arXiv subjects

Hirotaka Terai

Publications and source records attributed to Hirotaka Terai.

At least 19 recordsLinked to original sources

arXiv2026

Development of time-resolved fluorescence spectroscopy using heralded emission spectroscopy for organic light-emitting materials

Organic light-emitting materials are widely used in optoelectronics and bioimaging, but sometimes conventional fluorescence spectroscopy techniques can be limited by scattering, background fluorescence, photon pile-up and photobleaching. We report a quantum-spectroscopic approach using frequency-entangled, nondegenerate photon pairs generated via spontaneous parametric down-conversion with heralded detection, to perform photoluminescence lifetime measurements at low excitation intensities. We demonstrate this approach across three organic light-emitting material classes: the fluorescent dye Rhodamine 6G, an organic semiconducting conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), and a fluorescent protein mOrange. The intrinsic timing correlation between signal and idler photons enables coincidence gating that suppresses uncorrelated background counts. Across all materials and integration times of 500, 1000 and 3600 seconds, heralded detection yields signal-to-noise ratios 1.6-3.0 times higher than classical pulsed-laser excitation attenuated to match heralded photon counts in the fitting range. For MEH-PPV, this roughly halves the noise amplitude. Our measured lifetimes closely match long-integration classical pulsed-laser reference values.

quant-ph↗

arXiv2026

Provably efficient experimental classification of quantum data with quantum kernels

Analyzing quantum systems is essential for quantum information processing, but can require exponentially many measurements without exploiting the structure of the system or task. Quantum machine learning offers a promising way to reduce this cost by learning from quantum data. Here, we theoretically establish a framework for efficient quantum-state classification and experimentally demonstrate the resulting scheme on a programmable photonic processor. Our analysis links learning efficiency to the structure of quantum data, without relying on a physical description tailored to each task. We derive sufficient conditions for accurate classification with a number of measurements polynomial in system size. We apply this guarantee to a resource problem in continuous-variable (CV) quantum computing: classifying stellar rank, a measure of non-Gaussianity. Experimentally, we implement the classification of single-mode CV quantum states with stellar rank zero and one. This is enabled by developing a programmable CV photonic processor capable of preparing and processing diverse Gaussian and non-Gaussian states and performing virtual projection onto such states, going beyond the Gaussian-only capabilities of most existing photonic platforms. The experiment demonstrates accurate classification despite experimental imperfections. Our results establish a theory-guided and experimentally validated framework for quantum-state classification that can be implemented on quantum devices available today.

quant-ph↗

arXiv2026

Sidewall spacer passivated epitaxial NbN/AlN/NbN Josephson junctions for superconducting qubit applications

Epitaxial NbN/AlN/NbN Josephson junctions are promising alternatives to conventional Al/AlO$_x$/Al junctions for superconducting qubits due to their crystalline tunnel barriers and the high transition temperature of NbN. However, their integration into quantum circuits typically requires complex fabrication processes involving amorphous interlayer dielectrics, which can degrade qubit coherence times. To address these issues, we developed sidewall spacer passivated epitaxial NbN/AlN/NbN junctions using a fabrication process that reduces both the volume of amorphous dielectric material and fabrication complexity. The fabricated junctions exhibited a large gap voltage of approximately 5.6 mV and low subgap leakage currents. By varying the thickness of the AlN tunnel barrier, high-quality junctions were fabricated over a wide range of critical current densities ${\it J}_c$, from 0.3 to 10$^4$ A/cm$^2$. In addition, the comparison of the current-voltage characteristics before and after buffered hydrofluoric acid treatment indicated that the wet etching process did not degrade the electrical characteristics of the junctions. These results establish a scalable fabrication process for epitaxial Josephson junctions and provide a promising platform for high-coherence nitride-based superconducting qubits.

quant-ph↗

arXiv2026

Epitaxial NbN-junction-based self-shunted superconducting flux qubit with high anharmonicity

We report a demonstration of a self-shunted superconducting flux qubit (SSFQ) as an ultracompact, strongly anharmonic, and long-lifetime qubit by using the full epitaxially grown NbN/AlN/NbN Josephson junctions. The NbN-junction-based SSFQ achieved a high anharmonicity of 797 MHz as well as drastically reduced footprint of the qubit compared to capacitively shunted (C-shunt) flux qubits with a large shunt capacitor. The lifetime of the SSFQ reached 13 - 25 $μ$s, which is comparable to or relatively higher than that of NbN-based C-shunt flux qubits and Al-based flux qubits with more than 10 times smaller junctions. This indicates that the large epitaxial NbN-based junction can be adopted for SSFQ as well as merged-element transmon with the relatively large junctions. The results presented in this work will offer a new choice of the scalable qubits as well as the material platform toward the realization of large-scale superconducting quantum computers.

quant-ph↗

arXiv2026

Hybrid biphoton spectrometer for time-resolved quantum spectroscopy across visible and near-infrared regions

Joint spectral measurements are a powerful tool for characterising biphoton spectral correlation, which is crucial for quantum information and communication technologies. In these applications, highly pure biphoton states are essential in any time- and frequency-mode, often obviating the need for time-resolved measurements. Conversely, spectroscopy utilising entangled photon pairs is gaining significant attention for its ability to unveil molecular dynamics, a field that critically demands time-resolved capabilities. Here, we introduce a methodology for capturing a biphoton spectrum that comprises visible and near-infrared photons, resulting in a highly non-degenerate joint spectrum. Our system employs two non-scanning spectrographs: a fibre spectrometer for near-infrared photons and a delay-line-anode single-photon imager for visible photons. We successfully measure the joint spectral intensity by leveraging a time-tagging acquisition strategy. Furthermore, our approach uniquely enables time-resolved joint spectral measurements with respect to the laser synchronisation against the hundreds-of-picosecond instrument response function. Our methodology could advance heralded fluorescence spectroscopy using biphoton sources to investigate temporal dynamics of complex biological, chemical, and physical systems.

quant-ph↗

arXiv2026

Ultra-high-rate detection of entangled photon pairs

The high-rate detection of entangled photons is essential for advancing photonic quantum information processing. Although several experimental demonstrations have been reported, the achievable coincidence rates have so far remained limited. One of the main bottlenecks arises from the dead time of single-photon detectors, which constrains coincidence detection at high photon-pair generation rates. In this work, we employ 16-pixel superconducting nanowire single-photon detectors (SNSPDs) to mitigate the impact of detector dead time. Consequently, we achieve coincidence rates exceeding 3 million counts per second (Mcps) in two-photon interference and CHSH inequality experiments using 5-GHz clocked sequential time-bin entangled photon pair source. To the best of our knowledge, this is the first demonstration of multi-Mcps coincidence detection of entangled photons, paving the way for high-speed entangled-photon-based quantum information processing.

quant-ph↗

arXiv2026

Fast coherent control of a charge qubit on solid neon with a spin-qubit-compatible resonator

Electrons floating in vacuum provide a clean platform for quantum information processing owing to their isolation from material defects. In particular, electrons on solid neon have emerged as a promising qubit platform because of their potentially long coherence times. Here, toward spin-qubit realization, we couple a single electron on solid neon to a magnetic-field-compatible superconducting NbTiN nanowire resonator. We realize a charge qubit and demonstrate microwave readout and coherent control, with Rabi frequencies up to 76 MHz, an order of magnitude larger than in previous studies. Under strong driving, we observe a qubit frequency shift from nonlinear interactions with the intense microwave field. Deterministic electron trapping at an intended position remains challenging due to solid neon surface roughness; we characterize the electron's position from its differential coupling to distinct electrodes. Although not trapped at an intended position, our estimates indicate that spin-qubit demonstrations remain feasible.

cond-mat.mes-hall↗

arXiv2026

Quantum optical synthesis of high-dimensional ultrafast frequency-bin qudits

Frequency modes of light are one of the most promising platforms that provide access to high-dimensional quantum states amongst different photonic degrees of freedom capable of high-dimensionality, enabling robust, error-tolerant, and scalable quantum optical information systems. We demonstrate engineering of precisely controlled two-photon high-dimensional states entangled in frequency through time-domain Fourier optical synthesis. We generate and convert a continuous broadband frequency-entangled state into a large range of discrete frequency bins suitable for ITU standards, with spacings ranging from 12.5 GHz to 750 GHz, and observe spectral anticorrelations over 38 frequency bins, including intra-bin pure states at a 100 GHz bin spacing. We characterize the full quantum state dimensionality via Schmidt decomposition and observe lower bounds on the frequency-binned Hilbert-space dimensionalities of at least 289, formed by two entangled qudits with dimension 17. Furthermore, we demonstrate quantum nonlocality via frequency correlations in a transmission experiment over a campus-scale two-node fiber network. This work represents a crucial step towards building a versatile and relatively simple way of generating precisely controlled high-dimensional spectral qudits, with the potential of harnessing in wavelength-multiplexed quantum networks, high-dimensional information processing, and communication of quantum states specifically, and fiber-optic quantum remote sensing.

quant-ph↗

arXiv2026

Programmable non-Gaussian quantum light source with state and temporal-waveform tunability

A versatile quantum light source capable of programmably generating a variety of quantum light is a key enabler for photonic quantum technologies. In particular, independent control over both the output quantum state and its temporal waveform is essential for realizing diverse functionalities and enhancing processing performance. However, conventional sources of optical non-Gaussian states, a crucial resource for photonic quantum information processing, typically emit fixed states with predetermined temporal waveforms, lacking their programmability. Here, we propose a programmable non-Gaussian quantum light source that offers independent and arbitrary tunability of both the quantum state and the temporal waveform within a single platform. As a distinctive feature, our approach employs a heralding scheme in which these two properties are indirectly engineered to user-defined targets by manipulating the light in the heralding channel, thereby avoiding optical losses associated with direct manipulation of the heralded quantum light. We develop a prototype and demonstrate the generation of single-photon, Schrödinger cat, and two-photon states in a variety of unconventional temporal waveforms without degradation in state quality. This platform provides a versatile tool for tailoring quantum light to specific applications, significantly expanding the capabilities of photonic quantum technologies.

quant-ph↗

arXiv2026

Hybridization of pulse and continuous-wave based optical quantum computation

We propose a pulse and continuous wave (CW) hybrid architecture of continuous-variable measurement-based optical quantum computation utilizing the strengths of both pulsed and CW light. In this architecture, input and ancillary non-Gaussian quantum states necessary for fault-tolerance and universality are generated with pulsed light, whereas quantum processors including continuous-variable cluster states and homodyne measurement systems are operated with CW light. This architecture is expected to enable both generation of quantum states with shorter optical wavepackets for ultrafast computation and low-loss manipulation and measurement of these states. In this study, as a proof-of-principle, an ultrafast homodyne measurement using a CW local oscillator was performed on single-photon states generated with pulsed light. The measured single-photon state's temporal width was around 70 ps and the value of the Wigner function at the origin was $W(0,0) = -0.153\pm0.003$, which is highly non-classical. This will be a core technology for high-speed optical quantum information processing.

quant-ph↗

arXiv2026

Environment-Assisted Decoherence Suppression of Optical Non-Gaussian States

Optical loss is a common bottleneck in photonic quantum information processing, undermining the quantum advantage over classical approaches. Although several countermeasures, such as quantum distillation and error correction, have been proposed, they typically require experimentally demanding non-Gaussian operations. Here, we demonstrate a Gaussian-only scheme that suppresses loss-induced decoherence for general, unknown optical quantum states. By injecting a squeezed vacuum state into an environment of the loss channel and performing feedforward based on environmental monitoring, the scheme effectively suppresses loss-induced noise. Our programmable loop-based optical circuit allows us to implement the scheme for several types of loss-sensitive non-Gaussian states under various loss conditions for up to five steps, and directly compare the results with the unsuppressed case. Our results show that the scheme consistently mitigates state degradation, preserving higher fidelity and Wigner negativity than without suppression. This approach can be applied to mitigating a broad class of errors in optical systems and extending quantum memory lifetimes. Moreover, it is compatible with other loss-suppression techniques and extendable to physical platforms beyond optics, offering a promising route toward reducing the overhead required for fault-tolerant quantum information processing.

quant-ph↗

arXiv2026

Tuning Wave-Particle Duality of Quantum Light by Generalized Photon Subtraction

Wave--particle duality is a hallmark of quantum mechanics. For bosonic systems, there exists a continuum of intermediate states bridging wave-like Schrödinger cat states and particle-like Fock states. Such states have recently been recognized as valuable resources for enhancing fault-tolerant quantum computation (FTQC) with propagating light. Here we experimentally demonstrate tunable generation of these intermediate states by employing generalized photon subtraction (GPS). By detecting up to three photons from squeezed-light sources with a photon-number-resolving detector, we continuously control the balance between wave- and particle-like features. This approach allows us to construct a spectral family of quantum states with high generation rates, optimized according to the required fault-tolerance threshold. Our results establish GPS as a versatile toolbox for tailoring non-Gaussian resources, opening a pathway to efficient Gottesman--Kitaev--Preskill (GKP) qubit generation and addressing a central bottleneck in optical quantum computing.

quant-ph↗

arXiv2025

Distribution of non-Gaussian states in a deployed telecommunication fiber channel

Optical non-Gaussian states hold great promise as a pivotal resource for advanced optical quantum information processing and fault-tolerant long-distance quantum communication. Establishing their faithful transmission in a real-world communication channel, therefore, marks an important milestone. In this study, we experimentally demonstrate the distribution of such non-Gaussian states in a functioning telecommunication channel that connects separate buildings within the DTU campus premises. We send photon-subtracted squeezed states, exhibiting pronounced Wigner negativity, through 300 m of deployed optical fibers to a distant building. Using quantum homodyne tomography, we fully characterize the states upon arrival. Our results show the survival of the Wigner function negativity after transmission when correcting for detection losses, indicating that the established link can potentially facilitate the violation of Bell's inequality and enable quantum steering. This achievement not only validates the practical feasibility of distributing non-Gaussian states in real-world settings, but also provides an exciting impetus towards realizing fully coherent quantum networks for high-dimensional, continuous-variable quantum information processing.

quant-ph↗

arXiv2025

Waveguide-array-based multiplexed photonic interface for atom array

The growing demand for high-capacity quantum communication and large-scale quantum computing underscores the importance of networking quantum processing units via multiplexed photonic channels. A neutral atom array with multiplexed atom-photon entanglement is a promising platform for its realization. Here, we demonstrate a key multiplexed photonic interface guiding the photons from an atom array to a single-mode waveguide array fabricated on a glass-based photonic integrated circuit. Remarkable 10 channels out of the 32-channel waveguide array with 25 $μ$m pitch couple to photons from 10 sites of the atom array with Rydberg gate-enabled separation. Based on the observed correlation between the atomic states and the polarization of the photon with a visibility of 0.87, we anticipate its applicability to a large-scale multiplexed atom-photon entanglement generation for networking quantum processing units.

quant-ph↗

arXiv2025

Experimental entanglement swapping through single-photon $χ^{(2)}$ nonlinearity

In photonic quantum information processing, quantum operations using nonlinear photon-photon interactions are vital for implementing two-qubit gates and enabling faithful entanglement swapping. However, due to the weak interaction between single photons, the all-photonic realization of such quantum operations has remained out of reach so far. Herein, we demonstrate an entanglement swapping using sum-frequency generation between single photons in a $χ^{(2)}$-nonlinear optical waveguide. We show that a high signal-to-noise ratio~(SNR), stable sum-frequency-generation-based entanglement heralder with an ultralow-dark-count superconducting single-photon detector can satisfy the unprecedented SNR requirement indispensable for the swapping protocol. Furthermore, the system clock is enhanced by utilizing ultrafast telecom entangled photon-pair sources that operate in the GHz range. Our results confirm a lower bound 0.770(76) for the swapped state's fidelity, surpassing the classical limit of 0.5 successfully. Our findings highlight the strong potential of broadband all-single-photonic nonlinear interactions for further sophistication in long-distance quantum communication and photonic quantum computation.

quant-ph↗

arXiv2025

Low-loss polarization-maintaining router for single and entangled photons at a telecom wavelength

Photon polarization serves as an essential quantum information carrier in quantum information and measurement applications. Routing of arbitrarily polarized single photons and polarization-entangled photons is a crucial technology for scaling up quantum information applications. Here, we demonstrate a low-loss, noiseless, polarization-maintaining routing of arbitrarily polarized single photons and, crucially, multi-photon entangled states where the entanglement is encoded in orthogonal polarization bases, at the telecom L-band. Our interferometer-based router is constructed by optics with a low angle of incidence and cross-aligned electro-optic crystals, achieving the polarization-maintaining operation with a minimal number of optical components. We demonstrate the routing of arbitrarily-polarized heralded single photons with a 0.057 dB (1.3%) loss, a $>$ 22 dB switching extinction ratio, and $>$ 99% polarization process fidelity to ideal identity operation. Moreover, the high-quality router achieves the routing of two-photon N00N-type entangled states with a highly maintained interference visibility of $\approx$ 97%. The demonstrated router scheme preserving multi-photon polarization state paves the way toward polarization-encoded photonic quantum networks as well as multi-photon entanglement synthesis via spatial- and time-multiplexing techniques.

quant-ph↗

arXiv2025

High-$Q$ membrane resonators using ultra-high-stress crystalline TiN films

High-quality-factor ($Q$) mechanical resonators are essential components for precise sensing and control of mechanical motion at a quantum level. While amorphous materials such as SiN have been widely used in high-$Q$ mechanical resonators utilizing stress-induced dissipation dilution, crystalline materials have emerging potential to achieve higher quality factors by combining low intrinsic loss and high tensile stress. In this paper, we demonstrate high-Q membrane resonators using ultra-high-stress crystalline TiN. Our membrane resonator exhibits a tensile stress exceeding 2.3 GPa and a quality factor of $Q = 8.0 \times 10^6$ at 2.2 K. By estimating the dilution factor, we infer that our TiN resonator has a intrinsic quality factor comparable to that of SiN membrane resonators. With its ultra-high stress and crystalline properties, our TiN films can serve as a powerful tool for opto- and electromechanical systems, offering highly dissipation-diluted mechanical resonators.

cond-mat.mes-hall↗

arXiv2025

Scalable Optical Quantum State Synthesizer with Dual-Mode Resonator Memory

Optical quantum computing is a promising approach for achieving large-scale quantum computation. While Gaussian operations have been successfully scaled, the inherently weak nonlinearity in optics makes generating highly non-Gaussian states a critical challenge for universality and fault tolerance. Here, we propose and experimentally demonstrate a scalable method to generate optical non-Gaussian states with a resonator-based quantum memory that supports continuous-time storage and retrieval, in contrast to conventional loop-based memories. We introduce a dual-mode operation of the memory, enabling both storage and entangling functionalities within a single device. By employing a time-domain-multiplexed approach, we successfully demonstrate both cat and Gottesman-Kitaev-Preskill (GKP) breeding protocols in a scalable fashion, marking a key step toward quantum error correction. Our experiment also marks the first full demonstration of an optical resonator memory performing writing, storage, and readout operations. We validate the memory by storing squeezed single-photon states with up to 93% total efficiency, and measure an energy relaxation time $T_1 =$2.3$μ$s and dephasing time $T_ϕ =$0.96$μ$s. These results establish a scalable pathway to generating complex non-Gaussian states required for fault-tolerant optical quantum computing. Beyond computation, our techniques provide new tools for enhancing quantum communication, sensing, and metrology.

quant-ph↗