arXiv subjects

Ryotaro Inoue

Publications and source records attributed to Ryotaro Inoue.

16 recordsLinked to original sources

arXiv2025

A low-loss telecom-band nanofiber cavity for interfacing Yb atomic qubits

We demonstrate the fabrication of an optical nanofiber cavity designed for efficient interface with ytterbium (Yb) atoms at telecom-wavelength transitions. Replacing the conventional hydrogen-oxygen flame with a deuterium-oxygen flame in the heat-and-pull method suppresses hydroxyl-induced absorption losses and enables low-loss nanofiber production with minimal modifications to the existing fabrication system. Using this technique, we fabricate a nanofiber cavity at 1389 nm that exhibits an intrinsic round-trip loss of $0.31(2)\%$ and a finesse of $2.0(1)\times 10^{3}$. This performance corresponds to a projected cooperativity of 90 when interfaced with Yb atoms, indicating that the cavity is well suited for efficient atom-photon coupling at telecom wavelength transitions. Our results establish a practical route for developing fiber-integrated atom-photon interfaces in the telecom band, a critical step toward scalable quantum communication and distributed quantum computing.

quant-ph↗

arXiv2025

Sub-Recoil Transverse Momentum Width in a Cold Ytterbium Atomic Beam

We demonstrate the generation of a slow ytterbium atomic beam with a transverse momentum width of $0.44(6)$ times the photon recoil associated with Bragg diffraction, and a flux of $6.7(9) \times 10^6$ atoms/s. This is achieved by applying momentum filtering through a long-lived metastable state to atoms prepared in a slow beam via two-dimensional transverse laser cooling. The resulting narrow momentum distribution enables efficient quasi-Bragg diffraction, which we exploit to realize a Bragg interferometer. These results mark a significant step toward continuous, high-precision, and magnetically insensitive angular rate measurements using cold alkaline-earth(-like) atomic beams.

physics.atom-ph↗

arXiv2025

Taming Recoil Effect in Cavity-Assisted Quantum Interconnects

Photon recoil is one of the fundamental limitations for high-fidelity control of trapped-atom qubits such as neutral atoms and trapped ions. In this work, we derive an analytical model for efficiently evaluating the motion-induced infidelity in remote entanglement generation protocols. Our model is applicable for various photonic qubit encodings such as polarization, time bin, and frequency, and with arbitrary initial motional states, thus providing a crucial theoretical tool for realizing high-fidelity quantum networking. For the case of tweezer-trapped neutral atoms, our results indicate that operating in the bad-cavity regime with cavity decay rate exceeding atom-photon coupling rate, and near-ground-state cooling with motional quanta below 1, are desired to suppress the motion-induced infidelity sufficiently below the 1% level required for efficient quantum networking. Finite temperature effects can be mitigated efficiently by detection time filtering at the moderate cost of success probability and network speed. These results extend the understanding of infidelity sources in remote entanglement generation protocols, establishing a concrete path towards fault-tolerant quantum networking with scalable trapped-atom qubit systems.

physics.atom-ph↗

arXiv2024

Scalable Networking of Neutral-Atom Qubits: Nanofiber-Based Approach for Multiprocessor Fault-Tolerant Quantum Computer

Neutral atoms are among the leading platforms toward realizing fault-tolerant quantum computation (FTQC). However, scaling up a single neutral-atom device beyond $\sim 10^4$ atoms to meet the demands of FTQC for practical applications remains a challenge. To overcome this challenge, we clarify the criteria and technological requirements for further scaling based on multiple neutral atom quantum processing units (QPUs) connected through photonic networking links. Our quantitative analysis shows that nanofiber optical cavities have the potential as an efficient atom-photon interface to enable fast entanglement generation between atoms in distinct neutral-atom modules, allowing multiple neutral-atom QPUs to operate cooperatively without sacrificing computational speed. Using state-of-the-art millimeter-scale nanofiber cavities with the finesse of thousands, over a hundred atoms can be coupled to the cavity mode with an optical tweezer array, with expected single-atom cooperativity exceeding 100 for telecom-band transition of ytterbium atoms. This enables efficient time-multiplexed entanglement generation with a predicted Bell pair generation rate of 100 kHz while maintaining a small footprint for channel multiplexing. These proposals and results indicate a promising pathway for building large-scale multiprocessor fault-tolerant quantum computers using neutral atoms, nanofiber optical cavities, and fiber-optic networks.

quant-ph↗

arXiv2022

Bose-Einstein Condensation of Europium

We report the realization of a Bose-Einstein condensate of europium atoms, which is a strongly dipolar species with unique properties, a highly symmetric $[\mathrm{Xe}]\ 4f^7 6s^2\ {}^8\mathrm{S}_{7/2}$ electronic ground state and a hyperfine structure. By means of evaporative cooling in a crossed optical dipole trap, we produced a condensate of ${}^{151}$Eu containing up to $5\times 10^4$ atoms. The scattering length of ${}^{151}$Eu was estimated to be $a_s = 110(4)\, a_\mathrm{B}$ by comparing the velocities of expansion of condensates with different orientations of the atomic magnetic moments. We observed deformation of the condensate in the vicinity of the Feshbach resonance at $1.32\,\mathrm{G}$ with a width of $10\,\mathrm{mG}$.

cond-mat.quant-gas↗

arXiv2021

Understanding one-body losses in magnetically trapped metastable europium atoms

We report the measurement of one-body loss rates for magnetically trapped metastable europium atoms and the study of their loss mechanism. The loss of atoms observed in a magneto-optical trap is not fully understood because of the indivisibility of the loss regarding optical pumpings due to the presence of cooling laser beams. We magnetically trapped the atoms by directly loading from the magneto-optical trap and observed almost identical one-body loss rates of approximately $2.6\,\mathrm{s^{-1}}$ for two isotopes: $\mathrm{{}^{151}Eu}$ and $\mathrm{{}^{153}Eu}$. Our rate-equation-based model, combined with loss rate measurements carried out with repumpers, shows that blackbody radiation at room temperature drives E1 transitions and induces the observed losses.

physics.atom-ph↗

arXiv2021

High-Flux Cold Ytterbium Atomic Beam Source Using Two-Dimensional Laser Cooling with Intercombination Transition

We demonstrate a high-flux and low transverse temperature atomic beam of ytterbium by applying two-dimensional cooling using the ${}^1\mathrm{S}_0\text{-}{}^3\mathrm{P}_1$ intercombination transition to the cold atomic beam produced by the dipolar-allowed ${}^1\mathrm{S}_0\text{-}{}^1\mathrm{P}_1$ transition. The optimized transverse temperature of $11 \pm 9\,\mathrm{\mu K}$ and an atomic flux of $(7.5 \pm 1.0) \times 10^8\,\mathrm{atoms/s}$ are obtained for the atomic beam whose longitudinal velocity is $30\,\mathrm{m/s}$. The transverse temperature is maintained below $23\,\mathrm{\mu K}$, while the flux is above $6.5 \times 10^8\,\mathrm{atoms/s}$ in the longitudinal velocity range of $22 - 30\,\mathrm{m/s}$. We also discuss the feasibility of further narrowing the transverse momentum width to less than the recoil momentum using the momentum-selective optical transition between the ground state and the long-lived metastable state. The transverse momentum width of our atomic beam, which is narrower than 5.7 times the recoil momentum, is a good starting point for the proposed method.

physics.atom-ph↗

arXiv2021

Narrow-line magneto-optical trap for europium

We report on the realization of a magneto-optical trap (MOT) for europium atoms using a narrow-line cooling transition with a natural linewidth of 97 kHz. Our starting point is continuous capturing and cooling of optically pumped metastable europium atoms. We have employed simultaneous MOT for the metastable and ground-state atoms. The trapped metastable atoms are successively pumped back to the ground state and then continuously loaded to the narrow-line MOT, where up to $4.7\times10^7$ atoms are captured. A spin-polarized sample at a temperature of $6\,\mathrm{μK}$ and with a peak number density of $2.2\times10^{11}\,\mathrm{cm^{-3}}$ is obtained through the compression process, resulting in a phase space density of $3\times10^{-5}$.

cond-mat.quant-gas↗

arXiv2018

Magneto-optical trapping of optically pumped metastable europium

We demonstrate laser cooling and magneto-optical trapping of europium. The atoms are optically pumped to a metastable state and then loaded from an atomic-beam source via conventional Zeeman slowing and magneto-optical trapping techniques using a $J=13/2\leftrightarrow J=15/2$ quasi-cyclic transition. The trapped populations contained up to $1\times 10^7$ atoms, and a two-body loss rate is estimated as $1\times10^{-10}\,\mathrm{cm^3/s}$ from the non-exponential loss of atoms at high densities. We also observed leakage out of the quasi-cyclic transition to the two metastable states with $J=9/2$ and $11/2$, which is adequate to pump the laser-cooled atoms back to the $J=7/2$ ground state.

cond-mat.quant-gas↗

arXiv2017

Site-resolved imaging of a bosonic Mott insulator using ytterbium atoms

We demonstrate site-resolved imaging of a strongly correlated quantum system without relying on laser-cooling techniques during fluorescence imaging. We observed the formation of Mott shells in the insulating regime and realized thermometry on the atomic cloud. This work proves the feasibility of the noncooled approach and opens the door to extending the detection technology to new atomic species.

cond-mat.quant-gas↗

arXiv2016

Measuring the branching ratios from the $y^8 {\rm P}_{9/2}$ state to metastable states in europium

We measure the branching ratios from the $y^8{\rm P}_{9/2}$ excited state to six metastable states of europium through fluorescence spectroscopy of an atomic beam. The sum of the six branching ratios is estimated to be $1.05(2)\times 10^{-3}$. This research provides us with insightful information to determine the feasibility of using the $a^8{\rm S}_{7/2} - y^8{\rm P}_{9/2}$ transition in order to implement the Zeeman slowing for europium atoms in the ground state. Based on this result, we also propose a scheme for Zeeman slowing and magneto-optical trapping, using a specific metastable state which has a cyclic transition.

cond-mat.quant-gas↗

arXiv2015

Site-resolved imaging of ytterbium atoms in a two-dimensional optical lattice

We report a high-resolution microscope system for imaging ultracold ytterbium atoms trapped in a two-dimensional optical lattice. By using the ultraviolet strong transition combined with a solid immersion lens and high-resolution optics, our system resolved individual sites in an optical lattice with a 544-nm spacing. Without any cooling mechanism during the imaging process, the deep potential required to contain the atoms was realized using a combination of a shallow ground-state and a deep excited-state potentials. The lifetime and limitations of this setup were studied in detail.

cond-mat.quant-gas↗

arXiv2014

Injection locking of a high power ultraviolet laser diode for laser cooling of ytterbium atoms

We developed a high-power laser system at a wavelength of 399 nm for laser cooling of ytterbium atoms with ultraviolet laser diodes. The system is composed of an external cavity laser diode providing frequency stabilized output at a power of 40 mW and another laser diode for amplifying the laser power up to 220 mW by injection locking. The systematic method for optimization of our injection locking can also be applied to high power light sources at any other wavelengths. Our system, which does not depend on complex nonlinear frequency-doubling, has great importance for implementing transportable optical lattice clocks, and is also useful for investigations on condensed matter physics or quantum information processing using cold atoms.

physics.atom-ph↗

arXiv2013

Carrier flow and nonequilibrium superconductivity in superconductor-based light-emitting diode

Superconductor-based light-emitting diode (superconductor-based LED) in strong light-confinement regime are characterized as a superconductor-based three-terminal device, and its transport properties are quantitatively investigated. In the gate-controlled region, we confirm the realization of new-type Josephson field effect transistor (JoFET) performance, where the channel cross-sectional area of the junction is directly modulated by the gate voltage. In the current-injected region, the superconducting critical current of $μ$A order in the Josephson junction is found to be modulated by the steady current injection of pA order. This ultrahigh monitoring sensitivity of the radiative recombination process can be explained by taking into account the fact that the energy relaxation of the absorbed photons causes the conversion of superconducting pairs to quasiparticles in the active layer. Using quasiparticle density and superconducting pair density, we discuss the carrier flows together with the non-equilibrium superconductovity in the active layer and the superconducting electrodes, which take place for compensating the conversion.

cond-mat.supr-con↗

arXiv2013

Unconditional quantum-noise suppression via measurement-based quantum feedback

We demonstrate unconditional quantum-noise suppression in a collective spin system via feedback control based on quantum non-demolition measurement (QNDM). We perform shot-noise limited collective spin measurements on an ensemble of $3.7\times 10^5$ laser-cooled 171Yb atoms in their spin-1/2 ground states. Correlation between two sequential QNDMs indicates $-0.80^{+0.11}_{-0.12}\,\mathrm{dB}$ quantum noise suppression in a conditional manner. Our feedback control successfully converts the conditional quantum-noise suppression into the unconditional one without significant loss of the noise

quant-ph↗

arXiv2011

Transport properties of Andreev polarons in superconductor-semiconductor-superconductor junction with superlattice structure

Transport properties of a superconductor-semiconductor-superconductor (S-Sm-S) junction with superlattice structure are investigated. Differential resistance as a function of voltage shows oscillatory behavior under the irradiation of radio-frequency (RF) waves with the specific frequency of 1.77 GHz regardless of the superconducting materials and the junction lengths. Experimental data are quantitatively explained in terms of the coupling of superconducting quasiparticles with long-wavelength acoustic phonons indirectly excited by the RF waves. We propose that the strong coupling causes the formation of novel composite particles, Andreev polarons.

cond-mat.supr-con↗