{"id":"6f038c15-9a54-44e6-be1b-a2c43766e825","arxiv_id":"2509.07667","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":2,"one_line_summary":"A compact 20 cm C-band ground terminal with spectral, spatial and temporal filtering demonstrated over 120 dB noise suppression and stable 100 m free-space QKD.","lead":"The authors built a compact 20-centimeter telescope ground terminal that couples quantum signals into single-mode fiber and filters out daylight and neighboring telecom light. They report over 120 decibels of background suppression and stable key generation over a 100-meter outdoor free-space link.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The QKD demonstration never transmits the neighboring C-band classical channel through the FSO link, so the claimed crosstalk isolation is not validated under operating QKD conditions.","rationale":"The reader's conditional verdict already identified the fiber back-to-back classical channel as part of the weakest assumption, but the present review sharpens this into the single load-bearing gap: the crosstalk suppression and the QKD demonstration are separate experiments, so the integrated feasibility claim is not yet supported. This is not a numerical or stylistic issue; it is a missing experimental condition directly tied to the abstract's promise of \"daylight C-band satellite QKD\" with \"135 dB crosstalk noise isolation.\" The paper itself acknowledges the benign 100 m conditions and the need for adaptive optics for real satellite links, but the more immediate defect is that no QKD run includes the neighboring classical channel through the FSO terminal. If the proposed co-transmission test passes, the central claim would be substantially strengthened; if it fails, the terminal's advertised isolation would not have been demonstrated during key exchange. The reader's request for corrected numbers and error bars is still valid, but the simultaneous-operation test is the decisive condition. Since the reader's verdict was already CONDITIONAL, this concern does not move the verdict; it specifies the condition more precisely.","tokens_in":4541,"tokens_out":6990,"duration_ms":65313,"concrete_test":"Repeat the Figure 4a QKD run with the 1553.33 nm classical signal (through the 0.8 nm notch filter) launched from the same transmitter telescope and received through the same terminal and complete filtering chain simultaneously with the Clavis XG quantum signal at 1551.72 nm over the 100 m outdoor link. Record QBER, SKR, and visibility for 1200 s, and repeat under daylight at roughly 45° solar elongation if the daylight-noise claim is to be coupled to QKD. If QBER stays below 1% with the classical channel at the power used in the crosstalk test, the combined isolation claim is supported; if QBER or SKR degrade, the separate demonstrations do not establish the headline.","verdict_should_be":"UNCHANGED","load_bearing_attack":"In the QKD experiment (Figure 4a) the paper states that \"the classical communication between Alice and Bob is facilitated via a dedicated back-to-back fiber connection,\" while the 121 dB crosstalk suppression shown in Figure 2 was measured in a separate configuration with the classical signal at 1553.33 nm and the quantum passband at 1550.12 nm. The 100 m FSO link used for QKD therefore never carried the neighboring C-band classical channel through the same telescope, SMF-coupling module, and filtering chain while keys were being generated. The headline claim of \"135 dB crosstalk noise isolation\" (or \">120 dB crosstalk suppression\" in the Conclusions) is thus not demonstrated simultaneously with QKD; the paper's conclusion that it is the \"first experimental validation of quantum signal integration alongside neighbouring service wavelengths\" goes beyond what the experiment shows. A classical channel launched through the FSO path could introduce residual leakage, mode-field overlap, or back-reflections that are invisible in a fiber back-to-back test or in the separate crosstalk measurement. The 20 dB contribution attributed to SPAD gating is, moreover, an average-rate suppression produced by a 1% duty cycle and needs a timing-synchronization argument to count as isolation of a continuous neighboring-channel signal in a gated QKD receiver; the paper does not provide that argument. Because the two headline capabilities are validated in different configurations, the integrating feasibility claim is the weakest link in the paper.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports a compact C-band optical ground terminal for satellite quantum key distribution (SatQKD), built around an 8-inch telescope with SMF coupling and a three-stage filtering chain: spectral (notch and passband filters), spatial (single-mode fiber), and temporal (SPAD gating). The authors claim more than 120 dB daylight background suppression and 135 dB crosstalk isolation from a neighboring C-band classical channel, plus background noise rates of 50 cps at 45° solar elongation and 150 cps at 20°. They validate the terminal with a 100 m outdoor free-space QKD link using a Clavis XG system, reporting stable SKR around 4.2 kbps, visibility near 98%, and QBER below 1% over 1200 seconds. The central claim is that this demonstrates the feasibility of integrating satellite QKD with urban fiber segments and co-existing classical service wavelengths.","tokens_in":4769,"tokens_out":2573,"duration_ms":24241,"significance":"If the headline numbers are substantiated, the work is a practical engineering step toward daylight-capable, compact SatQKD ground stations in the telecom C-band. The use of COTS components and a small aperture is useful for constellation-based QKD architectures, and the direct measurement of suppression values is a strength. The paper also benefits from a clear experimental description and a stable QKD demonstration over an outdoor link. However, the significance is limited by the fact that the crosstalk suppression and the QKD demonstration are not performed in a single integrated configuration, and by inconsistencies in the reported suppression values. The engineering contributions are real, but the central 'first experimental validation of quantum signal integration alongside neighbouring service wavelengths' claim overreaches the present evidence.","major_comments":[{"comment":"The headline crosstalk suppression is not demonstrated simultaneously with the QKD run. The QKD experiment in Figure 4a uses a dedicated back-to-back fiber connection for classical communication, while the 121 dB crosstalk suppression in Figure 2 was measured in a separate configuration with the classical signal at 1553.33 nm and the quantum passband at 1550.12 nm; the QKD link, by contrast, operated at 1551.72 nm. Residual leakage, back-reflections, and mode-field overlap from a classical channel transmitted through the same telescope, SMF-coupling module, and filtering chain could degrade the isolation in ways invisible to the fiber back-to-back test or the separate crosstalk measurement. The paper should present an experiment in which the classical channel is launched through the FSO path (or through the same optical chain) while QKD is running, or explicitly rephrase the claim to indicate that the two capabilities were validated separately.","section":"Results and Discussion, Figures 2 and 4a"},{"comment":"The 20 dB contribution attributed to SPAD gating is an average-rate suppression from the 1% duty cycle, not a filter rejection ratio. For a continuous-wave neighboring-channel signal, gating reduces the detected count rate by the duty cycle, but whether this constitutes 'isolation' of the classical channel in a QKD receiver depends on the timing synchronization between the SPAD gates and the quantum signal. The manuscript does not provide the gate timing, synchronization scheme, or an argument that the classical signal is absent during the active gate windows. Since the 135 dB (or 121 dB) crosstalk figure includes this 20 dB, the paper should clarify the interpretation and either measure the suppression within the actual gating windows or remove the temporal contribution from the headline isolation figure.","section":"Results and Discussion, Figure 2 (temporal filtering)"},{"comment":"The reported suppression numbers are inconsistent across the manuscript. The abstract states '135 dB crosstalk noise isolation'; Results and Discussion reports 121 dB total crosstalk suppression (101 dB spectral plus 20 dB temporal) and 136 dB background sky radiance suppression; Conclusions states '>120 dB crosstalk suppression and >135 dB daylight noise reduction'. The daylight background suppression is also described as 'over 120 dB' in the Introduction. These discrepancies make it difficult for the reader to identify the measured values. The authors should reconcile the numbers and clearly specify which quantity is being reported in each location (e.g., spectral-only versus total, crosstalk versus daylight background).","section":"Abstract, Results and Discussion, Conclusions"}],"minor_comments":[{"comment":"The sentence 'Successful QKD over an outdoor 100m FSO validate its feasibility' has a subject-verb agreement error; 'validate' should be 'validates' (or the subject should be pluralized).","section":"Abstract"},{"comment":"The word 'supress' is misspelled; it should be 'suppress'. Similar typos appear elsewhere ('supressed' in Results and Discussion). A careful proofreading pass is recommended.","section":"Introduction"},{"comment":"The measurement method for the 101 dB spectral suppression is not described. It would be helpful to state whether this value was obtained with an optical spectrum analyzer, a power meter, or the SPAD itself, and what the uncertainty of the measurement is.","section":"Results and Discussion, Figure 2"},{"comment":"The values of 50 cps and 150 cps are reported in gated mode, but the duty cycle and the corresponding ungated rates are not stated. Clarifying the gate window and the effective noise rate in the gate would help readers compare with other systems.","section":"Results and Discussion (daylight noise)"},{"comment":"The dark count rate of the SPAD in free-running mode is given as ~500 cps, but the operating quantum efficiency is not stated for this measurement. Since the DCR depends on the bias and gating settings, this information should be included for reproducibility.","section":"Results and Discussion (Figure 3)"}],"recommendation":"major_revision","confidential_remarks":"The paper is a solid engineering report, and the underlying measurements appear plausible. The main reservation is that the central 'integration with neighbouring service wavelengths' claim is not supported by a single experiment where the classical channel traverses the same FSO path as the quantum signal. The authors should be encouraged to perform that integrated test or clearly limit their claim. The arithmetic inconsistency between 121 dB and 135 dB for crosstalk suppression should also be fixed; it is surprising that this slipped through the initial submission."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: this is a useful engineering demonstration, not a breakthrough. The 8-inch C-band terminal with combined spectral/spatial/temporal filtering is a real thing, and the measured numbers — 121 dB crosstalk suppression in the filtered configuration, daylight background of 50 cps at 45° elongation, and a stable 4.2 kbps QKD link over 100 m with QBER below 1% — are credible as standalone results. What the paper does well is show that COTS components can be packaged into a compact SMF-coupled terminal and still deliver the kind of isolation that satellite QKD ground segments need.\n\nThe weak spots are in how the claims are stitched together. The abstract says 135 dB crosstalk isolation; the Results say 121 dB; the Conclusion says over 120 dB. That needs fixing. More substantively, the crosstalk measurement and the QKD experiment are separate configurations: the classical channel in the QKD test is on a back-to-back fiber, not through the same 100 m FSO path as the quantum signal. So the headline 'first experimental validation of quantum signal integration alongside neighbouring service wavelengths' goes beyond what the data show. The >120 dB isolation is demonstrated in isolation, not while keys are being generated. That is the main caveat, and it is a legitimate one.\n\nTwo smaller points. The 20 dB contribution from SPAD gating is an average-rate suppression from a 1% duty cycle; that is fine as a statement, but the paper should say it explicitly and give at least a sentence on timing. And there are no error bars or measurement uncertainties anywhere, which makes it hard to know how precise the 50 cps and 121 dB numbers are.\n\nNone of this undermines the central capability: high suppression and low daylight noise are achievable with this terminal. The paper is honest enough to note the benign turbulence conditions and the need for adaptive optics in real orbit-to-ground links. It is a solid contribution to the satellite QKD ground-segment subfield, but it needs a revision that corrects the numbers, separates the two experiments, and softens the 'first' claim. I'd send it to peer review.","headline":"Compact C-band ground terminal with real filtering numbers, but the integration claim runs ahead of the experiment: crosstalk and QKD were validated separately.","tokens_in":5441,"tokens_out":3491,"would_cite":true,"duration_ms":30061,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"A compact 8-inch fiber-coupled ground terminal suppresses C-band daylight background by more than 120 dB and demonstrates QKD over a 100 m outdoor link.","keywords":["satellite quantum key distribution","optical ground terminal","daylight QKD","background noise suppression","free-space optical link","single-mode fiber coupling","C-band telecom wavelength","spectral filtering"],"falsifier":"With the telescope pointed at a clear daytime sky at 45 degrees solar elongation and the classical channel transmitting at 1553.33 nm, read the gated SPAD count after the full filtering chain: the central claim fails if the rate is not close to 50 cps or if the QBER on a 100 m link drifts above 1% when atmospheric turbulence is introduced.","tokens_in":4341,"feed_emoji":"🔭","tokens_out":6024,"duration_ms":52895,"temperature":0.7,"pith_summary":"The paper reports a compact 8-inch telescope ground terminal that couples free-space light into single-mode fiber and combines spatial, spectral, and temporal filtering to suppress background and crosstalk noise. It claims the terminal reduces daytime sky noise to about 50 counts per second and rejects a neighboring C-band classical channel by more than 120 dB, while still carrying phase-encoded quantum key distribution over a 100 m outdoor link. The point of the work is to show that small, fiber-coupled ground stations can run satellite quantum key distribution in daylight and alongside classical telecom signals, making integration with urban fiber networks plausible.","feed_headline":"Small telescope silences daylight noise for satellite QKD","feed_subtitle":"A 20-cm fiber-coupled terminal reaches 50 background counts per second while keeping QKD error below one percent.","key_machinery":"The load-bearing mechanism is the combination of single-mode-fiber spatial filtering with multi-stage spectral filtering and temporal gating. The telescope couples the received beam into SMF, whose small mode-field diameter restricts the detector field of view and makes diffuse background coupling almost independent of receiver optics; then a three-stage 0.8 nm band-stop filter cleans the classical channel before transmission, WDM modules combine and separate quantum and classical signals, a two-stage 0.2 nm passband filter at the receiver rejects solar radiance and leftover crosstalk, and SPAD gating at 1% duty cycle adds 20 dB of suppression.","core_discovery":"The paper reports that a compact, commercial-off-the-shelf-based ground terminal built around a 20 cm telescope and a custom single-mode-fiber coupling module can operate the quantum channel in the C-band during daylight while a classical channel sits at a neighboring wavelength. With a three-stage 0.8 nm band-stop filter on the classical signal, a two-stage 0.2 nm passband filter before the detector, and gated single-photon detection, the setup claims 121 dB of measured crosstalk suppression (101 dB from spectral filtering plus 20 dB from temporal gating), 136 dB of daylight background suppression, and background counts of about 50 cps at 45 degrees solar elongation and 150 cps at 20 degrees. On a 100 m outdoor free-space link under relatively stable conditions it achieved more than 10% coupling efficiency and ran phase-encoded QKD with secret key rate around 4.2 kbps, visibility near 98%, and QBER below 1% over 1200 seconds. The paper calls this the first experimental validation of quantum signals coexisting with neighboring service wavelengths for C-band satellite QKD.","pith_inferences":["The 100 m link ran with negligible turbulence and with the classical channel on a fiber back-to-back path, so the orbital case still needs demonstration; a natural next experiment is the same terminal on a several-kilometre elevated path with measured scintillation.","Because gated detection provides 20 dB of the crosstalk suppression at a 1% duty cycle, the lost collection efficiency must be traded against key rate; an ungated or higher-duty-cycle version would need deeper spectral filtering to keep the same isolation.","The coupling-efficiency number of about 10% already includes 3 dB telescope reflectivity loss; improving coatings could nearly double throughput without changing the filtering architecture.","The architecture seems compatible with swapping the fixed notch filter for a tunable one, which would let the same terminal adapt to different classical wavelength assignments in future missions."],"forward_implications":["Daytime background suppression is sufficient to reach the detector dark-count floor at solar elongation angles beyond roughly 70 degrees, so daylight QKD from a small terminal is not blocked by sky radiance alone.","A neighboring C-band classical channel can be rejected by more than 120 dB, so the quantum channel does not need a separate wavelength band or a dark fiber.","Single-mode-fiber coupling lets the detector sit in a shielded location, so keys can be distributed from a rooftop terminal into a passive urban fiber network.","A stable 100 m free-space link sustained QBER below 1% for 1200 seconds with a compact 8-inch terminal, supporting small-form-factor ground stations."],"supporting_citations":[{"why":"Establishes the space-QKD feasibility baseline that this terminal aims to make deployable and scalable.","marker":"[1]"},{"why":"Microsatellite-based real-time QKD demonstration that the compact ground-terminal design builds toward.","marker":"[2]"},{"why":"Supplies the Clavis XG QKD system used in the free-space transmission test and provides a daylight QKD precedent.","marker":"[4]"},{"why":"Public requirements for C-band ground stations that motivate the isolation and background-suppression targets.","marker":"[7]"},{"why":"Analysis used to argue that an SMF-limited field of view makes diffuse background coupling nearly independent of receiver optics.","marker":"[8]"},{"why":"Comparative study of small ground stations that identifies the orbital impairments requiring adaptive optics and more advanced tracking.","marker":"[9]"}],"fun_headline_variants":["20-cm telescope delivers daylight C-band QKD","Compact terminal silences daylight for satellite QKD","Daylight C-band QKD with 120 dB noise suppression","First daylight C-band QKD for satellite links"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The terminal was validated under relatively stable, low-turbulence conditions on a 100 m link, with the classical channel carried over fiber rather than through the free-space path, and the satellite case assumes the same noise suppression holds through an orbit-to-ground channel with beam wander, scintillation, Doppler shifts, and pointing errors.","fun_headline_variants_meta":{"raw":{"variants":["20-cm telescope delivers daylight C-band QKD","Compact terminal silences daylight for satellite QKD","Daylight C-band QKD with 120 dB noise suppression","First daylight C-band QKD for satellite links"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000484,"raw_usage":{"total_tokens":2323,"prompt_tokens":813,"completion_tokens":1510,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":429,"completion_tokens_details":{"reasoning_tokens":1447}},"tokens_in":429,"tokens_out":1510,"duration_ms":11355,"temperature":1.0,"reasoning_tokens":1447,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T16:11:38.187848+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"With the telescope pointed at a clear daytime sky at 45 degrees solar elongation and the classical channel transmitting at 1553.33 nm, read the gated SPAD count after the full filtering chain: the central claim fails if the rate is not close to 50 cps or if the QBER on a 100 m link drifts above 1% when atmospheric turbulence is introduced.","supporting_citations":[{"cited_title":"The-Eagle-1_QKD_protocol.pdf","cited_arxiv_id":null,"evidence_quote":"Public requirements for C-band ground stations that motivate the isolation and background-suppression targets."},{"cited_title":"Satellite -to- Ground QKD in Urban Environment: A Comparative Analysis of Small-Sized Optical Ground Stations,","cited_arxiv_id":null,"evidence_quote":"Comparative study of small ground stations that identifies the orbital impairments requiring adaptive optics and more advanced tracking."}],"review_version":2}