arXiv subjects

Masahiro Takeoka

Publications and source records attributed to Masahiro Takeoka.

At least 19 recordsLinked to original sources

arXiv2026

Device-independent conference key agreement from two-body correlators with balanced Dicke states

Device-independent conference key agreement (DI-CKA) enables $N > 2$ distant parties to establish a common secret key without detailed characterization of the source or measurement devices. Many existing protocols rely on full-body Bell correlators, whose measurement can be demanding for a large number of parties. Here, we show that two-body correlators alone suffice for DI-CKA using balanced Dicke states for every even $N \geq 4$. We construct two-body Bell inequalities and derive a tight analytical lower bound on the conditional von Neumann entropy by comparing their Bell operators with those of asymmetric Clauser-Horne-Shimony-Holt inequalities. In the ideal setting, certified key rates can approach $1-h[(N-2)/(4(N-1))]$, where $h$ is the binary entropy function. This rate remains bounded away from zero as $N \rightarrow \infty$, tending to $1-h(1/4) \approx 0.189$ bits per key-generation round. We show that the honest implementation of our protocol is invariant under collective phase noise. Moreover, a complementary full-body Bell construction certifies positive conference-key rates from correlations arbitrarily close to the local set.

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arXiv2026

Signatures of Narrowband and Ultrashort-Pulse-Like Probing in Coherent Anti-Stokes Raman Scattering Using Twin Beams and Sum-Frequency Generation Readout

We theoretically investigate coherent anti-Stokes Raman scattering (CARS) driven by bright twin beams with sum-frequency generation (SFG) readout. Expressing the SFG output in terms of quantum-field correlation functions yields multiple correlation structures with distinct properties associated with different interaction pathways on the bra and ket sides. Among them, frequency correlations between the CARS probe and SFG gating fields enable the two fields to act collectively as an effective narrowband probe, avoiding probe-induced spectral convolution. When this behavior appears on either the bra or ket side, the other retains Raman relaxation information as if probed by an ultrashort pulse.

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arXiv2026

A Compressive Sensing Inspired Monte-Carlo Method for Combinatorial Optimization

In this paper, we present a Monte-Carlo Compressive Optimization algorithm, a new method for black-box combinatorial optimization, which only assumes that the objective is compressible. The method relies on random queries to the objective function in order to estimate generalized moments. Next, a greedy algorithm from compressive sensing is repurposed to find the global optimum when not overfitting to the samples. We provide numerical results giving evidence that our method is competitive by comparing it with digital annealing. Moreover, we give theoretical justification for the success of the algorithm and analyze its properties. The practicality of our algorithm is enhanced by the ability to tune the heuristic parameters to the available computational resources. An end-to-end open-source implementation is available to use our method.

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arXiv2026

Low-nonlinearity advantage of hollow-core fiber for wavelength multiplexing of quantum entanglement and classical DWDM signals

Coexistence of classical and quantum light in optical fiber is a key requirement for scalable quantum networks. However, it is fundamentally limited by nonlinear crosstalk from intense classical signals. Hollow-core fiber (HCF), with its intrinsically low optical nonlinearity, offers a promising solution. Here, we experimentally demonstrate the coexistence of polarization-entangled photons with dense wavelength-division multiplexed (DWDM) classical signals with a total launch power of 11.2 dBm, and perform a direct comparison between equal-length single-mode fiber (SMF) and photonic-bandgap-based HCF deployed in a campus network. We observe that HCF suppresses background noise by approximately two orders of magnitude compared to SMF, while preserving high-fidelity entanglement distribution. Our results provide clear experimental evidence of the advantage of HCF and establish its potential as a transmission medium.

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arXiv2026

Remote Gottesman-Kitaev-Preskill Entanglement Generation between Trapped Ions

Distributing entanglement directly between error-corrected logical qubits is an important capability for scalable quantum networks. We propose a heralded protocol for generating remote entanglement between Gottesman--Kitaev--Preskill qubits encoded in the motional modes of spatially separated trapped ions, using the internal electronic states to interface the motional qubits with emitted photons. We derive the protocol for single-rail and polarization photonic encodings and evaluate the effects of finite GKP squeezing, photon loss, photon recoil, motional heating, and motional dephasing on the decoded logical state. Moreover, we numerically analysed the fidelity of distributed logical Bell-pair fidelity. We find that one can achieve more than 95% of fidelity in the realistic situation. The protocol combines motional-control and photonic-networking operations that have already been demonstrated in trapped-ion experiments.

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arXiv2026

Improving the Loss Tolerance of Heralded Photonic GHZ States for Long-Distance Device-Independent Conference Key Agreement

Heralded multipartite entanglement distribution is a key requirement for device-independent conference key agreement (DI-CKA) over lossy quantum networks. Although locally equivalent in the absence of loss, different single-rail photon-number encodings of Greenberger--Horne--Zeilinger (GHZ) states respond differently to photon loss. Here, we investigate the critical detection efficiencies for detection-loophole-free parity--CHSH violations of computational-basis GHZ states---a coherent superposition of the vacuum and an $n$-photon component---and of fixed-photon-number GHZ states, deriving exact analytical conditions for both. We show that for states that are not permutation symmetric, such as the latter, the assignment of measurement roles to physical modes affects loss tolerance. We introduce a star-network protocol employing heterogeneous sources to directly herald the loss-tolerant vacuum-$n$-photon GHZ states while retaining the favourable long-distance scaling $O(η_{\text{c}}^{n/2})$, where $η_{\text{c}}$ is the channel transmittance. For four users, we characterize the heralded state under photon loss and show that tunable source parameters allow genuine multipartite entanglement to persist at any finite channel distance. With ideal Pauli and displacement-based measurements, our protocol achieves positive DI-CKA key rates at lower detection efficiencies than previous schemes, while retaining comparable or greater rates and communication distances at high efficiency. Overall, our work improves the loss tolerance of heralded photonic GHZ states for DI-CKA both by directly heralding a more loss-tolerant encoding and by optimizing existing schemes. These results identify photon-number encoding, source architecture, and measurement-role assignment as key design parameters for loss-resilient multipartite quantum networks, offering a practical route toward near-term DI-CKA.

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arXiv2026

Efficient Simulation of Hybrid Continuous- and Discrete-Variable Quantum Circuits via Gaussian Decompositions

Hybrid quantum systems combining discrete-variable (DV) and continuous-variable (CV) subsystems arise across a broad range of physical platforms and enable quantum information processing beyond purely qubit-based designs. Their classical simulation, however, is challenging since computational costs of both CV and DV systems exponentially increase. In this paper, we introduce a simulation framework for CV--DV hybrid systems based on linear combination of Gaussian (LCoG) decomposition. We decompose the hybrid density matrix into blocks in a DV basis and represent each block as an LCoG expansion. Within the representation, Gaussian operations on the bosonic subsystem independently act on each Gaussian function. We derive analytic formulas describing how complex Gaussian functions transform under controlled Gaussian operations. We further extend the framework to controlled Gaussian channels with state-dependent Gaussian noise. The method avoids a Fock-space truncation for the CV subsystem, and the cost of updating each Gaussian term scales polynomially with the number of modes. The framework provides a general computational tool for analyzing hybrid quantum algorithms, quantum simulation protocols, and non-Gaussian state generation schemes. As an example, we simulate Gottesman--Kitaev--Preskill (GKP) state generation based on a cavity-QED system.

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arXiv2026

Symmetry-resolved tree tensor network analysis of Bell-state discrimination with ancilla-assisted passive linear optics

Ancillary photons allow passive linear-optical Bell-state discrimination to exceed the one-half probability of success limit achievable with vacuum auxiliary modes. For a fixed analyzer and ancillary resource, the discrimination probability is determined by the photon-counting patterns that occur uniquely for each Bell input. We develop a photon-number-resolved tree tensor network method for evaluating these detector supports in recursively structured analyzers. Photon-number conservation and mode symmetries separate most of the Bell-state outputs before the remaining detector support is examined. We establish a tree tensor network based construction to evaluate the remaining support without need of recalling all relevant photon detector patterns separately. We apply the method to recursive, product, and asymmetric ancillary states with up to $32$ optical modes and reproduce known analytical and literature benchmarks. For factorized ancillary resources with fixed photon numbers in halves of the analyzer and the symmetry property used in our analysis, we also derive an exact composition relation; for reflected asymmetric pairings the success probability is the arithmetic mean of the corresponding symmetric configurations. The complete enumeration for smaller systems, permanent-based amplitude calculations, and independent evaluations provide additional checks of the numerical results.

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arXiv2026

Privacy in continuous-variable distributed quantum sensing

Can a distributed network of quantum sensors estimate a global parameter while protecting every locally encoded value? We answer this question affirmatively by introducing and analysing a protocol for distributed quantum sensing in the continuous-variable regime. We consider a multipartite network in which an unknown local phase is imprinted at each node on a shared entangled Gaussian state. We show that the average phase can be estimated with high precision, exhibiting Heisenberg scaling in the total photon number, while individual phases are inaccessible. We further prove a no-go theorem showing that, for three or more parties, no finite-energy Gaussian probe can provide complete privacy of the average phase, meaning that all phase combinations orthogonal to the average remain entirely hidden. This identifies the two-party case as an exceptional Gaussian setting that can achieve complete privacy. We further investigate the impact of displacements and optical losses, revealing trade-offs between estimation accuracy and privacy. Finally, we benchmark the protocol against other continuous-variable resource states.

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arXiv2026

Enhancing noise robustness in device-independent conference key agreement with asymmetric parity-CHSH inequalities

Conference key agreement allows multiple remote parties to establish a shared secret key with information-theoretical security. In device-independent conference key agreement, security can be guaranteed with minimal assumptions on the devices used, provided that a violation of a Bell inequality is observed. However, implementations are extremely challenging because high detection efficiency is required to observe loophole-free Bell violations. Here, we enhance the robustness of device-independent conference key agreement by introducing a new family of multipartite Bell inequalities called the asymmetric parity-Clauser-Horne-Shimony-Holt (CHSH) inequalities. We derive a tight analytical lower bound on the conditional von Neumann entropy of the outcomes of one of the parties in a protocol based on this inequality, including noisy preprocessing. Using this bound, we analyze robustness to detection inefficiencies as well as local and global depolarizing noise. We show that the combination of the asymmetric parity-CHSH inequality and noisy preprocessing can significantly improve the robustness to imperfections.

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arXiv2026

Experimental zero-added-loss multiplexing Bell-pair source for long-haul quantum networks

Boosting the communication rate of quantum networks is a central challenge in quantum information science. Recently, an efficient entanglement distribution scheme employing quasi-deterministic Bell-pair sources based on time-frequency multiplexing, referred to as zero-added-loss multiplexing~(ZALM), has been proposed. Its implementation, however, requires high-fidelity entanglement swapping across densely multiplexed time-frequency modes, which has remained an experimental challenge. Here we demonstrate entanglement swapping across 16 parallel frequency modes with a high average fidelity of 93.9$\pm$\SI{1.4}{\%}. Notably, polarization-entangled photon pairs in each frequency mode are spectrally single-mode using only off-the-shelf 50-GHz dense wavelength-division multiplexing~(DWDM) filters, eliminating the need for additional narrowband filtering. Furthermore, in order to fully exploit the temporal degree of freedom, the pump pulse is operated with a repetition frequency of \SI{3.0}{GHz}. By combining the frequency and time multiplexing, the total swapping rate reaches 5.38$\pm$0.17\,\si{pairs\,s^{-1}}, which corresponds to the ZALM Bell-pair rate of \SI{8.2e2}{pairs\,s^{-1}}. Our results establish the key experimental capabilities required for ZALM and demonstrate a scalable route toward practical high-rate quantum repeaters and long-haul quantum networks.

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arXiv2026

Fault-tolerant modular quantum computing with surface codes using single-shot emission-based hardware

Fault-tolerant modular quantum computing requires stabilizer measurements across the modules in a quantum network. For this, entangled states of high quality and rate must be distributed. Currently, two main types of entanglement distribution protocols exist, namely emission-based and scattering-based, each with its own advantages and drawbacks. On the one hand, scattering-based protocols with cavities or waveguides are fast but demand stringent hardware such as high-efficiency integrated circulators or strong waveguide coupling. On the other hand, emission-based platforms are experimentally feasible but so far rely on Bell-pair fusion with extensive use of slow two-qubit memory gates, limiting thresholds to $\approx 0.16\%$. Here, we consider a fully distributed surface code using emission-based entanglement schemes that generate GHZ states in a single shot, i.e., without the need for Bell-pair fusions. We show that our optical setup produces Bell pairs, W states, and GHZ states, enabling both memory-based and optical protocols for distilling high-fidelity GHZ states with significantly improved success rates. Furthermore, we introduce protocols that completely eliminate the need for memory-based two-qubit gates, achieving thresholds of $\approx 0.19\%$ with modest hardware enhancements, increasing to above $\approx 0.24\%$ with photon-number-resolving detectors. These results show the feasibility of emission-based architectures for scalable fault-tolerant operation.

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arXiv2026

Fundamental limits of parameter estimation with heralded optical non-Gaussian states generated from Gaussian resources

Non-Gaussian states can exhibit large quantum Fisher information (QFI) in quantum sensing. In optical systems, however, its generation is often probabilistic via the boson-sampling type conditional operation and thus its generation rate is limited. This probabilistic generation of non-Gaussian resource should be taken into account for evaluation of the sensing performance. Then a natural question arising is whether the use of heralded probabilistic non-Gaussian states is better than that of the original deterministic Gaussian states for quantum sensing. In this paper, we answer to this question for single-parameter phase-estimation. By using photon-number conservation in passive linear optical systems, we show that heralded state preparation before parameter encoding can be mapped to a postselection problem after parameter encoding for phase estimation. This mapping allows the success probability of heralding to be included naturally in the metrological performance. We introduce an effective quantum Fisher information (EQFI), defined as the success-probability-weighted QFI of the heralded outputs, and prove that it cannot exceed the QFI of the original Gaussian inputs. The result highlights the importance of resource counting in quantum sensing toward better understanding of the resource efficient advantage of optical quantum sensing.

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arXiv2026

Information geometric quantification of effective privacy in quantum metrology

Privacy of a quantum metrological protocol concerns the extent to which single parameters can be kept inaccessible to an observer or to other users of the network. In this work, an information geometric framework is developed to quantify privacy and accessibility of functions of parameters effectively, that is, up to a finite accuracy in state discrimination. Both quantities are defined by measuring volumes in the parameter space induced by the underlying quantum states. This construction subsumes previous definitions of privacy based on the degeneracy of quantum Fisher information, naturally encompassing imperfect implementations. Using extended-GHZ states as a representative example of a quantum network scenario, privacy and accessibility are characterized by quantum correlations and accuracy, providing scaling laws depending on imperfect measurements and entanglement.

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arXiv2026

Private quantum network sensing with efficient multi-partite entanglement distribution via lossy channels

Quantum network sensing shows potential to enhance the estimation precision for functions of spatially distributed parameters beyond the shot noise limit. The key resource required for this task is possibly multi-partite quantum entanglement. Such protocols can also provide privacy, preventing sensitive information from leaking to unauthorized parties. The photonic entanglement is the most natural for this task; however, distributing it over long distances presents significant difficulties, mainly because of unavoidable loss in communication channels. The resource efficiency is also fundamental, both for precise network sensing and for private sensing In this research, we analyze a quantum network sensing protocol based on a recently proposed, efficient GHZ state distribution scheme. In comparison to conventional methods based on entanglement distribution, our protocol shows the decreasing loss-induced estimation error of certain functions of distributed parameters including their arbitrary linear combinations. Moreover, we consider a scenario in which one person can estimate linear combinations of distributed parameters without violating privacy of the other users. We show that an additional parameter can be used to hide the information about the target parameter from everyone, except the person who controls the parameter.

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arXiv2026

Linear-optical generation of hybrid GKP entanglement from small-amplitude cat states

Hybrid bosonic codes combining bosonic codes with photon states offer a promising pathway for fault-tolerant quantum computation. However, the efficient generation of such states in optical setups remains technically challenging due to the requirement for complex non-Gaussian resources. In this paper, we propose a novel scheme to efficiently generate hybrid entangled states between a GKP qubit and a photon-number state using small-amplitude cat states as the primary resource. We apply a breeding process using small-amplitude cat states to increase the non-Gaussianity of the input states. This method requires only linear optical elements and homodyne measurements. Furthermore, we demonstrate that this protocol can be extended to generate hybrid qudit states. This scheme has the potential to provide a resource-efficient and experimentally attractive route toward implementing hybrid quantum error correction.

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arXiv2026

Multipartite device-independent quantum key distribution using W states

Multipartite device-independent quantum key distribution (DI-QKD), also known as device-independent conference key agreement, enables more than two remote parties to share a common key with information-theoretic security even without trusting the devices. So far, several multipartite DI-QKD protocols have been proposed where Greenberger-Horne-Zeilinger (GHZ) states are used as multipartite entanglement. A natural question is then whether one can construct multipartite DI-QKD with the other type of multipartite entanglement. W state is of particular interest since it is intrinsically different from GHZ state and in some cases, easier to optically implement. In this paper, we show that multipartite DI-QKD is possible with W states. To this end, we construct Bell inequalities largely violated by W states, which can be used for the multipartite DI-QKD. Furthermore, we consider several different implementation scenarios. First, we analyze the minimum required detection efficiencies to extract finite amount of keys. Then we propose a long-distance multipartite DI-QKD protocol with single-photon interference and make detailed analyses with several physical implementation scenarios. We show that the protocol enables secret key distribution over longer distances than the existing multipartite DI-QKD protocols based on GHZ states. This study provides new insight about the relationship between multipartite entanglement and device-independent quantum information processing as well as opens an alternative path toward long-distance multipartite DI-QKD.

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arXiv2026

Demonstration of sequential processors with quantum advantage and analysis of classical performance limits

In this paper, we theoretically and experimentally analyze sequential processors with limited communication between parts. We compare the expressivity of sequential quantum and classical processors under the same constraints. They consist of three or four modules, each of which processes local data. The modules of the quantum processor are linked through one-qubit or one-qutrit communication, while those of the classical processor communicate through one bit or one trit. For the classical processor, we prove bounds on its performance in terms of inequalities on correlations of the output with a target function. We theoretically show that the quantum processor violates these inequalities. We show this violation experimentally on a silicon photonics setup. We describe how to find the classical bound on correlations with arbitrary target function by reducing the problem to the minimization of an Ising-type spin-glass Hamiltonian. Our theory is applicable in general problems, such as the low-rank binary matrix approximation.

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