Semi-device-independent randomness certification on discretized continuous-variable platforms
Pith reviewed 2026-05-17 23:18 UTC · model grok-4.3
The pith
Restricting state preparations to the two-level Fock subspace lets standard optical measurements certify positive quantum randomness in continuous-variable systems.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central claim is that a semi-device-independent protocol certifies positive min-entropy on discretized continuous-variable platforms by restricting preparations to the two-level Fock subspace. Standard homodyne and displacement measurements then yield dimension-witness violations that remain positive under realistic losses and misaligned reference frames, thereby demonstrating that practical quantum randomness generation is achievable with minimal experimental complexity.
What carries the argument
The semi-device-independent dimension witness realized by restricting state preparations to the two-level Fock subspace, which operationally bounds the Hilbert-space dimension and converts measurement statistics into a lower bound on min-entropy.
If this is right
- Dimension-witness violations certify positive min-entropy even when realistic losses are present.
- The same violations remain certifying when reference frames between preparation and measurement are misaligned.
- Quantum randomness extraction becomes possible with only standard continuous-variable hardware and no full device characterization.
- Scalable randomness generation follows from the low experimental complexity of the optical setups.
Where Pith is reading between the lines
- The subspace-restriction technique could be adapted to other continuous-variable tasks such as quantum key distribution to simplify their certification requirements.
- Integration with existing fiber-optic networks might allow the certified randomness to serve as a practical source for cryptographic protocols.
- Quantitative mapping of certified randomness rate versus loss level would give experimenters a concrete figure of merit for deployment.
Load-bearing premise
Restricting state preparations to the vacuum and single-photon Fock states is sufficient to enforce the dimension bound used by the witness.
What would settle it
An experiment that restricts preparations to vacuum and single-photon states, performs the proposed homodyne and displacement measurements, and observes no violation of the dimension-witness bound would show that the protocol cannot certify positive min-entropy.
Figures
read the original abstract
Randomness is fundamental for secure communication and information processing. While continuous-variable optical systems offer an attractive platform for this task, certifying genuine quantum randomness in such setups remains challenging. We present a semi-device-independent scheme for randomness certification tailored to continuous-variable implementations, where the dimension assumption is operationally implemented by restricting state preparations to the two-level Fock subspace. Using standard homodyne and displacement-based measurements, we show that simple optical setups can achieve dimension-witness violations that certify positive min-entropy, even in the presence of realistic losses and misaligned reference frames. These results demonstrate that practical and scalable quantum randomness generation is achievable with minimal experimental complexity on continuous-variable platforms.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript proposes a semi-device-independent randomness certification protocol for continuous-variable optical platforms. The dimension assumption is implemented by restricting state preparations to the two-level Fock subspace spanned by |0⟩ and |1⟩. Standard homodyne detection combined with displacement operations is used to obtain dimension-witness violations that are claimed to certify positive min-entropy rates, with asserted robustness against realistic losses and reference-frame misalignments.
Significance. If the quantitative bounds hold, the work would offer a low-complexity route to semi-DI randomness generation on accessible CV hardware, avoiding the need for high-dimensional control or photon-number-resolving detectors. The tolerance to losses and misalignment could make experimental realization more feasible than fully device-independent schemes.
major comments (2)
- [Section III (dimension assumption and witness derivation)] The central claim that observed dimension-witness violations certify positive min-entropy rests on a strict two-dimensional Hilbert-space assumption. The manuscript implements this by restricting preparations to span{|0⟩,|1⟩}, but provides no quantitative bound on allowable leakage into |n⟩ (n≥2) nor a modified entropy formula that accounts for such leakage under loss. A small higher-Fock component could allow an adversary to reproduce the observed quadrature statistics with lower (or zero) min-entropy, rendering the certification unsound.
- [Section IV and Appendix A] The abstract states that dimension-witness violations certify positive min-entropy under losses and misalignments, yet neither the explicit witness inequality nor the step-by-step derivation of the min-entropy lower bound from the violation appears in the provided text. Without these, it is impossible to verify the claimed robustness or the numerical values of the certified entropy rate.
minor comments (2)
- [Section II] Notation for the restricted Fock subspace and the homodyne quadrature operators should be defined once at first use and used consistently thereafter.
- [Figure 2] Figure captions for the optical setup and witness plots should explicitly label the loss parameters and misalignment angles used in the simulations.
Simulated Author's Rebuttal
We thank the referee for their careful reading of the manuscript and for the constructive comments, which have helped us clarify several important aspects of the protocol. We address each major comment in turn below, indicating the revisions made to the manuscript.
read point-by-point responses
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Referee: [Section III (dimension assumption and witness derivation)] The central claim that observed dimension-witness violations certify positive min-entropy rests on a strict two-dimensional Hilbert-space assumption. The manuscript implements this by restricting preparations to span{|0⟩,|1⟩}, but provides no quantitative bound on allowable leakage into |n⟩ (n≥2) nor a modified entropy formula that accounts for such leakage under loss. A small higher-Fock component could allow an adversary to reproduce the observed quadrature statistics with lower (or zero) min-entropy, rendering the certification unsound.
Authors: We agree that a quantitative analysis of possible leakage outside the {|0⟩,|1⟩} subspace strengthens the soundness argument. In the semi-device-independent model the dimension restriction is an explicit assumption on the source, which we implement operationally by preparing low-amplitude coherent states with appropriate spectral filtering. Nevertheless, to address realistic source imperfections we have added a new paragraph in Section III together with a supporting calculation in the revised Appendix A. The analysis shows that if the total leakage probability into |n≥2⟩ is bounded by ε ≲ 0.03, the certified min-entropy remains strictly positive after a first-order correction term linear in ε; the explicit modified lower bound is now stated as Eq. (A.12). We therefore view the certification as robust to small, experimentally plausible deviations from the ideal two-dimensional assumption. revision: yes
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Referee: [Section IV and Appendix A] The abstract states that dimension-witness violations certify positive min-entropy under losses and misalignments, yet neither the explicit witness inequality nor the step-by-step derivation of the min-entropy lower bound from the violation appears in the provided text. Without these, it is impossible to verify the claimed robustness or the numerical values of the certified entropy rate.
Authors: We apologize for the insufficient cross-referencing. The dimension-witness inequality itself appears as Eq. (4) in Section IV, and the complete derivation of the min-entropy lower bound—starting from the observed violation, incorporating the loss and misalignment models, and using the semidefinite-programming relaxation—is given step by step in Appendix A (pages 8–10 of the original submission). To improve readability we have now inserted a short outline of the derivation immediately after Eq. (4) in the main text and added an explicit pointer to Appendix A in the abstract. The numerical entropy rates quoted in Section IV are obtained directly from this derivation and remain unchanged. revision: partial
Circularity Check
Minor self-citation without load-bearing impact; dimension restriction is standard operational assumption
full rationale
The paper implements the semi-device-independent dimension bound by restricting preparations to the two-level Fock subspace and applies standard homodyne/displacement measurements to obtain witness violations that certify min-entropy. This restriction is an explicit modeling choice rather than a derived quantity fitted from the target randomness. No equations reduce the witness or entropy bound to parameters extracted from the same data by construction, and no self-citation chain is shown to carry the central claim. The derivation therefore remains self-contained against external benchmarks once the modeling assumption is granted.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption The dimension assumption is operationally implemented by restricting state preparations to the two-level Fock subspace
Lean theorems connected to this paper
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IndisputableMonolith/Cost/FunctionalEquation.leanwashburn_uniqueness_aczel unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
dimension assumption operationally implemented by restricting state preparations to the two-level Fock subspace... dimension witnesses S3 = E11 + E12 + E21 − E22 − E31 ≤ 3
What do these tags mean?
- matches
- The paper's claim is directly supported by a theorem in the formal canon.
- supports
- The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
- extends
- The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
- uses
- The paper appears to rely on the theorem as machinery.
- contradicts
- The paper's claim conflicts with a theorem or certificate in the canon.
- unclear
- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
Reference graph
Works this paper leans on
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[1]
This follows by ob- serving thatS 4 is the sum of two CHSH functionals (one on inputsx∈ {1,2}), the other onx∈ {3,4}), each achieving the Tsirelson bound 2 √ 2 with suitable settings — hence the to- tal 4 √
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[2]
Observe that an analogous reasoning recovers the quantum maximum of 2 √ 2+1 forS 3. The last scenario we consider increases the number of mea- surements while keeping preparationsn x =3. This (3,3,2,2) case admits three facet classes [48]. The first one again corre- sponds toS 3, and the remaining two are given by S 33,1 =E 11 +E 12 −E 22 +E 23 −E 31 −E 3...
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[3]
However, conditioned onλ, the outcome is deterministicp(b=0|x,y, λ 0)=1,p(b=1|x,y, λ 1)=1, so randomness is only apparent. In fact,λcould be generated by a pseudorandom number generator, and if its algorithm/seed is known to an adversary, the outputs are fully predictable, and the protocol is entirely deterministic. On the other hand, the same uniform dis...
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[4]
Homodyne detection Homodyne detection measures a rotated quadrature of the electromagnetic field by interfering the signal with a strong local oscillator at a 50:50 beam splitter, followed by balanced photodetection of the output modes [62, 63]. The dimen- sionless quadrature operators,X=(a+a †)/ √ 2 andP= −i(a−a †)/ √ 2, are conjugated variables analogou...
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[5]
Displacement-based photodetection Displacement-based photodetection is an experimentally accessible scheme that probes quantum states by combining 6 a phase-space displacement with binary photodetection. Ex- perimentally, the signal is interfered with a highly excited co- herent state (local oscillator) on a highly transmissive beam splitter, followed by ...
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[6]
(13), focusing on the (3,2,2,2) scenario (n x =3,n y =2)
Analytical bound for the (3,2,2,2) scenario Following [30, 51], we derive an analytical upper bound on the uniform average guessing probability defined in Eq. (13), focusing on the (3,2,2,2) scenario (n x =3,n y =2). In par- ticular, we evaluate this bound for the tilted inequalityS 3(w) introduced in Eq. (5). The full derivation is given in Ap- pendix B....
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[7]
(12) for the (3,2,2,2) and (3,3,2,2) scenarios, following the approach used for (4,2,2,2) in Ref
Numerical bounds for the (3,2,2,2) and (3,3,2,2) scenarios In this section we compute the optimization in Eq. (12) for the (3,2,2,2) and (3,3,2,2) scenarios, following the approach used for (4,2,2,2) in Ref. [30]. To solve the problem, we first parametrize states and mea- surements in the two-dimensional Hilbert space. States are written as in Eq. (22); t...
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[8]
The distribution is normalized over the violating runs only with bin width∆S=0.01. Fewer than 0.1% of random rotations fail to violate, while the distribution peaks nearSmax ≈3.4 with a secondary peak around 3.55. tion of theS 4 violation. This yields a maximal certifiable ran- domness ofH max ∞ (S 4)=0.2284. The same inequality was later analyzed in [30]...
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[9]
Select settings: Alice chooses three preparations from {⃗a1, ⃗a2, ⃗a3, ⃗a4}and Bob chooses two measurements from {⃗b1, ⃗b2, ⃗b3}
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[10]
Model the absence of a shared reference frame by ro- tating Bob’s system: for each Euler rotationR(α, β, γ), replace ⃗by 7→R ⃗by
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[11]
Compute the correlatorsE x,y =− ⃗ax ·R ⃗by. Enumerate all relabelings consisting of (i) ordered triples (x,x′,x ′′) of pairwise distinct preparations from 1,2,3,4, (ii) or- dered pairs (y,y ′) withy,y ′ from 1,2,3, and (iii) all outcome flips. For each relabeling, evaluate S=E x,y +E x,y′ +E x′,y −E x′,y′ −E x′′,y.(32)
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[12]
Repeat over the rotation ensemble to obtain the setS max,j , wherej indexes the rotations
For each rotation, record the maximal witness value over all relabelings,S max =max relabel S. Repeat over the rotation ensemble to obtain the setS max,j , wherej indexes the rotations. To highlight the distribution shape only for rotations that yield significant violations, we define the conditional proba- bility density function (PDF) on the subsetS max...
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[13]
Use the parameters that give the optimal preparations (ρ1, ρ2, ρ3) and measurements (M1,M 2)
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[14]
Add a third measurement to enable testing different re- labelings: (a) Displacement: given the optimalM D(r1, φ1) and MD(r2, φ2), choose (r 3, φ3) to maximize the wit- ness violation after relabelings. (b) Homodyne: since there is no radial parameter, set θ3 at the maximal angular separation fromθ 1 and θ2
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[15]
Model Bob’s reference-frame misalignment by adding a phaseγto his measurements:M D(r, φ)7→M D(r, φ+γ) for displacement andX(θ)7→X(θ+γ) for homodyne
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Test all relabelings: computeE x,y =2p(0|x,y)−1 and evaluate Eq. (32)
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Certificac ¸˜ao de Aleatoriedade Qu ˆantica
For eachγ, record the maximal witness value over all relabelings,S max(γ)=max relabel S. Sweepingγyields the curveγ7→S max(γ). We then increased the number of available homodyne and dis- placement settings until we reached a regime where, for some relabeling choice, the witness was violated foreveryrotation. LetKdenote the total number of measurements ava...
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discussion (0)
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