{"id":"caaa76f2-afa2-4fee-a75b-5f70ca5c92fe","arxiv_id":"2505.03914","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"The paper casts maximum-likelihood detection for RIS-assisted broadband wireless as a QUBO problem and solves it with Grover adaptive search initialized by an MMSE detector, reporting near-optimal BER at N=3 in simulation.","lead":"This paper combines a known quantum optimization algorithm called Grover adaptive search with a classical equalizer to detect signals in wireless systems aided by reconfigurable intelligent surfaces. It reports near-optimal error rates in small simulations, but the claimed speedup in query complexity is not directly measured.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Query-complexity reduction is asserted but never measured: no oracle query counts, scaling data, or complexity analysis support the 'substantially reducing query complexity' claim.","rationale":"The reader's verdict is CONDITIONAL, and my stress-test supports that. The strongest missing piece is not near-optimal BER (which is plausible from the toy simulations) but the quantitative complexity advantage, which is the headline numerical claim. The paper provides no query counts, no scaling with block length, and no accounting for the oracle's circuit cost. This is a load-bearing gap because if the query count is not actually reduced for realistic N, or if the per-query cost dominates, the central claim of the paper fails. I do not elevate to REJECT because the underlying idea is sound and the missing evidence is addressable by a straightforward measurement. I would keep CONDITIONAL, requiring the authors to provide query-complexity data and an oracle construction. The reader's weakest assumption partially overlaps with mine: they flagged both the oracle implementation and the applicability of GAS's query complexity; I focus on the latter as the more decisive missing analysis.","tokens_in":8605,"tokens_out":5212,"duration_ms":55665,"concrete_test":"Instrument the GAS implementation used for Fig. 2 to count oracle queries (Grover iterations) and total controlled-phase gates for the same channel realizations, for block lengths N = 3, 4, 5, 6 and the R = 4 RIS configuration. Compare the query count and gate count against exhaustive MLD's 2^N cost evaluations. If the query count is not substantially below 2^N (e.g., no clear reduction at N = 6) or the total gate count exceeds classical MLD's multiply-accumulate operations, the complexity claim collapses.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim has two parts: near-optimal BER and substantially reduced query complexity. The BER part is supported by N=3 BPSK simulations (Fig. 2), though only at toy scale. The query-complexity part is not supported by any simulation or analysis in the paper. Section V reports only BER curves; there is no figure, table, or equation giving the number of oracle queries used by GAS with MMSE initialization, nor any scaling study as N grows. The GAS query count from ref [18] applies to integer-valued cost functions and assumes the oracle marks states with E(b) < y_i. Here the cost is real-valued and encoded only approximately via Eq. (22)'s phase rotation (Fejer distribution), while threshold updates use exact classical evaluation. The number of marked states below the MMSE-initialized threshold is unknown; if this set is large, the Grover speedup O(sqrt(2^N/M)) degrades. Without measured query counts, the assertion of 'substantially reducing query complexity' is unverified. Furthermore, the per-query cost includes the omitted oracle circuit, so total complexity could still exceed classical MLD even if the query count appears smaller.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper proposes a hybrid quantum-classical detector for RIS-assisted broadband CPSC systems. It casts the MLD problem as a QUBO, solves it with Grover Adaptive Search (GAS), and uses an MMSE detector to initialize the GAS threshold. The authors extend GAS to real-valued cost functions via direct phase encoding, discard the quantum-evaluated cost in favor of exact classical evaluation, and simulate BER for N=3 BPSK under no-RIS, R=4, and R=8 configurations. The central claim is that the proposed detector achieves near-optimal MLD performance while substantially reducing query complexity relative to classical MLD.","tokens_in":8793,"tokens_out":5651,"duration_ms":56632,"significance":"If the complexity claim were supported, the paper would be a useful step toward practical quantum-assisted detection, particularly the MMSE threshold initialization and the extension of GAS to real-valued QUBO costs. The QUBO reformulation in Section IV-A and the MMSE filter equations in Section IV-C are standard and correct, and the statevector BER simulations at N=3 do show the expected behavior: the hybrid detector approaches MLD and outperforms MMSE. The paper also has the virtue of being explicit about the real-valued encoding issue. However, the paper's headline contribution is two-part, and only the BER half is evidenced; the query-complexity half is asserted without measurement, derivation, or scaling analysis.","major_comments":[{"comment":"The abstract and conclusion claim that the scheme 'substantially reduc[es] query complexity', but the paper reports no oracle-query counts, no Grover-iteration counts, and no scaling in block length N or RIS size R. The quantitative results are exclusively BER curves. Since the central complexity claim is never measured or derived for this system, it is unverified. The authors should either provide query-complexity measurements or a derivation (e.g., expected queries versus N and SNR for the proposed detector, conventional GAS, and classical MLD) or revise the claim.","section":"Section V, Figs. 2-3"},{"comment":"The real-valued direct encoding maps each coefficient to a phase theta = 2*pi*a/2^m and produces a Fejer distribution rather than an exact two's-complement integer. The oracle's sign-bit test therefore operates on an approximate value of E(b)-y_i, and the paper states that the quantum-evaluated cost is discarded in favor of exact classical computation. However, the oracle itself still relies on the approximate encoding to mark states, so it is not established that the oracle marks exactly the states with E(b) < y_i. Consequently, the GAS query-complexity result from [18] does not automatically transfer to this setting; a correctness argument for the oracle's marking condition is needed.","section":"Section IV-B, Eqs. (22)-(23)"},{"comment":"The MMSE-initialized threshold y_0 is claimed to reduce the number of marked states and hence the number of Grover iterations, citing [15]. This is plausible but unquantified: the number of marked states below y_0 is channel- and noise-dependent, and the paper gives no analysis or measurement of this quantity. In addition, the 'conventional GAS' baseline in Fig. 2 is not defined: which threshold initialization, encoding method, and cost-evaluation rule does it use? Without these details, the reported performance gain over vanilla GAS cannot be interpreted.","section":"Section IV-C and Fig. 2 baseline"},{"comment":"The conclusion claims 'practical scalability to realistic broadband scenarios with large block lengths and higher-order modulations', but all simulations use N=3 BPSK, a problem with only 8 candidate vectors. The paper provides no study of how qubit count, circuit depth, or query count scale with N or constellation size, so the scalability claim is unsupported and should either be demonstrated or removed.","section":"Section VI, conclusion"},{"comment":"The footnote states that the circuit construction for the Grover oracle is omitted. Because the oracle's circuit depth and gate count are part of the total computational cost, omitting the oracle construction makes the complexity comparison incomplete. The authors should provide the oracle circuit or at least a complexity model for its per-query cost, otherwise the 'reduced query complexity' claim is not enough to establish a reduction in overall complexity.","section":"Footnote 1"}],"minor_comments":[{"comment":"The MLD complexity is written as O(M N); exhaustive search over the transmitted symbol vector has complexity O(M^N), and the Grover speedup should accordingly be O(sqrt(M^N)). The current notation is misleading.","section":"Section I"},{"comment":"The text attributes Grover Adaptive Search to Gilliam et al. with citation [10], but reference [10] is Bulger et al.; the GAS reference appears later as [18]. Please correct the citation.","section":"Section I and References"},{"comment":"In Step 5-6 the algorithm compares 'y < y_i', but y is not defined in the pseudocode. It should be defined as the objective value of the measured sample b.","section":"Algorithm 1"},{"comment":"The first paragraph states that GAS is designed for integer-valued functions E: B^n -> Z, but Algorithm 1 declares E: B^n -> R and the following subsection discusses real-valued encoding. Please make the domain of the cost function consistent throughout.","section":"Section IV-B"},{"comment":"The effective channel is defined as a superposition of per-element convolutions, but the RIS phase phi_r is a scalar applied to the entire cascaded path; it may be worth clarifying that the per-tap phases are folded into the scalar in the effective channel model.","section":"Section III, Eq. (7)"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is a reasonable systems-level contribution, but the gap between the claimed query-complexity advantage and the evidence is large. The reader of the published version will expect either a complexity measurement/derivation or a softened claim. I would recommend major revision rather than rejection, since the BER component is plausible and the QUBO/MMSE setup is sound; the missing complexity analysis is substantial but fixable within the paper's scope."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Dear colleague,\n\nQuick take: this is a legitimate extension of known quantum search methods to a RIS-assisted broadband detection problem, and the BER part mostly checks out. But the abstract's headline claim — substantially reduced query complexity — is not supported by anything in the paper. That's a real gap, not a nitpick.\n\nWhat's new: the system model, RIS-assisted frequency-selective CPSC with GAS-based MLD, appears to be new. The QUBO derivation in Section IV-A is correct, the MMSE filter in the frequency domain is fine, and the N=3 BPSK simulations plausibly show the MMSE-initialized GAS detector approaching classical MLD and beating MMSE. The paper also honestly cites where the GAS machinery comes from [18] and the MMSE threshold idea from [15]; there is no circularity.\n\nWhere it's soft: (1) The complexity claim. The paper never reports a single query count, nor a scaling study, nor even an analytic bound for the proposed scheme. The GAS query complexity from [18] assumes integer cost functions and a known number of marked states; here the cost is real-valued, encoded approximately via the Fejer distribution, and the threshold is evaluated classically each iteration. That can change the query count, possibly for the worse. The omitted oracle circuit (Footnote 1) means per-query cost is unknown too. This is the core weakness. (2) The simulations are toy-scale: N=3, BPSK. That's fine for a proof-of-concept, but the conclusion's claim of practicality at larger block sizes and higher-order modulation is speculation. (3) The \"conventional GAS\" baseline in Fig. 2 is never specified (random threshold? which m?), so the comparison is hard to interpret. Minor.\n\nThe math is sound and the presentation is clear. The work is incremental but not trivial — applying GAS to a frequency-selective RIS channel is a reasonable next step in this literature. It deserves a serious referee. The referee should ask for either query counts/scaling or a toned-down complexity claim, and a proper baseline setup.\n\nTake it for what it is: a proof-of-concept for quantum-assisted detection in RIS-aided broadband, not a demonstration of a practical advantage. I'd not cite it in my own work right now, but it's worth a reading-group slot.\n\nRecommendation: send to peer review, conditional accept after the complexity claim is addressed.","headline":"A legitimate RIS-aided extension of GAS-based MLD with a solid BER proof-of-concept at N=3, but the central query-complexity reduction is asserted, not shown.","tokens_in":9366,"tokens_out":2315,"would_cite":false,"duration_ms":22230,"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 hybrid quantum-classical detector reaches near-optimal MLD accuracy with reduced query complexity.","keywords":["hybrid quantum-classical detection","maximum likelihood detection","QUBO","Grover adaptive search","reconfigurable intelligent surface","broadband wireless","single-carrier cyclic prefix","MMSE initialization"],"falsifier":"Implement the omitted oracle circuit for a longer block, say $N=8$ BPSK symbols, and count both the controlled-phase gates and the oracle queries to convergence; if the gate count grows exponentially in $N$, or the query count no longer beats classical MMSE/FFT equalization, the query-complexity claim fails.","tokens_in":8371,"feed_emoji":"⚛️","tokens_out":10029,"duration_ms":97149,"temperature":0.7,"pith_summary":"The paper proposes a hybrid quantum-classical detector for reconfigurable-intelligent-surface (RIS) aided broadband single-carrier systems. It rewrites maximum-likelihood detection (MLD) as a quadratic unconstrained binary optimization (QUBO) problem, solves that problem with Grover adaptive search (GAS), and initializes the search threshold with a classical minimum mean-squared error (MMSE) solution. The simulations show the hybrid detector nearly matches optimal MLD bit-error-rate performance at moderate-to-high signal-to-noise ratio and beats classical MMSE, while needing many fewer oracle queries than exhaustive MLD search. If correct, this would make near-optimal detection practical for frequency-selective RIS-aided channels.","feed_headline":"Quantum detector nears MLD accuracy with fewer queries","feed_subtitle":"For RIS-aided broadband links, a hybrid detector casts MLD as QUBO and seeds Grover search with an MMSE solution.","key_machinery":"The central object is Grover adaptive search (GAS), an iterative quantum optimization loop that encodes a binary cost function into quantum phases, amplifies states below a running threshold, and updates the threshold when a better solution appears. For real-valued costs, the paper uses direct phase encoding $\\theta=2\\pi a/2^m$ for each coefficient $a$, followed by the inverse quantum Fourier transform, which yields a Fejér-distributed estimate of the cost; following prior GAS practice, the quantum-evaluated cost is discarded and the exact cost is recomputed classically. The MMSE-initialized threshold is the second load-bearing component: it provides a deterministic, tight starting upper bound on the QUBO objective, reducing the number of marked states and hence the expected number of Grover iterations.","core_discovery":"On its own terms, the paper establishes that MLD for a RIS-aided frequency-selective channel can be cast as a QUBO over binary variables and solved by GAS with real-valued cost coefficients. The proposed detector replaces GAS's random initial threshold with the QUBO cost of the MMSE hard-decision estimate, giving a tight upper bound that limits the number of marked states and cuts the number of Grover iterations. The simulation evidence, for BPSK with block length $N=3$ and RIS sizes $R=0,4,8$, is that the hybrid detector's BER closely approaches classical MLD, particularly above 0 dB SNR, and consistently outperforms MMSE. The paper further claims this is the first quantum-assisted MLD treatment of a frequency-selective, RIS-supported propagation environment.","pith_inferences":["The paper counts oracle queries, not end-to-end cost; a testable extension is a full resource estimate that includes the omitted oracle circuit and the classical exact-cost recomputation performed each iteration.","The MMSE-initialization recipe transfers to other Grover-based detection problems where a cheap classical heuristic supplies a good upper bound, such as sphere decoding or NOMA joint detection.","The Fejér-distribution spread can push the quantum-evaluated cost below the true value, which is why exact costs are recomputed classically; quantifying the extra classical evaluations would show whether a bias-corrected quantum estimate could restore a full speedup.","Simulations at $N=3$ BPSK leave open the large-block regime; a natural next step is to simulate $N=8$ or $N=16$ blocks with explicit qubit and gate counts to test the scalability claim."],"forward_implications":["At moderate-to-high SNR, the hybrid detector's BER closely approaches classical MLD for RIS sizes $R=0,4,8$, making it a practical stand-in for optimal detection in that regime.","Initializing GAS with the MMSE threshold limits the number of marked states and thereby reduces the expected number of Grover iterations relative to a random threshold.","The paper concludes that the approach scales toward realistic broadband scenarios with larger block lengths and higher-order modulations, a claim that remains to be demonstrated beyond $N=3$ BPSK.","If the GAS query-complexity analysis holds with the MMSE threshold, the number of oracle queries needed for detection scales as the square root of the exhaustive-search space, i.e., $\\mathcal{O}(2^{N/2})$ for BPSK blocks, instead of $\\mathcal{O}(2^N)$.","The QUBO formulation applies to linearly modulated RIS-aided channels, which the paper argues includes BPSK and QPSK, so the framework is not tied to a single waveform."],"supporting_citations":[{"why":"supplies the Grover adaptive search framework and the real-valued encoding strategies the detector relies on","marker":"[18]"},{"why":"provides the higher-order GAS MIMO detector whose real-valued handling the paper follows, including discarding the quantum-evaluated cost","marker":"[16]"},{"why":"introduces the MMSE-threshold initialization used here to reduce Grover iterations","marker":"[15]"},{"why":"establishes the quantum-assisted maximum-likelihood multi-user detection line of work this paper extends","marker":"[12]"},{"why":"provides the original Grover search whose quadratic speedup motivates the query-complexity reduction","marker":"[9]"},{"why":"defines the classical exhaustive MLD baseline with exponential complexity that the hybrid detector is compared against","marker":"[4]"},{"why":"defines the classical MMSE detection baseline that the proposed detector consistently outperforms","marker":"[6]"}],"fun_headline_variants":["Quantum search speeds MLD for RIS-aided broadband","Hybrid detector: quantum MLD with fewer queries","Grover search tackles MLD in RIS systems","First quantum-assisted MLD for frequency-selective RIS","MMSE-seeded Grover cuts detection query load"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the real-valued Grover adaptive search oracle, whose circuit construction the paper explicitly omits, can be implemented with controlled-phase rotations at a cost that preserves GAS's query-complexity advantage, and that a three-symbol BPSK simulation is representative of broadband RIS-aided operation.","fun_headline_variants_meta":{"raw":{"variants":["Quantum search speeds MLD for RIS-aided broadband","Hybrid detector: quantum MLD with fewer queries","Grover search tackles MLD in RIS systems","First quantum-assisted MLD for frequency-selective RIS","MMSE-seeded Grover cuts detection query load"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000685,"raw_usage":{"total_tokens":3072,"prompt_tokens":876,"completion_tokens":2196,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":492,"completion_tokens_details":{"reasoning_tokens":2121}},"tokens_in":492,"tokens_out":2196,"duration_ms":16407,"temperature":1.0,"reasoning_tokens":2121,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T23:42:13.009106+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Implement the omitted oracle circuit for a longer block, say $N=8$ BPSK symbols, and count both the controlled-phase gates and the oracle queries to convergence; if the gate count grows exponentially in $N$, or the query count no longer beats classical MMSE/FFT equalization, the query-complexity claim fails.","supporting_citations":[{"cited_title":"Quantum algorithm for higher- order unconstrained binary optimization and MIMO maximum likelihood detection,","cited_arxiv_id":null,"evidence_quote":"provides the higher-order GAS MIMO detector whose real-valued handling the paper follows, including discarding the quantum-evaluated cost"},{"cited_title":"Fixed-complexity quantum-assisted multi-user detection for CDMA and SDMA,","cited_arxiv_id":null,"evidence_quote":"introduces the MMSE-threshold initialization used here to reduce Grover iterations"},{"cited_title":"Quantum search algorithms, quantum wireless, and a low-complexity maximum likelihood iterative quantum multi-user detector design,","cited_arxiv_id":null,"evidence_quote":"establishes the quantum-assisted maximum-likelihood multi-user detection line of work this paper extends"},{"cited_title":"A fast quantum mechanical algorithm for database search,","cited_arxiv_id":null,"evidence_quote":"provides the original Grover search whose quadratic speedup motivates the query-complexity reduction"},{"cited_title":"A reduced-complexity maximum-likelihood method for multiuser detection,","cited_arxiv_id":null,"evidence_quote":"defines the classical exhaustive MLD baseline with exponential complexity that the hybrid detector is compared against"},{"cited_title":"Diversity of MMSE MIMO receivers,","cited_arxiv_id":null,"evidence_quote":"defines the classical MMSE detection baseline that the proposed detector consistently outperforms"}],"review_version":1}