Quantum computers may one day solve problems that are effectively beyond the capacity of conventional supercomputers. Quantum communications may enable instantaneous, secure transmission of information across vast distances, and quantum sensors may provide previously unheard of sensitivities.
A global race is on to develop these new quantum information technologies, in which information will be stored in quantum bits, or qubits. In conventional digital technologies, a bit is either 0 or 1, whereas a qubit can represent both states at the same time because of a strange phenomenon of quantum physics called superposition. In theory, this will enable a massive increase in computing speed and capacity for certain types of calculations.
At UC Santa Cruz, materials scientists are working to develop novel materials that can serve as the foundation for quantum information technology, just as silicon chips paved the way for today's digital technologies. Several different systems for creating and manipulating qubits have been proposed and implemented, but for now they remain too cumbersome for real-world applications.
"Our focus as materials scientists is on what material we should use as the fundamental element to carry the information. Other researchers are more concerned with how to wire it up to make a device that can perform calculations, but we're focused on the material basis of the qubit," said Yuan Ping, assistant professor of chemistry and biochemistry at UC Santa Cruz.
2D materials
In particular, Ping and other UCSC researchers are focusing on defects in extremely thin materials, called two-dimensional (2D) materials. Defects or imperfections in the atomic structure of a material can function as qubits because information can be encoded in the spin states of their electrons. This phenomenon has been well studied in other types of materials, most notably the "nitrogen vacancy" or NV defect in diamond. But according to Ping, 2D materials offer significant advantages.
"Unlike diamond, 2D materials are relatively cheap and easy to make, they are scalable, and they are easy to integrate into a solid-state device," she said. "They are also stable at room temperature, which is important because a lot of the qubit systems implemented so far use superconductors that can only operate at very low temperatures."
There are a lot of different 2D materials, however, and a lot of ways to put defects into them. The possibilities are almost endless, and it's not practical to synthesize and test them all experimentally to see which have the best properties for quantum technologies.
That's where theorists like Ping come in. She is developing computational methods that can be used to predict the properties of defects in 2D materials reliably and efficiently. In December 2017, her team published a paper in Physical Review Materials establishing the fundamental principles for doing calculations to accurately describe charge defects, electronic states, and spin dynamics in 2D materials. (Her coauthors on the paper include postdoctoral fellow Feng Wu, graduate student Andrew Galatas, and collaborators Dario Rocca at University of Lorraine in France and Ravishankar Sundararaman at Rensselaer Polytechnic Institute.)
In July, Ping won a $350,000 grant from the National Science Foundation to further develop these computational methods.
"We're developing a reliable set of tools to predict the electronic structure, excited-state lifetime, and quantum-state coherence time of defects in 2D materials at a quantum mechanical level," Ping said. "We do calculations from first principles, meaning we don't need any input from experiments. Everything is predicted based on quantum mechanics."
Quantum weirdness
The world of quantum mechanics is notoriously counter-intuitive and hard to grasp. Concepts such as superposition and entanglement defy common sense, yet they have been demonstrated conclusively and are fundamental to quantum information technologies. Superposition, when a particle exists in two different states simultaneously, is often compared to a spinning coin, neither heads nor tails until it stops spinning. Entanglement creates a link between the quantum states of two particles or qubits, so it is as if the outcome of one spinning coin determined the outcome of another spinning coin.
A major challenge in exploiting these phenomena for quantum information technologies is their inherent fragility. Interaction with the environment causes a superposition to fall into one state or the other. Called decoherence, this can be caused by vibrations of the atoms in the material and other subtle effects.
"You want qubits to be well insulated from the environment to give longer coherence times," Ping said.
One 2D material that has shown promise for quantum technologies is ultrathin hexagonal boron nitride. Ping used her computational methods to investigate various defects in this material and identified a promising candidate for scalable quantum applications. This defect (a nitrogen vacancy adjacent to carbon substitution of boron) is predicted to have stable spin states well insulated from the environment and bright optical transitions, making it a good source for single photon emission and a good candidate for qubits.
"Quantum emitters, which can emit one photon at a time, are important for optically-based quantum information processing, information security, and ultrasensitive sensing," Ping said.
She works closely with experimentalists, helping to interpret their results and guide their efforts to create novel materials with desirable properties for quantum technologies. Her group is part of a large collaborative effort, the Quantum Information Science and Engineering Network (QISE-NET), funded by the National Science Foundation. Tyler Smart, a graduate student in Ping's group, is funded by QISE-NET and is working on a project at Argonne National Laboratory.
"He will be traveling to Chicago to present his research every few months," Ping said. "There are about 20 universities as well as national laboratories and industry partners in the network, meeting regularly and sharing ideas, which is important because it's a fast-moving field."
One of the National Science Foundation's 10 Big Ideas for Future NSF Investments is "The Quantum Leap: Leading the Next Quantum Revolution." The Department of Energy is also investing in this area, as are companies such as Google and Intel, hoping to exploit quantum mechanics to develop next-generation technologies for computing, sensing, and communications.
"They are all investing in it because it will take a lot of effort to develop this field, and the potential is so great," Ping said.
