MA3: Protected Qubits
Major Activity 3: Protected Qubits
Research Leads: Angela Kou (University of Illinois Urbana-Champaign), Robert McDermott (University of Wisconsin)
Error correction is necessary to achieving high performance in any quantum computing approach, including distributed schemes. The standard error correction strategy involves tremendous overhead in physical qubits and logic gate depth. HQAN researchers will pursue an alternative technique that is less demanding: protected qubits. Three approaches will be pursued in parallel—Majorana bound states in superconducting Josephson junctions, proximitized holes in strained germanium, and π-periodic superconducting metamaterials. The common challenges in these systems will be overcome by leveraging interdisciplinary and inter-institutional teams. HQAN researchers will work towards advancing these nascent technologies to functionality and integration into the HQAN superconducting network testbeds.
MA3 focuses on advancing superconducting qubit designs that naturally resist noise and support scalable fault-tolerant computing. A key milestone was the development of a tunable fluxonium qubit that protects against both charge and flux noise. This circuit achieves long coherence times through energy-level engineering, providing a hardware-efficient approach to protected qubit design [1].
To further enhance robustness, HQAN researchers demonstrated Floquet engineering of fluxonium molecules to dynamically isolate two-level subspaces. This approach led to exponential suppression of dephasing, stabilizing the qubit against common noise sources [2].
In support of modular architectures, HQAN has also developed theoretical designs for superconducting components that enable directional control of quantum signals. These include a three-terminal current splitter and a non-reciprocal superconducting diode, both of which aim to minimize back-action and crosstalk between circuit elements while preserving coherence. These tools could prove critical for scaling up complex quantum processors with clean signal routing and architectural flexibility [3, 4].
In parallel, HQAN researchers proposed novel approaches for realizing and identifying Majorana bound states and parity effects in chiral p-wave Josephson junctions. These signatures could serve as the basis for future topological protection in superconducting [5].
On the semiconductor front, new work explored techniques for avoiding valley excitations in silicon quantum wells by omnidirectional electron shuttling. This design strategy offers a promising route to minimize decoherence in spin-based quantum dots, particularly in Si/SiGe architectures [6].
Together, these efforts define a multilayered approach to building robust quantum platforms, spanning superconducting and semiconductor systems, with built-in mechanisms for noise protection, coherence preservation, and modular scalability.
Project Descriptions
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Majorana qubits in S–TI–S junctions: Bezryadin, Eckstein, Levchenko, McDermott, Schuster, Vishveshwara Project Contact: Van Harlingen The goal of this project to develop a quantum processor node based on the braiding of Majorana bound states in networks of lateral superconductor–topological insulator–superconductor (S–TI–S) Josephson junctions formed by fabricating superconductor electrodes on the surface of a 3D-topological insulator. A dispersive readout scheme will be used to readout the parity of Majorana pairs and verify the existence of these states in this system. Ultimately, if Majorana bound states exist, they can be manipulated by applying magnetic fields and currents, which will enable the exchange and hybridization of pairs. |
Image Credit: Van Harlingen Lab |
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Majorana qubits in proximitized, strained Ge:Friesen, Hughes, Levchenko, McDermott, Vishveshwara Project Contact: Eriksson The focus of this project is working toward realizing Majorana modes using proximitzed, strained Ge. The long-term goals of this project are to make use of the strong spin-orbit coupling in Ge and the ability to work with planar heterostructures—as opposed to nanowires—to provide flexibility in how Majorana modes can be manipulated and, ultimately, braided. Present activity focuses experimentally on demonstrating a robust and clean proximity effect in Ge that can be controlled by an electrostatic gate, and on the theoretical side it focuses on simulations and designs aimed at maintaining a low-disorder, single-mode wire in this system. |
Image Credit: Eriksson Lab |
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Topological protection enabled by quantum metamaterials: Bezryadin, Ioffe Schuster, Van Harlingen Project Contact: McDermott HQAN researchers are pursuing an engineered approach to topologically protected qubits based on composite Josephson elements that are p-periodic in phase. Each p-element is constructed from conventional Josephson junctions and inductors, without the need for novel topological materials. When a small superconducting island is connected to ground via a pi-element, there exist near degenerate low lying eigenstates that are superpositions of either even numbers or odd numbers of Cooper pairs on the island. Since the p-element only permits tunneling of pairs of Cooper pairs, fluctuations between these two eigenstates are suppressed, and this system can be used to implement a protected qubit.
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Image Credit: McDermott Lab |