CAREER: Encoding ultra-coherent logical qubits in spin ensembles

The construction and control of large-scale quantum systems is one of the grand challenges in contemporary science. The ability to do so would enable us to unlock fundamentally new technological capabilities in the form of quantum simulation and computation. Once available, these technologies are widely anticipated to lead to solutions to so-far unsolvable problems across a multitude of fields such as materials science, drug discovery, or computer science. As such, quantum technology has exceptional potential to benefit society. The critical challenge for building suitable quantum devices lies in the inherent fragility of quantum states: we must find physical quantum systems that represent the logical units of a quantum machine, quantum bits or qubits, with long, intrinsic coherence times (i.e., the time that a quantum state can be kept intact) while at the same time maintaining the ability to control them. This proposal aims to find the means to effectively enhance the available coherence in one of the most promising platforms for quantum computation, superconducting qubits, by developing quantum memories based on naturally extremely coherent spins. Through the design of specifically tailored coupling circuits, this proposal will realize efficient interfaces between superconducting qubit devices and ensembles of spins and allow storage and retrieval of quantum states that are suited for quantum computation. In superconducting circuits, a leading platform for quantum computation, coherence limits are set by materials and methods to fabricate the circuits. The central research goal of this project is the experimental realization of the storage of logically encoded qubits in long-lived spin ensembles. Using state-of-the-art parametric couplers, this work aims to efficiently transfer quantum information from superconducting qubits to harmonic oscillator modes realized by the ensembles. This capability will result in the creation of logical qubit states in the form of so-called cat-qubits. The research aims to provide a pathway for the storage of error-correctable quantum states with orders-of-magnitude improvements compared to superconducting qubits and cavities. Research objectives are: (1) Encoding cat states in ensembles of Ytterbium dopants through parametrically induced strong coupling between spins and a superconducting qubit using Josephson parametric couplers; and the storage of multiple cat states on the millisecond timescale in the spin ensemble using spin echoes. (2) Realization of new parametric couplers based on nonlinear kinetic inductance in superconducting thin films; and the quantum control of spin ensemble modes in externally applied magnetic fields using these couplers. The proposed research will result in new insights across fundamental and applied science, as well as quantum engineering and quantum computing. It will further the understanding of driven, nonlinear quantum systems and limits of gate speeds; aim to demonstrate the feasibility of achieving net benefits in hybrid quantum systems; and significantly advance available storage times of logical qubit states for improved quantum computation. 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. NSF Award ID: 2541984 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Wolfgang Pfaff | Institution: University of Illinois at Urbana-Champaign, URBANA, IL | Award Amount: $550,000 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2541984 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2541984.html