{"id":"fc224c28-f1c4-42db-a206-942349e20dd3","arxiv_id":"1908.05022","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"A measurement-device-independent coherence witness using test-state tomography and decoy states is proposed and demonstrated in a time-bin optical system, yielding a coherence lower bound of 0.25 per detected signal.","lead":"The paper shows a way to certify quantum coherence in an unknown state even when the measurement device cannot be trusted, and demonstrates it in an optical experiment. A generalist might read it because it provides a practical tool for quantum randomness generation and other coherence-based tasks under realistic device flaws.","discovery_kind":"new_method","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The protocol's core claim depends on an unproven squashing/support assumption: an arbitrary untrusted measurement need not admit a qubit POVM, so the tomography may not determine the coherence bound for states outside the test-state subspace.","rationale":"The reader's weakest_assumption is exactly the concern we identify: the protocol assumes the unknown state lies in the same qubit support as the test states, and the justification relies on a squashing model. We agree that this is the most load-bearing soft spot. The Eq. (7) typo and the QRNG finite-size caveat are real but secondary: they affect numerical accuracy and the randomness application, not the existence of the MDICW protocol. The squashing/support assumption is what makes the protocol work; if it fails, the central claim—that an untrusted measurement can be characterized and a coherence bound certified with one setting—does not hold for a class of devices the paper appears to cover. The paper explicitly acknowledges the support assumption and the absence of a dimension assumption in MDIEW, so this is not a hidden inconsistency; it is a boundary on the result. The proposed counterexample test would determine whether the boundary is real for generic non-squashable measurements, and the verdict remains conditional as the reader stated.","tokens_in":17811,"tokens_out":8900,"duration_ms":83958,"concrete_test":"Construct an explicit counterexample: choose a qutrit two-outcome POVM {M0,M1} on H=span{|0>,|1>,|2>} (with |2> orthogonal to the qubit subspace) whose four test-state probabilities match Eq. (2) for some qubit POVM parameters, but for which tr(M1 ρ) for a state ρ with support on |2> differs from the value predicted by the tomographed qubit POVM. Compute the coherence lower bound via Eq. (25) using only the four test-state probabilities and the (incorrectly inferred) constraint on ρ; compare with the true relative-entropy coherence of the reduced state in the {|0>,|1>} basis. If the method reports a positive lower bound for a state whose reduced qubit is incoherent, or a bound larger than the actual coherence, the squashing/support assumption is violated and the central claim fails in a concrete regime.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The load-bearing assumption is the qubit-support/squashing condition stated in the remarks after Eq. (7): \"we assume that the unknown state is on the same support of the test states. This assumption comes from the squashing model.\" The protocol tomographs the untrusted measurement using four qubit test states (Eqs. (1)-(2)), which determines only the two-outcome qubit POVM projected onto span{|0>,|1>}. For a truly arbitrary untrusted measurement, the probability tr(M1 ρ) in the constraint (4) is not fixed by that tomography when ρ has support outside this subspace. The paper asserts that \"we can always squash the unknown state into the subspace of {τ}\" and that an incoherent squasher preserves the bound. But a squashing map that preserves outcome statistics for all input states is not a universal property of measurements; Ref. [41] establishes squashing models only for specific detector classes. Without a proof that the device in question is squashable, the coherence lower bound from the dual optimization Eq. (25) is not justified for the original state. The summary itself concedes the limitation by listing \"a new MDICW scheme without the dimension assumption\" as future work, which confirms that the headline \"remove the requirements on the measurement device\" is conditional on this dimension/squashing assumption.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript introduces a measurement-device-independent coherence witness (MDICW) protocol in which Alice prepares four Pauli eigenstates to tomograph an untrusted measurement, then lower-bounds the relative entropy of coherence of an unknown state rho by solving a convex optimization with constraints derived from the tomography. A decoy-state method is added to estimate single-photon contributions from weak coherent states, and the protocol is demonstrated experimentally in a time-bin optical system. The experiment reports a coherence lower bound of 0.25 per detected signal state for the state |+i>, while an equal mixture of |+i> and |-i> yields no witnessed coherence; a QRNG application is also demonstrated.","tokens_in":18056,"tokens_out":10504,"duration_ms":104505,"significance":"The idea of extending measurement-device-independent certification from entanglement to coherence is timely, and the decoy-state enhancement is a practical contribution that improves the tightness of the bound. The experimental implementation is careful, with active phase stabilization, intensity optimization, and a control experiment that supports the convexity of coherence. The central caveat is that the protocol is not fully independent of the measurement dimension: it assumes the unknown state lies in the qubit support of the test states, or that the measurement admits a qubit squashing model. Within that scope, the scheme provides a valid way to certify coherence without trusting the calibration of the measurement basis, and the experimental results are consistent with the theory.","major_comments":[{"comment":"The protocol assumes that the unknown state rho lies in the same support as the four test states, an assumption justified only by an appeal to squashing models. For a genuinely untrusted measurement device, four qubit test states determine the POVM only on span{|0>,|1>}; the constraint (4) is then not fixed for a rho with support outside this subspace. The statement 'we can always squash the unknown state into the subspace of {tau}' is not a theorem for arbitrary POVMs; squashing models are known to exist only for specific detector classes (Ref. [41]). Without a proof that the relevant device admits an incoherent squashing map, the lower bound from Eq. (25) does not apply to the original state. The authors should either provide such a proof for a clearly specified detector class, or explicitly restrict the claims to qubit states in the known subspace and state this as a limitation in the abstract and introduction.","section":"Protocol description and remarks after Eq. (7)"},{"comment":"Equation (7) of the main text is missing the factor e^mu in the second term of the lower-bound expression: it reads p_mu(1|j) nu^2/mu^2, while the correct term, as given in Eq. (28) of the Supplemental Material, is p_mu(1|j) e^mu nu^2/mu^2. As printed, the lower bound is looser and could overestimate p_1(1|j), and hence the coherence bound. Please correct the main-text formula and confirm that the reported experimental bound used the correct expression.","section":"Eq. (7)"},{"comment":"Equation (5) in the main text omits the factor k = ln 2 that appears in the dual problem in Eq. (22) of the Supplemental Material. Without this factor, the objective value is in nats rather than bits, which would make the reported coherence bound and the QRNG rate calculation inconsistent. Please clarify the convention used and align the main text with the supplement.","section":"Eq. (5)"}],"minor_comments":[{"comment":"The paper reports that no coherence is witnessed for the mixed state rho', but the actual numerical lower bound from the optimization is not given; please state the computed value (e.g., a non-positive lower bound) to make the claim quantitative.","section":"Control experiment"},{"comment":"The Supplement states that finite-size effects in randomness quantification are ignored; this should be noted in the main text wherever the 320 kbps rate is quoted, to avoid overstating the rigor of the QRNG demonstration.","section":"Randomness generation section"},{"comment":"It would help to state explicitly that the computational basis is the time-bin basis and that |+i> is the equal-superposition time-bin state, so that the coherence basis is unambiguous throughout the main text.","section":"Experimental setup and Table II"}],"recommendation":"major_revision","confidential_remarks":"The experimental work appears careful and the qubit-subspace version of the protocol is a valid contribution. The main issue is the dimension/squashing assumption, which the authors themselves acknowledge in the summary; the revision should make the scope of the claims precise and fix the equation typos. I would not recommend rejection, as the protocol is useful and the limitations are addressable within the manuscript's scope."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Here's my take on 1908.05022. The paper does something genuinely useful: it adapts measurement-device-independent tomography to certify coherence, using four test states to reconstruct the untrusted POVM and a convex dual to lower-bound relative-entropy coherence. Adding the decoy-state method to tighten the bound with weak coherent sources is a sensible and material improvement. The experiment is careful and the control is convincing: the unknown |+i> state gives a coherence bound of 0.25, the mixed state gives zero, and the phase stabilization is solid. Credit where due—this is a competent piece of work.\n\nThe soft spots are real but not fatal. The biggest is the dimension assumption. The tomography only pins down the POVM on the qubit subspace spanned by the test states. To extend the bound to an arbitrary input, the paper invokes a squashing model and claims we can 'always squash' the unknown state into that subspace. That is too strong: squashing models are established only for specific detector classes, not universally. The paper does list 'a new MDICW scheme without the dimension assumption' as future work, so the honesty is there, but the abstract's claim of removing requirements on the measurement device oversells it. The protocol is measurement-device-independent in the sense of not trusting the POVM, but it is not dimension-independent. The distinction should be made explicit upfront.\n\nSecond, the decoy-state bound in Eq. (7) of the main text is missing an e^mu factor on the p_mu term that appears in the supplement's Eq. (28). This looks like a typo rather than a substantive error—the experiment presumably used the supplement's expression—but the main text should be corrected.\n\nThird, the QRNG rate of 320 kbps and the min-entropy of 6.4e-3 per pulse are quoted without finite-size randomness quantification; the paper acknowledges this. That is acceptable for a demonstration, but not a security claim.\n\nThe citation pattern is fine, mostly building on the authors' own MDI-QRNG work and decoy-state QKD. No red flags.\n\nWho is this for? People working on MDI quantum information tasks, coherence resource theory, and QRNG source certification. It is a reasonable incremental contribution with a real experiment. I would send it to a serious referee; the issues are fixable in revision. I would not desk-reject it.","headline":"A sound and useful MDI coherence-witness scheme with a clean experiment, but the 'no assumptions' claim is qualified by a dimension/squashing assumption the paper itself concedes.","tokens_in":18636,"tokens_out":4664,"would_cite":true,"duration_ms":47603,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"A measurement-device-independent coherence witness certifies and lower-bounds quantum coherence using only one measurement setting, without trusting the measurement device.","keywords":["quantum coherence","coherence witness","measurement-device-independent","decoy-state method","time-bin encoding","quantum random number generation","POVM tomography","relative entropy of coherence"],"falsifier":"Run the same protocol with a detector known to violate the squashing model—for example, one whose efficiency depends on photon number or on which time bin arrives—and compare the certified lower bound with the true relative entropy of coherence obtained from full state tomography of $\\rho$; finding a case where the certified bound exceeds the true coherence would refute the device-independent claim.","tokens_in":17599,"feed_emoji":"⚛️","tokens_out":8074,"duration_ms":78040,"temperature":0.7,"pith_summary":"Conventional coherence witnesses can be fooled: if an adversary rotates the measurement basis, an incoherent state can look coherent. This paper proposes a measurement-device-independent coherence witness (MDICW) that removes all assumptions about the measurement device by first tomographing it with trusted test states and then solving a convex optimization to lower-bound the coherence of the unknown state. The protocol needs only one measurement setting and treats loss and double clicks as zero, making it loss tolerant. The decoy-state method is added to tighten the bound when weak coherent states replace ideal single photons. An experiment with time-bin encoding certifies a coherence lower bound of 0.25 per detected signal state, while a control mixture shows no coherence.","feed_headline":"Untrusted detectors still certify quantum coherence","feed_subtitle":"A decoy-state protocol bounds coherence with one measurement setting and powers a 320 kbps quantum random number generator.","key_machinery":"The central object is the effective qubit POVM $\\{M_0,M_1\\}$ obtained from tomography: $M_1 = a_1(I + n_x\\sigma_x + n_y\\sigma_y + n_z\\sigma_z)$, with $a_1$ and $(n_x,n_y,n_z)$ fixed by the four measured probabilities $p(1|j)$. The coherence bound comes from the dual form of a convex optimization over states, $\\max_\\lambda [-\\|\\sum_i \\Pi_i \\exp(-I-\\lambda M_1)\\Pi_i\\| - \\lambda \\mathrm{tr}(\\rho M_1)]$, which lower-bounds the relative entropy of coherence. The decoy-state inequalities provide upper and lower bounds on the single-photon probabilities $p_1(1|j)$ that feed into this optimization, and a squashing model is invoked to justify treating the detector as a two-outcome qubit POVM.","core_discovery":"The paper claims that coherence can be witnessed and quantified even when the measurement device is completely untrusted and biased. In the proposed MDICW scheme, Alice prepares the four Pauli eigenstates $\\{|0\\rangle,|1\\rangle,|+\\rangle,|+i\\rangle\\}$ to reconstruct the effective two-outcome POVM $\\{M_0,M_1\\}$ of the detector from conditional click probabilities, then computes the largest lower bound on the relative entropy of coherence of the unknown state $\\rho$ that is compatible with the measured probability $\\mathrm{tr}(\\rho M_1)$ and the decoy-state constraints. The main experimental result is a certified lower bound of $0.25$ per detected signal state for an unknown time-bin qubit using only a $Y$-basis measurement, and a control experiment mixing $|+i\\rangle$ and $|-i\\rangle$ yields no coherence, illustrating convexity. The same setup operated as a quantum random number generator produces $320$ kbps of random bits that pass the NIST test suite.","pith_inferences":["Swapping the roles of the trusted and untrusted parties would turn this into a source-independent coherence witness, certifying coherence when the source is suspect but the measurement is trusted.","If the squashing assumption is dropped, the same tomography idea could be extended to higher-dimensional effective POVMs; more test states would be needed, and the bound would likely be looser.","The control experiment suggests a practical test for nonclassicality in noisy mixtures: any mixture that still yields a positive bound must have at least one coherent component with enough coherence to survive mixing.","The same idea might be pushed toward a fully device-independent setting by replacing the trusted test states with an untrusted Bell-state measurement, at the cost of more complexity."],"forward_implications":["Coherence can be certified and lower-bounded without calibrating or trusting the measurement device, so basis misalignment or deliberate bias cannot produce a false positive coherence witness.","The decoy-state method makes the witness practical under channel loss: in the experimental conditions with 13.13 dB loss, the no-decoy version could not quantify coherence at all, while the decoy version gave a positive bound.","The same coherence lower bound converts directly into certified randomness, yielding a measurement-device-independent quantum random number generator with 320 kbps output in the demonstration.","Because the protocol treats loss and double-click events as zero, it remains valid for lossy channels and inefficient detectors.","Because the optimization only requires convexity of the coherence measure, the same witness structure can bound other convex coherence measures, not just relative entropy."],"supporting_citations":[{"why":"defines a valid coherence witness and its relation to robustness of coherence, the quantity the protocol extends.","marker":"[19]"},{"why":"defines the relative entropy measure of coherence used as the objective in the optimization.","marker":"[3]"},{"why":"supplies the four-parameter qubit POVM form used for tomography from the four test states.","marker":"[25]"},{"why":"provides the dual convex optimization that converts the coherence lower-bound problem into a tractable form.","marker":"[26]"},{"why":"gives the decoy-state bounds on single-photon probabilities that tighten the tomography under weak coherent states.","marker":"[28]"},{"why":"is the measurement-device-independent entanglement witness that inspires the overall MDI approach.","marker":"[33]"},{"why":"provides the MDI-QRNG framework and the loss-tolerant treatment of loss and double-click events as zero.","marker":"[36]"},{"why":"supplies the squashing model that justifies restricting the untrusted measurement to an effective qubit POVM.","marker":"[41]"}],"fun_headline_variants":["Coherence certified despite untrusted detectors","Decoy states make coherence witness detector-independent","Bounds coherence with biased, untrusted measurement","Quantum coherence witness works with device flaws"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the unknown state $\\rho$ lies in the same two-dimensional support as the four test states, justified only through a squashing model—a theoretical reduction of the detector to a two-outcome qubit measurement; if the real detector is not squashable, or $\\rho$ has components outside that subspace, the tomography and the coherence bound stop applying.","fun_headline_variants_meta":{"raw":{"variants":["Coherence certified despite untrusted detectors","Decoy states make coherence witness detector-independent","Bounds coherence with biased, untrusted measurement","Quantum coherence witness works with device flaws"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000127,"raw_usage":{"total_tokens":1064,"prompt_tokens":844,"completion_tokens":220,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":460,"completion_tokens_details":{"reasoning_tokens":167}},"tokens_in":460,"tokens_out":220,"duration_ms":3065,"temperature":1.0,"reasoning_tokens":167,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T13:25:17.408589+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Run the same protocol with a detector known to violate the squashing model—for example, one whose efficiency depends on photon number or on which time bin arrives—and compare the certified lower bound with the true relative entropy of coherence obtained from full state tomography of $\\rho$; finding a case where the certified bound exceeds the true coherence would refute the device-independent claim.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"defines the relative entropy measure of coherence used as the objective in the optimization."},{"cited_title":"Ringbauer, T","cited_arxiv_id":null,"evidence_quote":"supplies the four-parameter qubit POVM form used for tomography from the four test states."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"provides the dual convex optimization that converts the coherence lower-bound problem into a tractable form."},{"cited_title":"Zheng, Z","cited_arxiv_id":null,"evidence_quote":"gives the decoy-state bounds on single-photon probabilities that tighten the tomography under weak coherent states."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"is the measurement-device-independent entanglement witness that inspires the overall MDI approach."}],"review_version":1}