A photon is a particle of electromagnetic radiation. It is most widely recognized as a light particle but ranges across the whole electromagnetic spectrum, from the longest radio waves to the shortest gamma rays. Since its discovery in the early twentieth century, the photon has contributed greatly to the understanding of the fundamental physics, was used in the multitude of experiments to prove the dualistic nature of light, served as a basis for the conceptualization of the quantum mechanics theory and has found its application in the wide variety of human life, ranging from theoretical concepts of analogous computation and information techniques to the basic utilization of its zero rest mass and speed.
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The idea of light as a sum of particles has predated the wave hypothesis for several centuries, mainly because of Sir Isaac Newton’s influential Hypothesis of Light of 1675. However, certain properties of light, primarily the refraction and diffraction, and, to a lesser degree, birefringence, aligned poorly with this idea, and, as a result, the wave nature of light has been suggested instead.
This hypothesis, confirmed experimentally by Thomas Young and August Fresnel and further strengthened by the detection of radio waves by Heinrich Hertz, finally became generally accepted in the late nineteenth century. Paradoxically, the wave nature also did not explain some of the characteristics of light. For instance, some chemical reactions require the light of certain frequencies to be set in action. If the light is assumed to be a wave, the frequency does not determine the energy of the light beam.
Instead, the intensity is the only relevant parameter. However, the intensity of light does not matter when it comes to triggering the said reaction. Besides, the photoelectric effect, which was well studied and documented at the time, produced similar results, where the electrons produced by exposing a metal plate to the light source differed in their energy depending on the frequency of the source, not the intensity.
Thus, in the early twentieth century, Max Planck suggested that the energy carried by light was transmitted by discreet amounts, called “packets.” This was partially confirmed by the experiments of black-body radiation studies, where the energies observed inside a cavity at a certain temperature exhibited an uneven distribution, forming a bell curve, with a peak in certain frequencies and a decline in higher and lower values (Bortz 24).
This bell curves aligned well with the similar curve of the velocities of molecules of the ideal gas, where certain median velocities dominated while both higher and lower values occurred significantly less frequently (Bortz 24). This correlation has drawn the attention of Albert Einstein, who, in 1905, has suggested that light waves are carrying energy in discreet amounts, or quantized, much like particles. The quantization of energy was initially thought of as a result of some material obstacle that interrupted the energy flow until Einstein has suggested that it was an intrinsic property of radiation itself (Bortz 26).
Later, Einstein has expanded on the law of black-body radiation, stating that the quanta suggested by Planck must have momentum, which means they are particles. The term “photon” was applied to them as late as 1928 by Arthur Compton, who also contributed greatly to the discovery of light particles by demonstrating the validity of Einstein’s theories in what is now known a “Compton effect” – a mathematical analysis of the scattering angle and wavelength shift of x-rays (Bortz 28).
The subsequent development of the studies of photon further enhanced the understanding of the nature of electromagnetic radiation and gave way to the discovery of its quantum properties. The current understanding of a photon’s life cycle is as follows: the photon is formed when a certain amount of energy is released from an atom (i.e. a transition from one discreet level of energy to the other).
This most commonly happens when the heat is applied to a body, which frees the energy in the form of radiation (visible light is the most recognizable example). At this point, a photon is released and travels in a certain direction at a speed of light. A photon has zero mass, so it can travel infinite distances until it is absorbed, effectively transferring its energy to other atom and disintegrating in the result. Alternatively, if the energy of the photon is high enough, it can create an electron and a positron without colliding with a quantum system (i.e. an atom) by splitting into a particle and an anti-particle (positron) which, in turn, disintegrate upon collision and produce a photon (Zettili 17).
However, the most important development in understanding the quantum properties of a photon has occurred when the double-slit experiment has been conducted to confirm the wave-particle duality of the EM radiation, suggested by Einstein. The photons exhibit a certain behavior, characteristics of waves, such as diffraction. At the same time, it does not divide when it encounters a beam splitter, thus suggesting its particle nature (Paul 6).
At the same time, it does not behave strictly like a point-like particle, as its trajectory is not influenced by an electromagnetic field as predicted (Zettili 588). Most interestingly, the photon, like any quantum particle, demonstrates the uncertainty principle, where the position of a photon’s arrival is determined, among other things, by the fact of the measurements conducted at the splitting point (the slit). This effect has led to the establishment of the quantum theory, which views the particle as being in a superposition, i.e. more than one state at once. Besides contributing greatly to the fundamental theoretical physics, the quantum characteristics of a photon are potentially useful in a range of practical applications.
The most widely recognized area of application of the quantum mechanics pertinent to photons is commonly known as quantum computing. The theory suggests the usage of photons as carriers of information. Whereas the traditional electronic computers, which are binary, carry the information by assigning the transistor one of the two defined states (commonly recognized as 0 and 1), the quantum computation device makes use of the superposition of light particles, which may be in two states simultaneously.
These data packets, termed qubits, allow for the much faster computation capabilities of the device. Additionally, some of the functions of the current computers, mostly those of non-deterministic and probabilistic nature, require complex simulations to be performed, while the quantum computer would have an intrinsic capability for them (Hirvensalo 5). Finally, the physical characteristics of such machines, most prominently their size, would be superior compared to the available technology. Currently, the development of quantum computers is still in its theoretical stage, with few actual working computations performed on a small scale (Hirvensalo 2). Nevertheless, the benefits of quantum computing are already recognized by investors in both the private and state sector.
Another possible area where the quantum properties of photons are expected to yield superior results in quantum information. While the concept of the information carried by the qubits instead of bits is valuable primarily for the field of theoretical physics, several theories exist of practical applications. Most notable of these are superdense coding and quantum teleportation. The former allows storing twice the amount of classical information by using one qubit as two classical bits.
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The latter significantly speeds up the transfer of information. However, both rely on the quantum entanglement, the concept which allows the extrapolation of parameters of a photon by measuring the same parameters of its Bell pair, and which is still in the stage of early development (Marinescu 329). Thus, while certain progress was made, and both concepts have been experimentally proven to work, none of them is close to practical implementation.
Other characteristics of a photon, such as its speed and zero rest mass, have found application in multiple fields of human activity. Optical telecommunication, for example, is a widely accepted method of transferring information that ranges from macroscopic to an atomic scale. Photochemistry is a branch of chemistry that utilizes properties of electromagnetic radiation (usually of a visible spectrum) to trigger chemical reactions. Finally, photons offer a wide variety of monitoring capabilities, from the optical sensors in robotics to the medical equipment relying on the radiation of varying wavelengths, from ultraviolet to hard x-rays.
The impact of a photon’s discovery can not be overestimated. Since the early twentieth century, it has not only contributed greatly to the understanding of the fundamental physical laws and allowed for the formation of the quantum theory but has found itself in a variety of applications. While the majority of these applications are currently theoretical in origin, these implementations will doubtlessly serve to advance the computation and information transfer in the nearest future.
Bortz, Alfred. The Photon, New York: The Rosen Publishing Group, 2004. Print.
Hirvensalo, Mika. Quantum Computing, New York: Springer Science & Business Media, 2013. Print.
Marinescu, Dan. Classical and Quantum Information, New York: Academic Press, 2011. Print.
Paul, Harry. Introduction to Quantum Theory, Cambridge, UK: Cambridge University Press, 2008. Print.
Zettili, Nouredine. Quantum Mechanics: Concepts and Applications, New York: John Wiley & Sons, 2009. Print.