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How Can Photons Be Counted and How Is Light Trapped as Matter?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

How do you count photons?

How do you count photons?

Light Trapped in Matter

To be fair, saying that photons are weird is an understatement. They are massless yet have momentum. They can be emitted and absorbed by electrons depending on the circumstances of the collision between them. Moreover, they act like both a wave and a particle. However, new science is showing that they may have properties that we never imagined possible. What we do with these new facts is uncertain for now, but the possibilities of any emerging field are endless.

Measuring Photon Properties Without Destroying Them

Light’s interactions with matter are rather simple upon first glance. When they collide, the electrons surrounding nuclei will absorb them and transform their energy, increasing the orbital level of the electron. Of course, we can find out the amount of the increase of energy and from there compute the number of photons that were destroyed. To try to save them without this happening is difficult because they need something to both contain them and not eliminate them into energy. But Stephan Ritter, Andreas Reiserer, and Gerhard Rempe of the Max Planck Institute of Quantum Optics in Germany were able to accomplish this seemingly impossible feat. It had been accomplished for microwaves but not for visible light until the Planck team (Emspak).

The basic experiment by the Max Planck Institute

The basic experiment by the Max Planck Institute

The Experiment

To achieve this, the team used a rubidium atom and put it between mirrors that were 1/2000 of a meter apart. Then quantum mechanics settled in. The atom was put into two superposition states, one being in the same resonance as the mirrors and the other not. Now, laser pulses were fired that allowed single photons to hit the outside of the first mirror, which was double reflective. The photon would either pass through and reflect off the back mirror without difficulty (if the atom was not in phase with the cavity) or the photon would encounter the front mirror and not go through (when in phase with the cavity). If the photon happened to pass through the atom when in resonance, it would alter the timing of when the atom entered phase again because of the phase difference the photon would enter based off wave properties. By comparing the superposition state of the atom to its current phase, scientists could then figure out if the photon had passed by (Emspak, Francis).

Implications? Plenty. If fully mastered, it could be a huge leap in quantum computing. Modern electronics rely on logic gates to send commands. Electrons do this currently, but if photons could be enlisted, then we could have many more logic sets because of the superposition of the photon. But it is critical to know certain information about the photon that we normally can only gather if it is destroyed, thus defeating its use in computing. By using this method, we can learn properties of the photon, such as polarization, which would allow for more types of bits, called qubits, in quantum computers. This method will also allow us to observe potential changes that the photon may go through, if any (Emspak, Francis).

Light as Matter and What May Come of It

Interestingly, rubidium was used in another photon experiment that helped shape the photons into a type of matter never before seen, for light is massless and should not be able to form bonds of any sort. A team of scientists from Harvard and MIT was able to take advantage of several properties to make the light act like molecules. First, they created an atom cloud made of the rubidium, which is a “highly reactive metal.” The cloud was chilled to a nearly motionless state, otherwise known as a low-temperature state. Then, after the cloud was placed inside a vacuum, two photons were launched together into the cloud. Because of a mechanism known as the Rydberg blockade (“an effect that prevents photons from exciting nearby atoms at the same time”), the photons came out from the other end of the cloud together and acted like a single molecule without actually colliding with one another. Some potential applications of this include data transmission for quantum computers and crystals that are composed of light (Huffington, Paluspy).

In fact, light as a crystal was discovered by Dr. Andrew Houck and his team from Princeton University. To accomplish this, they gathered 100 billion atoms worth of superconducting particles to form an "artificial atom," which when put near a superconducting wire that had photons going through it gave those photons some of the properties of the atoms courtesy of quantum entanglement. And because the artificial atom is like a crystal in behavior, so too will the light act like that (Freeman).

Lightsabers: a possible future with light as matter?

Lightsabers: a possible future with light as matter?

Now that we can see light acting like matter, can we capture it? The process from before only let light pass through to measure its properties. So how could we gather a group of photons for study? Alex Kruchkov from the Swiss Federal Institute of Technology has not only found a way to do this but also for a special construct called the Bose-Einstein Condensate (BEC). This is when a group of particles gain a collective identity and act like a huge wave all together as the particles get colder and colder. In fact, we are talking about temperatures around a millionth of a degree above zero Kelvin, which is when particles have no motion. However, Alex was able to show mathematically that a BEC made of photons could actually happen at room temperatures. This alone is amazing, but even more impressive is that BECs can only be constructed with particles that have mass, something a photon does not have. Some experimental evidence of this special BEC was found by Jan Klaers, Julian Schmitt, Frank Vewinger, and Martin Weitz, all from the Bonn University in Germany in 2010. They used two mirror surfaces, creating a “micro-cavity” to push the photons into behaving as if they had mass (Moskvitch).

Simulated photon orbits inside hexagonal boron nitride

Simulated photon orbits inside hexagonal boron nitride

Bending the Path

Can we use material to bend the paths of photons into orbits? You betcha. A team led by Michael Folger (University of California) and team found that if layered boron and nitrogen atoms arranged into hexagonal lattices had light introduced to them, the path of the photon isn't scattered but instead becomes fixed and creates a resonance pattern, creating lovely images. They start to act like phonon polaritons and seemingly violate the known rules of reflection by forming these closed loops, but how? It deals with EM disturbances via the atomic structures acting like a containment field, with the orbiting photons creating concentrated regions that appear as tiny spheres to scientists. Possible uses for this could include improved sensor resolutions and enhanced color filtration (Brown).

Of course I would be at fault if I did not mention a special method for making matter out of light: gamma-ray bursts. The outpouring of deadly radiation can also be the birth of matter. In 1934, Gregory Briet and John Wheeler detailed the process of gamma ray conversion into matter and eventually the mechanism was named after them but both felt at the time that testing their idea would be impossible based on the required energies. In 1997, a multi-photon Briet-Wheeler process was done at the Stanford Linear Accelerator Centre when high-energy photons underwent many collisions until electrons and positrons were created. But Oliver Pike of Imperial College London and his team have a possible set-up for a more direct Briet-Wheeler process with the hope of creating particles that normally require the high energy of the Large Hallidron Collider. They want to use a high-intensity laser emitted into a small piece of gold which releases a "radiation field" of gamma rays. A second high-intensity laser is fired into a small gold chamber called a hohlraum which is typically used to help fuse hydrogen but in this case would fill with X-rays produced by the laser exciting the electrons of the chamber. The gamma-rays would enter one side of the hohlraum and once inside collide with the X-rays and produce electrons and positrons. The chamber is designed so that if anything is created it has only one end to exit from, making the recording of data easier. Also, it requires less energy than what occurs in a gamma-ray burst. Pike hasn't tested this yet and awaits access to a high-energy laser, but the homework on this rig is promising (Rathi, Choi).

Continuing Discoveries

Some even say that these experiments will help find a new link between light and matter. Now that scientists have the ability to measure light without destroying it, push photons into acting like a particle and even help them act like they have mass, scientific knowledge will surely benefit and help illuminate discoveries of the unknown.

Works Cited

Brown, Susan. "Trapped light orbits within an intriguing material." innovations report, 17 Jul. 2015. Web. 06 Mar. 2019.

Choi, Charles Q. "Turning Light into Matter May Soon Be Possible, Physicists Say." HuffingtonPost. Huffington Post, 21 May. 2014. Web. 23 Aug. 2015.

Emspak, Jesse. “Photons Seen Without Being Destroyed for First Time.” HuffingtonPost. Huffington Post, 25 Nov. 2013. Web. 21 Dec. 2014.

Fransis, Matthew. “Counting Photons Without Destroying Them.” ars technica. Conte Nast., 14 Nov. 2013. Web. 22 Dec. 2014.

Freeman, David. "Scientists Say They've Created a Freaky New Form of Light." HuffingtonPost. Huffington Post, 16 Sept. 2013. Web. 28 Oct. 2015.

Huffington Post. “New Form of Matter Made of Photons Behaves Like Star Wars Lightsabers, Scientists Say.” Huffington Post. Huffington Post, 27 Sept. 2013. Web. 23 Dec. 2014.

Moskvitch, Katia. “New State of Light Revealed With Photon-Trapping Method.” HuffingtonPost. Huffington Post. 05 May 2014. Web. 24 Dec. 2014.

Paluspy, Shannon. "How to Make Light Matter." Discover Apr. 2014: 18. Print.

Rathi, Akshat. "'Supernova in a Bottle' Could Help Create Matter From Light." ars technica. Conte Nast., 19 May 2014. Web. 23 Aug. 2015.

© 2015 Leonard Kelley