Qubit ZooCurated map of Zoo entries in the Superconducting family.
Entries
| Entry | Type | Status |
|---|---|---|
| tunable-coupler | coupling | demonstrated |
| binomial-codes | encoding | demonstrated |
| circuit-qed | infrastructure | demonstrated |
| 0-pi-qubit | qubit | demonstrated |
| bifluxon-qubit | qubit | demonstrated |
| blochnium | qubit | demonstrated |
| bosonic-qubit | qubit | demonstrated |
| cat-codes | qubit | demonstrated |
| cooper-pair-box-charge-qubit | qubit | demonstrated |
| cos2phi-qubit | qubit | demonstrated |
| dual-rail-superconducting-qubit | qubit | demonstrated |
| flux-qubit | qubit | demonstrated |
| fluxonium | qubit | demonstrated |
| gkp-codes | qubit | demonstrated |
| gmon | qubit | demonstrated |
| heavy-fluxonium-qubit | qubit | demonstrated |
| kerr-cat-qubit | qubit | demonstrated |
| mergemon | qubit | demonstrated |
| phase-qubit | qubit | demonstrated |
| transmon | qubit | demonstrated |
| unimon-qubit | qubit | demonstrated |
| xmon | qubit | demonstrated |
Composition
- coupling: 1
- encoding: 1
- infrastructure: 1
- qubit: 19
Conceptual anchors
- charge-noise-in-superconducting-qubits and charge-noise-sweet-spot explain the CPB → transmon → protected-superconducting arc.
- josephson-junction-as-nonlinear-element is the common circuit primitive behind nearly every qubit in this family.
- jaynes-cummings-in-circuits, dispersive-readout-mechanism, and resonator-as-quantum-bus are the shared circuit-QED infrastructure layer.
- bosonic-code-hierarchy explains why
cat-codes,gkp-codes, andbinomial-codesare related but not interchangeable.
Four-branch reading map
| Branch | What it is really optimizing | Best entry points | Evergreen lens |
|---|---|---|---|
| Baseline circuit-QED branch | Fast microwave control, scalable readout, and tunable two-qubit gates in a weakly anharmonic artificial atom stack | transmon, xmon, gmon, tunable-coupler, circuit-qed | josephson-junction-as-nonlinear-element, dispersive-readout-mechanism, resonator-as-quantum-bus |
| Protected-circuit branch | Suppress sensitivity before decoding by reshaping the circuit landscape or exploiting duality | fluxonium, heavy-fluxonium-qubit, blochnium, 0-pi-qubit, cos2phi-qubit, bifluxon-qubit | charge-noise-sweet-spot, coherence-time-hierarchy, noise-bias-and-asymmetric-error-channels |
| Bosonic / oscillator branch | Push error correction partly into a long-lived cavity mode instead of a single anharmonic junction mode | bosonic-qubit, cat-codes, kerr-cat-qubit, gkp-codes, binomial-codes | bosonic-code-hierarchy, noise-bias-and-asymmetric-error-channels, erasure-error-vs-pauli-error |
| Rail / erasure branch | Turn dominant relaxation or leakage events into flagged faults that the decoder can exploit | dual-rail-superconducting-qubit | erasure-error-vs-pauli-error, threshold-theorem |
A useful routing rule is that circuit-qed is infrastructure, not a peer qubit. Enter it when the live question is about readout, coupling, resonators, or cavity-mediated control. Enter transmon, fluxonium, blochnium, or the bosonic-code notes when the live question is what degree of freedom is actually carrying the logical information.
Baseline lineage and role separation
The baseline branch is easy to misread as a flat list of competing qubits. It is better understood as a lineage plus two architecture layers:
| Entry | Role in the graph | What changed relative to its nearest neighbor |
|---|---|---|
| cooper-pair-box-charge-qubit | charge-qubit ancestor | exposed the coherent Josephson-charge degree of freedom, but also the charge-dispersion problem that motivated the transmon |
| transmon | dominant encoding baseline | moved to large so charge dispersion falls exponentially while usable anharmonicity remains |
| xmon | planar layout descendant | kept the transmon Hamiltonian and changed capacitor geometry and connectivity for 2D integration |
| gmon | packaged qubit-plus-coupler architecture | added a flux-tunable interaction path to Xmon-style qubits so coupling can cross through zero rather than remain always on |
| tunable-coupler | reusable coupling component | abstracts the interaction element away from any one qubit layout; compare it on ON/OFF ratio, parasitic , and gate path, not as an encoding |
| circuit-qed | readout and bus infrastructure | supplies resonators, dispersive measurement, Purcell engineering, and mediated coupling around the qubit |
| mergemon | compact materials / participation variant | kept transmon-like encoding but merged most shunt capacitance into the junction, trading footprint against concentrated dielectric participation |
| unimon-qubit | distributed-mode alternative | placed one junction inside a grounded resonator mode, avoiding an isolated charge island and obtaining a flux sweet spot with positive anharmonicity |
| phase-qubit | historical control-and-measurement branch | used a shallow tilted-washboard well and state-selective escape to demonstrate tomography and multi-qubit entanglement, but destructive readout and the absence of a flux sweet spot made it a poor processor endpoint |
| flux-qubit | persistent-current branch | encoded in opposite circulating-current states, preserving strong magnetic coupling and large anharmonicity while trading against flux-noise sensitivity |
This separation prevents three category errors: treating xmon as a new encoding instead of a transmon layout, treating tunable-coupler or circuit-qed as peer qubits, and treating mergemon as a protection mechanism rather than a compactness-and-participation trade. When the comparison is about the carrier Hamiltonian, start with cooper-pair-box-charge-qubit, transmon, unimon-qubit, phase-qubit, or flux-qubit. When it is about processor wiring, start with xmon, gmon, tunable-coupler, or circuit-qed.
What the historical branches actually proved
The charge, phase, and flux branches should not be read as three obsolete labels on the way to the transmon. Each established a different piece of the superconducting-computing stack, and each failed or survived for a different architectural reason.
| Branch | Durable proof | Limiting architecture trade | Where the idea survives |
|---|---|---|---|
| cooper-pair-box-charge-qubit | coherent control of a circuit-level charge degree of freedom | a narrow charge sweet spot could not prevent strong dephasing across the wider operating manifold | transmon keeps the same Hamiltonian but flattens charge dispersion across the band |
| phase-qubit | fast control, single-shot tunneling readout, state tomography, and two- then three-qubit entanglement in a strongly anharmonic well | measurement destroys the stored state, while strong bias dependence and no flux sweet spot left the best cited device at and | current-biased junctions remain useful nonlinear systems and threshold detectors, but not a competitive processor-qubit branch |
| flux-qubit | coherent superposition of macroscopically distinct persistent currents with strong magnetic coupling | the same large persistent current that makes coupling easy also exposes the device to flux noise | C-shunt descendants, fluxonium-adjacent circuit design, and flux-qubit quantum annealing preserve parts of the branch |
The decisive transmon transition was therefore more than a coherence-record race. It combined broad charge-dispersion suppression with nondestructive dispersive readout and a reusable circuit-qed coupling/readout stack. The phase qubit remains important as the branch that proved sophisticated control and entanglement before those processor-level ingredients converged; the flux qubit remains a distinct lineage rather than merely another abandoned precursor.
Hardware-assisted fault-tolerance routes
transmon,xmon,gmon,flux-qubit, and the broader circuit-QED stack are the baseline branch: improve raw gate and measurement fidelity, then hand off to surface-code-logical-qubit or color-code-logical-qubit for architecture-level scaling.bosonic-qubit,cat-codes,kerr-cat-qubit,gkp-codes, andbinomial-codesare the oscillator branch: the hardware itself already starts behaving like a logical primitive, so route first through bosonic-code-hierarchy and then ask whether the win comes from general cavity QEC or from biased-noise protection.0-pi-qubit,cos2phi-qubit,bifluxon-qubit,blochnium, andheavy-fluxonium-qubitare the protected-circuit branch, but they are not interchangeable. The bifluxon uses odd-charge Aharonov-Casher interference in a split CPB to conserve fluxon parity; the others buy protection through different circuit symmetries or parameter regimes.- unimon-qubit belongs near the baseline/protected boundary: a single junction embedded in a multimode resonator yields strong anharmonicity at a flux sweet spot, but it is not a parity-protected code.
dual-rail-superconducting-qubitis the erasure-conversion branch: the dominant relaxation event is supposed to leave the codespace in a flagged way, so compare it through erasure-error-vs-pauli-error rather than only through bare gate fidelity.
Protected-circuit routing
heavy-fluxonium-qubitis the nearest-term coherence-first branch. It still lives squarely inside familiar fluxonium control and readout practice, but uses disjoint-support wavefunctions and sweet-spot operation to suppress matrix elements before the error ever reaches the decoder.blochniumis the duality branch. Read it when the interesting claim is quasicharge physics, large anharmonicity, and flux-dispersion flattening, not maximal passive protection in the 0-π sense.0-pi-qubitandcos2phi-qubitare the symmetry-engineered protection branch. Both are experimentally demonstrated, but neither yet eliminates active error correction: the cos(2φ) route strongly suppresses charge-coupled relaxation while present interference-based devices remain flux-noise limited.dual-rail-superconducting-qubitshould sit adjacent to this branch, not inside it: the payoff is still lower logical overhead, but via flagged leakage and erasure structure rather than suppressed in-code-space matrix elements.
Boundary with the super-semi family
Route superconducting descendants to super-semi-moc only when the semiconductor weak link or Andreev physics becomes part of the qubit’s defining control story. If the central claim is still protected superconducting-circuit design, cavity coupling, or oscillator-level encoding, keep it here even when the fabrication stack overlaps materially with hybrid devices.
Scope boundary
This facet currently mixes bare qubits, bosonic encodings, and enabling infrastructure. Its table is exhaustive within the facet, while the Evergreen links distinguish those different conceptual layers.