As part of the Co-design Center for Quantum Advantage (C2QA), a DOE National Quantum Information Science Research Center led by Brookhaven Lab, scientists from the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have demonstrated a qubit, whose architecture is more amenable to mass production can perform comparably to qubits currently dominating the field.
Scientists did a series of mathematical analyses to create a guide for making qubits more easily, ensuring that these key parts of quantum computers can be made reliably and strongly.
Recently, scientists have been trying to improve how long qubits hold onto quantum information, a property called coherence tied to the quality of a qubit’s junction.
Their primary focus is on superconducting qubits, which have two layers separated by an insulator, known as an SIS junction (superconductor-insulator-superconductor). However, manufacturing these junctions reliably and with the precision needed for mass-producing quantum computers is quite challenging. Making SIS junctions is genuinely an art.
Recently, scientists have been trying to improve how long qubits hold onto quantum information, a property called coherence tied to the quality of a qubit’s junction.
Their primary focus is on superconducting qubits, which have two superconducting layers separated by an insulator, known as an SIS junction (superconductor-insulator-superconductor). However, manufacturing these junctions reliably and with the precision needed for mass-producing quantum computers is quite challenging.
Making SIS junctions is truly an art. In this study, scientists investigated the impact of this architectural change, trying to understand the performance tradeoffs of switching to constriction junctions.
The most common type of superconducting qubit works best when the junction between the two superconductors allows only a tiny amount of current to pass. The insulator in the SIS (superconductor-insulator-superconductor) sandwich blocks most current but is thin enough for a bit of current to flow through a process called quantum tunneling.
While the SIS design is ideal for today’s superconducting qubits, it’s challenging. Researchers found that replacing the SIS with a constriction—typically allowing more current—can still work for qubits. Their analysis showed it’s possible to reduce the current through a constriction to suitable levels for superconducting qubits, but this approach requires using less conventional superconducting metals.
Liu said, “The constriction wire would have to be impractically thin if we used aluminum, tantalum, or niobium. Other superconductors that do not conduct as well would let us fabricate the constriction junction at practical dimensions.”
However, constriction junctions act differently than SIS junctions, so scientists looked into the effects of this design change.
For superconducting qubits to function, they need some nonlinearity, which allows them to operate between just two energy levels. Superconductors don’t naturally show this nonlinearity; the qubit junction introduces it.
Since superconducting constriction junctions are more linear than traditional SIS junctions, they must be better suited for qubit designs. Nevertheless, researchers discovered that they can adjust the nonlinearity of constriction junctions by choosing specific superconducting materials and carefully designing the size and shape of the junction.
However, constriction junctions act differently than SIS junctions, so scientists looked into the effects of this design change.
For superconducting qubits to function, they need some nonlinearity, which allows them to operate between just two energy levels. Superconductors don’t naturally show this nonlinearity; the qubit junction introduces it.
Since superconducting constriction junctions are more linear than traditional SIS junctions, they must be better suited for qubit designs. Nevertheless, researchers discovered that they can adjust the nonlinearity of constriction junctions by choosing specific superconducting materials and carefully designing the size and shape of the junction.
This exciting work points materials scientists towards specific targets based on the device requirements.
Liu said, “For example, the scientists identified that for qubits operating between 5 and 10 gigahertz, which is typical for today’s electronics, there need to be specific tradeoffs between the material’s ability to carry electricity, determined by its resistance, and the junction’s nonlinearity.”
Charles Black, co-author of the paper that was recently published in the Physical Review A, said, “Certain combinations of material properties just aren’t workable for qubits operating at 5 gigahertz,” said Black. But with materials that meet the criteria outlined by the Brookhaven scientists, qubits with constriction junctions can operate similarly to qubits with SIS junctions.”
Liu and Black and their C2QA colleagues are exploring materials that meet the specifications in their recent paper. They’re particularly interested in superconducting transition metal silicides, which are already used in semiconductor manufacturing.
Their research demonstrated that it’s possible to address the challenges associated with constriction junctions, allowing them to take advantage of the simpler qubit fabrication process.
This work reflects C2QA’s core co-design principle, as Liu and Black are developing a qubit architecture that meets the needs of quantum computing while aligning with existing electronics manufacturing capabilities.
Journal Reference:
- Mingzhao Liu (刘铭钊) and Charles T. Black. Performance analysis of superconductor-constriction-superconductor transmon qubits. Phys. Rev. A. DOI: 10.1103/PhysRevA.110.012427