arXiv subjects

Yuichi Igarashi

Publications and source records attributed to Yuichi Igarashi.

7 recordsLinked to original sources

arXiv2026

Change in bit-flip times of Kerr parametric oscillators caused by their interactions

We experimentally investigate how interactions between Kerr parametric oscillators (KPOs) degrade their bit-flip times, where a bit flip is defined as a transition between the two degenerate ground states of a KPO. Interactions between KPOs cause quantum states of KPOs to leak outside the computational subspace, leading to bit flips. Bit flips degrade fidelity and pose a significant problem for KPO-based quantum information processing. We performed an experiment in which a weak microwave signal is injected into one KPO to emulate photon injection from another KPO, and find that the bit-flip time decreases by an order of magnitude due to induced excitations, depending on the frequency and power of the injected signal. Methods to mitigate the decrease in bit-flip times caused by interactions between KPOs are discussed, including adjusting the pump frequencies, coherent-state amplitudes, and couplings between KPOs. These findings provide valuable insights for scaling up KPO-based quantum computers.

quant-ph↗

arXiv2025

Four-body interactions in Kerr parametric oscillator circuits

We theoretically present new unit circuits of Kerr parametric oscillators (KPOs) with four-body interactions, which enable the scalable embedding of all-to-all connected logical Ising spins using the Lechner-Hauke-Zoller (LHZ) scheme. These unit circuits enable four-body interactions using linear couplers, making the circuit fabrication and characterization much simpler than those of conventional unit circuits with nonlinear couplers. Numerical calculations indicate that the magnitudes of the coupling constants can be comparable to those in conventional circuits. On the basis of this theory, we designed a four-KPO circuit and experimentally confirmed the four-body correlation by measuring the pump-phase dependence of the parity of the four-KPO states. We show that the choice of the pump frequencies are important not only to enable the four-body interaction, but to cancel the effects of other unwanted interactions. Using the circuit, we demonstrated the quantum annealing based on the LHZ scheme, where the strength of the interaction between the logical Ising spins is mapped to the local field and controlled by a coherent drive applied to each KPO.

quant-ph↗

arXiv2025

Resonance fluorescence spectra of a driven Kerr nonlinear resonator

Resonance fluorescence spectra of a driven Kerr nonlinear resonator is investigated both theoretically and experimentally. When the Kerr nonlinear resonator is driven strongly such that the induced Rabi frequency is comparable to or larger than the Kerr nonlinearity, the system cannot be approximated as a two-level system. We theoretically derive characteristic features in the fluorescence spectra such as the decrease of the center-peak intensity and the asymmetric sideband peaks in the presence of finite dephasing. Those features are consistently explained by the population of the initial dressed state and its transition matrix element to the final dressed state of the transition corresponding to each peak. Finally, we experimentally measure the resonance fluorescence spectra of a driven superconducting Kerr nonlinear resonator and find a quantitative agreement with our theory.

quant-ph↗

arXiv2023

Spectroscopy of flux-driven Kerr parametric oscillators by reflection coefficient measurement

We report the spectroscopic characterization of a Kerr parametric oscillator (KPO) based on the measurement of its reflection coefficient under a two-photon drive induced by flux modulation. The measured reflection spectra show good agreement with numerical simulations in term of their dependence on the two-photon drive amplitude. The spectra can be interpreted as changes in system's eigenenergies, transition matrix elements, and the population of the eigenstates, although the linewidth of the resonance structure is not fully explained. We also show that the drive-amplitude dependence of the spectra can be explained analytically by using the concepts of Rabi splitting and the Stark shift. By comparing the experimentally obtained spectra with theory, we show that the two-photon drive amplitude at the device can be precisely determined, which is important for the application of KPOs in quantum information processing.

quant-ph↗

arXiv2016

Imaging of Iso-frequency Contours via Resonance-Enhanced Scattering in Near-Pristine Photonic Crystals

The iso-frequency contours of a photonic crystal are important for predicting and understanding exotic optical phenomena that are not apparent from high-symmetry band structure visualizations. Here, we demonstrate a method to directly visualize the iso-frequency contours of high-quality photonic crystal slabs that shows quantitatively good agreement with numerical results throughout the visible spectrum. Our technique relies on resonance-enhanced photon scattering from generic fabrication disorder and surface roughness, so it can be applied to general photonic and plasmonic crystals, or even quasi-crystals. We also present an analytical model of the scattering process, which explains the observation of iso-frequency contours in our technique. Furthermore, the iso-frequency contours provide information about the characteristics of the disorder and therefore serve as a feedback tool to improve fabrication processes.

physics.optics↗

arXiv2015

Experimental Observation of Large Chern numbers in Photonic Crystals

Despite great interest in the quantum anomalous Hall phase and its analogs, all experimental studies in electronic and bosonic systems have been limited to a Chern number of one. Here, we perform microwave transmission measurements in the bulk and at the edge of ferrimagnetic photonic crystals. Bandgaps with large Chern numbers of 2, 3, and 4 are present in the experimental results which show excellent agreement with theory. We measure the mode profiles and Fourier transform them to produce dispersion relations of the edge modes, whose number and direction match our Chern number calculations.

physics.optics↗

arXiv2015

Spawning rings of exceptional points out of Dirac cones

The Dirac cone underlies many unique electronic properties of graphene and topological insulators, and its band structure--two conical bands touching at a single point--has also been realized for photons in waveguide arrays, atoms in optical lattices, and through accidental degeneracy. Deformations of the Dirac cone often reveal intriguing properties; an example is the quantum Hall effect, where a constant magnetic field breaks the Dirac cone into isolated Landau levels. A seemingly unrelated phenomenon is the exceptional point, also known as the parity-time symmetry breaking point, where two resonances coincide in both their positions and widths. Exceptional points lead to counter-intuitive phenomena such as loss-induced transparency, unidirectional transmission or reflection, and lasers with reversed pump dependence or single-mode operation. These two fields of research are in fact connected: here we discover the ability of a Dirac cone to evolve into a ring of exceptional points, which we call an "exceptional ring." We experimentally demonstrate this concept in a photonic crystal slab. Angle-resolved reflection measurements of the photonic crystal slab reveal that the peaks of reflectivity follow the conical band structure of a Dirac cone from accidental degeneracy, whereas the complex eigenvalues of the system are deformed into a two-dimensional flat band enclosed by an exceptional ring. This deformation arises from the dissimilar radiation rates of dipole and quadrupole resonances, which play a role analogous to the loss and gain in parity-time symmetric systems. Our results indicate that the radiation that exists in any open system can fundamentally alter its physical properties in ways previously expected only in the presence of material loss and gain.

physics.optics↗