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Quantum bits with Josephson junctions

T0 review · 2 major / 3 minor · reviewed 2026-08-14 · deepseek-v4-flash

Pith's one-line read This review argues that the Josephson junction's cosine potential turns a superconducting circuit into an addressable qubit, opening new regimes of quantum computing and quantum optics.

desk verdict Pedagogically solid review of superconducting qubits, but Eq. (23) has a units error in the phase-qubit Hamiltonian that a referee should fix. read the letter →

arxiv 1908.09558 v1 pith:ZCVPWYMH submitted 2019-08-26 quant-ph cond-mat.mes-hall

classification quant-phcond-mat.mes-hall
keywords JosephsonjunctionqubitssuperconductingcircuitscircuitQEDtransmonqubitchargefluxartificialatomsquantuminformationprocessing
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The chapter sets out to explain the basics of superconducting quantum circuits built from Josephson junctions and to show why these circuits matter for both quantum information processing and quantum optics. Its central lesson is that the Josephson junction supplies the one ingredient a harmonic $LC$ resonator lacks: a cosine potential energy that makes the energy-level spacing anharmonic, so a two-level subspace can be addressed without leaking into higher states. Working through the standard quantization of electrical circuits, the chapter derives Hamiltonians for the three basic designs—the charge qubit (Cooper-pair box), the flux qubit, and the phase qubit—and then shows how refinements such as the transmon extended coherence times from nanoseconds to hundreds of microseconds. It closes by arguing that these 'artificial atoms' have reached coupling strengths, selection rules, and atom sizes inaccessible with natural atoms, from ultrastrong light-matter coupling to multi-point 'giant' atoms.

What carries the argument

The central object is the Josephson junction itself, modeled as an ideal lumped element: a capacitance $C_J$ in parallel with a nonlinear inductance carrying energy $E_J(1-\cos\varphi)$, where $\varphi$ is the phase difference across the junction. The cosine potential is the load-bearing piece, because unlike the quadratic potential of an $LC$ resonator it produces unequal spacings between energy levels, and that anharmonicity is what makes a two-level qubit subspace addressable without exciting higher states. The organizing parameter of the whole family of qubits is the ratio $E_J/E_C$, which determines whether the charge number $n$ (charge-qubit regime) or the phase $\varphi$ (flux- and phase-qubit regimes) is the well-defined variable; the transmon exploits the large-ratio regime, where sensitivity to charge noise falls exponentially in $\sqrt{E_J/E_C}$ while the anharmonicity $\omega_{12}-\omega_{01} = -E_C/\hbar$ falls only linearly.

What would settle it

Take a transmon and tune $E_J/E_C$ over two decades by changing the flux through its SQUID loop while measuring the $|0\rangle\leftrightarrow|1\rangle$ and $|1\rangle\leftrightarrow|2\rangle$ transition frequencies: the chapter's Eqs. (25)-(26) predict exactly $\omega_{12}-\omega_{01} = -E_C/\hbar$, so a systematic deviation of the measured anharmonicity from this value, or a charge-noise sensitivity that fails to fall exponentially with $\sqrt{E_J/E_C}$, would show that the ideal single-phase junction model is incomplete.

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Extended reading notes

Core claim

The chapter's claim, stated on its own terms, is that a Josephson junction described by the Josephson equations $I_J = I_c \sin \varphi$ and $\dot\varphi = (2e/\hbar)V$ acts as a nonlinear inductance with energy $E_J(1-\cos\varphi)$, and that once such a junction is embedded in a superconducting circuit and quantized through node fluxes, a Legendre transformation, and canonical commutation relations, the resulting Hamiltonian describes an anharmonic multi-level system whose two lowest states form a qubit. The ratio $E_J/E_C$ between Josephson energy and charging energy decides which variable is well defined: the charge qubit lives at $E_J/E_C \ll 1$ with Hamiltonian $H = 4E_C(n-n_g)^2 - E_J\cos\varphi$, while the flux and phase qubits live at $E_J/E_C \gg 1$. The chapter further claims that refinements of these designs—most notably the transmon, which shunts the junction with a large capacitance to reach $E_J/E_C \sim 10^2$—have pushed coherence times into the hundreds of microseconds and have fulfilled the five DiVincenzo criteria (fabrication of many qubits, initialization, universal gates, readout, and sufficient coherence) to a reasonable degree in experiments. Finally, it claims that the same circuits, regarded as artificial atoms, have demonstrated ultrastrong and deep-strong coupling to photons, $\Delta$-type three-level transitions that natural atoms cannot have, and giant atoms coupled to a field at multiple wavelength-separated points.

Load-bearing premise

The load-bearing premise is that a Josephson junction can be treated as a single ideal electrical element described by the two Josephson equations, with one phase value across it and no dissipative losses; if quasiparticle dissipation or a phase that varies across the junction becomes significant at the operating frequencies and temperatures, every qubit Hamiltonian derived from this starting point would need revision.

Editorial extensions

If this is right

  • If the presented circuit quantization is accurate, all five DiVincenzo criteria have been fulfilled to a reasonable degree in experiments with Josephson-junction qubits, although scaling the control electronics to large systems remains an outstanding engineering challenge that the chapter itself states.
  • The transmon trade-off follows from the chapter's perturbation theory: charge-noise sensitivity drops exponentially in $\sqrt{E_J/E_C}$ while the anharmonicity $\omega_{12}-\omega_{01} = -E_C/\hbar$ drops only linearly, which is why the transmon became the dominant design.
  • Since the normalized coupling strength scales as $g \propto \alpha^{1/2}$ (or $\alpha^{-1/2}$) in circuit QED rather than $\alpha^{3/2}$, ultrastrong coupling with $\eta > 0.1$ and even deep strong coupling with $\eta > 1$ are reachable, and the Jaynes-Cummings model must give way to the full quantum Rabi Hamiltonian with its counter-rotating terms.
  • A Josephson-junction qutrit tuned away from its symmetry point has all three transitions active (the $\Delta$-type configuration), which enables population inversion, weak-signal amplification, and microwave frequency up- and down-conversion.
  • Giant atoms coupled to a waveguide at two points spaced $\lambda/2$ apart are protected from decaying into the waveguide, giving a frequency-dependent relaxation rate that can be engineered on a chip.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the giant-atom interference picture is correct, the same decay-free points and frequency-dependent relaxation should appear in any platform where an atom couples to a bosonic field at several points separated by distances comparable to the wavelength; a waveguide QED experiment with a different type of atom would test whether the mechanism is generic or specific to superconducting circuits.
  • The chapter's reliance on the ideal lumped-junction model in Sec. 2 points to a precise boundary for all its Hamiltonians: deviations from the $E_J(1-\cos\varphi)$ potential should first appear in the higher energy levels, so monitoring the $|2\rangle\leftrightarrow|3\rangle$ transition of a transmon as the operating frequency approaches the junction plasma frequency could reveal the model's break
  • The exponential-protection-versus-linear-anharmonicity trade-off identified for the transmon suggests a general design heuristic: push the protected operating ratio as far as fabrication tolerances allow, because the noise sensitivity falls much faster than the anharmonicity that limits gate speed.
  • Since the paper shows that selection rules can be switched on and off by tuning through a sweet spot, an implicit extension is to modulate that control parameter in time, using parity as a dynamic switch for gating, amplifying, or converting signals at chosen frequencies.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 3 minor

Summary. This is a review chapter on Josephson-junction qubits. It introduces circuit quantization, derives the Hamiltonians of the charge, flux, and phase qubits, then discusses refined designs (transmon, fluxonium, etc.), quantum computing with these qubits, and their use in quantum optics and atomic physics. The stated aim is to explain the basic physics of superconducting quantum circuits and to show how they open new prospects for quantum information processing and quantum optics.

Significance. If the equations are corrected, this is a competent and useful pedagogical review of a mature field. The treatment of the Cooper-pair box, flux qubit, transmon, dispersive readout, and ultrastrong coupling follows standard literature and cites the primary sources extensively. The chapter is free of parameter fitting and makes no unfalsifiable predictions; as a review, that is appropriate. The main value is as a reference for newcomers; the main risk is that readers will reproduce the displayed Hamiltonians, so the prefactor errors in Eqs. (23), (35), and (36) must be fixed before the accuracy claim in the introduction can stand.

major comments (2)
  1. [Sec. 3.3, Eq. (23)] The kinetic term of the phase-qubit Hamiltonian is written as (2π/Φ0) p²/(2C_J), but the text defines the momentum by Q = 2ep/ħ, i.e., p = ħQ/(2e). With this definition, the charging energy is Q²/(2C_J) = (2e/ħ)² p²/(2C_J) = (2π/Φ0)² p²/(2C_J). As printed, the prefactor is missing one power of 2π/Φ0 and the first term does not have the correct dimensions of energy. A reader using Eq. (23) to compute the phase-qubit level spacing or transition frequency will obtain quantitatively wrong results. Please correct the prefactor to (2π/Φ0)², or redefine p consistently and adjust the text.
  2. [Sec. 6.1, Eqs. (35) and (36)] The Jaynes-Cummings and Rabi Hamiltonians are written as ħω01 σz, but with the Pauli-matrix convention used in Sec. 3.2 (σz = |e⟩⟨e| − |g⟩⟨g|, eigenvalues ±1), the qubit term should be (ħω01/2) σz. As written, the two-level transition energy is 2ħω01, which shifts the resonance conditions and all subsequent dynamics discussed in Sec. 6. Please introduce an explicit convention for σz or add the missing factor 1/2 in both Eq. (35) and Eq. (36).
minor comments (3)
  1. [Abstract] The abstract gives the book title as "Fundamentals and Physics and Applications of the Josephson Effect"; the reference list in Ref. [1] gives "Physics and Applications of the Josephson Effect". Please make the title consistent.
  2. [Sec. 2] The chapter adopts the lumped-element Josephson-junction model (a single phase φ, no quasiparticle dissipation) without explicitly stating its range of validity. A one-sentence remark that this approximation requires the junction to be small compared with the relevant wavelength and that quasiparticle effects are neglected would help the intended readership.
  3. [Sec. 5.4, Eq. (33)] The three-qubit product state in Eq. (33) is written without normalization factors in the individual qubit states; this is harmless but could be clarified by using (α|0⟩ + β|1⟩)⊗3.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity; the chapter is a standard review whose derivations are self-contained textbook material benchmarked against external literature.

full rationale

This is a review chapter that quantizes circuits from the standard Josephson relations I_J = I_c sin(phi) and d(phi)/dt = (2e/hbar)V (Sec. 2, Eqs. (11)-(12)) to obtain Lagrangians and Hamiltonians (Eqs. (14), (18)-(21), (23)) by Legendre transformation and canonical commutation. No parameter is fitted from the quantities being 'predicted'; the Josephson energy E_J is an input, not a fitted output. The transmon formulas (24)-(26) are quoted from Koch et al. 2007, an external, independently published derivation, and the cited experiments provide external benchmarks. Author self-citations occur (e.g., Refs. [2], [22], [144], [161]), but they are introductory reading references and contextual pointers, not arguments that make a conclusion true by definition. The only concrete defect visible in the derivation chain is a dimensional/prefactor error in Eq. (23): with Q = 2ep/hbar, the charging term should be (2*pi/Phi_0)^2 p^2/(2C_J), so the printed prefactor (2*pi/Phi_0) is one power short. That is a correctness or typo issue, not a circularity issue; it does not mean the phase-qubit Hamiltonian was assumed in order to derive it. The chapter makes no independent quantitative predictions that reduce to its own inputs, so it receives a circularity score of 0.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

This is a review, so the central claim rests on standard physics assumptions about Josephson junctions and circuit quantization. No free parameters are fitted and no new entities are introduced. The assumptions listed are all common in superconducting qubit theory and are not ad hoc to this paper.

assumptions (4)
  • domain assumption Josephson equations: I_J = I_c sin φ and dφ/dt = (2e/ħ)V
    Invoked in Sec. 2 to derive the Josephson junction Lagrangian and all subsequent qubit Hamiltonians.
  • standard math Lumped-element circuit model with node fluxes as generalized coordinates and canonical quantization [Φ, ∂L/∂Φ'] = iħ
    Used in Sec. 2 to quantize the circuits.
  • domain assumption Flux quantization condition Φ_ext = mΦ0 around a superconducting loop
    Used in Sec. 2, Eq. (4).
  • domain assumption Two-level approximation for the qubit subspace and validity of the rotating-wave approximation for weak coupling
    Used in Sec. 5.1.4 and Sec. 6.2 to justify Jaynes-Cummings and dispersive readout.

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Cite this review

Pith. "Pith review of Quantum bits with Josephson junctions." pith.science (2026). https://pith.science/paper/ZCVPWYMH

@misc{pith2026190809558,
  author       = {Pith},
  title        = {Pith review of: Quantum bits with Josephson junctions},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/ZCVPWYMH}},
  note         = {Machine review of arXiv:1908.09558}
}
read the original abstract

Already in the first edition of this book (Barone and Paterno, "Fundamentals and Physics and Applications of the Josephson Effect", Wiley 1982), a great number of interesting and important applications for Josephson junctions were discussed. In the decades that have passed since then, several new applications have emerged. This chapter treats one such new class of applications: quantum optics and quantum information processing (QIP) based on superconducting circuits with Josephson junctions. In this chapter, we aim to explain the basics of superconducting quantum circuits with Josephson junctions and demonstrate how these systems open up new prospects, both for QIP and for the study of quantum optics and atomic physics.

Discussion (0). Continue with ORCID to comment.

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