quantum simulation

Back in the 1950s, theoretical physicists postulated that the kinds of particles we actually see in nature are just the tip of the iceberg. Many other types of particles with weird properties, which they termed paraparticles, were popping out of the math as theoretical possibilities. But as physicists discovered more about the fundamental particles seen in nature, they found no evidence for paraparticles.

In 2016 Cinthia Alderete, then a graduate student in theoretical physics, discovered a way to simulate paraparticles in which ions and light come together to put on a paraparticle play. To direct this dramatic reenactment, Alderete made the switch from theory to experiment and moved from Mexico to the United States, collaborating with the group of Norbert Linke, a member of the NSF Quantum Leap Challenge Institute for Robust Quantum Simulation and a former Fellow of the Joint Quantum Institute. Together, they brought to life an obscure theoretical curiosity from the past.

The newest JQI Fellow manipulates interactions between atoms and photons in novel, well-controlled ways to simulate difficult-to-observe quantum phenomena.

One of the most exciting applications of quantum computers will be to direct their gaze inwards, at the very quantum rules that make them tick. Quantum computers can be used to simulate quantum physics itself, and perhaps even explore realms that don’t exist anywhere in nature. But even in the absence of a fully functional, large-scale quantum computer, physicists can use a quantum system they can easily control to emulate a more complicated or less accessible one. Now, researchers have coached their ultracold atoms to do a new dance, adding to the growing toolkit of quantum simulation.

There is a heated race to make quantum computers deliver practical results. But this race isn't just about making better technology—usually defined in terms of having fewer errors and more qubits, which are the basic building blocks that store quantum information. At least for now, the quantum computing race requires grappling with the complex realities of both quantum technologies and difficult problems.

One of the mind-bending ideas that physicists and mathematicians have come up with is that space itself—not just objects in space—can be curved. When space curves (as happens dramatically near a black hole), sizes and directions defy normal intuition. Understanding curved spaces is important to expanding our knowledge of the universe, but it is fiendishly difficult to study curved spaces in a lab setting (even using simulations). A previous collaboration between researchers at JQI explored using labyrinthine circuits made of superconducting resonators to simulate the physics of certain curved spaces. In particular, the team looked at hyperbolic lattices that represent spaces—called negatively curved spaces—that have more space than can fit in our everyday “flat” space. Our three-dimensional world doesn’t even have enough space for a two-dimensional negatively curved space. Now, in a paper published in the journal Physical Review Letters on Jan. 3, 2022, the same collaboration between the groups of JQI Fellows Alicia Kollár and Alexey Gorshkov, who is also Fellow of the Joint Center for Quantum Information and Computer Science, expands the potential applications of the technique to include simulating more intricate physics. They’ve laid a theoretical framework for adding qubits—the basic building blocks of quantum computers—to serve as matter in a curved space made of a circuit full of flowing microwaves. Specifically, they considered the addition of qubits that change between two quantum states when they absorb or release a microwave photon—an individual quantum particle of the microwaves that course through the circuit. 

Thanks to Einstein, we know that our three-dimensional space is warped and curved. And in curved space, normal ideas of geometry and straight lines break down, creating a chance to explore an unfamiliar landscape governed by new rules. Spaces that have different geometric rules than those we usually take for granted are called non-Euclidean. Physicists are interested in new physics that curved space can reveal, and non-Euclidean geometries might even help improve designs of certain technologies. One type of non-Euclidean geometry that is of interest is hyperbolic space. Even a two-dimensional, physical version of a hyperbolic space is impossible to make in our normal, “flat” environment. But scientists can still mimic hyperbolic environments to explore how certain physics plays out in negatively curved space. In a recent paper in Physical Review A, a collaboration between Kollár’s research group and JQI Fellow Alexey Gorshkov’s group presented new mathematical tools to better understand simulations of hyperbolic spaces. The research builds on Kollár’s previous experiments to simulate orderly grids in hyperbolic space by using microwave light contained on chips. Their new toolbox includes what they call a “dictionary between discrete and continuous geometry” to help researchers translate experimental results into a more useful form. With these tools, researchers can better explore the topsy-turvy world of hyperbolic space.

To help overcome the challenges of working directly with phonons in physical materials, JQI Fellow Victor Galitski, JQI postdoctoral researcher Colin Rylands and their colleagues have cast photons in the role of phonons in a classic story of phonon-driven physics.

Topological materials have captured the interest of many scientists and may provide the basis for a new era in materials development. On April 10, 2020 in the journal Science Advances, physicists working with Andreas Elben, Jinlong Yu, Peter Zoller and Benoit Vermersch, including JQI Fellow Mohammad Hafezi and former JQI postdoctoral researcher Guanyu Zhu (currently a research staff member at IBM T. J. Watson Research Center), presented a new method for identifying and characterizing topological invariants on various experimental platforms, testing their protocol in a quantum simulator made of neutral atoms.

In nuclear physics, like much of science, detailed theories alone aren’t always enough to unlock solid predictions. There are often too many pieces, interacting in complex ways, for researchers to follow the logic of a theory through to its end. But simulations have helped researchers explore many challenging questions. Now, quantum simulators (which exploit quantum effects like superposition and entanglement) promise to bring their power to bear on many problems that have refused to yield to simulations built atop classical computers—including problems in nuclear physics. But to run any simulation, quantum or otherwise, scientists must first determine how to faithfully represent their system of interest in their simulator. They must create a map between the two.

Quantum behavior plays a crucial role in novel and emergent material properties, such as superconductivity and magnetism. Unfortunately, it is still impossible to calculate the underlying quantum behavior, let alone fully understand it. Scientists of QuTech, the Kavli Institute of Nanoscience in Delft and TNO, in collaboration with ETH Zurich and the University of Maryland, have now succeeded in building an "artificial material" that mimics this type of quantum behavior on a small scale. In doing so, they have laid the foundations for new insights and potential applications. Their work is published today in Nature.