Single Photon (2024)

Single Photon (2024)

Contents

History

It started in 1900 when Max Planck postulated that the heat energy in a black body is quantized (as discrete particles now called photons) depending on the frequency , and is given by the formula E = h, where h = 6.625x10-27 erg-sec is known as the Planck's constant. This idea was verified by Einstein in 1905 with the Photo-Electric Effect (see Figute 01). Planck then went on to derive a correct formula for "Black Body Radiation" based on the quantized photons.

Figure 01 Photo-Electric Effect [view large image]

Detection of single photon is facilitated by the development of "quantum optics" which deals with the interaction between photons, and atoms / molecules. Photons have been used to test many of the counter-intuitive predictions of quantum mechanics, such as entanglement and teleportation, and are a useful resource for quantum information processing.

The development of sensitive "single photon detectors" capable of measuring single photons was essential. Early devices, such as photomultiplier tubes, allowed for the detection of individual photons and paved the way for more sophisticated experiments. Single photon source is unavailable before 1974. The latest advance is the nanowires (see "On-chip indistinguishable photons using III-V nanowire/SiN hybrid integration" and Figure 02). III-V materials = (old)group III (new 13) and V (new 15) elements of the periodic table) are essential in modern electronics and photonics due to their superior electronic and optical properties. SiN is a waveguide. The invention of the laser and developments in non-linear optics enabled the generation of single photons and entangled photon pairs with high precision and control. Techniques like spontaneous parametric down-conversion (SPDC) became standard for producing entangled photons. Advances in materials science and nanotechnology have led to the development of reliable single photon sources, such as quantum dots and defect centers in diamonds, which can emit single photons on demand. Single photon physics is integral to quantum computing and quantum information science. Photons are used as qubits in optical quantum computers and for transmitting information in quantum networks.

Figure 02 Nanowires News [view large image]

Latest news on nanowires technology in 2024 (from "On-chip ...") illustrated pictorially below : within a waveguide

	The role of excitation timing jitter, spectral diffusion, and pure dephasing in limiting visibilities over the temporal extent of the photon is investigated using additional measurements of the coherence and line-width of the emitted photons (see Figure 03).
Figure 03 Single Photon Generation [view large image]	BTW, "on-chip" means that the component is built on the chip (IC) itself, i.e. on the same silicon substrate together with things like transistors, resistors, capacitors, coils...etc, and the mass of a QD ~ 10-18 gm

Courtesy of Copilot (in Italic) :
Superconducting Nanowire Single-Photon Detectors (SNSPDs) offer several advantages, making them highly effective for various applications in quantum technology and photonics. Here are some of the key benefits:

" High Detection Efficiency: SNSPDs can achieve near-unity detection efficiency, meaning they are capable of detecting almost every photon that reaches them.

" Low Dark Count Rate: They have a low rate of false signals (dark counts), which is crucial for applications requiring high signal-to-noise ratios. " Fast Timing Resolution: With the ability to resolve the timing of photon detection down to picoseconds, SNSPDs are suitable for high-speed applications. " Short Recovery Times: They can quickly recover between detection events, allowing for high counting rates, which is beneficial for communication and computing applications. " Broad Spectral Range: SNSPDs can detect photons across a wide range of wavelengths, from infrared to visible light.

Figure 04 Nanowires Detector [view large image]

" Low Timing Jitter: The precision in timing measurements is high, which is important for accurate time-correlated photon counting.

These characteristics make SNSPDs ideal for use in quantum computing, secure communication, and precise timing applications, among others. See Figure 04.

Control

	On-chip propagation of the emitted photons. On-chip splitting of a photon into different branches. On-chip recombining of two photons from different SPSs- resulting in photon interference.
Figure 05 Single Photon Circuit [view large image]

Applications

Application of single photon can be categorized into 2 eras :

1. In the 20 century to theoretical physics
2. In the 21 century to quantum technology

1. Theoretical physics :
   • Feynman diagram is a pictorial representation of the mathematical formulation describing the interaction of subatomic particles. Usually it involves a fermion (for matter, the solid lines in Figure 06) and a boson (carrying out the interaction, the wavy line in the same figure).

     	The boson can be virtual (appearing only for a fleeting moment in between 2 fermions according to the uncertainty principle as in Figure 06,a), or real as the the Compton scattering in Figure 06,b. The idea has been generalized to all the fundamental interactions (weak and strong, see "Standard Model") shown in Figure 06,c.
     Figure 06 Feynman Diagrams [view large image]

   • Feynmman diagrams are used extensively to calculate the various processes (see a list provided by ChatGPT below in Italic) from the interaction between a photon and charged particle in Quantum ElectroDynamics (QED).
     
     The key processes calculated by QED :
     1. Electron-Photon Scattering (Compton Scattering):
     o This process involves the scattering of a photon by an electron. The electron absorbs the photon, is scattered, and then emits another photon. The calculations involve determining the probabilities of different scattering angles and energy distributions of the scattered particles.
     2. Electron-Electron Scattering (Bhabha Scattering):
     o When two electrons (or an electron and a positron) scatter off each other, QED calculates the probabilities of different scattering angles and energy exchanges between the particles.
     3. Electron-Positron Annihilation:
     o When an electron and a positron collide, they can annihilate each other, producing one or more photons. QED provides the framework to calculate the cross-sections and energy distributions of the resulting photons.
     4. Photon Emission and Absorption:
     o QED describes how electrons emit and absorb photons, including processes like bremsstrahlung (braking radiation) and the photoelectric effect. It calculates the rates at which these processes occur and the resulting energy distributions.
     5. Vacuum Polarization:
     o This process involves the interaction of a photon with the vacuum, which can temporarily produce virtual electron-positron pairs. QED calculates the effects of these virtual pairs on the propagation of photons and the modification of the electromagnetic field.
     6. Electron Self-Energy:
     o This refers to the interaction of an electron with its own electromagnetic field, leading to corrections in the electron's mass and charge. QED provides the tools to calculate these self-energy corrections.
     7. Lamb Shift:
     o The Lamb shift is a small difference in energy levels of hydrogen atoms that cannot be explained by the Dirac equation alone. QED calculations of the interactions between the electron and the vacuum account for this energy shift.
     8. Radiative Corrections:
     o QED calculates higher-order corrections to basic interaction processes, such as vertex corrections, which refine the predictions of scattering amplitudes and cross-sections.
     9. Magnetic Moment of the Electron (Anomalous Magnetic Dipole Moment):
     o QED provides highly accurate calculations of the electron's magnetic moment, including corrections that explain the slight deviation from the Dirac value (g-2 factor).
     



2. Quantum technology :
   Single photons have several practical applications, particularly in the fields of quantum technology. Here are some of the key applications by courtesy of ChatGPT (in Italic) :
   
   1. Quantum Computing
   Single photons are essential in optical quantum computing, where quantum information is encoded in the quantum states of photons. This includes polarization, phase, and path. Quantum computing using photons can potentially solve certain problems much faster than classical computers.
   " Linear Optical Quantum Computing (LOQC): This approach uses linear optical elements (like beam splitters and phase shifters), single-photon sources, and detectors to perform quantum computations.
   " Photonic Quantum Gates: Implementing logic gates at the quantum level using the interaction of single photons.
   2. Quantum Cryptography
   Quantum cryptography, particularly Quantum Key Distribution (QKD), utilizes single photons to securely transmit encryption keys. The most famous protocol, BB84, relies on the quantum properties of photons to detect eavesdropping.
   " Secure Communication: Any attempt to intercept the quantum key alters the state of the photons, alerting the communicating parties to the presence of an eavesdropper.
   " Entanglement-Based QKD: Uses entangled photon pairs to ensure security, with protocols like E91.
   3. Quantum Sensing and Metrology
   Single-photon techniques enhance the precision of measurements and sensing applications.
   " Single-Photon Detectors: Highly sensitive detectors used in LIDAR (Light Detection and Ranging) and other sensing technologies.
   " Quantum Lithography: Using entangled photons to achieve resolutions beyond the diffraction limit of classical light.
   4. Quantum Networks
   Building large-scale quantum networks involves the transmission and manipulation of single photons across long distances.
   " Quantum Repeaters: Devices that extend the range of quantum communication by entangling photons over long distances.
   " Quantum Internet: A global network of quantum computers and other quantum devices interconnected via single-photon communication channels.
   5. Quantum Imaging
   Imaging techniques that utilize single photons can surpass classical imaging limits.
   " Quantum Ghost Imaging: Using correlated photon pairs to create images with fewer photons, which is useful in low-light scenarios.
   " Single-Photon LIDAR: Provides high-resolution 3D imaging for applications such as autonomous vehicles and topographical mapping.
   6. Biological and Medical Applications
   Single-photon technologies have applications in biological imaging and medical diagnostics.
   " Single-Molecule Detection: Using single-photon counting techniques to detect and analyze individual molecules, crucial for biological research.
   " Fluorescence Microscopy: Enhances the sensitivity and resolution of fluorescence microscopy, aiding in cellular and molecular biology studies.
   

   7. Single-Photon Sources and Detectors The development of reliable single-photon sources and detectors is critical for all these applications. " Single-Photon Sources: Devices like quantum dots, color centers in diamonds, and spontaneous parametric down-conversion (SPDC) sources. " Single-Photon Detectors: Advanced detectors such as superconducting nanowire single-photon detectors (SNSPDs) and avalanche photodiodes (APDs) offer high efficiency and low noise. Conclusion The application of single photons in various fields is rapidly advancing due to their

   Figure 07 Quantum Technology [view large image]

   fundamental role in quantum technologies. As research and development continue, we can expect significant breakthroughs and new applications that leverage the unique properties of single photons to transform industries and expand our understanding of the quantum world.

   
   Meanwhile from Copilot :
   
   
   • Detecting single photons presents several technical challenges due to their unique quantum properties and the precision required. Some of the main challenges include:
     
     1. Detection Efficiency: This refers to the probability of a photon being detected by the detector. High efficiency is crucial for applications like quantum cryptography and quantum computing.
     2. Timing Resolution: Many applications require precise timing information. For instance, time-correlated single photon counting (TCSPC) or quantum cryptography need accurate timing to function properly.
     3. Spatial Resolution: The ability to determine the exact location where a photon is detected is important, especially in imaging applications.
     4. Spectral Resolution: Differentiating photons of different wavelengths or energies is necessary for applications like spectroscopy or quantum communication.
     5. Environmental and Operating Conditions: Single-photon detectors must often operate under a wide range of temperatures and environmental conditions, which can affect their performance.
     6. Dark Counts: These are false counts that occur in the absence of actual photon events, which can lead to errors in data interpretation.
     7. Afterpulsing: This is a phenomenon where a previous detection event triggers a false count, leading to inaccuracies.
     8. Quantum Efficiency: The ratio of detected photons to the number of incident photons is a measure of a detector's quantum efficiency, which is a critical parameter for many applications.
     
     These challenges are being addressed through advancements in materials science, electronics, and quantum physics, leading to the development of more sophisticated single-photon detectors.
   
   
   • Common types of single-photon detectors include:
     
     " Photomultiplier Tubes (PMTs): These are highly sensitive detectors that amplify the signal of incoming photons using a series of dynodes to produce a measurable current.
     " Single-Photon Avalanche Diodes (SPADs): SPADs operate in a mode where a single photon can trigger a measurable avalanche current, making them suitable for single-photon detection.
     " Superconducting Nanowire Single-Photon Detectors (SNSPDs): These detectors use a thin film of superconducting material that becomes resistive upon the absorption of a photon, allowing for the detection of single photons with high efficiency and low jitter.
     " Transition Edge Sensors (TES): TES detectors are superconducting devices that can resolve the energy of individual photons and are capable of photon-number-resolving, which is a unique feature.
     " Scintillation Counters: These detectors use a scintillating material to convert incoming photons into a flash of light, which is then detected by a photomultiplier tube.
     
     Each type of detector has its own advantages and is suited for different applications based on factors like efficiency, timing resolution, and operating conditions.
     
     End of Copilot contribution
     
     
   • III-V materials are a class of semiconductor compounds made from elements in (old) groups III and V of the periodic table. These materials are widely used in advanced electronic and optoelectronic devices due to their unique electrical and optical properties. Here's a more detailed explanation:
     
     Definition and Examples
     " Group III Elements: These include boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl).
     " Group V Elements: These include nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi).
     Common III-V Compounds
     Some of the most commonly used III-V compounds are:
     " Gallium Arsenide (GaAs): Widely used in microwave frequency integrated circuits, infrared light-emitting diodes, laser diodes, and solar cells.
     " Indium Phosphide (InP): Used in high-speed and high-frequency electronics, as well as in photonic devices like lasers and detectors.
     " Gallium Nitride (GaN): Known for its high thermal conductivity and used in light-emitting diodes (LEDs), power electronics, and high-frequency, high-power transistors.
     " Aluminum Gallium Arsenide (AlGaAs): Often used in conjunction with GaAs for laser diodes and photodetectors.
     
     Properties of III-V Materials
     " Direct Bandgap: Many III-V materials have direct bandgaps, making them highly efficient for light emission and absorption. This is why they are preferred for optoelectronic devices like LEDs and laser diodes.
     " High Electron Mobility: III-V semiconductors generally have higher electron mobility compared to silicon, which allows for faster electronic devices.
     " High Thermal Conductivity: Some III-V materials, like GaN, have high thermal conductivity, making them suitable for high-power and high-temperature applications.
     
     Applications
     1. Optoelectronics: III-V materials are crucial in the fabrication of LEDs, laser diodes, and photodetectors due to their efficient light-emission properties.
     2. High-Frequency Electronics: Devices like high-electron-mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs) use III-V materials for applications in radio frequency (RF) and microwave communications.
     3. Solar Cells: Multi-junction solar cells made from III-V compounds achieve high efficiencies by utilizing different bandgaps to capture a broader spectrum of sunlight.
     4. Power Electronics: GaN and other III-V materials are used in power transistors and diodes due to their ability to handle high voltages and currents with high efficiency.
     
     Summary
     III-V materials are essential in modern electronics and photonics due to their superior electronic and optical properties. They enable the development of advanced devices that are faster, more efficient, and capable of operating under demanding conditions compared to traditional silicon-based technologies.
     
     

Classical Theory

Electromagnetic Wave (Classical Concept) : An electromagnetic wave is described classically by Maxwell's equations. These equations govern the behavior of electric and magnetic fields in space and time. When solving Maxwell's equations for free space (no charges or currents), you obtain wave-like solutions that represent the propagation of electric and magnetic field disturbances - these are the classical electromagnetic waves.

Figure 08 Maxwell Equations [view large image]

	The EM wave and formulas are shown in Figure 09 (u = E or B), where T = period, and = wave-length. Polarization refers to the orientation of the oscillations of the electric or magnetic field vector. Phase refers to the relative position (usually the origin) of the wave at a given point in time and space.
Figure 09 EM Wave [view large image]	Even though polarization is a classical concepts, it plays a crucial roles in the context of single photons and quantum information.

2. Frequency and Wavelength
Classical Concept - Frequency (or wavelength) is a fundamental property of waves, describing the number of oscillations per unit time (or the distance between successive peaks of the wave). In classical optics, it determines the color of light.
Quantum Concept - For photons, frequency (or wavelength) is directly related to the photon's energy via Planck's relation E = h (where E is energy, h is Planck's constant, and is frequency). The energy levels of photons can be used in various quantum processes, such as in quantum dots, atomic transitions, and in distinguishing different quantum states.
3. Wave Vector (Momentum)
Classical Concept - The wave vector k (see Figure 09) describes the direction of propagation of a wave and its spatial frequency.
Quantum Concept - In quantum mechanics, the wave vector of a photon is directly related to its momentum p = h/ according to the De Broglie relation. This relationship is used in various quantum phenomena, including diffraction, interference, and the momentum transfer in interactions with atoms and molecules.
4. Coherence
Classical Concept - Coherence refers to the fixed phase relationship between the electric field vectors at different points in space and time. It is a measure of the predictability of the phase of the wave over time and is crucial in phenomena such as interference and diffraction.
Quantum Concept - In quantum optics, coherence relates to the quantum state of light. Single-photon sources often strive to produce photons that are coherent, which is essential for many quantum information processes. The coherence properties of single photons are critical in applications like quantum cryptography and quantum computing.
5. Interference and Diffraction
Classical Concept - Interference occurs when two or more waves superimpose, leading to a pattern of constructive and destructive interference. Diffraction involves the bending and spreading of waves around obstacles and through slits.
Quantum Concept - For single photons, interference and diffraction are explained by the superposition principle in quantum mechanics. Experiments like the double-slit experiment demonstrate that even single photons exhibit interference patterns, highlighting the wave-particle duality of light. These principles are exploited in various quantum technologies, such as quantum interference devices and quantum lithography.
6. Refraction and Dispersion Classical Concept - Refraction is the bending of light as it passes from one medium to another, and dispersion is the spreading of light into its component wavelengths due to different refractive indices for different wavelengths.
Quantum Concept - In the quantum realm, refraction and dispersion still apply, but they are interpreted through the interaction of photons with the atomic structure of the medium. These effects are essential in designing optical components for quantum devices, such as waveguides and photonic crystals that manipulate single photons for quantum communication and computing.
7. Energy Levels and Transitions
Classical Concept - Classically, energy levels and transitions in atoms and molecules are described by electronic states and the absorption/emission of light at specific wavelengths.
Quantum Concept - In quantum mechanics, photons are emitted or absorbed during transitions between discrete energy levels of atoms and molecules. This quantization is fundamental to processes like spontaneous emission, stimulated emission (which is the basis for lasers), and absorption spectroscopy. Quantum optics uses these principles to control and measure quantum states of light and matter.

Summary
These classical concepts, when applied to single photons in the quantum domain, provide a framework for understanding and manipulating the fundamental properties of light at the quantum level. They enable the development of advanced quantum technologies and deepen our understanding of the wave-particle duality of light. The interplay between classical and quantum descriptions enriches both fields and drives innovation in photonics and quantum information science.

Here's a comment about the Schrödinger Equation :

It is a fundamental equation in quantum mechanics that describes how the quantum state of a physical system changes over time. However, it is non-relativistic and primarily applies to particles with mass. The situation with photons, which are massless particles that always travel at the speed of light, is quite different. Hence, the wave function should not be associated to photons. Relativistic means the formulation is invariant under the Lorentz transformation of space-time.

Figure 10 Wave Function [view large image]

Figure 10 shows the Schrödinger Equation where , V the potential acting on the particle, and m is the mass. The wave function could be bound (Figure 10,a) or free (for V = 0).

Photons are better described within the framework of quantum field theory, where they are understood as excitations of the electromagnetic field. Such approach aligns with the principles of relativity and accurately describes the behavior of light and other electromagnetic radiation. See Standard Model.

There is more confusion with the entanglement of 2 photons in Spontaneous parametric down-conversion (SPDC, see Figure 11). After pondering the problem for a whole day, here's the discussion with ChatGPT (which did not offer a satisfactory explanation previously) : I think the confusion about the massless photon in entanglement is due to the different facets it manifests in different circumstances. For example, it is an electromagnetic wave in diffraction; it is a massless boson in the calculation of cross-section for many QED processes, and then in entanglement like SPDC it is the byproduct of a composite system like the pump photon. What do you think?

Figure 11 SPDC [view large image]

ChatGPT :

You're touching on some interesting and nuanced aspects of how photons are understood in different contexts in physics. Indeed, the concept of the photon can appear different depending on the framework and the phenomenon being considered. Let's break down these facets:

Electromagnetic Wave: In classical electromagnetism, light is described as an electromagnetic wave. This wave picture explains phenomena such as interference and diffraction. The wave properties of light are described by Maxwell's equations, which treat light as continuous waves of electric and magnetic fields oscillating in space and time.

Massless Boson: In the realm of quantum field theory, specifically Quantum Electrodynamics (QED), the photon is treated as a massless gauge boson. It is the quantum of the electromagnetic field, mediating the electromagnetic force between charged particles. This picture is crucial for calculating cross-sections in particle interactions and understanding processes like electron-photon scattering (Compton scattering) or the creation of electron-positron pairs.

Entanglement and SPDC (Spontaneous Parametric Down-Conversion): In quantum optics, especially in experiments involving entanglement, photons are often described in terms of their quantum states. SPDC is a process where a single photon (the pump photon) interacts with a nonlinear crystal to produce a pair of entangled photons (signal and idler photons). Here, the focus is on the quantum correlations between the produced photons, and the description relies heavily on quantum mechanics and the wavefunction (?) of the system.

The confusion can arise because these descriptions, while fundamentally connected, emphasize different properties of the photon:

Wave-particle duality: Photons exhibit both wave-like and particle-like properties. The wave description is often more intuitive for explaining diffraction and interference, while the particle description is essential for explaining phenomena like the photoelectric effect or photon-photon interactions.

Context-dependent properties: In different experimental setups or theoretical frameworks, different properties of photons are more relevant. In diffraction, wave properties dominate; in particle physics, the role of photons as force carriers is central; and in quantum optics, the focus is on their quantum state and entanglement properties.

Composite systems and entanglement: In entanglement scenarios, especially those involving SPDC, photons are part of a larger quantum system. The entangled state is a property of the composite system rather than the individual photons, emphasizing the non-local and holistic nature of quantum mechanics.

In summary, the different manifestations of photons—electromagnetic waves, massless bosons, and entangled quantum states—are all correct but context-dependent descriptions. They reflect the multifaceted nature of photons as understood through classical, quantum mechanical, and quantum field theoretical lenses. This multi-perspective understanding is what makes photons such a rich and fascinating subject in physics.

Standard Model

The physics of a single photon is a part of the Standard Model of elementary particles :
" Photon: The photon is described as a gauge boson, specifically the force carrier of the electromagnetic interaction. It is massless and mediates the electromagnetic force between charged particles. This description comes from Quantum Electrodynamics (QED), which is the quantum field theory of the electromagnetic force.

Gauge Theory and Quantum Electrodynamics (QED)
" Gauge Theory: The electromagnetic force is described by a U(1) gauge theory. The photon is the gauge boson associated with this U(1) symmetry. This theoretical framework successfully explains the interactions of photons with charged particles through the exchange of virtual photons.
" QED: Quantum Electrodynamics provides the detailed mathematical formulation of how photons interact with charged particles. It incorporates principles of quantum mechanics and special relativity to describe these interactions.

Single Photon Physics :
When we talk about the physics of a single photon in various contexts, we are often considering:
" Quantum Optics: This field studies the quantum mechanical properties of light and its interaction with matter. It includes the behavior of single photons in experiments, such as interference, entanglement, and quantum state manipulation.
" Quantum Information Science: Single photons are used as qubits or information carriers in quantum communication and quantum computing. Relationship Between Classical and Quantum Descriptions
" Classical Electromagnetic Waves: Described by Maxwell's equations, which are derived from the classical theory of electromagnetism.
" Single Photon Quantum States: Described by quantum mechanics and specifically by QED within the Standard Model. This includes phenomena like superposition and entanglement, which are purely quantum mechanical and have no classical counterpart.

Summary
" Standard Model: Single photon is a fundamental particle within the Standard Model, described as a massless gauge boson of the electromagnetic force.
" Gauge Theory (QED): Provides the quantum field theoretical framework that explains how photons interact with charged particles.
" Single Photon Physics: Studied within quantum optics and quantum information science, utilizing the principles of quantum mechanics to explore and exploit the unique properties of photons.
So, the physics of a single photon is part of the Standard Model of elementary particles, which describes it as an entity within Gauge Theory, specifically through Quantum Electrodynamics (QED).

In general :
Quantum Electrodynamics (QED) is a part of the Standard Model of particle physics :
It is the quantum field theory that describes how light (photons) and matter (specifically electrons and positrons) interact. It is one of the most precise and successful theories in physics, providing extremely accurate predictions for phenomena such as the electron's magnetic moment and the Lamb shift in hydrogen.
Its role in the Standard Model -
" Gauge Theory: QED is a gauge theory based on the U(1) symmetry group. This means it describes the interactions of charged particles through the exchange of photons, the gauge bosons of the electromagnetic force.
" Interactions: In QED, interactions between charged particles are mediated by virtual photons, which can be represented in Feynman diagrams. These diagrams provide a visual and mathematical representation of the interactions.

The Standard Model :
It is the comprehensive theory that describes the fundamental particles and their interactions, except for gravity. It includes:
" Electromagnetic Force: Described by QED, with photons as the gauge bosons.
" Weak Force: Described by the Glashow-Weinberg-Salam theory, with W and Z bosons as the gauge bosons.
" Strong Force: Described by Quantum Chromodynamics (QCD), with gluons as the gauge bosons.

Components of the Standard Model :
1. Electromagnetic Interaction (QED):
" Gauge boson: Photon ()
" Governs interactions between charged particles like electrons and positrons.

	2. Weak Interaction: " Gauge bosons: W and Z bosons " Responsible for processes like beta decay in nuclei. 3. Strong Interaction (QCD): " Gauge bosons: Gluons " Governs interactions between quarks and gluons within protons, neutrons, and other hadrons. 4. Matter Particles: " Fermions: Quarks and leptons, organized into three generations.
Figure 12 Standard Model [view large image]	All the particles in SM are point-like without internal structure. The photon is in the U(1) Unitary Group (see Figure 12).