Half-Quantum Step Toward Quantum Advantage

Physicists show unconventional pairing in a new type of superconductor that could be useful for quantum computing

A flux qubit, a ring having electric current flowing clockwise and counterclockwise simultaneously with an external field represented as white rings with arrows.
Image courtesy of Yufan Li, Johns Hopkins University
A famous metaphor for a qubit is Schrodinger’s hypothetical cat that can be both dead and alive. A flux qubit, a ring made of superconducting material, can have electric current flowing clockwise and counterclockwise simultaneously with an external field.

The Science

Superconductors are materials that have no electrical resistance below a critical temperature. They typically push away magnetic fields, as if they are surrounded by an anti-field shield. But to use superconductors as qubits (the unit of a quantum computer), scientists had to surround them with magnetic fields. Researchers recently measured a surprising effect for a new type of superconductor: bismuth palladium (β-Bi2Pd). Even when there was no magnetic field around this superconductor, they found it existed between two states. That’s a necessary requirement for creating a qubit. This superconductor may host a Majorana fermion, an exotic quasiparticle. Scientists have proposed the idea of Majorana fermions, but have not observed them in experiments. 

The Impact

Classical computers process information using binary states (0 and 1), called bits. Quantum computers use quantum bits (qubits). However, a qubit can be both 0 and 1 at the same time. Qubits enable quantum computers to perform certain calculations at speeds many times faster than a classical computers. However, large-scale quantum computers need qubits that are much more stable than those currently available. Scientists are researching many different possible approaches to qubits, including photons, trapped ions, loops of superconducting material, and Majorana fermions. Majorana fermions are a promising candidate for stable qubits.  


Scientists are currently looking for material systems that can support long-lived, coherent quantum phenomena that can be used for the development of qubits for future quantum computers.  One actively investigated system is based on Majorana fermions. In condensed matter physics, Majorana fermions are quasiparticles that are their own antiparticles, a fascinating quantum property. Because they always come in pairs, entangled Majorana quasiparticles could store quantum information at two discrete locations. For example, it could store data at opposite ends of one-dimensional wires. Scientists have suggested that Majorana fermions might exist in a spin-triplet superconductor (a superconductor in which pairs of electrons align their spins in parallel, resulting in a net total spin). In this research, scientists observed a half-quantum flux or half-quantum step when measuring the influence of a magnetic field on patterned rings of thin films of β-Bi2Pd. This observation proves unconventional Cooper pairing of electrons. The half-quantum flux was first observed in high-temperature copper-oxide superconductors. Other experiments reported in the literature are also consistent with spin-triplet pairing in this material. Spin-triplet superconductors have the necessary but not sufficient potential to host topologically-protected Majorana fermions. These quasiparticles could serve as a platform for the development of stable qubits for quantum computers with long coherence times and robustness toward atomic perturbations. What makes the half-quantum flux superconducting ring especially attractive is that such a field-free qubit device may enable practical applications of flux qubits for quantum computing.


Chia-Ling Chien
Department of Physics and Astronomy, Johns Hopkins University


This work was supported by the U.S. Department of Energy (DOE), Basic Energy Sciences, including the SHINES Energy Frontier Research Center. E-beam lithography was conducted at the University of Delaware Nanofabrication Facility (UDNF) and the NanoFab laboratory of NIST (CNST).


Li, Yufan, Xu, Xiaoying, Lee, M. -H., Chu, M. -W., and Chien, C. L. "Observation of half-quantum flux in the unconventional superconductor β-Bi2Pd." Science366, 238-241 (2019) [DOI: 10.1126/science.aau6539]

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Johns Hopkins Researchers Discover Superconducting Material That Could Someday Power Quantum Computer: Johns Hopkins University press release

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