Quantum Information Meets Quantum Matter: Long Range Entanglement and Dynamics Across Quantum Phase Transitions

Ohio State University, The, Columbus OH

Investigators

Abstract

Nontechnical Summary This award supports a program that integrates theoretical and computational research, education, and outreach at the intersection of quantum materials and quantum information. One of the major goals of quantum materials and matter is to understand how new kinds of organizations of electrons and atoms emerge from simple interactions, such as magnetism and superconductivity. Quantum information on the face of it is a very different discipline which deals with information coded in qubits rather than the classical bits, 0 and 1. This program brings together ideas of entanglement between qubits, explores their existence in quantum magnets, and elucidates their unique properties. Quantum systems show unusual properties not exhibited by classical systems, for example a single electron can behave both as a particle and a wave. An even more mind boggling and “weird” property, observable for two electrons or two photons (quanta of light), is quantum entanglement. If two electrons are created in a superposition of quantum states—say (1up, 2down) + (1down, 2up) where up and down indicate the direction of the angular momentum of the spinning electron. Now when one qubit is sent to observer A and the other to B, far apart so they cannot communicate with each other, something truly unbelievable happens: If A detects a spin up electron, B necessarily finds an electron with opposite spin. The two spins are entangled. Their precise spin orientations are revealed only when one of them is observed, at which point the spin of the other also gets precisely determined, even though physically very far from the observed electron. Recently, such entanglement was spectacularly demonstrated between photons separated by over 1200 km. Such entanglement occurs not just between two spins but between billions of spins that exist in magnetic materials with frustration. The emergent state of matter is called a quantum spin liquid. Frustration can arise due to competing interactions and lattice geometry preventing the spins from ordering and forming a magnet. It may appear that with no ordering all is lost, but no! the entangled soup is useful for creating a special kind of qubit called a topological qubit in which information can be stored in a non-local manner and remains protected from environmental effects that can scramble the information they contain. The goal of this project is to put together the theoretical foundation for quantum entanglement and its signatures in quantum materials. The research activity will go hand in hand with an education and outreach program. A holistic course on quantum information meets quantum matter that integrates the standard material with simulations, experiments, and current day research will be developed. The PI is a founder of the Scientific Thinkers program for elementary schools whose motto is “Meet a scientist, Be a scientist, Think like a scientist”. By developing videos on experiments using easily available materials, the engagement of volunteers from Ohio State with the school children and teachers will be greatly enhanced. The PI will develop the Culture Change in Physics (CHIP) program by creating recordings of women in physics and by connecting with the National Society of Black Physicists. Technical Summary This award supports theoretical and computational research and education in quantum materials, quantum matter, and related phenomena. Quantum systems show unusual non-classical properties, including quantum coherence, superposition and interference. But the most mind boggling and “weird” of all is quantum entanglement. Two-particle entanglement has been verified experimentally primarily using photons. This project aims toward the next frontier of multi-particle entanglement in quantum spin liquids (QSLs) consisting of billions of entangled spins, thereby unifying quantum information and quantum matter. The excitations in QSLs are fractionalized and are strong candidates for topological quantum computing. The two main thrusts that will be investigated are: (1) the physical nature of non-local correlations imprinted by long range entanglement due to quantum statistics, interactions, and topological order; and (2) the real-time dynamics of fractionalized quasiparticles and how they create novel entanglement patterns in QSLs. By now quantum phase transitions (QPT) between non-topological phases such as the superfluid-Mott transition in the Bose Hubbard model are well understood. This research activity will focus on understanding the QPT in the Kitaev model on a honeycomb lattice between a gapped and gapless QSL driven by a magnetic field and between two topologically ordered gapped phases that harbor very different excitations as a function of exchange anisotropy. Topologically ordered phases harbor unusual fractionalized excitations, whose emergent exchange statistics, abelian and non-abelian, are richer than the standard bosons and fermions found in nature. The aim will be to make testable predictions for experimental probes by coupling an ancilla to the system and by using two-point correlated noise spectroscopy to probe quantum entanglement. The research will go hand in hand with an education and outreach program. A holistic course on quantum information meets quantum matter that integrates the standard material with simulations, experiments, and current day research will be developed. The PI is a founder of the Scientific Thinkers program for elementary schools whose motto is “Meet a scientist, Be a scientist, Think like a scientist”. By developing videos on experiments using easily available materials, the engagement of volunteers from Ohio State with the school children and teachers will be greatly enhanced. The PI will develop the Culture Change in Physics (CHIP) program by creating recordings of women in physics and by connecting with the National Society of Black Physicists. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.