arXiv subjects

Shigeki Takeuchi

Publications and source records attributed to Shigeki Takeuchi.

At least 19 recordsLinked to original sources

arXiv2026

Infrared photonics for healthcare: A roadmap for proactive and predictive health management

The field of infrared (IR) photonics is currently undergoing remarkable progress, moving rapidly towards practical sensing applications demanded by medical therapy and diagnostics (theranostics). The Developments can be divided into three main categories: (i) novel devices and measurement concepts including advanced updates of classical approaches that push medical sensing into the spotlight; (ii) new demonstrations of photonic integrated circuit (PIC-)based IR devices enabling highly miniaturized sensors for point-of-care application as well as medical and wellness wearables; and (iii) technologically-mature IR demonstrators that enable first medical sensing and treatment applications. This roadmap paper provides a consolidated overview of this highly dynamic and interdisciplinary research field with a focus on the major roadblocks that limit the widespread adoption of IR photonics in large-scale medical diagnostics. Special attention is given to the ambivalence between the molecular-level spectroscopic interpretation and a broader health-state assessment, highlighting the need for a common framework. Additionally, the paper discusses the critical importance of unified measurement standards, calibration protocols, and medical certification processes to ensure the validity of experimental results, reproducibility, and clinical trust, particularly when novel experimental techniques and AI algorithms are involved. Perspectives from major past and current contributors to application-oriented IR photonics will be provided.

physics.app-ph↗

arXiv2025

Bright Single-Photon Emission from Individual Tin-Vacancy Centers in Multi-Cone Diamond Waveguides

Diamonds containing color centers have recently gathered significant attention for photonic quantum technologies, including quantum sensing, photonic quantum computers, and quantum networks. Among the various color centers, tin-vacancy (SnV) centers are particularly promising due to the high emission efficiency from the zero-phonon line and due to their long spin coherence times. However, the extraction of photons from diamond remains a key challenge. Here we demonstrate high photon extraction from a single SnV center incorporated in a diamond nanopillar with tapered sidewalls and a multi-cone structure. A sharp emission peak with a full width at half maximum (FWHM) of $6\,$nm was observed at a wavelength of $619\,$nm. Furthermore, the second-order correlation function exhibited an antibunching dip well below $g^{(2)}(0) = 0.5$, indicating single-photon emission. Remarkably, the emitter achieved a high saturation count rate of approximately $9\,$Mcps. These results establish our nanopillar platform as a promising candidate for bright and stable quantum sources and sensors based on SnV centers in diamond.

quant-ph↗

arXiv2021

The Anomalous Formation of Irradiation Induced Nitrogen-Vacancy Centers in 5-Nanometer-Sized Detonation Nanodiamonds

Nanodiamonds containing negatively charged nitrogen-vacancy (NV$^-$) centers are versatile room-temperature quantum sensors in a growing field of research. Yet, knowledge regarding the NV-formation mechanism in very small particles is still limited. This study focuses on the formation of the smallest NV$^-$-containing diamonds, 5 nm detonation nanodiamonds (DNDs). As a reliable method to quantify NV$^-$ centers in nanodiamonds, half-field signals in electron paramagnetic resonance (EPR) spectroscopy are recorded. By comparing the NV$^-$ concentration with a series of nanodiamonds from high-pressure high-temperature (HPHT) synthesis (10 - 100 nm), it is shown that the formation process in 5 nm DNDs is unique in several aspects. NV$^-$ centers in DNDs are already formed at the stage of electron irradiation, without the need for high-temperature annealing. The effect is explained in terms of "self-annealing", where size and type dependent effects enable vacancy migration close to room temperature. Although our experiments show that NV$^-$ concentration generally increases with particle size, remarkably, the NV$^-$ concentration in 5 nm DNDs surpasses that of 20 nm-sized nanodiamonds. Using Monte Carlo simulations, we show that the ten times higher substitutional nitrogen concentration in DNDs compensates the vacancy loss induced by the large relative particle surface. Upon electron irradiation at a fluence of $1.5 \times 10 ^{19}$ e$^-$/cm$^2$, DNDs show a 12.5-fold increment in the NV$^-$ concentration with no sign of saturation. These findings can be of interest for the creation of defects in other very small semiconductor nanoparticles beyond NV-nanodiamonds as quantum sensors.

cond-mat.mtrl-sci↗

arXiv2021

Quantum Fourier-transform infrared spectroscopy in the fingerprint region

Harnessing the quantum interference of photon-pair generation processes, infrared quantum absorption spectroscopy (IRQAS) can extract the infrared optical properties of a sample through visible or near-infrared photon detection without the need for an infrared optical source or detector, which has been an obstacle for higher sensitivity and spectrometer miniaturization. However, experimental demonstrations have been limited to wavelengths shorter than 5 um or in the terahertz region, and have not been realized in the so-called fingerprint region of 1500- 500 cm^-1 (6.6 to 20 um), which is commonly used to identify chemical compounds or molecules. Here we report the experimental demonstration of quantum Fourier transform infrared (QFTIR) spectroscopy in the fingerprint region, by which both absorption and phase spectra (complex spectra) can be obtained from Fourier transformed quantum interferograms obtained with a single pixel visible-light detector. As demonstrations, we obtained the transmittance spectrum of a silicon wafer at around 10 um (1000 cm^-1) and complex transmittance spectrum of a synthetic fluoropolymer sheet, polytetrafluoroethylene, in the wavelength range of 8 to 10.5 um (1250 to 950 cm^-1), where absorption due to symmetric and asymmetric stretching modes of C-F bonds is clearly observed. The signal-to-noise ratio per unit spectral width and unit probe light intensity of our QFTIR spectroscopy method outperforms conventional FTIR spectroscopy by a factor of 10^2. These results open the way for new forms of spectroscopic devices based on quantum technologies.

physics.optics↗

arXiv2021

Frequency correlated photon generation at telecom band using silicon nitride ring cavities

Frequency entangled photon sources are in high demand in a variety of optical quantum technologies, including quantum key distribution, cluster state quantum computation and quantum metrology. In the recent decade, chip-scale entangled photon sources have been developed using silicon platforms, offering robustness, large scalability and CMOS technology compatibility. Here, we report the generation of frequency correlated photon pairs using a 150-GHz silicon nitride ring cavity. First, the device is characterized for studying the phase matching condition during spontaneous four-wave mixing. Next, we evaluate the joint spectrum intensity of the generated photons and confirm the photon pair generation in a total of42 correlated frequency mode pairs, corresponding to a bandwidth of 51.25 nm. Finally, the experimental results are analyzed and the joint spectral intensity is quantified in terms of the phase matching condition.

quant-ph↗

arXiv2019

Direct optical excitation of an NV center via a nanofiber Bragg-cavity: A theoretical simulation

Direct optical excitation of a nitrogen-vacancy (NV) center in nanodiamond by light via a nanofiber is of interest for all-fiber-integrated quantum applications. However, the background light induced by the excitation light via the nanofiber is problematic as it has the same optical wavelength as the emission light from the NV center. In this letter, we propose using a nanofiber Bragg cavity to address this problem. We numerically simulate and estimate the electric field of a nanodiamond induced by excitation light applied from an objective lens on a confocal microscope system, a nanofiber, and nanofiber Bragg-cavities (NFBCs). We estimate that by using a nanofiber, the optical excitation intensity can be decreased by roughly a factor of 10 compared to using an objective lens, while for an NFBC with a grating number of 240 (120 for one side) on a nanofiber the optical excitation intensity can be significantly decreased by roughly a factor of 100. Therefore, it is expected that the background light inside a nanofiber can be significantly suppressed.

quant-ph↗

arXiv2017

Serial-parallel conversion for single photons with heralding signals

We present serial-parallel conversion for a heralded single photon source (heralded SPS). We theoretically show that with the heralding signal, the serial-parallel converter can route a stream of n photons to n different spatial modes more efficiently than is the case without using a heralding signal. We also experimentally demonstrate serial-parallel conversion for two photons generated from a heralded SPS. We achieve a conversion efficiency of 0.533 \pm 0.003, which exceeds the maximum achievable efficiency of 0.5 for serial-parallel conversion using unheralded photons, and is double the efficiency (0.25) for that using beamsplitters. When the losses in the optical converter are corrected for, the efficiency of the current setup can be increased up to 0.996 \pm 0.006.

quant-ph↗

arXiv2017

Consistent Nonparametric Different-Feature Selection via the Sparsest $k$-Subgraph Problem

Two-sample feature selection is the problem of finding features that describe a difference between two probability distributions, which is a ubiquitous problem in both scientific and engineering studies. However, existing methods have limited applicability because of their restrictive assumptions on data distributoins or computational difficulty. In this paper, we resolve these difficulties by formulating the problem as a sparsest $k$-subgraph problem. The proposed method is nonparametric and does not assume any specific parametric models on the data distributions. We show that the proposed method is computationally efficient and does not require any extra computation for model selection. Moreover, we prove that the proposed method provides a consistent estimator of features under mild conditions. Our experimental results show that the proposed method outperforms the current method with regard to both accuracy and computation time.

stat.ML↗

arXiv2017

Nonlocal Position Changes of a Photon Revealed by Quantum Routers

Since its publication, Aharonov and Vaidman's three-box paradox has undergone three major advances: i). A non-counterfactual scheme by the same authors in 2003 with strong rather than weak measurements for verifying the particle's subtle presence in two boxes. ii) A realization of the latter by Okamoto and Takeuchi in 2016. iii) A dynamic version by Aharonov et al. in 2017, with disappearance and reappearance of the particle. We now combine these advances together. Using photonic quantum routers the particle acts like a quantum "shutter." It is initially split between Boxes A, B and C, the latter located far away from the former two. The shutter particle's whereabouts can then be followed by a probe photon, split in both space and time and reflected by the shutter in its varying locations. Measuring the former is expected to reveal the following time-evolution: The shutter particle was, with certainty, in boxes A+C at t1, then only in C at t2, and finally in B+C at t3. Another branch of the split probe photon can show that boxes A+B were empty at t2. A Bell-like theorem applied to this experiment challenges any alternative interpretation that avoids disappearance-reappearance in favor of local hidden variables.

quant-ph↗

arXiv2017

Implementation of a quantum controlled-SWAP gate with photonic circuits

Quantum information science addresses how the processing and transmission of information are affected by uniquely quantum mechanical phenomena. Combination of two-qubit gates has been used to realize quantum circuits, however, scalability is becoming a critical problem. The use of three-qubit gates may simplify the structure of quantum circuits dramatically. Among them, the controlled-SWAP (Fredkin) gates are essential since they can be directly applied to important protocols, e.g., error correction, fingerprinting, and optimal cloning. Here we report a realization of the Fredkin gate for photonic qubits. We achieve a fidelity of 0.85 in the computational basis and an output state fidelity of 0.81 for a 3-photon Greenberger-Horne-Zeilinger state. The estimated process fidelity of 0.77 indicates that our Fredkin gate can be applied to various quantum tasks.

quant-ph↗

arXiv2017

Coupling quantum emitters in 2D materials with tapered fibers

Realization of integrated photonic circuits on a single chip requires controlled manipulation and integration of solid-state quantum emitters with nanophotonic components. Previous works focused on emitters embedded in a three-dimensional crystals -- such as nanodiamonds or quantum dots. In contrast, in this work we demonstrate coupling of a single emitter in a two-dimensional (2D) material, namely hexagonal boron nitride (hBN), with a tapered optical fiber and find a collection efficiency of the system is found to be 10~\%. Furthermore, due to the single dipole character of the emitter, we were able to analyse the angular emission pattern of the coupled system via back focal plane imaging. The good coupling efficiency to the tapered fiber even allows excitation and detection in a fully fiber coupled way yielding a true integrated system. Our results provide evidence of the feasibility to efficiently integrate quantum emitters in 2D materials with photonic structures.

physics.optics↗

arXiv2016

Non-linear excitation of quantum emitters in two-dimensional hexagonal boron nitride

Two-photon absorption is an important non-linear process employed for high resolution bio-imaging and non-linear optics. In this work we realize two-photon excitation of a quantum emitter embedded in a two-dimensional material. We examine defects in hexagonal boron nitride and show that the emitters exhibit similar spectral and quantum properties under one-photon and two-photon excitation. Furthermore, our findings are important to deploy two-dimensional hexagonal boron nitride for quantum non-linear photonic applications.

physics.optics↗

arXiv2016

Ultrathin fiber-taper coupling with nitrogen vacancy centers in nanodiamonds at cryogenic temperatures

We demonstrate cooling of ultrathin fiber tapers coupled with nitrogen vacancy (NV) centers in nanodiamonds to cryogenic temperatures. Nanodiamonds containing multiple NV centers are deposited on the subwavelength 480-nm-diameter nanofiber region of fiber tapers. The fiber tapers are successfully cooled to 9 K using our home-built mounting holder and an optimized cooling speed. The fluorescence from the nanodiamond NV centers is efficiently channeled into a single guided mode and shows characteristic sharp zero-phonon lines of both neutral and negatively charged NV centers. The present nanofiber/nanodiamond hybrid systems at cryogenic temperatures can be used as NV-based quantum information devices and for highly sensitive nanoscale magnetometry in a cryogenic environment.

quant-ph↗

arXiv2016

0.54 μm resolution two-photon interference with dispersion cancellation for quantum optical coherence tomography

Quantum information technologies harness the intrinsic nature of quantum theory to beat the limitations of the classical methods for information processing and communication. Recently, the application of quantum features to metrology has attracted much attention. Quantum optical coherence tomography (QOCT), which utilizes two-photon interference between entangled photon pairs, is a promising approach to overcome the problem with optical coherence tomography (OCT): As the resolution of OCT becomes higher, degradation of the resolution due to dispersion within the medium becomes more critical. Here we report on the realization of 0.54 $μ$m resolution two-photon interference, which surpasses the current record resolution 0.75 $μ$m of low-coherence interference for OCT. In addition, the resolution for QOCT showed almost no change against the dispersion of a 1 mm thickness of water inserted in the optical path, whereas the resolution for OCT dramatically degrades. For this experiment, a highly-efficient chirped quasi-phase-matched lithium tantalate device was developed using a novel $`$nano-electrode-poling$'$ technique. The results presented here represent a breakthrough for the realization of quantum protocols, including QOCT, quantum clock synchronization, and more. Our work will open up possibilities for medical and biological applications.

quant-ph↗

arXiv2015

Highly Efficient Coupling of Nanolight Emitters to a Ultra-wide Tunable Nanofibre Cavity

Solid-state microcavities combining ultra-small mode volume, wide-range resonance frequency tuning, as well as lossless coupling to a single mode fibre are integral tools for nanophotonics and quantum networks. We developed an integrated system providing all of these three indispensable properties. It consists of a nanofibre Bragg cavity (NFBC) with the mode volume of under 1 micro cubic meter and repeatable tuning capability over more than 20 nm at visible wavelengths. In order to demonstrate quantum light-matter interaction, we establish coupling of quantum dots to our tunable NFBC and achieve an emission enhancement by a factor of 2.7.

physics.optics↗

arXiv2014

An Entanglement-Enhanced Microscope

Among the applications of optical phase measurement, the differential interference contrast microscope is widely used for the evaluation of opaque materials or biological tissues. However, the signal to noise ratio for a given light intensity is limited by the standard quantum limit (SQL), which is critical for the measurements where the probe light intensity is limited to avoid damaging the sample. The SQL can only be beaten by using {\it N} quantum correlated particles, with an improvement factor of $\sqrt{N}$. Here we report the first demonstration of an entanglement-enhanced microscope, which is a confocal-type differential interference contrast microscope where an entangled photon pair ({\it N}=2) source is used for illumination. An image of a Q shape carved in relief on the glass surface is obtained with better visibility than with a classical light source. The signal to noise ratio is 1.35$\pm$0.12 times better than that limited by the SQL.

quant-ph↗

arXiv2014

Anomaly detection in reconstructed quantum states using a machine-learning technique

The accurate detection of small deviations in given density matrices is important for quantum information processing. Here we propose a new method based on the concept of data mining. We demonstrate that the proposed method can more accurately detect small erroneous deviations in reconstructed density matrices, which contain intrinsic fluctuations due to the limited number of samples, than a naive method of checking the trace distance from the average of the given density matrices. This method has the potential to be a key tool in broad areas of physics where the detection of small deviations of quantum states reconstructed using a limited number of samples are essential.

quant-ph↗

arXiv2013

Dispersion cancellation in high resolution two-photon interference

The dispersion cancellation observed in Hong-Ou-Mandel (HOM) interference between frequency-entangled photon pairs has been the basis of quantum optical coherence tomography and quantum clock synchronization. Here we explore the effect of phase dispersion on ultranarrow HOM dips. We show that the higher-order dispersion, the line width of the pump laser, and the spectral shape of the parametric fluorescence have a strong effect on the dispersion cancellation in the high-resolution regime with several experimental verifications. Perfect dispersion cancellation with a linewidth of 3μm is also demonstrated through 25 mm of water.

quant-ph↗