A Brief Overview of Superconducting Circuit Qubit Types

Superconducting-based quantum bits can be classified into two main types:

  • Qubits based on superconducting circuits using Josephson Junctions, sometimes referred to as artificial atoms. A single qubit size is about the micrometer (\( \mu m \)) scale. This article focuses on this type of qubit.
  • Topological qubits, using superconductors as the substrate, with nanowires placed on the surface. Due to the proximity effect, the nanowires are influenced by the substrate's superconductivity. Applying a magnetic field to break time-reversal symmetry forms special Majorana states at both ends of the nanowires. This topic may be discussed in the future. \(^a\)
\(^a\)10.1038/s41586-022-05585-1
Superconducting circuits made of Josephson Junctions can refer to the figure below, showing different superconducting qubit types:

The A series represents the most basic unit to form a single qubit. The arrows indicate the conceptual extension directions. I categorize them into four main types:

  • Type A: A1 represents a single Josephson Junction (JJ) as a qubit, an earlier qubit design.
  • Type B: A2โ€“A8, based on a single JJ shunted with a capacitor (C-shunt). Later designs aimed to increase coherence time by connecting large inductors in parallel or reducing capacitance to mitigate noise effects.
  • Type C: A9โ€“A10, qubits with modifications on one side of the JJj, making the ground state phase difference non-zero. These can be achieved through complex circuit designsa,b,c, replacing the insulating layer with magnetic materialsd,i, or using s-wave with d-wave heterostructure JJsg,h.
  • Type Hybrid: A11, representing superconducting qubits combined with spin quantum dots (Spin Quantum Dot, Spin QD)k. Other types include combinations with superfluid heliume,f, ion traps, and more.l
\(^a\)10.1038/s41534-019-0231-2
\(^b\)10.1088/1367-2630/aab7cd
\(^c\)10.1088/1367-2630/ab09b0
\(^d\)10.1038/s43246-024-00659-1
\(^e\)10.1103/PhysRevA.101.012336
\(^f\)10.1038/s41467-019-13335-7
\(^g\)10.1109/TASC.2019.2892078
\(^h\)10.1088/1361-6668/ab7053
\(^i\)10.1103/PhysRevApplied.8.054028
\(^j\)10.1088/0953-2048/21/3/034011
\(^k\)10.1038/s41567-023-02071-x
\(^l\)arXiv: 1204.2137
(A8) https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr201209fa2.html

The B series represents examples of Type A qubits paired with control circuits, using different control circuit techniques. More details can be found in Early Types of Superconducting Qubits.
The C series represents examples of Type B qubits paired with control circuits.

Originally written in Chinese by the author, these articles are translated into English to invite cross-language resonance.

Peir-Ru Wang Superconducting Qubits (Elementary)