Quantum Computing - What Is Quantum Chromodynamics (QCD)?

Quantum Chromodynamics (QCD) is a physics theory that explains interactions mediated by the strong force, one of the four basic forces of nature. 

It was developed as an analogue for Quantum Electrodynamics (QED), which describes interactions owing to the electromagnetic force carried by photons. 

The theory of the strong interaction between quarks mediated by gluons, the basic particles that make up composite hadrons like the proton, neutron, and pion, is known as quantum chromodynamics (QCD). 

QCD is a non-abelian gauge theory with the symmetry group SU, which is a form of quantum field theory (3). 

The color attribute is the QCD equivalent of electric charge. 

Gluons are the theory's force carriers, exactly as photons are in quantum electrodynamics for the electromagnetic force. 

The hypothesis is an essential aspect of particle physics' Standard Model. 

Over the years, a considerable amount of experimental data supporting QCD has accumulated. 

How does the QCD scale work? 

The quantity is known as the QCD scale in quantum chromodynamics (QCD). 

When the energy-momentum involved in the process permits just the up, down, and strange quarks to be produced, but not the heavier quarks, the value is for three "active" quark flavors. 

This is equivalent to energies less than 1.275 GeV. 

Who was the first to discover quantum chromodynamics? 

One of the founders of quantum chromodynamics, Harald Fritzsch, remembers some of the backdrop to the theory's development 40 years ago. 

What is the Quantum Electrodynamics (QED) Theory? 

Quantum electrodynamics (QED) is the quantum field theory of charged particles' interactions with electromagnetic fields. 

It mathematically defines not just light's interactions with matter, but also the interactions of charged particles with one another. 

Albert Einstein's theory of special relativity is integrated into each of QED's equations, making it a relativistic theory. 

Because atoms and molecules are mainly electromagnetic in nature, all of atomic physics may be thought of as a test bed for the hypothesis. 

Experiments using the behavior of subatomic particles known as muons have been some of the most exact tests of QED. 

This sort of particle's magnetic moment has been found to accord with theory to nine significant digits. 

QED is one of the most effective physics theories ever established, with such great precision. 

Recent Developments In The Investigation Of QCD

A new collection of papers edited by Diogo Boito, Instituto de Fisica de Sao Carlos, Universidade de Sao Paulo, Brazil, and Irinel Caprini, Horia Hulubei National Institute for Physics and Nuclear Engineering, Bucharest, Romania, and published in The European Physical Journal Special Topics brings together recent developments in the investigation of QCD. 

The editors explain in a special introduction to the collection that,

the divergence of perturbation expansions in the mathematical descriptions of a system can have important physical consequences because the strong force — carried by gluons between quarks, forming the fundamental building blocks of matter — described by QCD has a much stronger coupling than the electromagnetic force. 

The editors note out that, with to developments in so-called higher-order loop computations, this has become more significant with recent high-precision calculations in QCD. 

"The fact that perturbative expansions in QCD are divergent greatly influences the renormalization scheme and scale dependency of the truncated expansions," write Boito and Caprini, "which provides a major source of uncertainty in the theoretical predictions of the standard model."

"One of the primary problems for precision QCD to meet the needs of future accelerator facilities is to understand and tame this behavior." 

A cadre of specialists in the subject discuss these and other themes pertaining to QCD, such as the mathematical theory of revival and the presence of infrared (IR) and ultraviolet (UV) renormalons, in the special edition. 

These issues are approached from a range of perspectives, including a more basic viewpoint or phenomenological approach, and in the context of related quantum field theories.

~ Jai Krishna Ponnappan

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Further Reading

Diogo Boito et al, Renormalons and hyperasymptotics in QCD, 

The European Physical Journal Special Topics (2021).

DOI: 10.1140/epjs/s11734-021-00276-w

Quantum Physics Shapes the Laser

 

The Laser—Ever More Abstract Theory, Ever More Technology.

However, atomic energy may be used for peaceful purposes, such as in nuclear power plants. Quantum physics has also shaped a number of other extremely helpful technologies, the most well-known of which is the laser. 

In their motions around the atomic nucleus, electrons can spontaneously hop from one orbit to another, according to quantum theory as expressed in Bohr's atomic model. These are the "quantum jumps or leaps" 

In reality, quantum jumps underpin all of nature's most essential systems for producing light, including chemical reactions like burning (radiation emitted by accelerated charged particles, such as bremsstrahlung which generates X-rays, are a relatively insignificant source of light). 

But, exactly, how do these leaps happen? 

When an electron jumps to a higher energy level, it absorbs the energy of an incoming light particle (photon); when the electron jumps down to a lower level, it releases a photon. So far, everything has gone well. 

But where do light particles originate and where do they end up? 

Another issue is that single quantum jumps are not causal processes that can be anticipated exactly. Instead, they are instantaneous processes that take place outside of time. 

What exactly does that imply? 

When a light switch is turned on, it turns on the light for a short period of time. In other words, the effect takes a fraction of a second to appear. When an electron leaps, however, no time passes at all, not even a fraction of a fraction of a second. 

There is no direct trigger for an electron to jump back to its ground state, and we can't pinpoint a certain moment or time period when it happens. 

These quantum problems prompted Einstein to delve more into the subject of light absorption and emission in atoms in 1917. 

The quantized emission of photons from black substances is described by Planck's radiation formula. Einstein was able to derive another “amazingly easy derivation” of the rule of spontaneous light emission from purely theoretical considerations, as he noted himself. In addition, he discovered an entirely new mechanism that he dubbed "induced light emission." 

This is the emission of photons from adequately prepared (“excited”) atoms that is prompted by another incoming photon rather than occurring spontaneously. The energy produced in this way is released into the electromagnetic field, resulting in the generation of another photon. The triggering photon is still present. 

In an atmosphere where many atoms are stimulated, i.e., many electrons are at a higher energy level, a chain reaction of electrons hopping to lower levels can occur, implying simultaneous emission of light. 

The unique aspect here is that each of the freshly produced photons has the same properties as the others: they all oscillate with the same phase, travel in the same direction, and have the same frequency and polarization (direction of oscillation). 

As a result, a very bright light with attributes equal to those of its constituent photons emerges from a few photons that start the chain reaction. 

A “coherent light wave” is another term used by physicists. Physicists only succeeded in experimentally proving and technologically realizing the stimulated emission of photons that Einstein had described in 1917 on purely theoretical grounds in the 1950s and 1960s. It served as the foundation for the laser, another important quantum technology of the twentieth century. 

A laser is made in two steps: 

1. first, electrons in a material are encouraged to leap to higher energy levels by light radiation, an electric current, or other processes (physicists call this “pumping”). 
2. Then, into the medium, light particles with the same energy (frequency) as the electrons' excitation energy are transmitted, causing the electrons to jump back to their ground state. 

As a result, they emit photons that are identical replicas of the entering photons. The laser's name comes from this process: Light Amplification by Stimulated Emission of Radiation. Even with the laser, scientists were unsure about the exact nature of the processes involved for a long time. 

Only the quantum theory of the electromagnetic field, or quantum electrodynamics, would be able to explain the atomic quantum jumps of electrons and the related spontaneous generation and destruction of light quanta. 

Even more complex mathematics was required for this description than for the basic quantum mechanics. 

The laser once again demonstrates a basic aspect of quantum physics: even the most abstract and non-descriptive theories may yield very practical practical applications.

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