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New device produces “solid light,” hope for big answers in quantum mechanics

by
Scope Correspondent

One very small device may hold very big promise for answering some of the most complex questions in physics. A team led by researchers at Princeton University has developed a system that can force light into a solid state. They are hopeful that their device could lead to the discovery of new forms of light energy, and that it might pave the way toward new answers in the mysterious realm of quantum mechanics.

It is hard to imagine crystallized light, and with good reason. Photons, the tiny particles that make up light energy, “…are massless particles…in nature, photons don’t interact at all,” says co-first author Dr. Darius Sadri, a postdoctoral fellow in condensed matter physics at Princeton. “[What] we’ve managed to do is figure out how to get photons to interact with each other in a strong way.”

The process occurs on what Raftery describes as a “tiny little chip”—it is two by five millimeters, and coated in superconducting metal. The chip has two sites at which photons can exist, and between which the light particles move back and forth. Photons can escape or “leak” out at either site, which changes the number of photons in the system. The researchers then expose these photons to what they call an “artificial atom,” which is actually billions of atoms that work as one while in a superconducting state. When many photons interact at once with this simulated atom, it is as if they are interacting with each other—something that would never happen in nature. And when the number of photons falls low enough, they become “trapped,” freezing together like a solid.

The process of forcing photons to interact is not unprecedented, but the design of this experimental system sets this study apart. The chip is “based on solid state devices,” so it “gives [researchers] more control” in manipulating the system experimentally, Sadri wrote in a September 19 email. This allows Sadri and his colleagues to easily “scale up” their experimentation. James Raftery, a graduate student in electrical engineering at Princeton and co-first author on the paper, says he sees this work as the first step towards larger systems, in which one could “really talk about creating…interesting phases of light.”

This particular project also provides valuable information about quantum systems, which can often elude experimentation. The quantum realm is difficult to think about, much less model. The laws of quantum mechanics govern very small particles, such as the photons on the chip. On a quantum scale, these photons will have a dual nature; they are both particles (like tiny bowling balls) and waves (like sound passing through air). Quantum processes are often described in terms of probabilities, as opposed to deterministic outcomes. Dr. Lu Sham, Emeritus Professor of physics at the University of California, San Diego, describes the strangeness of this concept with a coin toss. In a classical world, heads or tails represent the two possible outcomes of the toss. But in a quantum world, “you can make a so-called ‘superposition’ of two states, up and down. So how do you explain that?” Sham says. “Basically, language fails us, because the language is built on classical concepts.”

Language is not the only thing that fails to clarify quantum mechanics—most technology does as well. Like our vocabulary, our best computers are built with and for classical mechanics. Dr. Aram Harrow, Assistant Professor of Physics at the Massachusetts Institute of Technology, writes, “[C]lassical computers appear to not be able to efficiently simulate quantum systems.” This limits researchers’ ability to fully explore many quantum principles. “The basic theory of quantum mechanics is mostly known, but working out its consequences can be incredibly difficult,” Harrow wrote in a September 19 email. “It’s like the difference between knowing the rules of chess and knowing how to win.”

Because of these limitations, researchers dream of building a quantum computer—one constructed with both quantum rules and quantum bits (or “qubits”). Harrow explains that unlike normal computer bits, which have deterministic states of “0” or “1,” a qubit has amplitudes, similar to the probability of being “0” or “1.” Qubits can compute with wave-like interactions that are much more flexible than binary ones; Harrow estimates that a hundred qubits could be more powerful than a thousand or a million classical bits. The “artificial atom” that allows photons to interact in Sadri and Raftery’s system is made up of the same components as a qubit. This leaves Sadri hopeful that his team’s future research will focus on how to optimize qubits to store quantum information, which he says is “one of the bottlenecks in trying to build a quantum computer right now.”

While the Princeton team’s device is only a first step toward the quantum computers of the future, it is currently a fully fledged “quantum simulator.” It can simulate just one quantum system, making it a specialized, less “universal” version of a quantum computer. But it does provide experimental evidence of the system it models—specifically, a quantum system that is in flux, or a state of “nonequilibrium.” A better understanding of systems like this could “shed light on fundamental questions about how quantum systems thermalize and the formation of the early Universe,” writes Dr. Mohammad Hafezi in a September 8 Physics Viewpoint on this experiment.

Harrow believes that this work stands to significantly benefit the field. “For quantum mechanics, having a quantum computer—even an non-universal one like the one developed by the Princeton team—will greatly extend the range of quantum systems that we can simulate,” writes Harrow, “[A]nd thereby [this helps us] understand the consequences of quantum mechanics.”

 

“Observation of a Dissipation-Induced Classical to Quantum Transition” by J. Raftery et al was published September 8 in the journal Physical Review X.  It is available at http://journals.aps.org/prx/abstract/10.1103/PhysRevX.4.031043

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