Interacting photons could make this Star Wars fantasy a reality
As you read this sentence, infinitesimally small particles of light are bouncing around at infinitely fast speeds, transferring these words you see on the magazine page or mobile screen directly to your eyes. These particles are called photons, and they are responsible for the most beautiful sunsets, the most fantastic paintings, and the most gorgeous snippets of nature. There is one property, however, that unites photons from all these diverse settings: the particles do not interact with each other.
Unlike particles with mass, which come together to form atoms and a whole host of other structures, photons at their most elementary level do not seem to interact at all. This property explains the following phenomenon: you shine two flashlights in a dark room so that their beams cross, and you see nothing special in the area where the photons intersect. The streams of light are two ships in the night, passing by without even knowing the other exists.
Vladan Vuletic and Mikhail Lukin beg to differ. Vuletic, Professor of Physics at the Massachusetts Institute of Technology, and Lukin, Professor of Physics at Harvard University, have been studying how to make these particles interact with astounding success. A 2013 paper by the two professors detailed the first weak interactions created between two photon molecules—a dimer—but their more recent paper, published in Science, eproves the existence of three photons strongly bound together: a trimer. This refinement reflects improvements in their experimental system and brings us closer to futuristic quantum inventions most scientists never thought would be possible.
The reason for Vuletic and Lukin’s groundbreaking success lies in their elegant experimental design and the properties photons acquire when interacting with matter. “When photons travel in space they are just photons,” Vuletic said. “But when photons travel in a medium, they can be absorbed by atoms and reemitted by atoms.” Lukin, Vuletic, and the rest of their team at the MIT-Harvard Center for Ultracold Atoms took advantage of this unique property. When photons are in this absorbed state, they are able to weakly interact, but the presence and power of this attraction is dependent upon what matter they are passing through. To facilitate long-range interaction between photons, Vuletic and Lukin used a cloud of supercooled rubidium atoms as the medium. Once a photon is reemitted from the atom, having passed through the rubidium cloud, it becomes a “quasi-particle”, having gained certain properties of attraction and repulsion from the atom.
Vuletic described this process as akin to two boats on a lake. On the metaphorical lake of the rubidium cloud, the interaction between the photon boats and the rubidium water creates waves that ripple out and interact with the other photon boat, creating an attraction between the two even though they never directly interact. If the rubidium lake is not present—if these boats are grounded on a dry lake bed—there is no way for the photon boats to affect each other and they never interact.
Imagine the journey of a single photon through Lukin and Vuletic’s experimental setup. The massless particle is first emitted from a weak laser beam, racing at near-light speed towards the ultracooled cloud of rubidium atoms, which is chilled to a temperature just above absolute zero to prevent confounding collisions. Once it hits the cloud, it is absorbed by one of the near-immobile rubidium atoms, now living as part of the atom. Then, within millionths of a second of absorption, the photon quasi-particle is reemitted by the atom, having gained a small fraction of an electron’s mass from its time inside the atom. The quasi-particle continues its journey through the cloud, now traveling about 100,000 times slower. At some point during this journey, the quasi-particle will happen upon another quasi-particle and attract, potentially even picking up a third quasi-particle before exiting the cloud and being measured by researchers.
The researchers looked at the formation of these dimer and trimer photons using a measurement called phase shift, which records the changes in the frequency of photon oscillation before and after exiting the rubidium cloud. This phase shift measurement is an indicator of how strongly the photons are bound: the larger the phase shift, the stronger the interaction. According to the math, the phase shift of a trimer photon structure should be about four times greater than that of a dimer photon structure, because there are more avenues of interaction in a trimer structure. The researchers observed, however, that the trimer’s phase shift was only three times larger than that of the dimer. Vuletic found that this lack of efficiency was actually due to repulsion, despite the trimer structure’s strong attractions. “There’s a weaker, but still there, three photon repulsion at the same time, and so that makes the binding a little bit weaker and the phase a little bit weaker,” he said.
The findings of this paper raise the question of whether larger photon structures do actually exist in nature, contrary to current scientific consensus. Although Vuletic and Lukin had to set up extremely specific conditions for photons to interact, the fact that it is possible and that there are different types of photon interaction suggests that this might not be a solely artificial phenomenon after all. Vuletic suggested that his research has changed light’s fundamental properties, but maybe these properties aren’t so fundamental after all.
There are also more tangible uses of this work. Although this newly-discovered ability of strong photon interaction might initially seem unexciting to non-physicists, it could lead to groundbreaking applications in the field of quantum technology. It might be a little while before lightsabers are a reality, but many scientists are excited about the prospect of applying this discovery to quantum computing. Quantum computers, which theoretically would be able to instantly perform calculations a modern supercomputer could not even dream of, rely on the entanglement of bits of information. Entanglement, a connection between two quantum particles that instantaneously links them regardless of distance, is often fickle; one of the main challenges facing the development of a quantum computer today is how to set up entangled systems consistently and accurately. The strong, structured attraction of photons this research supports is a potential way to reliably achieve this entanglement.
Vuletic, however, is most excited about the potential of his discovery to revolutionize quantum communication. “Quantum communication is the idea that you can send messages absolutely securely protected by quantum mechanics if you use individual photons,” Vuletic said. Because photons are fundamental particles, meaning they cannot be split into anything smaller, it would be practically impossible to interfere with or to steal a signal sent using a photon. The distance and strength of this signal, however, depends on the ability of the sending and receiving centers of the single photon—so called quantum gates—to induce binding and phase shifts. With no phase shift, Vuletic says, a signal cannot be sent more than 50 miles. With the phase shift achieved in this research, you would be able to have light speed communication between Boston and L.A, provided there are intermediate amplification stations for the signal. Achieving phase shift twice to three times as large would allow a photon signal to be theoretically sent across the entire universe using intermediate stations.
It follows, then, that Lukin and Vuletic are constantly trying to increase the phase shift of these photon interactions. While this research only found direct evidence of trimer structures, the researcher’s lab also has indirect evidence of larger-scale photon interactions that could lead to greater phase shifts—a lake with four, five, or even more photon boats. Vuletic thinks that these larger interactions could be repulsive as well as attractive—rather than sticking together as trimers do, larger photon states could repel like ping pong balls bouncing off one another. If their team succeeds in finding photon states that create even larger phase shifts, the implications for quantum technology are literally and figuratively limitless.