arXiv subjects

Shohei Kiryu

Publications and source records attributed to Shohei Kiryu.

7 recordsLinked to original sources

arXiv2026

Modular optical fault-tolerant quantum computation via an interface between verified resources and online logical operations

We introduce a factory--processor interface architecture for modular optical fault-tolerant quantum computation that separates code-dependent encoded-resource preparation from reusable online logical processing on a common Gaussian optical processor. Beyond encoded resources, the interface specifies logical outputs, code-to-mode mapping, measurement roles, frame updates, verification records, and resource-injection boundaries. As a concrete realization, we combine an offline verified Bell-resource factory based on a recursively concatenated $C_4$ code with physical Gottesman--Kitaev--Preskill (GKP) qubits with an online time-domain measurement-based quantum computation (MBQC) processor. Under the specified additive-displacement-noise model and fixed postselection policy, simulations support the finite-level logical Bell-error ordering $P_5<P_4<P_3<P_2$ at 12.25 dB of effective GKP displacement-noise quality, both with and without the prescribed additional post-injection displacement noise. At 15.6 dB and $L=2$, a separate fixed-sample evaluation gives one-sided 95$\%$ Clopper--Pearson upper bounds below $10^{-5}$ for both conditions under the binomial model. The interface supports address-tracked injection, transversal CNOT and logical Bell-measurement constructions, and logical SWAP and Hadamard constructions with tracked code permutations. Given advance resource preparation and sufficient routing parallelism, increasing the concatenation level expands parallel encoded-resource width without increasing the protected online sequence length of these constructions. These results identify a modular route to lower logical error rates while retaining a common Gaussian MBQC operation set with shallow online processing.

quant-ph↗

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.

quant-ph↗

arXiv2026

Constant-rate quantum codes with low-weight stabilizers and full logical Clifford actions via transversal and fold-transversal gates

Low-space-overhead fault-tolerant quantum computation requires not only high-rate quantum error-correcting codes but also space-efficient implementations of logical operations. Transversal and fold-transversal gates are promising since they limit error propagation and require no additional qubits. However, the logical operations they enable are typically restricted, and a central challenge is to construct codes that combine a complete set of logical Clifford gates with favorable code parameters. In this work, we introduce a family of quantum codes with an asymptotically constant encoding rate and sublogarithmically growing stabilizer weight, while supporting the entire logical Clifford group using only transversal and fold-transversal gates. Our construction is based on classical codes whose code spaces are absolutely irreducible Steinberg modules of their Tanner-graph automorphism groups. Taking hypergraph products of these classical codes yields quantum codes for which all logical Clifford operations can be synthesized from a fixed set of transversal and fold-transversal gates. Moreover, the slow growth of the stabilizer weight enables a high error-correcting performance in small instances. These results provide a path toward fault-tolerant quantum computation with low space overhead using transversal and fold-transversal gates.

quant-ph↗

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.

quant-ph↗

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.

quant-ph↗

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.

quant-ph↗

arXiv2025

Linear optical quantum computing with a hybrid squeezed cat code

In recent years, squeezed cat codes with resilience to specific types of loss have been proposed as a step toward realizing fault-tolerant optical quantum computers. However, error correction for squeezed cat codes requires a strong nonlinearity, which makes its implementation challenging with current technology. We propose a novel hybrid code that combines the squeezed cat code and the polarization qubit. First, we propose a generation method and a universal gate set that can be implemented with a linear optical system. Then, we show the superiority of the hybrid squeezed cat code over the hybrid cat code and the squeezed cat code through numerical simulations. These results demonstrate that the hybrid squeezed cat code is a promising candidate as a new resource for optical quantum information processing.

quant-ph↗