arXiv subjects

Atsushi Okamoto

Publications and source records attributed to Atsushi Okamoto.

10 recordsLinked to original sources

arXiv2026

Hong-Ou-Mandel test to verify indistinguishability of the states emitted from a quantum key distribution transmitter implementing decoy Bennett-Brassard 1984 protocol

Quantum Key Distribution (QKD) systems require rigorous verification of device properties to ensure implementation security. A critical requirement is the indistinguishability of transmitted pulses encoded by different modulation patterns, as distinguishability through non-encoded degrees of freedom could enable undetected eavesdropping. We present a practical method for testing pulse indistinguishability in QKD transmitters based on Hong-Ou-Mandel (HOM) interference. We establish the theoretical equivalence between the SWAP test and HOM measurement for characterizing quantum state fidelity, demonstrating that HOM visibility directly relates to the trace of density matrix products for phase-randomized weak coherent pulses. We experimentally validated this approach using a high-speed QKD transmitter implementing the decoy BB84 protocol with time-bin encoding at 1.25 GHz. HOM interference was measured between adjacent pulses prepared in different Bennett-Brassard 1984 states (X0, X1, Y0, Y1) using superconducting nanowire single-photon detectors. The observed HOM visibility was approximately 0.3 across all state combinations, with no statistically significant differences detected. These results confirm that modulation does not compromise pulse indistinguishability in our transmitter. The HOM test provides a practical, quantum-optical method for security certification of QKD systems without requiring assumptions about specific degrees of freedom, using only standard fiber-optic components and single-photon detectors.

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↗

arXiv2022

Deep learning generates custom-made logistic regression models for explaining how breast cancer subtypes are classified

Differentiating the intrinsic subtypes of breast cancer is crucial for deciding the best treatment strategy. Deep learning can predict the subtypes from genetic information more accurately than conventional statistical methods, but to date, deep learning has not been directly utilized to examine which genes are associated with which subtypes. To clarify the mechanisms embedded in the intrinsic subtypes, we developed an explainable deep learning model called a point-wise linear (PWL) model that generates a custom-made logistic regression for each patient. Logistic regression, which is familiar to both physicians and medical informatics researchers, allows us to analyze the importance of the feature variables, and the PWL model harnesses these practical abilities of logistic regression. In this study, we show that analyzing breast cancer subtypes is clinically beneficial for patients and one of the best ways to validate the capability of the PWL model. First, we trained the PWL model with RNA-seq data to predict PAM50 intrinsic subtypes and applied it to the 41/50 genes of PAM50 through the subtype prediction task. Second, we developed a deep enrichment analysis method to reveal the relationships between the PAM50 subtypes and the copy numbers of breast cancer. Our findings showed that the PWL model utilized genes relevant to the cell cycle-related pathways. These preliminary successes in breast cancer subtype analysis demonstrate the potential of our analysis strategy to clarify the mechanisms underlying breast cancer and improve overall clinical outcomes.

cs.LG↗

arXiv2018

Tracking Quantum Error Correction

To implement fault-tolerant quantum computation with continuous variables, the Gottesman--Kitaev--Preskill (GKP) qubit has been recognized as an important technological element. We have proposed a method to reduce the required squeezing level to realize large scale quantum computation with the GKP qubit [Phys. Rev. X. {\bf 8}, 021054 (2018)], harnessing the virtue of analog information in the GKP qubits. In the present work, to reduce the number of qubits required for large scale quantum computation, we propose the tracking quantum error correction, where the logical-qubit level quantum error correction is partially substituted by the single-qubit level quantum error correction. In the proposed method, the analog quantum error correction is utilized to make the performances of the single-qubit level quantum error correction almost identical to those of the logical-qubit level quantum error correction in a practical noise level. The numerical results show that the proposed tracking quantum error correction reduces the number of qubits during a quantum error correction process by the reduction rate $\left\{{2(n-1)\times4^{l-1}-n+1}\right\}/({2n \times 4^{l-1}})$ for $n$-cycles of the quantum error correction process using the Knill's $C_{4}/C_{6}$ code with the concatenation level $l$. Hence, the proposed tracking quantum error correction has great advantage in reducing the required number of physical qubits, and will open a new way to bring up advantage of the GKP qubits in practical quantum computation.

quant-ph↗

arXiv2018

High-threshold fault-tolerant quantum computation with analog quantum error correction

To implement fault-tolerant quantum computation with continuous variables, the Gottesman-Kitaev-Preskill (GKP) qubit has been recognized as an important technological element. However,it is still challenging to experimentally generate the GKP qubit with the required squeezing level, 14.8 dB, of the existing fault-tolerant quantum computation. To reduce this requirement, we propose a high-threshold fault-tolerant quantum computation with GKP qubits using topologically protected measurement-based quantum computation with the surface code. By harnessing analog information contained in the GKP qubits, we apply analog quantum error correction to the surface code.Furthermore, we develop a method to prevent the squeezing level from decreasing during the construction of the large scale cluster states for the topologically protected measurement based quantum computation. We numerically show that the required squeezing level can be relaxed to less than 10 dB, which is within the reach of the current experimental technology. Hence, this work can considerably alleviate this experimental requirement and take a step closer to the realization of large scale quantum computation.

quant-ph↗

arXiv2017

Analog quantum error correction with encoding a qubit into an oscillator

To implement fault-tolerant quantum computation with continuous variables, Gottesman-Kitaev-Preskill (GKP) qubits have been recognized as an important technological element. However, the analog outcome of GKP qubits, which includes beneficial information to improve the error tolerance, has been wasted, because the GKP qubits have been treated as only discrete variables. In this paper, we propose a hybrid quantum error correction approach that combines digital information with the analog information of the GKP qubits using the maximum-likelihood method. As an example, we demonstrate that the three-qubit bit-flip code can correct double errors, whereas the conventional method based on majority voting on the binary measurement outcome can correct only a single error. As another example, a concatenated code known as Knill's C4/C6 code can achieve the hashing bound for the quantum capacity of the Gaussian quantum channel. To the best of our knowledge, this approach is the first attempt to draw both digital and analog information from a single quantum state to improve quantum error correction performance.

quant-ph↗

arXiv2015

Entanglement Generation by Communication using Phase-Squeezed Light with Photon Loss

In order to implement fault-tolerant quantum computation, entanglement generation with low error probability and high success probability is required. We have proposed the use of squeezed coherent light as a probe to generate entanglement between two atoms by communication, and shown that the error probability is reduced well below the threshold of fault-tolerant quantum computation [Phys. Rev. A. {\bf 88}, 022313 (2013)]. Here, we investigate the effect of photon loss mainly due to finite coupling efficiency to the cavity. The error probability with the photon loss is calculated by the beam-splitter model for homodyne measurement on probe light. Optimum condition on the amplitude of probe light to minimize the error probability is examined. It is shown that the phase-squeezed probe light yields lower error probability than coherent-light probe. A fault-tolerant quantum computation algorithm can be implemented under 0.59 dB loss by concatenating five-qubit error correction code.

quant-ph↗

arXiv2014

Evaluation of the phase randomness of the light source in quantum key distribution systems with an attenuated laser

The phase randomized light is one of the key assumptions in the security proof of Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol implemented with an attenuated laser. Though the assumption has been believed to be satisfied for conventional systems, it should be reexamined for current high speed QKD systems. The phase correlation may be induced by the overlap of the optical pulses, the interval of which decreases as the clock frequency. The phase randomness was investigated experimentally by measuring the visibility of interference. An asymmetric Mach-Zehnder interferometer was used to observe the interference between adjacent pulses from a gain-switched distributed feedback laser diode driven at 10 GHz. Low visibility was observed when the minimum drive current was set far below the threshold, while the interference emerged when the minimum drive current was close to the threshold. Theoretical evaluation on the impact of the imperfect phase randomization provides target values for the visibility to guarantee the phase randomness. The experimental and theoretical results show that secure implementation of decoy BB84 protocol is achievable even for the 10-GHz clock frequency, by using the laser diode under proper operating conditions.

quant-ph↗

arXiv2004

Anti-ExB flow field associated with a vortex formation in a partially ionized plasma

A high-density magnetized plasma has been studied for understanding of plasma dynamics in partially ionized plasmas. Ion flow field has been obtained experimentally, and is shown to be associated with a vortex formation. The most remarkable result is that the direction of rotation is opposite to that of the ExB drift. Measurement of neutral density profile reveals that there is a steep density gradient of the neutrals around the vortex, suggesting that the generation of inward momentum of the neutrals due to the density gradient. The momentum is transfered to ion with charge-exchange collision, and cause effective force on the ion. The present experiment shows that this effective force may dominate the ambipolar-electric field and drive the anti-ExB vortical motion of ions.

physics.plasm-ph↗

arXiv2004

Measurement of electron sheath thickness and collection region of electric probe using laser photodetachment signals

A new type of laser photodetachment (LPD) technique has been developed for the measurement of electron sheath thickness around an electrostatic probe and for the measurement of the length of collection region of photodetached electrons (PDE). When a thickness of the sheath formed around an electrostatic probe is thicker than about 0.1 mm, modification of the temporal evolution of the LPD signal is observed. By making use of the modification, we evaluated sheath thickness around a cylindrical probe in the existence of the magnetic field of 15 mT. It was found that the thickness of electron sheath along the magnetic field was comparable to the calculated plane-parallel Child-Langmuir sheath thickness when the probe-bias voltage was high. Furthermore, by inserting a small screening object in the laser beam channel, we can measure the sheath thickness from the modification of LPD signal. From the variation of the signal intensity as scanning the screening object perpendicular to the laser beam channel, we can observe the collection region of photodetached electrons, because this procedure changes the relative displacement between shadow and the probe electrode.

physics.plasm-ph↗