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REVIEW 4 major objections 4 minor 74 references

Fabrication of low-loss Josephson parametric devices

T0 review · 4 major / 4 minor · reviewed 2026-08-11 · deepseek-v4-flash

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Pith's one-line read The authors claim a refined surface-treatment and fabrication recipe yields Josephson parametric devices with internal quality factors above 10^5 at the single-photon level, confirmed by squeezed-state purity.

desk verdict The JPC internal quality factor is a credible record; the JPA claim needs an independent measurement or a softer claim. read the letter →

arxiv 2412.11280 v3 pith:EIDYIBM3 submitted 2024-12-15 quant-ph cond-mat.supr-con

K. E. Honasoge , M. Handschuh , W. K. Yam , S. Gandorfer , D. Bazulin , N. Bruckmoser , L. Koch , A. Marx
show 2 more authors
R. Gross K. G. Fedorov
This is my paper · ORCID
classification quant-phcond-mat.supr-con
keywords Josephsonparametricamplifierconverterinternalqualityfactorsingle-photonregimemicrowavesqueezingsuperconductingfabricationsurfacetreatment
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 paper reports a fabrication recipe for two kinds of flux-tunable Josephson microwave devices, a parametric converter and a parametric amplifier, that reaches internal quality factors above $10^{5}$ when probed with roughly one photon in the resonator. Internal loss is the main barrier to clean operation of these devices, because it injects noise and degrades squeezed microwave states. The authors extract the quality factor from calibrated scattering measurements and independently confirm it by fitting the measured purity of squeezed vacuum states generated by the amplifier. If the claim is correct, tunable Josephson circuits no longer have to be much lossier than fixed-frequency superconducting resonators.

What carries the argument

The devices are a Josephson parametric converter (two half-wavelength coplanar-waveguide resonators coupled through an inductively shunted Josephson ring modulator) and a Josephson parametric amplifier (a quarter-wavelength resonator shorted to ground through a dc-SQUID). The argument is carried by two independent probes of loss. One is a calibrated scattering-parameter analysis in which cable delay, background, impedance mismatch, and Fano distortion are removed so that the corrected complex resonance circle can be fit to extract the total and external loss rates. The other is a two-bath input-output model of the amplifier whose quadrature variances depend on pump power through the internal loss rate; fitting measured squeezing and purity gives $Q_\mathrm{int}$ without using the scattering constraint. On the fabrication side, piranha and buffered-oxide etches plus argon-ion milling clean the substrate-metal, metal-air, and metal-metal interfaces that otherwise host lossy defects.

What would settle it

Fabricate a second JPA with a much smaller coupling capacitance so that Qext is comparable to Qint, then fit its background-corrected reflection circle without imposing the converter-derived bound; a directly fitted Qint below 100,000 would contradict the paper's central claim.

Watch

Extended reading notes

Core claim

The central claim is that a combination of wet-chemical surface treatments, e-beam-defined Al/AlOx/Al Manhattan junctions, and optimized Ar+ milling for the Nb-Al contact produces Josephson parametric devices whose internal quality factors exceed 100,000 at the single-photon level. For the converter, fits to background-corrected reflection data give $Q_\mathrm{int}$ between $1.1\times10^5$ and $1.34\times10^5$. For the strongly overcoupled amplifier, the same scattering analysis is constrained by assuming the converter loss rate applies and yields $1.1\times10^5$ to $1.5\times10^5$; a separate multi-parameter fit to the measured squeezing and purity returns $Q_\mathrm{int}=1.26\times10^5$, consistent with the scattering estimate. The authors state that these are the highest internal quality factors reported so far for such tunable Josephson devices.

Load-bearing premise

The amplifier's internal quality factor is assumed to equal the converter's because both are fabricated and measured under identical conditions; the amplifier's own reflection data, being strongly overcoupled, cannot determine Qint on its own.

Editorial extensions

If this is right

  • Tunable Josephson circuits can be operated at internal loss rates comparable to fixed-frequency superconducting resonators, removing a long-standing gap.
  • Squeezed microwave states from such amplifiers should reach higher purity at a fixed squeezing level, since purity degrades roughly as the internal loss rate.
  • Josephson parametric converters with $Q_\mathrm{int}$ above $10^5$ become practical low-noise frequency converters and quantum memories at the single-photon level.
  • The fabrication recipe, especially the surface treatment and argon milling steps, can be carried over to other flux-tunable superconducting circuits to raise their coherence times.
  • The maximum squeezing level achievable, which scales inversely with internal loss, increases correspondingly once $Q_\mathrm{int}$ exceeds $10^5$.

Reading between the lines

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

  • The paper does not show it, but a direct test of the amplifier claim would be to fabricate a series of JPAs with different coupling capacitors and fit each reflection curve without imposing the converter-derived bound; agreement would make the JPA $Q_\mathrm{int}$ fully standalone.
  • An unstated consequence is that the same surface-treatment and bandaging steps might improve flux-tunable qubits, whose coherence times currently lag fixed-frequency qubits, though junction operating energies and impedances differ.
  • The two-bath input-output fit treats internal loss as a single lumped channel; splitting $Q_\mathrm{int}$ into substrate, interface, and quasiparticle contributions would show which part of the $10^5$ ceiling remains to be attacked.
  • The purity-versus-pump-power curve used to confirm $Q_\mathrm{int}$ depends on the added pump-noise model, so an independent check using a weakly coupled JPA would remove that coupling as well.
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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

4 major / 4 minor

Summary. The paper reports fabrication and cryogenic characterization of Josephson parametric converter (JPC) and Josephson parametric amplifier (JPA) devices, claiming internal quality factors Q_int exceeding 10^5 in the single-photon regime. The JPC is a λ/2 resonator coupled through a Josephson ring modulator; the JPA is a λ/4 resonator terminated by a dc-SQUID. The authors extract Q_int from reflection scattering data using circle fits and background corrections, and additionally analyze squeezing and purity of squeezed vacuum states generated by the JPA to support the extracted loss rates. The central claim is that both device types reach record-low internal loss, comparable to fixed-frequency resonators.

Significance. If substantiated, the claimed Q_int > 10^5 for tunable Josephson parametric devices would be a meaningful advance: low internal loss directly improves squeezed-state purity, amplifier noise performance, and the fidelity of microwave quantum operations. The paper provides a detailed fabrication protocol, including surface treatments and argon milling steps, which is valuable for reproducibility. The JPC analysis follows a standard circle-fit procedure to scattering data and is credible. However, the JPA-specific evidence is weaker: the scattering-based JPA Q_int relies on an explicit assumption that JPC and JPA have equal internal loss, and the written constraint bounding Q_l^{-1} - Q_ext^{-1} is mathematically impossible as stated. The squeezing fit, presented as confirmation, uses Q_int as a free parameter, so it does not independently establish the JPA loss rate without a sensitivity or identifiability analysis. The paper would be strengthened by correcting the constraint and providing such an analysis.

major comments (4)
  1. [§IV, JPA scattering analysis] The constraint used in the constrained minimization is written as 1 × 10^{-5} < Q_l^{-1} − Q_ext^{-1} < 5 × 10^{-6}. Since Q_int^{-1} = Q_l^{-1} − Q_ext^{-1} and 1 × 10^{-5} > 5 × 10^{-6}, no value satisfies both inequalities; the feasible region is empty. The quoted JPA Q_int = 1.1 × 10^5–1.5 × 10^5 from scattering therefore cannot be a valid output of the procedure as described. Please correct the bound or re-analyze without it, and state clearly what constraint was actually enforced.
  2. [§IV, paragraph beginning 'Our JPA is deliberately designed'] The JPA scattering Q_int is not independently measured. The text acknowledges that the JPA is strongly overcoupled and that an accurate Q_int estimate is difficult, then assumes that JPC and JPA samples fabricated and measured under identical conditions have the same Q_int. The JPC and JPA differ in nonlinear element (JRM vs dc-SQUID), resonator type (λ/2 vs λ/4), and the presence of a separate pump line, so equal internal loss is an untested assumption rather than a consequence of identical processing. A series of resonators with varying external coupling, or an independent loss measurement, is needed to support the JPA scattering value.
  3. [§IV, squeezing analysis, Eq. (8)–(10)] The squeezing and purity fits are presented as confirmation of the JPA Q_int, but Q_int (κ_int) is one of four free fit parameters, together with χ^(2), n'_J, and δ, while T_att and T_mxc are only bounded to 10–50 mK. Agreement of a multi-parameter fit does not by itself establish Q_int unless the fit is shown to be sensitive to it. Please provide profile likelihoods, confidence intervals, or another identifiability check for κ_int. Without such an analysis, the stated 'excellent agreement' does not confirm the scattering-derived value.
  4. [Abstract and Conclusion] The abstract states that Q_int values 'for both Josephson parametric converters and Josephson parametric amplifiers in the single-photon regime' are obtained 'by fitting the scattering data.' For the JPA, the scattering fit is constrained by the JPC-based assumption, and the only device-specific JPA estimate comes from the squeezing fit with free κ_int. The wording overstates the independence of the JPA scattering result and should be revised to match the actual analysis.
minor comments (4)
  1. [§IV, thermal bath paragraph] There is a typo in the sentence defining the thermal photon numbers: 'Tmsc' should presumably be 'Tmxc' to match the earlier notation.
  2. [§IV, temperature treatment] The text says 'Using previously determined values of κ_ext and experimentally measured temperatures' but then states that T_att and T_mxc are constrained within 10–50 mK and fitted. Please clarify whether these temperatures are measured or treated as free parameters with bounded ranges.
  3. [Fig. 5(b)] The caption states that red stars refer to both JPC and JPA devices, but the figure does not appear to distinguish them. Please add separate markers or labels so the reader can identify which point corresponds to which device.
  4. [§IV, JPC coupling regime] For the JPC, the text states Q_ext < Q_int (slightly overcoupled) and later reports Q_ext ≈ 3.9–4.1 × 10^4 and Q_int ≈ 1.1–1.34 × 10^5; this is consistent, but it would be helpful to state the coupling efficiency Q_ext/Q_l explicitly for the JPA as well, since that quantity is used in Fig. 5(b).

Circularity Check

2 steps flagged · score 6.0 of 10

JPA scattering Qint is imported from JPC by an assumed constraint, and the squeezing 'confirmation' refits κint as a free parameter.

  1. fitted input called prediction [Section IV, JPA scattering analysis, paragraph beginning 'Our JPA is deliberately designed...']
    "As all our JPC and JPA samples are fabricated and measured under identical conditions, we assume that the values obtained for the JPC are good first-order approximations for the internal quality factor of the JPA. By relying on this assumption, we constrain the least square fits of the complex scattering coefficient SMC21 with Eq. (6) using the bound 1 × 10−5 < Q−1 l − Q−1 ext < 5 × 10−6."

    By Eq. (5), Q_l^{-1} - Q_ext^{-1} is exactly Q_int^{-1}. The stated bound is therefore a prior range on the JPA internal loss rate taken from the JPC measurement, so the constrained scattering fit cannot independently determine JPA Qint. The extracted JPA Qint range 1.1-1.5e5 is the JPC prior returned by the fit, making the scattering-based JPA Qint claim reduce to the identical-conditions assumption rather than to JPA scattering data. Moreover, as written the inequality has an empty feasible region because 1e-5 > 5e-6, so the stated constraint is not even a well-defined minimization bound.

  2. fitted input called prediction [Section IV, squeezing analysis, paragraph beginning 'Using previously determined values of κext...']
    "Using previously determined values of κext and experimentally measured temperatures, we simultaneously fit the squeezing and purity data to Eq. 8 by minimizing a least-square deviation. The quantities κint, χ(2), n′J, and δ are treated as free fit parameters, while the temperatures Tatt and Tmxc are constrained within the experimentally observed range of 10 mK to 50 mK. ... Most importantly, we extract the internal quality factor Qint = 1.26 × 105, which is in excellent agreement with the estimations derived from the earlier analysis of the scattering data."

    The squeezing dataset is separate from the scattering data, so this is not a pure identity with the JPC prior. However, Qint enters as one of four free fit parameters (κint, χ(2), n′J, δ), with the two bath temperatures only bounded, not fixed. A good fit to the squeezing and purity curves therefore shows model consistency, not that the data force the reported Qint. Calling the resulting Qint an 'excellent agreement' and a confirmation of the scattering estimate effectively renames a fitted parameter as an independent verification; no profile-likelihood or identifiability check is shown to demonstrate that κint is the quantity actually pinned down by the data.

full rationale

The JPC scattering analysis is self-contained and standard: Qint is obtained from the loaded and external quality factors using Eq. (5) and an independent least-squares fit. The circularity concerns the JPA claim. The JPA is strongly overcoupled, and the paper explicitly concedes that an accurate scattering estimate of its Qint is difficult. Instead of measuring a coupling-strength series, the authors impose a constraint derived from the assumption that JPC and JPA Qint values are the same; the resulting JPA Qint range is then the prior in a constrained fit, not an independent measurement. The squeezing analysis is a separate dataset, which prevents the overall circularity from being complete, but κint is a free parameter in that model, so the agreement is not a parameter-free prediction and does not by itself establish the JPA Qint. The use of Ref. [13] for the squeezing model is a self-citation but is not load-bearing for the fitted Qint value: it supplies the model rather than the numerical result. Overall, the headline claim of record JPA Qint is partially circular because the scattering route imports the value from the JPC and the squeezing route fits it; only the JPC Qint is independently measured from scattering. The written constraint is also inconsistent as an inequality, further undermining the JPA scattering result as stated.

Assumptions & free parameters 6 free parameters · 4 assumptions · 0 invented entities

The central claim depends on a set of fitted parameters in the squeezing model (Qint, chi^(2), n'_J, delta, and bath temperatures) and on the domain assumption that JPC and JPA share the same internal loss. No new physical entities are introduced. The JPA scattering fit additionally depends on a hand-set prior derived from the JPC measurement.

free parameters (6)
  • JPA internal quality factor Qint (squeezing fit) = 1.26e5
    Extracted by simultaneous least-squares fit to squeezing level and purity versus pump power; Qint is a free parameter, not a direct measurement.
  • Nonlinear susceptibility chi^(2) = 840 MHz
    Free fit parameter in the squeezing model.
  • Pump-noise prefactor n'_J = 0.0069
    Phenomenological parameter accounting for pump photon number fluctuations.
  • Pump-noise scaling delta = 0.047
    Fit parameter in the gain-dependent noise model.
  • Bath temperatures Tatt and Tmxc = 31 mK and 10 mK
    Constrained within the experimentally observed range of 10 to 50 mK and then fitted in the squeezing model.
  • JPA Qint prior for scattering fit = Implied range roughly 1e5 to 2e5
    The authors constrain Q_l^{-1} - Q_ext^{-1} with the bound 1e-5 to 5e-6, imposing a Qint range based on the assumed JPC value; as written the inequality is inconsistent.
assumptions (4)
  • domain assumption JPC and JPA samples are fabricated and measured under identical conditions, so JPC Qint approximates JPA Qint.
    Invoked in Section IV to justify constraining JPA scattering fits; if false, the JPA scattering-derived Qint is not independently measured.
  • domain assumption Single-port reflection scattering of a driven dissipative harmonic oscillator is described by Eq. (6) after background correction.
    Standard input-output formalism; used to fit S21 and extract Qext and Qint.
  • domain assumption The squeezing output variances obey Eqs. (8) through (10) with thermal baths and a gain-dependent noise model.
    Model from Ref. [13]; used to fit squeezing and purity and extract Qint.
  • standard math Planck thermal occupancy at Tatt and Tmxc describes the input bath variances.
    Standard thermal noise model used in the squeezing analysis.

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Cite this review

Pith. "Pith review of Fabrication of low-loss Josephson parametric devices." pith.science (2026). https://pith.science/paper/EIDYIBM3

@misc{pith2026241211280,
  author       = {Pith},
  title        = {Pith review of: Fabrication of low-loss Josephson parametric devices},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/EIDYIBM3}},
  note         = {Machine review of arXiv:2412.11280}
}
abstract

Superconducting circuits incorporating Josephson elements represent a promising hardware platform for quantum technologies. Potential applications include scalable quantum computing, microwave quantum networks, and quantum-limited amplifiers. However, progress in Josephson junction-based quantum technologies is facing the ongoing challenge of minimizing loss channels. This is also true for parametric superconducting devices based on nonlinear Josephson resonators. In this work, we report on the fabrication and characterization of low-loss Josephson parametric devices operated in the GHz frequency range, showing record internal quality factors. Specifically, we achieve internal quality factors $Q_\mathrm{int}$ significantly exceeding $10^5$ for both Josephson parametric converters and Josephson parametric amplifiers in the single-photon regime by fitting the scattering data. We confirm the extracted $Q_\mathrm{int}$ values by analyzing purity of squeezed vacuum states generated by these devices. These low-loss devices mark a significant step forward in realizing high-performance quantum circuits, enabling further advancements in superconducting quantum technologies.

Figures

Figures reproduced from arXiv: 2412.11280 by the authors.

Figure 1
Figure 1. FIG. 1. Design renderings and experimental images of fabricated key components of the Josephson parametric converter (JPC) [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2. Fabrication procedure for Josephson parametric de [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3. Experimental setup, measured flux dependencies of [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figures from the paper (3 more)
Figure 4
Figure 4. Figure 4: FIG. 4. Analysis of the internal quality factors of Josephson parametric devices. (a) By an additional background contribution [PITH_FULL_IMAGE:figures/full_fig_p006_4.png]
Figure 5
Figure 5. Figure 5: FIG. 5. (a) Internal JPC quality factors [PITH_FULL_IMAGE:figures/full_fig_p007_5.png]
Figure 6
Figure 6. Figure 6: FIG. 6. Squeezing level and purity of squeezed vacuum states [PITH_FULL_IMAGE:figures/full_fig_p008_6.png]

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