## About

1,211

Publications

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Introduction

PDF files of all of our publications are available online in
http://dml.riken.jp/pub/
A few news about our research group are posted here:
http://dml.riken.jp/outreach/
Our research work is interdisciplinary and explores the interface between condensed matter, atomic physics, optics, quantum optics, nano-science, quantum information, and computing.
We also study opto-mechanics, hybrid quantum electro-mechanical systems, quantum devices, quantum nano-electronics, and quantum emulators.

Additional affiliations

January 2002 - present

January 1990 - present

## Publications

Publications (1,211)

Simulating quantum mechanics is known to be a difficult computational
problem, especially when dealing with large systems. However, this difficulty
may be overcome by using some controllable quantum system to study another less
controllable or accessible quantum system, i.e., quantum simulation. Quantum
simulation promises to have applications in t...

We review basic physics and novel types of optical angular momentum. We start
with a theoretical overview of momentum and angular momentum properties of
generic optical fields, and discuss methods for their experimental
measurements. In particular, we describe the well-known longitudinal (i.e.,
aligned with the mean momentum) spin and orbital angul...

Exploiting the interplay between gain, loss and the coupling strength between different optical components creates a variety of new opportunities in photonics to generate, control and transmit light. Inspired by the discovery of real eigenfrequencies for non-Hermitian Hamiltonians obeying parity–time (PT) symmetry, many counterintuitive aspects are...

Ultrastrong coupling between light and matter has, in the past decade, transitioned from a theoretical idea to an experimental reality. It is a new regime of quantum light–matter interaction, which goes beyond weak and strong coupling to make the coupling strength comparable to the transition frequencies in the system. The achievement of weak and s...

In the past 20 years, impressive progress has been made both experimentally and theoretically in superconducting quantum circuits, which provide a platform for manipulating microwave photons. This emerging field of superconducting quantum microwave circuits has been driven by many new interesting phenomena in microwave photonics and quantum informa...

Superposition of trajectories, which modify quantum evolutions by superposing paths through interferometry, has been utilized to enhance various quantum communication tasks. However, little is known about its impact from the viewpoint of open quantum systems. Thus, we examine this subject from the perspective of system-environment interactions. We...

We describe a method to identify a dynamical critical phenomenon in the quantum Rabi model consisting of a three-level atom and a cavity; i.e., a sudden change of the photon number distribution in its equilibrium dynamics during a quantum phase transition. We show that this critical phenomenon can be interpreted as a sudden change of steady-state o...

The quantum Hall effect, fundamental in modern condensed matter physics, continuously inspires new theories and predicts emergent phases of matter. Analyzing the quantum Hall effect and other intriguing quantum topological phenomena by testing the bulk-edge correspondence remains challenging on quantum simulation platforms. We experimentally demons...

In the past years, many efforts have been made to study various noteworthy phenomena in both parity-time ($\mathcal{PT}$) and anti-parity-time ($\mathcal{APT}$) symmetric systems. However, entanglement dynamics in $\mathcal{APT}$-symmetric systems has not previously been investigated in both theory and experiments. Here, we investigate the entangle...

Superradiant phase transitions (SPTs) are important for understanding light-matter interactions at the quantum level [1-3], and play a central role in criticality-enhanced quantum sensing [4]. So far, SPTs have been observed in driven-dissipative systems [5-10], but where the emergent light fields did not show any nonclassical characteristic due to...

We develop a discrete fermion approach for modelling the strong interaction of an arbitrary system interacting with continuum electronic reservoirs. The approach is based on a pseudo-fermion decomposition of the continuum bath correlation functions, and is only limited by the accuracy of this decomposition. We show that to obtain this decomposition...

Single‐Photon Blockade In article number 2100430, Hui Jing, Şahin K. Özdemir, Franco Nori, Ran Huang and colleagues show that a purely quantum effect, known as single‐photon blockade, emerges in a non‐Hermitian microring resonator with chiral exceptional points induced by two nanotips. A striking feature of this photon blockade is that it emerges a...

Tensor product states (TPS), which enable decomposing the states of large-dimensional Hilbert space in terms of lower-dimensional elementary tensors, are fundamental tools to capture the quantum nature of condensed matter phenomena. Here, we show how TPS can naturally emerge in the synthetic space of operator moments of bosonic systems described by...

Quantum steering is an important correlation in quantum information theory. A recent work [Nat. Commun. 12, 2410 (2021)] showed that quantum steering is also beneficial to quantum metrology. Here, we extend the exploration of this steering-enhanced quantum metrology from a noiseless regime to a superposition of noisy phase shifts in quantum channel...

Quantum walks have wide applications in quantum information, such as universal quantum computation, so it is important to explore properties of quantum walks thoroughly. We propose a novel method to implement discrete-time quantum walks (DTQWs) using only a single qubit, in which both coin and walker are encoded in the two-dimensional state space o...

The Exceptional Points (EPs) of non-Hermitian Hamiltonians (NHHs) are spectral degeneracies associated with coalescing eigenvalues and eigenvectors which are associated with remarkable dynamical properties. These EPs can be generated experimentally in open quantum systems, evolving under a Lindblad equation, by postselecting on trajectories that pr...

The ongoing exploration of the ambiguous boundary between the quantum and the classical worlds has spurred substantial developments in quantum science and technology. Recently, the nonclassicality of dynamical processes has been proposed from a quantum-information-theoretic perspective, in terms of witnessing nonclassical correlations with Hamilton...

We study a single-photon band structure in a one-dimensional coupled-resonator optical waveguide that chirally couples to an array of two-level quantum emitters (QEs). The chiral interaction between the resonator mode and the QE can break the time-reversal symmetry without the magneto-optical effect and an external or synthetic magnetic field. As a...

Quantum optics with giant emitters has shown a new route for the observation and manipulation of non-Markovian properties in waveguide-QED. In this paper we extend the theory of giant atoms, hitherto restricted to the perturbative light-matter regime, to deal with the ultrastrong coupling regime. Using static and dynamical polaron methods we addres...

Non‐hermitian spectral degeneracies, known as exceptional points (EPs), feature the simultaneous coalescence of both eigenvalues and the associated eigenstates of a system. A host of intriguing EP effects and their applications have been revealed in the classical realm, such as loss‐induced lasing, single‐mode laser, and EP‐enhanced sensing. Here,...

We show that an atom can be coupled to a mechanical oscillator via quantum vacuum fluctuations of a cavity field enabling energy transfer processes between them. In a hybrid quantum system consisting of a cavity resonator with a movable mirror and an atom, these processes are dominated by two pair-creation mechanisms: the counter-rotating (atom-cav...

Multipartite entangled states are significant resources for both quantum information processing and quantum metrology. In particular, non-Gaussian entangled states are predicted to achieve a higher sensitivity of precision measurements than Gaussian states. On the basis of metrological sensitivity, the conventional linear Ramsey squeezing parameter...

Studies have shown that the Hilbert spaces of non-Hermitian systems require non-trivial metrics. Here, we demonstrate how evolution dimensions, in addition to time, can emerge naturally from the geometric formalism. Specifically, the Hamiltonian can be interpreted as a Christoffel symbol-like operator, and Schroedinger's equation as a parallel tran...

We reveal universal connections between three important phenomena in classical wave physics: (i) the ponderomotive force acting on the medium particles in an oscillatory wavefield, (ii) the Stokes drift of free medium particles in a wave field, and (iii) the canonical wave momentum in a medium. We analyse these phenomena for (a) longitudinal sound...

Topological phases feature robust edge states that are protected against the effects of defects and disorder. These phases have largely been studied in conservatively coupled systems, in which non-trivial topological invariants arise in the energy or frequency bands of a system. Here we show that, in dissipatively coupled systems, non-trivial topol...

Since the pioneering works by Landau, Zener, St\"{u}ckelberg, and Majorana (LZSM), it has been known that driving a quantum two-level system results in tunneling between its states. Even though the interference between these transitions is known to be important, it is only recently that it became both accessible, controllable, and useful for engine...

While the squeezing of a propagating field can, in principle, be made arbitrarily strong, the cavity-field squeezing is subject to the well-known 3 dB limit, and thus has limited applications. Here, we present a novel method to beat this limit using a fully quantum degenerate parametric amplifier (DPA). Specifically, we show that by {\it simply} ap...

We propose a general approach to implement nonadiabatic geometric single- and two-qubit gates beyond the rotating wave approximation (RWA). This protocol is compatible with most optimal control methods used in previous RWA protocols; thus, it is as robust as (or even more robust than) the RWA protocols. Using counter-rotating effects allows us to a...

A quantum dipole interacting with an optical cavity is one of the key models in cavity quantum electrodynamics (cavity-QED). To treat this system theoretically, the typical approach is to truncate the dipole to two levels. However, it has been shown that in the ultrastrong-coupling regime, this truncation naively destroys gauge invariance. By trunc...

We propose an all-optical approach to achieve optical nonreciprocity on a chip by quantum squeezing one of two coupled resonator modes. By parametric pumping a χ^{(2)}-nonlinear resonator unidirectionally with a classical coherent field, we squeeze the resonator mode in a selective direction due to the phase-matching condition, and induce a chiral...

Cavity optomechanical (COM) sensors, as powerful tools for measuring ultraweak forces or searching for dark matter, have been implemented to date mainly using linear COM couplings. Here, quantum force sensing is explored by using a quadratic COM system which is free of bistability and allows accurate measurement of mechanical energy. We find that t...

Matrix quantum mechanics plays various important roles in theoretical physics, such as a holographic description of quantum black holes, and it underpins the only practical numerical approach to the study of complex high-dimensional supergravity theories. Understanding quantum black holes and the role of entanglement in a holographic setup is of pa...

Feynman path integrals provide an elegant, classically-inspired representation for the quantum propagator and the quantum dynamics, through summing over a huge manifold of all possible paths. From computational and simulational perspectives, the ergodic tracking of the whole path manifold is a hard problem. Machine learning can help, in an efficien...

Entanglement of light and multiple vibrations is a key resource for multi-channel quantum information processing and memory. However, entanglement generation is generally suppressed, or even fully destroyed, by dark modes formed by multiple vibrational modes coupled to a common optical mode. Here we propose how to generate both \emph{bipartite} and...

Cat states, which were initially proposed to manifest macroscopic superpositions, play an outstanding role in fundamental aspects of quantum dynamics. In addition, they have potential applications in quantum computation and quantum sensing. However, cat states are vulnerable to dissipation, which puts the focus of cat-state generation on higher spe...

The study of the impact of noise on quantum circuits is especially relevant to guide the progress of Noisy Intermediate-Scale Quantum (NISQ) computing. In this paper, we address the pulse-level simulation of noisy quantum circuits with the Quantum Toolbox in Python (QuTiP). We introduce new tools in qutip-qip , QuTiP's quantum information processin...

Spin is a fundamental yet nontrivial intrinsic angular momentum property of quantum particles or fields, which appears within relativistic field theory. The spin density in wave fields is described by the theoretical Belinfante-Rosenfeld construction based on the difference between the canonical and kinetic momentum densities. These quantities are...

While classical chaos has been successfully characterized with consistent theories and intuitive techniques, such as with the use of Lyapunov exponents, quantum chaos is still poorly understood, as well as its relation with multi-partite entanglement and information scrambling. We consider a benchmark system, the kicked top model, which displays ch...

We present a canonical derivation of an influence superoperator which generates the reduced dynamics of a Fermionic quantum system linearly coupled to a Fermionic environment initially at thermal equilibrium. We use this formalism to derive a generalized Lindblad master equation (in the Markovian limit) and a generalized version of the hierarchical...

Phonon lasers, exploiting coherent amplifications of phonons, have been a cornerstone for exploring quantum phononics, imaging nanomaterial structures, and realizing force sensors or phonon frequency combs. Single-mode phonon lasers, governed by dispersive optomechanical coupling, have been recently demonstrated via levitating a nanoparticle using...

We introduce jump-time unraveling as a distinct description of open quantum systems. As our starting point, we consider quantum jump trajectories, which emerge, physically, from continuous quantum measurements, or, formally, from the unraveling of Markovian quantum master equations. If the stochastically evolving quantum trajectories are ensemble-a...

Phase transitions of thermal systems and the laser threshold were first connected more than forty years ago. Despite the nonequilibrium nature of the laser, the Landau theory of thermal phase transitions, applied directly to the Scully-Lamb laser model (SLLM), indicates that the laser threshold is a second-order phase transition, associated with a...

The prototypical system constituted by a two-level atom interacting with a quantized single-mode electromagnetic field is described by the quantum Rabi model (QRM). The QRM is potentially valid at any light-matter interaction regime, ranging from the weak (where the decay rates exceeds the coupling rate) to the deep strong coupling (where the inter...

A primary motivation for studying topological matter regards the protection of topological order from its environment. In this work, we study a topological emitter array coupled to an electromagnetic environment. The photon-emitter coupling produces nonlocal interactions between emitters. Using periodic boundary conditions for all ranges of environ...

We propose a new protected logic qubit called pokemon, which is derived from the 0-$\pi$ qubit by harnessing one capacitively shunted inductor and two capacitively shunted Josephson junctions embedded in a superconducting loop. Similar to the 0-$\pi$ qubit, the two basis states of the proposed qubit are separated by a high barrier, but their wave f...

The paradigm of second-order phase transitions (PTs) induced by spontaneous symmetry breaking (SSB) in thermal and quantum systems is a pillar of modern physics that has been fruitfully applied to out-of-equilibrium open quantum systems. Dissipative phase transitions (DPTs) of second order are often connected with SSB, in close analogy with well-kn...

Three-dimensional (3D) topological states resemble truly localised, particle-like objects in physical space. Among the richest such structures are 3D skyrmions and hopfions, that realise integer topological numbers in their configuration via homotopic mappings from real space to the hypersphere (sphere in 4D space) or the 2D sphere. They have recei...

We propose a mechanism to engineer an n-photon blockade in a nonlinear cavity with an n-photon parametric drive λ(â†n+ân). When an n-photon-excitation resonance condition is satisfied, the presence of n photons in the cavity suppresses the absorption of the subsequent photons. To confirm the validity of this proposal, we study the n-photon blockade...

We theoretically study the dynamics of an optomechanical system, consisting of a passive optical mode and an active mechanical mode, in the PT- and broken-PT-symmetric regimes. By fully analytical treatments for the dynamics of the average displacement and particle numbers, we reveal the phase diagram under different conditions and the various regi...

We study single-photon band structure in a one-dimensional (1D) coupled-resonator optical waveguide (CROW) which chirally couples to an array of two-level quantum emitters (QEs). The chiral interaction between the resonator mode and the QE can break the time-reversal symmetry without the magneto-optical effect. As a result, a nonreciprocal single-p...

Non-Hermitian parity-time ($\mathcal{PT}$) and anti-parity-time ($\mathcal{APT}$)-symmetric systems exhibit novel quantum properties and have attracted increasing interest. Although many counterintuitive phenomena in $\mathcal{PT}$- and $\mathcal{APT}$-symmetric systems were previously studied, coherence flow has been rarely investigated. Here, we...

For first-order topological semimetals, non-Hermitian perturbations can drive the Weyl nodes into Weyl exceptional rings having multiple topological structures and no Hermitian counterparts. Recently, it was discovered that higher-order Weyl semimetals, as a novel class of higher-order topological phases, can uniquely exhibit coexisting surface and...

In this work, we derive exact solutions of a dynamical equation, which can represent all two-level Hermitian systems driven by periodic N-step driving fields. For different physical parameters, this dynamical equation displays various phenomena for periodic N-step driven systems. The time-dependent transition probability can be expressed by a gener...

The Leggett-Garg inequality (LGI) distinguishes nonmacrorealistic channels from macrorealistic ones by constraining the experimental outcomes of the underlying system. In this work, we propose a class of channels which, initially, cannot violate the LGI (in the form of the temporal Bell inequality) but can violate it after the application of stocha...

The dark-mode effect is a stubborn obstacle for ground-state cooling of multiple degenerate mechanical modes optomechanically coupled to a common cavity-field mode. Here we propose an auxiliary-cavity-mode scheme for simultaneous ground-state cooling of two degenerate or near-degenerate mechanical modes by breaking the dark mode. We find that the i...

The paradigm of second-order phase transitions (PTs) induced by spontaneous symmetry breaking (SSB) in thermal and quantum systems is a pillar of modern physics that has been fruitfully applied to out-of-equilibrium open quantum systems. Dissipative phase transitions (DPTs) of second order are often connected with SSB, in close analogy with well-kn...