MA3: Topologically Protected Qubits

Major Activity 3: Topologically Protected Qubits  

Research Leads: Angela Kou (University of Illinois Urbana-Champaign), Robert McDermott (University of Wisconsin) and Dale Van Harlingen (University of Illinois Urbana-Champaign)

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: topologically 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. 

Project Descriptions

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.  

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Image Credit: Van Harlingen Lab

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.  


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Image Credit: Eriksson Lab

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.  



SEM image of an engineered approach to topologially protected qubits based on composite Josephson elements that are p-periodic in phase

Image Credit: McDermott Lab