in the use of superconducting magnets). In the context of quantum mechanics, the waveŠparticle duality of energy and matter and the uncertainty principle provide a unified view of the behavior of photons, electrons, and other atomic-scale objects.

The mathematical formulations of quantum mechanics are abstract. A mathematical function, the wave function, provides information about the probability amplitude of position, momentum, and other physical properties of a particle. Mathematical manipulations of the wave function usually involve braŠket notation, which requires an understanding of complex numbers and linear functionals. The wavefunction formulation treats the particle as a quantum harmonic oscillator, and the mathematics is akin to that describing acoustic resonance. Many of the results of quantum mechanics are not easily visualized in terms of classical mechanics. For instance, in a quantum mechanical model, the lowest energy state of a system, the ground state, is non-zero as opposed to a more "traditional" ground state with zero kinetic energy (all particles at rest). Instead of a traditional static, unchanging zero energy state, quantum mechanics allows for far more dynamic, chaotic possibilities, according to John Wheeler.

The earliest versions of quantum mechanics were formulated in the first decade of the 20th century. About this time, the atomic theory and the corpuscular theory of light (as updated by Einstein) first came to be widely accepted as scientific fact; these latter theories can be viewed as quantum theories of matter and electromagnetic radiation, respectively.

Early quantum theory was significantly reformulated in the mid-1920s by Werner Heisenberg, Max Born and Pascual Jordan (matrix mechanics); Louis de Broglie and Erwin Schršdinger (wave mechanics); and Wolfgang Pauli and Satyendra Nath Bose (statistics of subatomic particles). Moreover, the Copenhagen interpretation of Niels Bohr became widely accepted. By 1930, quantum mechanics had been further unified and formalized by the work of David Hilbert, Paul Dirac and John von Neumann with a greater emphasis placed on measurement in quantum mechanics, the statistical nature of our knowledge of reality, and philosophical speculation about the role of the observer.

Quantum mechanics has since permeated throughout many aspects of 20th-century physics and other disciplines including quantum chemistry, quantum electronics, quantum optics, and quantum information science. Much 19th-century physics has been re-evaluated as the "classical limit" of quantum mechanics and its more advanced developments in terms of quantum field theory, string theory, and speculative quantum gravity theories. Read More

In the News ...

New method of quantum entanglement vastly increases how much information can be carried in a photon   PhysOrg - June 30, 2015
A team of researchers has demonstrated a new way to harness light particles, or photons, that are connected to each other and act in unison no matter how far apart they are - a phenomenon known as quantum entanglement. In previous studies, photons have typically been entangled by one dimension of their quantum properties - usually the direction of their polarization. In the new study, researchers demonstrated that they could slice up and entangle each photon pair into multiple dimensions using quantum properties such as the photons' energy and spin. This method, called hyperentanglement, allows each photon pair to carry much more data than was possible with previous methods.

Quantum Cheshire Cat effect may be explained by standard quantum mechanics   PhysOrg - June 13, 2015
"A grin without a cat" is how Lewis Carroll describes the Cheshire Cat's mysterious way of disappearing while leaving its grin behind in his 1865 classic, Alice in Wonderland. The fanciful character raises a question that has captured physicists' attention over the past few years: can an object be separated from its properties? However, in a new paper published in the New Journal of Physics has questioned this interpretation of the results. Instead of a particle being separated from its properties, they suggest that the results can be explained by a standard quantum effect, quantum interference, in which an individual particle interferes with itself due to its wave-like properties.

Strange behavior of quantum particles may indicate the existence of other parallel universes   PhysOrg - June 6, 2015
Quantum mechanics is a strange realm of reality. Particles at this atomic and subatomic level can appear to be in two places at once. Because the activity of these particles is so iffy, scientists can only describe what's happening mathematically by "drawing" the tiny landscape as a wave of probability.

Game theory elucidates the collective behavior of bosons   PhysOrg - April 28, 2015
When scientists explore the mysterious behavior of quantum particles, they soon reach the limits of present-day experimental research. There are two kinds of quantum particles in nature: fermions and bosons. Whether a particle is a fermion or a boson depends on its intrinsic angular momentum or spin. For fermions, the spin is always half-integer valued and the most prominent example is the electron. Bosons, on the other hand, always exhibit integer spins. Such is the case for photons, for example, but also whole atoms may be bosons. Bosons are social beasts that like to be on the same wavelength Š or, as physicists put it, they like to be in the same quantum state.

The beginning of everything: A new paradigm shift for the infant universe   PhysOrg - November 29, 2012
A new paradigm for understanding the earliest eras in the history of the universe has been developed by scientists at Penn State University. Using techniques from an area of modern physics called loop quantum cosmology, the scientists now have extended analyses that include quantum physics farther back in time than ever before - all the way to the beginning. The new paradigm of loop quantum origins shows, for the first time, that the large-scale structures we now see in the universe evolved from fundamental fluctuations in the essential quantum nature of "space-time," which existed even at the very beginning of the universe over 14 billion years ago. The achievement also provides new opportunities for testing competing theories of modern cosmology against breakthrough observations expected from next-generation telescopes.

Quantum test pricks uncertainty   BBC - September 8, 2012

Pioneering experiments have cast doubt on a founding idea of the branch of physics called quantum mechanics. The Heisenberg uncertainty principle is in part an embodiment of the idea that in the quantum world, the mere act of observing an event changes it. But the idea had never been put to the test, and a team writing in Physical Review Letters says "weak measurements" prove the rule was never quite right. That could play havoc with "uncrackable codes" of quantum cryptography. Quantum mechanics has since its very inception raised a great many philosophical and metaphysical debates about the nature of nature itself.

Quantum Entanglement Holds DNA Together, Say Physicists   Technology Review - January 11, 2012

A new theoretical model suggests that quantum entanglement helps prevent the molecules of life from breaking apart. There was a time, not so long ago, when biologists swore black and blue that quantum mechanics could play no role in the hot, wet systems of life. Since then, the discipline of quantum biology has emerged as one of the most exciting new fields in science

Vibration rocks for entangled diamonds   PhysOrg - December 16, 2011

You can take two diamonds - not quite everyday objects, but at least simple and recognizable - and put them in such a state: in particular a superposition of a state of one diamond vibrating and the other not, and vice versa.

Two Diamonds Linked by Strange Quantum Entanglement   Live Science - December 1, 2011
Scientists have linked two diamonds in a mysterious process called entanglement that is normally only seen on the quantum scale. Entanglement is so weird that Einstein dubbed it "spooky action at a distance." It's a strange effect where one object gets connected to another so that even if they are separated by large distances, an action performed on one will affect the other. Entanglement usually occurs with subatomic particles, and was predicted by the theory of quantum mechanics, which governs the realm of the very small. But now physicists have succeeded in entangling two macroscopic diamonds, demonstrating that quantum mechanical effects are not limited to the microscopic scale.

One clock with two times: When quantum mechanics meets general relativity   PhysOrg - October 20, 2011

According to general relativity, time flows differently at different positions due to the distortion of space-time by a nearby massive object. A single clock being in a superposition of two locations allows probing quantum interference effects in combination with general relativity.

Near-Perfect Particle Measurement Achieved   Live Science - July 14, 2011

The mind-bending laws of quantum mechanics say we can't observe the smallest particles without affecting them. Physicists have now caused the smallest-ever disturbance while making a quantum measurement - in fact, almost the minimum thought to be possible. This disturbance is called back-action, and it is one of the hallmarks of quantum mechanics, which governs the actions of the very small. It arises from the supposition that before a measurement is made, particles exist in a sort of limbo state, being neither here nor there while retaining the possibility of either.

Bridging the gap to quantum world   BBC - June 3, 2009
Scientists have "entangled" the motions of pairs of atoms for the first time. Entanglement is an effect in quantum mechanics, a relatively new branch of physics that is based more in probability than in classical laws. It describes how properties of two or more objects can be inextricably linked over "vast" distances. The results, published in Nature, further bridge the gap between the world of quantum mechanics and the laws of everyday experience. This is the first time entanglement has been seen in a so-called "mechanical system". The phenomenon suggests that a measurement performed on one object can affect the measurement on another object some distance away.

Einstein's 'Spooky Physics' Gets More Entangled   Live Science - June 3, 2009
Quantum entanglement is just spooky - even Einstein thought so. As if particles (as in particle physics) have telepathic empathy. The theory of quantum mechanics predicts that two or more particles can become "entangled" so that even after they are separated in space, when an action is performed on one particle, the other particle responds immediately. Scientists still don't know how the particles send these instantaneous messages to each other, but somehow, once they are entwined, they retain a fundamental connection. This bizarre idea riled Einstein so much he called it "spooky action at a distance."