arXiv subjects

Shohei Imai

Publications and source records attributed to Shohei Imai.

9 recordsLinked to original sources

arXiv2026

Heralded ultrafast generation of macroscopic quantum states in matter with bright squeezed vacuum light

We show that bright squeezed vacuum light, combined with a single-shot quadrature measurement of the post-interaction light, enables the ultrafast generation of macroscopic quantum states in matter. Although in the weak-coupling regime multiphoton quantum light leaves the unconditional matter state as a classical mixture due to light--matter entanglement, quadrature-based heralding prepares the matter in a Gaussian-weighted quantum superposition of laser-driven matter states. For an ensemble of resonantly electric-dipole-coupled two-level systems, this heralding dynamics acts as a Gaussian filter with respect to the electric polarization, with brighter squeezed-vacuum light accelerating the preparation of the zero-eigenvalue Dicke state. Counter-rotating terms further drive a stroboscopic transition from this Dicke state to a cat-like state. Our results open a route to ultrafast engineering of macroscopic quantum matter with strong-field quantum light.

quant-ph↗

arXiv2026

Generating collective spin cat states via photon-number measurements near the Dicke critical point

We propose a method for generating collective spin cat states in a cavity-coupled atomic ensemble by exploiting strong light-matter entanglement and anti-squeezing associated with the superradiant phase transition. We numerically and analytically demonstrate that the cat states can be heralded by photon-number measurement on the ground state of the Dicke model. The near-critical regime enhances both the cat-state size and the probability of obtaining larger photon-number outcomes, and outcomes with larger photon numbers yield even larger cat states. We also show that a thermodynamic-limit analysis clarifies the generation mechanism and connects it to a natural light-matter analogue of generalized photon subtraction for optical cat-state generation. These results suggest that exploiting criticality in strongly coupled light-matter systems could open new directions for matter-based many-body quantum technologies.

quant-ph↗

arXiv2025

Macroscopic Schr\"{o}dinger-cat states of nonequilibrium electrons induced by cat-state optical driving and projective measurements on the light field

We show that projective measurements on quantum light can induce macroscopic cat states in many-electron systems driven by such light. Here we investigate the quantum dynamics of $N$ independent two-level electrons interacting with Schr\"{o}dinger-cat or -kitten states of light. Without measurement, a macroscopic cat state of the electrons appears only in an ultrashort time window. In contrast, we demonstrate that photon-number parity or quadrature projective measurements can restore a macroscopic cat state in nonequilibrium electrons, even in the thermodynamic limit. These dynamics are captured by an external-field approximation, in which the electronic system evolves into a Rabi-oscillation cat state. Our results highlight the need for precise quantum measurement techniques for light to control macroscopic quantum states of matter driven by quantum light.

quant-ph↗

arXiv2025

Electron dynamics induced by quantum cat-state light

We present an effective theory for describing electron dynamics driven by an optical external field in a Schr\"{o}dinger's cat state. We show that the reduced electron density matrix evolves as an average over trajectories $\{\rho_\alpha\}$ weighted by the Sudarshan--Glauber $P$ distribution $P(\alpha)$ in the weak light--matter coupling regime. Each trajectory obeys an equation of motion, $\mathrm{i} \partial_t\rho_\alpha=\mathcal{H}_{\alpha} \rho_\alpha-\rho_\alpha\mathcal{H}_{\alpha}$, where an effective Hamiltonian $\mathcal{H}_{\alpha}$ becomes non-Hermitian due to quantum interference of light. The optical quantum interference is transferred to electrons through the asymmetric action between the ket and bra state vectors in $\rho_{\alpha}$. This non-Hermitian dynamics differs from the conventional one observed in open quantum systems, described by $\mathrm{i} \partial_t\rho=\mathcal{H}\rho-\rho \mathcal{H}^\dagger$, which has complex conjugation in the second term. We confirm that the reduced, trajectory-resolved effective theory agrees with full electron-photon simulations for the few-electron Dicke model, thereby validating the interferential non-Hermitian description in the weak-coupling regime.

quant-ph↗

arXiv2024

High harmonic generation from electrons moving in topological spin textures

High harmonic generation (HHG) is a striking phenomenon, which reflects the ultrafast dynamics of electrons. Recently, it has been demonstrated that HHG can be used to reconstruct not only the energy band structure but also the geometric structure characterized by the Berry curvature. Here, we numerically investigate HHG arising from electrons coupled with a topological spin texture in a spin scalar chiral state where time reversal symmetry is broken. In this system, a sign change in scalar chirality alters the sign of the Berry curvature while keeping the energy band structure unchanged, allowing us to discuss purely geometrical effects on HHG. Notably, we found that, when the optical frequency is significantly lower than the energy gap, the sign of scalar chirality largely affects the longitudinal response parallel to the optical field rather than the transverse response. Our analysis suggests that this can be attributed to interband currents induced by the recombination of electron-hole pairs whose real-space trajectories are modulated by the anomalous velocity term.

cond-mat.mes-hall↗

arXiv2024

Quantum many-body scars with unconventional superconducting pairing symmetries via multibody interactions

We present a systematic framework to construct model Hamiltonians that have unconventional superconducting pairing states as exact energy eigenstates, by incorporating multibody interactions (i.e., interactions among more than two particles). The multibody interactions are introduced in a form of the local density-density coupling in such a way that any pair configuration in real space has a constant interaction energy by canceling the two-body and multibody interactions. Our approach is applicable to both spinless and spinful models in any spatial dimensions and on any bipartite lattices, facilitating an exhaustive extension of Yang's $s$-wave $\eta$-pairing state to various other unconventional pairing symmetries ($p$-wave, $d$-wave, $f$-wave, etc.). Particularly, the constructed eigenstates have off-site pairs with finite center-of-mass momentum, which leads to superconducting states with either even-parity and spin-triplet or odd-parity and spin-singlet symmetry. We verify that the two-dimensional spinful Hubbard model on a square lattice with the multibody interactions has the spin-triplet $d$-wave pairing state as an energy eigenstate, which can be regarded as a quantum many-body scar state as evidenced from the numerical analysis of the pair correlation function, entanglement entropy, and level statistics. We also discuss other examples, including spin-triplet $f$-wave pairing states on a honeycomb lattice and spin-singlet $p$-wave pairing states in a one-dimensional chain. These findings open up the possibility of realizing nonequilibrium unconventional superconductivity in a long-lived manner protected against thermalization.

cond-mat.supr-con↗

arXiv2023

Theory for Fourier-limited attosecond pulse generation in solids

The generation of ultrashort light pulses is essential for the advancement of attosecond science. Here, we show that attosecond pulses approaching the Fourier limit can be generated through optimized optical driving of tunneling particles in solids. We propose an ansatz for the wave function of tunneling electron-hole pairs based on a rigorous expression for massive Dirac fermions, which enables efficient optimization of the waveform of the driving field. It is revealed that the dynamic sign change in the effective mass due to optical driving is crucial for shortening the pulse duration, which highlights a distinctive property of Bloch electrons that is not present in atomic gases, i.e., the periodic nature of crystals. These results show the potential of utilizing solid materials as a source of attosecond pulses.

cond-mat.mes-hall↗

arXiv2022

Energy-band echoes: Time-reversed light emission from optically driven quasiparticle wavepackets

The at-will control of quantum states is a primary goal of quantum science and technology. The celebrated Hahn echo exemplifies such quantum-state control based on a time-reversal process in a few-level system. Here, we propose a different echo phenomenon associated with the energy-band structure in quantum many-body systems. We show that the dynamics of quasiparticle wavepackets can be reversed by a driving electric-field pulse, yielding echoes with the time-reversed waveform of the optical excitation pulse when the quasiparticles recombine. The present echoes are observed not only in band insulators but also in correlated insulators, including a Mott insulator and a spontaneously-broken-symmetry charge-ordered insulator, in one- and higher-dimensional systems, irrespective of the integrability of the models. Analytical expressions reveal the conditions under which the echoes appear, and they also indicate that the frequency of the echo pulses reflects the dispersion relation for quasiparticles such as electron-hole pairs, doublon-holon pairs, and kink-antikink pairs. These findings provide a framework for all-optical momentum-resolved spectroscopy of the quasiparticles in quantum many-body systems.

cond-mat.str-el↗

arXiv2019

High-Harmonic Generation in a Correlated Electron System

High-harmonic generation (HHG) in crystalline solids have been examined so far on the basis of one-body energy-band structures arising from electron itineracy in a periodic potential. Here, we show emergence of HHG signals which are attributed to dynamics of many-body states in a low-dimensional correlated electron system. An interacting fermion model and its effective pseudo-spin model on a one-dimensional dimer-type lattice are analyzed. Observed HHG signals in a spontaneously symmetry-broken state, where charge densities are polarized inside of dimer units, show threshold behavior with respect to light amplitude and are interpreted in terms of tunneling and recombination of kink-antikink excitations in an electric field.

cond-mat.str-el↗