{"id":"f8ff29e2-d88c-4ae9-a575-2f2c833b4199","arxiv_id":"2603.11817","paper_version":2,"verdict":"ACCEPT","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"low","formal_verification":"none","parameter_count":3,"one_line_summary":"A piezoelectrically tunable Zerodur-PZT-Kovar Fabry-Perot cavity with CTE zero-crossing at 4.9 °C achieves 4×10^{-13} fractional frequency instability at 1 s under realistic operating conditions.","lead":"A research team built a piezo-tunable Fabry-Perot cavity whose thermal expansion cancels near 5 °C, reaching 4×10^{-13} fractional frequency stability at 1 s. The design removes the usual need for external length locks in many atom-cavity experiments such as superradiant lasers.","discovery_kind":"new_method","skeptic_critique":{"model":"grok-4.5","headline":"No significant objection identified","rationale":"The reader correctly isolates residual temperature control after locating T0 as the softest link, yet the paper already supplies the necessary multi-condition evidence (filtered vs unfiltered PZT, oven on/off, ADEV vs HDEV) that keeps this residual below the demonstrated floor. No stronger load-bearing flaw is present; the experimental claim therefore stands and the ACCEPT verdict needs no adjustment.","tokens_in":8665,"tokens_out":366,"duration_ms":4450,"concrete_test":"Re-acquire a multi-hour frequency time series at the fixed T0 set-point while logging the out-of-loop PT100; compute the residual thermal contribution a_tot·(T-T0)·L_eff and subtract it from the beat-note record. If the residual Allan deviation at 1–100 s remains ≤4×10^{-13}, the temperature-regulation premise is confirmed.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim is an experimental demonstration that a composite PZT-tunable Zerodur cavity operated at its measured CTE zero-crossing T0=(4.9±0.5)°C reaches 4×10^{-13} fractional frequency instability at 1 s (and stays mid-10^{-13} with the atomic oven on). Multi-condition Allan and Hadamard data (Fig. 4) already separate PZT voltage noise, residual temperature fluctuations, and oven-induced gradients; the out-of-loop temperature stability (<2 mK) together with the fitted a_tot is consistent with the observed floor once the set-point is fixed at T0. No internal inconsistency or untested assumption appears that would overturn the claim under the reported conditions.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.5","summary":"The manuscript reports a piezoelectrically tunable Fabry–Perot cavity built around a 50 mm Zerodur spacer, ring PZTs and Kovar washers, designed for an ytterbium superradiant-laser experiment. By operating at the measured CTE zero-crossing temperature T0 = (4.9 ± 0.5) °C of the composite stack, the free-running cavity reaches a fractional frequency instability of 4 × 10^{-13} at 1 s (Allan deviation) when the PZTs are either disconnected or low-pass filtered, and remains in the mid-10^{-13} range under realistic atomic-oven conditions. Optical, thermal and electrical characterizations (finesse, PZT voltage noise, temperature regulation, CTE mapping) are presented, together with multi-condition Allan and Hadamard deviations referenced to an H-maser via an optical frequency comb.","tokens_in":8877,"tokens_out":1052,"duration_ms":8499,"significance":"The work addresses a genuine practical bottleneck: the simultaneous need for cavity tunability and passive length stability at the 10^{-13} level required by active optical clocks and related cQED experiments. The multi-condition stability data (PZTs on/off/filtered, oven on/off, T0 versus room temperature) provide a clear experimental demonstration that external length feedback can be dispensed with for many atom-cavity applications. The design is transferable to other precision experiments that require both tunability and thermal-noise-limited performance (e.g., frequency-dependent squeezing, cavity-enhanced spectroscopy). Strengths include the transparent separation of noise sources and the direct comparison against an independent H-maser reference.","major_comments":[{"comment":"The finesse after bake-out is only 6920 ± 40 (decay time 411 ns), attributed to possible NEG contamination. For the intended Yb SR laser the cavity must support high atom-cavity cooperativity; the manuscript should quantify whether this finesse still meets the design requirements or whether a re-coating / re-bake path is planned. Without that assessment the claim that the cavity is “ideal for ultra-stable superradiant lasers” rests on an incomplete optical characterization.","section":null},{"comment":"The residual thermal contribution is asserted to lie below the observed 4 × 10^{-13} floor once the set-point is fixed at T0. While the out-of-loop temperature stability (< 2 mK) and the fitted a_tot = (4.8 ± 0.9) \times 10^{-8} K^{-2} are consistent with this statement, an explicit calculation of the expected frequency noise from the measured temperature spectrum (or a short-term temperature Allan deviation converted via a_tot) would make the argument quantitative rather than qualitative.","section":null}],"minor_comments":[{"comment":"Figure 3: the parabolic fit and the extraction of T0 and a_tot would be clearer if the raw frequency-versus-temperature data points were tabulated or if the fit residuals were shown.","section":null},{"comment":"Section 4.2: the mechanical resonances at 110 kHz and 10 MHz are mentioned; a short spectrum of the beat-note with and without the DC filter would strengthen the claim that the filter fully suppresses them.","section":null},{"comment":"Equation (1) and the subsequent discussion of α_PZT(T) rely on literature values whose temperature range (0–50 °C) is only partially overlapping with the operating point near 5 °C; a brief remark on the extrapolation uncertainty would be useful.","section":null},{"comment":"Typographical consistency: “5°C” versus “5 °C”, and occasional missing spaces around units (e.g., “50-mm”, “2mm-thick”).","section":null},{"comment":"The abstract states “around 5 °C” while the body reports (4.9 ± 0.5) °C; aligning the two statements would avoid minor confusion.","section":null}],"recommendation":"minor_revision","confidential_remarks":"The paper is a solid experimental demonstration that fits well in a physics.optics or applied-physics venue. The finesse degradation is the only point that could become a reviewer sticking point if the journal is strongly cQED-oriented; otherwise the work is ready after minor polishing. No concerns about novelty or citation practices."},"author_rebuttal":null,"desk_editor":{"model":"grok-4.5","letter":"This is a clean experimental paper that does what it says: a composite Zerodur + PZT + Kovar cavity with optical access for atoms, voltage-tunable over FSRs, and free-running fractional frequency stability of 4×10^{-13} at 1 s when held at its measured CTE zero-crossing of (4.9±0.5) °C. Under realistic oven-on conditions it stays mid-10^{-13}. That combination is the actual new result; pure ultra-stable cavities and pure piezo-tunable cavities already exist, but this engineered stack places the zero near a convenient temperature while keeping the optical and mechanical features needed for cQED/SR work.\n\nWhat they do well is the measurement campaign. Allan and Hadamard deviations are shown with PZTs disconnected, filtered, and unfiltered, at T0 versus room temperature, and with the atomic oven on/off. The reference chain (PDH lock → OFC → H-maser) is quantified, the low-pass filter that kills PZT voltage noise is described, and the temperature-step data that locate the zero-crossing are shown with a simple parabolic fit. The thermal-noise floor estimate (~3×10^{-15}) and acceleration-sensitivity simulation are secondary but consistent. Citations to NPL, Möhle, Numata/Kessler, etc., are appropriate; no circularity.\n\nSoft spots are real but secondary. Finesse dropped from ~21 k to ~7 k after bake-out (possible NEG contamination); that is a practical nuisance for atom-cavity coupling but does not undercut the stability claim. Residual temperature stability (<2 mK out-of-loop) plus the fitted a_tot is assumed to keep thermal expansion below the observed floor once the set-point is fixed at T0; the multi-condition data make that assumption plausible rather than load-bearing. No active length feedback is used, which is the point of the paper.\n\nThis is for people building SR lasers, cQED systems, or frequency-dependent squeezing who need both tunability and mid-10^{-13} free-running performance without an external lock. The math and data are solid enough that a serious editor should send it to referees. I would cite it if I were designing a similar cavity, and I would bring it to reading group for the practical engineering details.","headline":"Solid experimental demo of a piezo-tunable Zerodur cavity that hits mid-10^{-13} free-running stability at a measured CTE zero near 5 °C, with multi-condition data that actually support the claim for SR-laser use.","tokens_in":9479,"tokens_out":588,"would_cite":true,"duration_ms":4615,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.5","headline":"A piezo-tunable Fabry-Perot cavity can cancel thermal expansion near 5 °C and hold fractional frequency stability of 4×10^{-13} at one second, without external length locks.","keywords":["Fabry-Perot cavity","coefficient of thermal expansion","piezoelectric tuning","frequency stability","superradiant laser","cavity QED","Zerodur","Pound-Drever-Hall"],"falsifier":"Measure the Allan deviation of the same cavity while deliberately detuning the set-point by a few kelvin from the claimed zero-crossing; if the short-term floor rises by the amount predicted by the fitted CTE slope a_tot, the thermal-cancellation claim is confirmed; if it does not, the temperature-cancellation mechanism is not the dominant stabilizer.","tokens_in":9602,"feed_emoji":"🔬","tokens_out":939,"duration_ms":7789,"temperature":0.7,"pith_summary":"Metrology cavities are built for extreme length stability; atom-cavity experiments need the resonance frequency to be tunable onto a narrow atomic line. Those two requirements usually fight each other, so most precision atom-cavity setups add external length-feedback systems. This paper shows that a carefully chosen stack of materials (Zerodur spacer, PZT rings, Kovar washers) produces a composite cavity whose overall thermal-expansion coefficient crosses zero near 5 °C. Operated at that temperature, with heavy electrical filtering of the piezos, the free-running cavity reaches 4×10^{-13} fractional frequency stability at one second and stays in the mid-10^{-13} range even when an atomic oven is running. That level is already low enough that a future ytterbium superradiant laser would not be limited by cavity noise, and it removes the need for continuous external length stabilization in many cavity-QED experiments.","feed_headline":"Piezo cavity cancels thermal expansion at 5 °C, hits 4e-13 stability","feed_subtitle":"Tunable Fabry-Perot stays stable without external length locks, ready for superradiant lasers","key_machinery":"The composite thermal-expansion coefficient α_tot(T) formed by the length-weighted sum of Zerodur, PZT and Kovar contributions; its measured zero-crossing near 5 °C is the operating point that suppresses thermal length noise while the piezos still provide multi-FSR tunability.","core_discovery":"A piezoelectrically tunable composite Fabry-Perot cavity whose coefficient of thermal expansion vanishes at (4.9 ± 0.5) °C achieves a free-running fractional frequency instability of 4 × 10^{-13} at 1 s integration time (and remains in the 10^{-13} range under realistic oven conditions), eliminating the need for external length stabilization in many atom-cavity experiments.","pith_inferences":["The same zero-crossing strategy could be retuned for other wavelengths simply by changing the relative thicknesses of the PZT and Kovar layers.","If the residual oven-induced gradients can be reduced further, the free-running floor may reach the thermal-noise limit of a few times 10^{-15} already estimated in the paper.","The design suggests that many existing piezo-tunable cavities could be re-operated at a carefully chosen temperature rather than rebuilt."],"forward_implications":["Superradiant lasers targeting 10^{-18} fractional instability can run free of continuous cavity-length locks.","Other cavity-QED platforms that need both high finesse and atomic resonance can adopt the same passive temperature set-point instead of active feedback.","Frequency-dependent squeezing cavities or space-borne interferometers that require occasional tuning can use the same material stack for long-term passive stability.","Feed-forward or PID correction of residual temperature drifts can be added later without redesigning the spacer."],"fun_headline_variants":["Piezo cavity zeros CTE near 5 °C for 4e-13 free-running stability","Tunable Fabry-Perot hits 4×10^{-13} at 1 s without external locks","Composite cavity cancels thermal expansion at 5 °C, stays 10^{-13}","Piezo-tunable FP cavity free-runs at 4e-13 after CTE null at 5 °C","Stable atom-ready cavity: CTE vanishes at 5 °C, no length feedback"],"cache_read_input_tokens":128,"weakest_assumption_plain":"Once the set-point sits at the zero-crossing temperature, residual temperature fluctuations of a few millikelvin are assumed small enough that the residual thermal-expansion noise stays below the observed 4 × 10^{-13} floor.","fun_headline_variants_meta":{"raw":{"variants":["Piezo cavity zeros CTE near 5 °C for 4e-13 free-running stability","Tunable Fabry-Perot hits 4×10^{-13} at 1 s without external locks","Composite cavity cancels thermal expansion at 5 °C, stays 10^{-13}","Piezo-tunable FP cavity free-runs at 4e-13 after CTE null at 5 °C","Stable atom-ready cavity: CTE vanishes at 5 °C, no length feedback"]},"model":"grok-4.5","effort":"low","cost_usd":0.003816,"raw_usage":{"total_tokens":1182,"prompt_tokens":723,"num_sources_used":0,"completion_tokens":128,"cost_in_usd_ticks":38160000,"prompt_tokens_details":{"text_tokens":723,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":331,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":723,"tokens_out":128,"duration_ms":3727,"temperature":1.0,"reasoning_tokens":331,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-07-14T22:33:50.352675+00:00","model_set":{"reader":"grok-4.5"},"falsifier":"Measure the Allan deviation of the same cavity while deliberately detuning the set-point by a few kelvin from the claimed zero-crossing; if the short-term floor rises by the amount predicted by the fitted CTE slope a_tot, the thermal-cancellation claim is confirmed; if it does not, the temperature-cancellation mechanism is not the dominant stabilizer.","supporting_citations":[],"review_version":1}