{"id":"c3b39121-1376-422d-a4b2-a7206f3cc5bd","arxiv_id":"2507.01205","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"A passive Sagnac-like interferometer with Faraday mirrors maintains 95.3% interferometric visibility over 72 hours, offering drift-free phase and polarization stabilization for Twin-Field QKD.","lead":"This paper reports a fiber-optic interferometer that holds two pulses in a fixed phase and polarization relationship for 72 hours without active feedback, using Faraday mirrors in a Sagnac-like loop. The idea targets Twin-Field QKD networks, where stable interference between distant users is the key to generating secret keys over long distances.","discovery_kind":"new_method","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The 72-h CW spool test omits the active AM/PM stages, so the central TF-QKD claim rests on the unverified assumption that Faraday-mirror compensation survives modulator PDL and pulsed operation.","rationale":"The reader's weakest assumption correctly identifies that the demonstration is not a full TF-QKD test: it uses CW light, no amplitude or phase modulators, and a stable laboratory spool rather than deployed fiber. My read agrees and sharpens the concern to a specific mechanism: in the proposed architecture, the active modulators sit inside the Faraday-mirror round-trip, so their PDL and modulation-dependent birefringence directly affect the polarization compensation that the scheme relies on. The paper's own Eq. (11) models PBS extinction but omits modulator PDL, and Section IV explicitly says no modulators were used. Thus the measured 95.3% +/- 0.47% visibility is solid evidence for the passive configuration, but the extrapolation to a working TF-QKD link requires an unverified assumption. This does not make the paper's argument internally inconsistent, and it is an addressable limitation rather than a fundamental error. The reader's CONDITIONAL verdict is therefore appropriate, and no verdict change is needed.","tokens_in":9715,"tokens_out":16561,"duration_ms":223261,"concrete_test":"Re-run the Fig. 3(b) setup with a pulsed laser, the AM and PM stages placed as in Fig. 1 and driven with SNS-style modulation, and a fast polarimeter at Charlie monitoring the polarization extinction after the final PBS. Measure net visibility over at least 24 h and compare with the CW baseline, recording visibility separately for Alice-only, Bob-only, and both-modulating cases. If the modulated-pulse visibility remains within 1-2 percentage points of the CW baseline and the returning-pulse extinction exceeds 25 dB, the modulator-PDL concern is settled; if visibility drops or extinction degrades significantly, the passive-stabilization claim is limited to the unmodulated regime.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Section IV explicitly states that the experiment was performed in CW mode with 'no modulators implemented' on a 22-km laboratory spool. In the proposed TF-QKD use (Section II), each signal pulse passes through an AM and a PM in one user station, first on an unmodulated outbound pass and then on the modulated return pass after the Faraday mirror. Because the return pass sees the orthogonal polarization, any polarization-dependent loss (PDL) or state-dependent transmission in the modulators enters the round-trip Jones chain and is not exactly cancelled by the Faraday mirror. The paper's visibility model (Eq. 11) accounts only for PBS extinction η and static misalignment ξ; it does not include modulator PDL, modulation-transient birefringence, or pulsed-operation gating effects. The CW experiment therefore supports passive phase and polarization stabilization for the specific passive configuration tested, but it does not establish that the same visibility is maintained when the actual TF-QKD encoding stages are active and the fiber is a deployed link with independent, faster birefringence fluctuations. This is the load-bearing gap: without a modulated, pulsed demonstration, the central claim that the scheme is ready for TF-QKD remains conditional.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper proposes a modified Sagnac interferometer that combines Faraday mirrors and polarizing beamsplitters to passively stabilize both phase and polarization fluctuations in a fiber-based Twin-Field QKD (TF-QKD) setup. The authors present a formal description of the round-trip propagation, report an experimental demonstration on a 22-km fiber spool in continuous-wave (CW) mode with no amplitude or phase modulators, and observe a net interferometric visibility of about 95.3% over 72 hours with a standard deviation of 0.476%. They also model the effects of Rayleigh backscattering and PBS extinction, discuss the influence of a fitted polarization alignment factor, and outline an extension of the scheme to multi-user star, bus, and mixed topologies.","tokens_in":9917,"tokens_out":12948,"duration_ms":145531,"significance":"If the scheme works as claimed under realistic TF-QKD operating conditions, it would remove the need for active polarization control in TF-QKD links, which is a practically important simplification. The 72-hour stability data with a standard deviation below 0.5% is a solid experimental contribution, and the explicit comparison with a standard Sagnac interferometer clearly illustrates the benefit of the passive polarization compensation. The paper also provides a useful conceptual extension to multi-user networks. However, the significance is qualified by the fact that the experiment omits the amplitude and phase modulators and the pulsed, single-photon-level operation that are essential to actual TF-QKD; the quantitative visibility models rely on a parameter fitted to the same data; and the experiment uses a laboratory spool rather than a deployed fiber link. These caveats mean the central claim is best read as a proof-of-principle for the passive core rather than a full validation for TF-QKD.","major_comments":[{"comment":"The 72-hour stability demonstration was performed in continuous-wave mode with no amplitude or phase modulators implemented (Section IV, first paragraph), whereas the TF-QKD use described in Section II requires Alice and Bob to apply AM and PM modulation on the return pass only, after the pulse has been reflected by the Faraday mirror. Because the Faraday-mirror round-trip compensation is only exact for reciprocal propagation that is identical in the forward and backward directions, the asymmetric insertion of the modulated AM/PM stages on the return pass can introduce polarization-dependent loss and birefringence that are not cancelled by the Faraday mirror. The visibility model, Eq. (11), includes only PBS extinction eta and static misalignment xi, and does not account for modulator PDL or modulation-induced birefringence. The experimental data therefore support the stability of the passive configuration tested, but they do not establish that the same visibility is maintained when the actual TF-QKD encoding stages are active, and the claim that the setup passively stabilizes phase and polarization 'for Twin-Field QKD' is conditional on this untested assumption.","section":"Section IV and Section II"},{"comment":"The alignment factor xi is estimated 'from the behavior of the single photon detection counts over time', i.e., from the same data used to report the visibility. Since xi is a free parameter fitted to the measured data, the agreement between Eq. (11) and the observed visibility is not a predictive test of the noise model. Moreover, using the stated values (t^2 approx 0.074, xi = 0.97, eta = 0.0156) in Eq. (11) yields V approx 98.7%, whereas the measured average net visibility is 95.3%; the discrepancy is not discussed, so the claim that the visibility is 'primarily limited' by the two modeled mechanisms is not quantitatively supported.","section":"Section V.B"}],"minor_comments":[{"comment":"The text says 'we measured an attenuation of 11.3 dB ... corresponding to t approx 0.074'. Since t is defined as the one-way channel transmission in Eq. (7), the correct statement is t^2 approx 0.074 (or equivalently t approx 0.27); otherwise Eq. (11) does not yield the reported visibility of approximately 98.7%.","section":"Section V.B"},{"comment":"The sentence 'the polarization state of the optical pulses that return to Charlie always correspond to the horizontal state |V⟩' is internally contradictory; it should read 'vertical state |V⟩' (or be otherwise made consistent with the notation for horizontal and vertical polarizations).","section":"Section II"},{"comment":"The text refers to figures with no number in two places ('as shown in Fig. .' and 'in Fig. .'); the figure reference should be completed (likely Fig. 2) and the corresponding caption should be checked.","section":"Section III"},{"comment":"The procedure for subtracting dark counts to obtain the reported 'net' visibilities is not described; the reader only learns in Section V that dark counts were subtracted and that the raw visibility was 91.4%. Please state the formula or method used for this subtraction.","section":"Section IV"},{"comment":"The abstract reports the experimental visibility result without mentioning that the measurement was performed in CW mode with no modulators; given that this is an important scope limitation, the abstract (or at least the conclusions) should explicitly state that the demonstration is for the passive core and that the active encoding stages were not included.","section":"Abstract"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is a solid proof-of-principle demonstration of passive polarization stabilization in a Sagnac-like interferometer, and the 72-hour data are convincing for the specific setup tested. The main concern is that the title and abstract promise relevance to TF-QKD while the experiment omits the amplitude/phase modulators and pulsed operation that are integral to the protocol. I believe the paper can be made publishable by either adding an experimental demonstration with active modulators (even in a proof-of-principle form) or by explicitly reframing the claims and conclusions to state that the validation covers only the passive stabilization core, with the modulator-related effects left as a theoretical open point. The quantitative visibility model also needs to be reconciled with the measured value rather than left with an unexplained 3% discrepancy."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Net take: the 72-hour, 95.3% net visibility result on a 22-km lab spool is real evidence that combining Faraday mirrors with a Sagnac-like layout passively holds polarization and phase against slow drift. The comparison with the standard Sagnac, which swung from 97% to 1% over the same window, makes the point cleanly. That part deserves credit and is genuinely useful for practical TF-QKD work.\n\nWhat's actually new is the integration itself: plug-and-play Faraday mirrors into a Sagnac-like TF-QKD topology, which prior plug-and-play TF-QKD [39] lacked. The multi-user star/bus extension is a sensible proposal, even if it's only sketched. The modeling of the visibility ceiling from Rayleigh backscattering and PBS extinction is plausible and matches the observed values.\n\nNow the soft spots, in proportion. The stress-test note nails the main one: the experiment runs CW with no modulators, and the proposed TF-QKD use puts AM/PM stages in the round-trip path where modulator PDL is not exactly canceled by the Faraday mirror. The paper's own visibility model omits that. The authors are honest about the lab conditions, and they argue pulsed gating would filter noise photons, but they don't test it. So the central claim that this stabilizes a working TF-QKD link is conditional. The multi-user network is also unvalidated. These are addressable limitations, not fatal errors.\n\nThe fitted parameter xi is data-derived, but it only explains the visibility ceiling and doesn't feed back into the stability claim, so the circularity burden is low.\n\nWho's this for? Groups building TF-QKD links or networks who want to remove active polarization tracking. It deserves a serious referee, not a desk reject. My recommendation: accept with revisions, and the revisions should either add a pulsed/modulated test or reframe the claims as component-level validation of the passive stabilization scheme.","headline":"A solid passive-stabilization engineering result for TF-QKD, with the main gap being that the experiment stops short of a modulated, pulsed, deployed-fiber demonstration.","tokens_in":10450,"tokens_out":1971,"would_cite":false,"duration_ms":121503,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":["81P94"],"pacs":["03.67.Hk"],"model":"deepseek-v4-flash","headline":"A Sagnac-like interferometer with Faraday mirrors passively stabilizes both phase and polarization in Twin-Field QKD, holding net interferometric visibility at 95.3% for 72 hours of continuous operation.","keywords":["Twin-Field QKD","Sagnac interferometer","Faraday mirror","passive polarization stabilization","phase stabilization","plug-and-play QKD","quantum key distribution","fiber interferometry"],"falsifier":"Operate the modified Sagnac setup in pulsed mode with the amplitude and phase modulators active and single-photon-level returns, while applying rapid, non-reciprocal polarization perturbations, for example squeezing or bending a short section of the fiber at frequencies comparable to or faster than the round-trip time; if the net visibility drops by more than the roughly 0.5% level seen in the 72-hour spool test, the assumption that the two counter-propagating pulses experience identical fiber transformations is violated.","tokens_in":9520,"feed_emoji":"⚛️","tokens_out":6742,"duration_ms":67259,"temperature":0.7,"pith_summary":"This paper sets out to show that Twin-Field QKD's two hardest practical problems — keeping the phase reference stable and keeping the two interfering pulses in the same polarization — can both be solved passively, without any active feedback. It proposes a modified Sagnac interferometer that borrows the plug-and-play trick of Faraday mirrors: any polarization rotation picked up in the fiber is undone on reflection, so the pulses always recombine with matching polarizations, while the Sagnac geometry continues to cancel path-length fluctuations. A 72-hour experiment over 22 km of fiber backs this up: net visibility held at 95.3% with a 0.47% standard deviation, whereas a standard Sagnac loop drifted between 97% and 1%. The paper also shows the configuration extends to multi-user star, bus, and mixed networks. If the stabilization survives pulsed, single-photon-level operation, it would remove an entire class of active-control hardware from practical TF-QKD.","feed_headline":"Passive Sagnac variant stabilizes QKD phase and polarization","feed_subtitle":"Faraday mirrors and a common optical path cancel drift without active control, holding 95.3% visibility over 72 hours.","key_machinery":"The central object is the modified Sagnac configuration: a Sagnac-like interferometer in which a beam splitter at Charlie's station sends two pulse halves toward Alice and Bob, each of whom reflects them with a Faraday mirror after passing them through a polarizing beamsplitter. The Faraday mirror rotates polarization by 90 degrees on reflection, so any unitary polarization rotation picked up in the fiber is undone on the return trip; the PBSs then route each returning pulse to the opposite arm, making the two halves traverse exactly the same optical path before recombining. What the machinery does is convert the polarization-matching requirement, which in a standard Sagnac setup would reduce visibility, into an automatic property of the round trip, while preserving the Sagnac phase-cancellation property.","core_discovery":"The paper claims that adding Faraday mirrors to a Sagnac-style beamsplitter network converts it into a modified Sagnac interferometer in which polarization drift is compensated passively, so the two counter-propagating pulse halves always return to the beamsplitter in the same polarization state. Because the two halves still travel the same optical path in opposite directions, path-length and phase fluctuations cancel as in a standard Sagnac loop, while the Faraday mirrors guarantee polarization reciprocity. The claim is supported by a 72-hour continuous-wave experiment over 22 km of fiber showing a net visibility of 95.3% with a standard deviation of 0.47%, while a standard Sagnac loop under the same conditions fluctuated between roughly 97% and 1%. The paper further claims that the configuration generalizes to multi-user networks with star, bus, or mixed topologies using slow optical switches.","pith_inferences":["Inference: If the passive stability carries over to pulsed single-photon operation, TF-QKD field links could drop their fast active phase-locking feedback loops, simplifying the hardware and reducing the electronics an adversary could attack.","Inference: The same Faraday-mirror round-trip idea should transfer to any first-order interferometric protocol whose visibility is limited by polarization mismatch, not only TF-QKD.","Inference: A direct stress test would compare the modified loop against a standard Sagnac loop under deliberately applied fast birefringence perturbations; the modified loop should stay flat while the standard loop degrades, isolating the polarization-compensation mechanism."],"forward_implications":["Twin-Field QKD links built on this configuration can run for days without active polarization tracking, since phase and polarization drift are both compensated by the same optical path.","The same passive stabilization extends to multi-user star, bus, and mixed networks: any pair of users can be connected by configuring slow optical switches, without high-speed switching.","The visibility ceiling in the current implementation is set by Rayleigh backscattering and PBS extinction ratio or polarization misalignment, both of which the paper argues can be raised with polarization-maintaining fiber and gated detection in pulsed operation.","Because the information-carrying modulation is applied only at the last pass, the twin-field key-rate advantage proportional to the square root of the channel transmission is preserved.","The scheme is compatible with hybrid fiber and free-space networks, since a free-space channel only affects the branch in use."],"supporting_citations":[{"why":"Introduces the plug-and-play Faraday-mirror setup whose round-trip polarization compensation is the core new ingredient of the proposed scheme.","marker":"[38]"},{"why":"Demonstrates the original Sagnac-based TF-QKD configuration that this work modifies by adding Faraday mirrors.","marker":"[30]"},{"why":"Extends Sagnac TF-QKD to multiuser networks, providing the baseline for the proposed star and bus topologies.","marker":"[31]"},{"why":"Quantifies Rayleigh backscattering in long-fiber Sagnac interferometers, used here to model the measured visibility ceiling.","marker":"[32]"},{"why":"A prior plug-and-play TF-QKD proposal that lacks passive phase stabilization, distinguishing this work's contribution.","marker":"[39]"},{"why":"Provides the PBS characterization and extinction-ratio values used in the polarization-misalignment visibility estimate.","marker":"[40]"},{"why":"Supplies the Rayleigh backscattering coefficient used in the numerical visibility estimate.","marker":"[41]"},{"why":"Establishes the TF-QKD protocol and its square-root transmission rate advantage, which motivates the need for a stable distributed phase reference.","marker":"[20]"}],"fun_headline_variants":["Passive Sagnac+Faraday mirrors give 72h 95.3% QKD visibility","Passive Sagnac setup locks QKD phase and polarization for 72h","No active control: Sagnac+Faraday mirrors stabilize QKD","Passive Sagnac achieves 95.3% QKD visibility for 72 hours"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The demonstration ran in continuous-wave mode on a 22-km fiber spool in the laboratory, without the amplitude and phase modulators, single-photon-level pulses, or deployed-fiber disturbances of a real Twin-Field QKD session; the passive stabilization claim for a working link assumes the Faraday-mirror round trip stays exactly reciprocal under those conditions.","fun_headline_variants_meta":{"raw":{"variants":["Passive Sagnac+Faraday mirrors give 72h 95.3% QKD visibility","Passive Sagnac setup locks QKD phase and polarization for 72h","No active control: Sagnac+Faraday mirrors stabilize QKD","Passive Sagnac achieves 95.3% QKD visibility for 72 hours"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001638,"raw_usage":{"total_tokens":6466,"prompt_tokens":855,"completion_tokens":5611,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":471,"completion_tokens_details":{"reasoning_tokens":5519}},"tokens_in":471,"tokens_out":5611,"duration_ms":40622,"temperature":1.0,"reasoning_tokens":5519,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-06T20:56:51.722220+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Operate the modified Sagnac setup in pulsed mode with the amplitude and phase modulators active and single-photon-level returns, while applying rapid, non-reciprocal polarization perturbations, for example squeezing or bending a short section of the fiber at frequencies comparable to or faster than the round-trip time; if the net visibility drops by more than the roughly 0.5% level seen in the 72-hour spool test, the assumption that the two counter-propagating pulses experience identical fiber transformations is violated.","supporting_citations":[{"cited_title":"Ribordy, J.-D","cited_arxiv_id":null,"evidence_quote":"Introduces the plug-and-play Faraday-mirror setup whose round-trip polarization compensation is the core new ingredient of the proposed scheme."},{"cited_title":"Mandil, L","cited_arxiv_id":null,"evidence_quote":"Quantifies Rayleigh backscattering in long-fiber Sagnac interferometers, used here to model the measured visibility ceiling."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"A prior plug-and-play TF-QKD proposal that lacks passive phase stabilization, distinguishing this work's contribution."},{"cited_title":"Calliari, P","cited_arxiv_id":null,"evidence_quote":"Provides the PBS characterization and extinction-ratio values used in the polarization-misalignment visibility estimate."},{"cited_title":"Derickson, Fiber optic test and measurement (Pren- tice Hall, 1998)","cited_arxiv_id":null,"evidence_quote":"Supplies the Rayleigh backscattering coefficient used in the numerical visibility estimate."}],"review_version":1}