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And Let There Be “Molecules” of Light

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Light is a phenomenon that happens daily. It is a physical occurrence. We use lamps to read, we use headlights to drive in unsafe situations, we use it to grow plants,  and we use it as a form of energy to heat and cook. In nature, the sun, stars, and fluorescence provide us with sources of light and energy. Light is also a source of energy for our bodies. Light enters our bodies through our eyes and activates chemicals in our pituitary gland. This reaction gives us energy and creates our moods.


We are all familiar with the writings of Genesis. Genesis gives an account of the beginning of our universe. One statement in this account is “Let there be light” Light, as much as time, has been around since the beginning of our universe.It is believed that light was produced in the second stage of the Big Bang. The second stage of the Big bang includes quark-gluon interaction. This quark-gluon interaction is also evidence to support the color force studied by quantum chromodynamics.The color force is the interaction that binds protons and neutrons together in the nuclei of atoms. This is controlled by a particle called the gluon. A gluon is a subatomic particle that binds quarks together. A quark is any of a number of subatomic particles carrying a fractional electric charge. A good reference for quantum chromodynamics is “The Quark and The Jagur”by Murray Gell-Mann. Light, as defined by Merriam Webster dictionary, is “the form of energy that makes it possible to see things”


Through experiments and discoveries, we have learned to harness light and experiment with it.Descartes, a greek philosopher, is credited for the discovery of the mechanical  properties of light. Benjamin Franklin experimented with the electrical properties of light. Thomas Edison gave us the light bulb. Charles Townes and Arthur Schawlow gave us the maser. Gordon Gould invented the first laser. The maser and the laser both use stimulated radiation. The laser and maser use an artificial form of spontaneous emission. This artificial form is called stimulated emission. Because of stimulated radiation, we can see that light is energy. In nature, spontaneous emission is found through the natural phenomena of fluorescence.

Spontaneous emission is the process by which a quantum system such as an atom or molecule is in an excited state. It undergoes a transition to a state with a lower energy (the ground state) and emits a quanta of energy.

Stimulated emission is the process by which an an excited state interacts with an electromagnetic wave of a certain frequency. Because of this, the state may drop to a lower energy level, transferring its energy to that field.

Light has also been instrumental in the discovery of new particles. This has been done by using particle colliders. One of these particles is the Higgs Boson at CERN.


Since Descartes, several scientists have studied this phenomena and have  become more curious about light. This curiosity has led to discoveries about the physical properties of light.. One popular property of light is that it is electromagnetic radiation. Another property is that it can be quantized. When light is quantized, it is called a photon. Huygens, Newton, Hooke, Young, Fresnel, Faraday, Maxwell, Hertz, Planck, Lewis, Einstein, Lenard, Schrodinger, De Broglie, and Feynman have all contributed to the modern theories of light.

Huygens explained how wave theory accounts for geometrical optics. Geometrical optics describes light propagation in terms of “rays”.

Newton”s theory of light was based on Descartes. Though, Descartes was responsible for discovering the mechanical properties of light; he believed in corpuscularism. This philosophy discusses corpuscles ( or particles) and their interaction. Newton took Descartes’idea and developed the corpuscular theory of light. This theory states that light is a stream of particles.

Hooke investigated the phenomenon of refraction.Refraction is the phenomenon of light or radiation being deflected as it passes obliquely through the interface through a mediums of varying density.He suggested that light’s vibrations could be perpendicular to the direction of propagation. A quantum system called “Hooke’s atom”is named for him. It is an artificial helium-like atom. In this system,  the coulombic electron-nucleus interaction potential is replaced by a harmonic potential. The harmonic potential used to describe the electron-nucleus interaction is a consequence of Hooke’s law. Hooke’s law states that the stress applied to an object is proportional to the strain produced.

Young studied the wave theory of light. He proved that the property of interference existed in waves of light. This was done by doing the double slit experiment. This experiment displays the wave-particle nature of matter. In regards to interference, the property of superposition is displayed. This property affirms the existence of entanglement.Entanglement involves properties of quantum states correlating with each other in quantum systems.

Fresnel was responsible for diffraction.  Diffraction is the process by which a beam of light or radiation is spread out as a result of passing through either a narrow aperture or edge. This is accompanied by interference between the waveforms produced.

Faraday also established that magnetism could affect rays of light and that there was an underlying relationship between the two phenomena.

Maxwell proposed that light was a product of the interaction or entanglement of the electric and magnetic fields.  An electric field is a region around a charged particle or object within which a force would be exerted on other charged particles or objects. Magnetic fields are produced by electric currents. It is an area around a moving electric charge within which the force of magnetism acts. An insight to this is that-if the photon, light or a quantum of electromagnetic energy, is a product of two fields interacting then other particles would be products of the interactions of other fields.

Hertz ,through experimentation, proved that transverse free space electromagnetic waves can travel over some distance. Hertz was also responsible for giving us the concept of frequency. Frequency is the rate at which a vibration occurs that constitutes a wave,

Planck discovered the concept of blackbody radiation. Blackbody radiation is the  electromagnetic radiation that is radiated from an ideal black body. A black body is an object that is capable of absorbing all of the electromagnetic radiation that falls on it. The distribution of energy in the radiated spectrum of a black body depends on temperature .

Gilbert Lewis , who created the lewis dot structures for chemistry, also studied photochemistry. Photochemistry is the branch of chemistry concerned with the chemical effects of light. Because of this, he coined the word “photon” A photon is a particle representing a quantum of light or other electromagnetic radiation. It carries energy proportional to the radiation frequency but has zero rest mass. It is also an elementary particle and a force carrier. A force carrier is the product of  forces between particles arising from the exchange of other particles.

Einstein is credited for the photoelectric effect. The photoelectric effect happens when electrons are emitted from matter. The electrons are emitted because the matter absorbs energy from electromagnetic radiation. This electromagnetic radiation has a short wavelength.

Lenard, through experimenting with cathode rays,  discovered that the energy of the electron depends on the wavelength of light.

Schrodinger studied that light was coherent. This proves that light has the property of superposition.

De Broglie used the concept of frequency in mathematical relations to define wavelengths. There is another type of frequency. This is called angular frequency. Angular frequency is also known as angular speed. It measures the rotation rate.

Feynman studied how light and matter interacted. He created the theory of quantum electrodynamics. This a quantum field theory that deals with the electromagnetic field and its interaction with electrically charged particles.


Physicists make Photons Interact PW-2013-09-26-Johnston-photos courtesy of stockphoto

Physicists make Photons Interact PW-2013-09-26-Johnston-photos courtesy of stockphoto

Using some sophisticated equipment,  Scientists have attempted to create  “molecules” made from two photons. This study basically examines light as a molecule, bound states, the strong interaction, the conservation of mass and energy,and the interaction between light and matter.

A team led by Mikhael Lukin at Harvard University and Vladan Vuletićat the Massachusetts Institute of Technology has created strong interactions between photons.1

Their experiment was created by firing pairs of photons through an ultracold atomic gas composed of Rubidium atoms. Rubidium is the chemical element of atomic number 37, a rare soft silvery reactive metal of the alkali metal group. An ultracold atom is an atom that is controlled at temperatures close to 0 kelvins or temperature of absolute zero.  The attraction of the photons with the ultracold atomic gas caused the photons to become coherent or stick together. This means that quantum entanglement occurred between the photons. Quantum entanglement is a form of quantum chaos. it is a type of organized “chaos” In entanglement, states become entangled and share each other’s properties. However,these states “disentangle”when observation of the system occurs.

The team was also able to show that the photons in each pair were entangled in terms of their polarization. The researchers did this by firing pairs of photons with a specific polarization into the gas. As the photons travel through the medium, their polarizations change. By measuring the correlation between the polarizations of the photons, the team was able to show that the photons had been entangled when they formed a molecule.

Blue laser light with a carefully chosen wavelength of 479 nm was used. This modified the Rubidium atoms so that a photon can share a part of its energy with several atoms and create a collective “Rydberg state”. This state is basically a Rydberg atom. A Rydberg atom is an atom in a highly excited state in which one electron has the sufficient energy to escape. However, the electron is shared among several atoms. When a Rydberg state forms, however, it becomes impossible for more Rydberg states to be created nearby, thanks to a process called the Rydberg blockade.This Rydberg state propagates through the gas like a sluggish photon with a non-zero mass and when the collective state reaches the opposite edge of the gas cloud, the photon re-emerges at its original energy.

In regards to the second photon, the region of the Rydberg state has a different index of refraction than the rest of the gas. The index of refraction describes how light or radiation travels through a medium.It is measured by the ratio of the speed of light in a vacuum to the speed of light in a substance. This causes the second photon to stay close to the other photon  as they travel together through the gas.When two photons are fired into the gas in quick succession, this creates a bound state of two photons(or molecule)  travelling through the atomic gas.

To monitor this tendency to stay together, the team measured the time interval between the detection of the first and second photons in a pair. Instead of seeing the second photon overtake the slower Rydberg-state photon, the two tend to emerge from the gas together. “It’s a photonic interaction that’s mediated by the atomic interaction, which makes these two photons behave like a molecule,” says Lukin. “So when they exit the medium, they’re much more likely to do so together than as single photons.”

A correlation between the polarizations of the photons was measured. The team was able to show that the photons had been entangled when they formed a molecule.

Getting photons to stick together is almost impossible because they normally pass through each other.As a general rule, they do not interact. However, a photon has a electromagnetic field. As stated earlier, this is an interaction of the electric and magnetic fields. Because these fields “entangle” photons can influence other photons around them and effectively interact with each other. This effect is very small. If the using proper medium, a radioactive element, then the effect is significant as electromagnetism produces radiation.

While photons are very good at transmitting quantum bits of information over long distances, the fact that they do not normally interact with each other makes it difficult to create all-optical logic gates. “What it will be useful for we don’t know yet; but it’s a new state of matter, so we are hopeful that new applications may emerge as we continue to investigate these photonic molecules’ properties,” says Lukin.

Light in a box. Photons confined within the photonic molecule are restricted to a set of discrete energies, like electrons in a real molecule. APS Physics

Light in a box. Photons confined within the photonic molecule are restricted to a set of discrete energies, like electrons in a real molecule. APS Physics

A similar experiment was done in the late 1990’s.It was a test done by a collaboration led by Alfred Forchel, of the University of Würzburg in Germany, and Thomas Reinecke, of the Naval Research Laboratory in Washington, DC.2 To make a photonic molecule, the group constructed two blocks of gallium arsenide, approximately 3 × 3 × 1 µm in size, to act as light-confining resonators. The size and shape of the whole structure tended to enhance specific frequencies of this light as it reflected within the structure–essentially the same effect that causes electrons confined in atoms and molecules to assume a set of discrete energy levels. A fraction of the light escaped, which allowed the team to measure the intensity of the photonic molecule’s light over a range of wavelengths.

Reinecke and his colleagues suspected that the splitting was due to the photonic atom states transforming into photonic molecule states, just as the energy levels of two hydrogen atoms split when they bind to become a molecule.

These patterns could be deduced by measuring the intensity of light emitted at different angles.

The Teachings of Photon Interaction

This experiment confirms many concepts in physics. First. it demonstrates the concept of  the conservation law of energy and matter. This law basically states that matter nor energy are neither destroyed but change form. This experiment shows that matter and and energy can co-exist.

These experiments show us what goes on in nature. They allow us to look at nature’s quantum mechanics.

It also demonstrates quantum chromodynamics. It shows the quark-gluon relationship. This relationship was also responsible for the beginning of our universe. This relationship is called the strong interaction It also defines the practical nature of quantum mechanics.Quantum mechanics studies the interaction between particles and matter. It also affirms entanglement. Entanglement is the process of systems interacting with each other. For a photon, the systems that interact are the electric and magnetic fields. Entanglement confirms that multi-level quantum systems can exist. One such system is the qubit; the quantum bit. It also affirms that a molecule is a bound state. A bound state is a composite of two or more building blocks (particles or bodies) that behave  as a single object.

The photonic molecule is just the first step toward fabricating even more complicated structures, atom by atom, in a way that is impossible to do with electronic building blocks.

The result of such a close relationship is a “photonic molecule,”whose optical modes bear a strong resemblance to the electronic states of a diatomic molecule like hydrogen.

Creating interactions between photons is not just for lab experiments but for practical application. It could also lead to faster and more energy-efficient computers that use light pulses instead of electrical pulses to process information. Currently, light pulses are converted to electrical pulses for processing and then back again. This costly and time consuming..The discovery of a “molecule”of light could allow both conventional and quantum computers to encode and process information using photons. This would be of greater benefit for quantum computers. Quantum computers  use quantum bits(qubits) to process information. If we are able to create molecules of light, perhaps, we could use the photon to create qubits. Not only could this discovery be used to advance computers, but perhaps create better mobile systems and energy sources.


Joel Taylor About the Author: Joel Taylor grew up in Siskiyou County, California. He attended Yreka High School in Yreka, California. After that, he attended College of the Siskiyous in Weed, California and then transferred to Southern Oregon University in Ashland, Oregon where he graduated with a Bachelor's of Science in Physics. Since then, he has had his own tutoring and research company. He has also worked at the Maryland Science Center in Baltimore, Maryland. Joel is also a member of the American Physical Society and American Institute of Physics. Follow on Twitter @Joel_a_taylor.

The views expressed are those of the author and are not necessarily those of Scientific American.

Comments 5 Comments

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  1. 1. marco-sum 4:17 pm 10/14/2013

    Not to mention calling Descartes a Greek philosopher, but despite of that, it is a very informative article.

    Link to this
  2. 2. Bill_Crofut 6:11 pm 10/14/2013

    Re: “It is believed that light was produced in the second stage of the Big Bang.”

    It seems acceptable to believe that light was produced in a “big-bang.” As an alternative, it’s my belief that light was called into existence on the First Day of
    Creation (Genesis 1:3).

    Link to this
  3. 3. Paul108 5:18 am 10/16/2013

    “Benjamin Franklin experimented with the electrical properties of light.” Are you thinking of lightning rather than light?

    Link to this
  4. 4. phoenix91125 11:17 am 10/16/2013

    I have visited this article several times as it is very interesting and great teaching material for this week. I have seen some very negative comments toward the author. Perhaps that might be a bit of a jealous shot at the author, However, though the internet is a place to express your opinions freely as the authors do here on Scientific American, it is non=productive to take pot shots and be negative towards the author(s), These blogs are meant for informative purposes and not for judgement.

    Link to this
  5. 5. phoenix91125 11:21 am 10/16/2013

    I want to add one more thing, I was just on twitter and even the armed forces appreciate this article as well as a teacher in the Arctic Tundra. Do not go judging people for their writing. Learn from it and use it. Yes, everyone has their own opinion but take it as that.

    Link to this

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