Search

A nonperturbative approach is developed to analyze superconducting circuits coupled to quantized electromagnetic continuum within the framework of the functional renormalization group. The formalism allows us to determine complete physical pictures of equilibrium properties in the circuit quantum electrodynamics (cQED) architectures with high-impedance waveguides, which have recently become accessible in experiments. We point out that nonperturbative effects can trigger breakdown of the supposedly effective descriptions, such as the spin-boson and boundary sine-Gordon models, and lead to qualitatively new phase diagrams. The origin of the failure of conventional understandings is traced to strong renormalizations of circuit parameters at low-energy scales. Our results indicate that a nonperturbative analysis is essential for a comprehensive understanding of cQED platforms consisting of superconducting circuits and long high-impedance transmission lines., Comment: 16 pages, 8 figures

The ground-state properties and excitation energies of a quantum emitter can be modified in the ultrastrong coupling regime of cavity quantum electrodynamics (QED) where the light-matter interaction strength becomes comparable to the cavity resonance frequency. Recent studies have started to explore the possibility of controlling an electronic material by embedding it in a cavity that confines electromagnetic fields in deep subwavelength scales. Currently, there is a strong interest in realizing ultrastrong-coupling cavity QED in the terahertz (THz) part of the spectrum, since most of the elementary excitations of quantum materials are in this frequency range. We propose and discuss a promising platform to achieve this goal based on a two-dimensional electronic material encapsulated by a planar cavity consisting of ultrathin polar van der Waals crystals. As a concrete setup, we show that nanometer-thick hexagonal boron nitride layers should allow one to reach the ultrastrong coupling regime for single-electron cyclotron resonance in a bilayer graphene. The proposed cavity platform can be realized by a wide variety of thin dielectric materials with hyperbolic dispersions. Consequently, van der Waals heterostructures hold the promise of becoming a versatile playground for exploring the ultrastrong-coupling physics of cavity QED materials., 6+5 pages, 3+3 figures

Dissipative quantum phase transition has been widely believed to occur in a Josephson junction coupled to a resistor despite a lack of concrete experimental evidence. Here, on the basis of both numerical and analytical nonperturbative renormalization group (RG) analyses, we reveal breakdown of previous perturbative arguments and defy the common wisdom that the transition always occurs at the quantum resistance $R_{Q} \!=\! h/(4e^2)$. We find that RG flows in nonperturbative regimes induce nonmonotonic renormalization of the charging energy and lead to a qualitatively different phase diagram, where the insulator phase is strongly suppressed to the deep charge regime (Cooper pair box), while the system is always superconducting in the transmon regime. We identify a previously overlooked dangerously irrelevant term as an origin of the failure of conventional understandings. Our predictions can be tested in recent experiments realizing high-impedance long superconducting waveguides and would provide a solution to the long-standing controversy about the fate of dissipative quantum phase transition in the resistively shunted Josephson junction., 6+12 pages, 4+6 figures, to appear in PRL

Autorzy: Abe K., Adrich P., Aihara H., Akutsu, R., Alekseev I., Ali A. , Ameli F., Anghel I., Anthony L. H. V., Antonova M. , Araya A., Asaoka Y., Ashida Y., Aushev V., Ballester F., Bandac I., Barbi M., Barker G. J., Barr G., Batkiewicz-Kwasniak M., Bellato M., Berardi V., Bergevin M., Bernard L., Bernardini E., Berns L., Bhadra S., Bian J., Blanchet A., Blaszczyk F. d. M., Blonde A., Boiano A., Bolognesi S., Bonavera L., Booth N., Borjabad S., Boschi, T., Bose D., Boyd S . B., Bozza C., Bravar A., Bravo-Berguño D., Bronner C., Brown L., Bubak Arkadiusz, Buchowicz A., Buizza Avanzini M., Cafagna F. S., Calabria N. F., Calvo-Mozota J. M., Cao S., Cartwright S.L., Carroll A., Catanesi M. G., Cebriàn, S., Chabera M., Chakraborty, S., Checchia C., Choi J.H., Choubey S., Cicerchia M., Coleman J., Collazuol G., Cook L., Cowan G., Cuen-Rochin, S., Danilov M., Díaz López G., De la Fuente E., de Perio P., De Rosa G., Dealtry T., Densham C. J., Dergacheva A., Deshmukh N., Devi M. M., Di Lodovico F., Di Meo, P., Di Palma I., Doyle T. A., Drakopoulou E., Drapier O., Dumarchez J., Dunne P., Dziewiecki M., Eklund L., El Hedri S., Ellis J., Emery S., Esmaili A., Esteve R., Evangelisti A., Feely M., Fedotov S., Feng J., Fernandez P., Fernández-Martinez E., Ferrario P., Ferrazzi,B., Feusels T., Finch A., Finley C., Fiorentini A., Fiorillo G., Fitton M., Frankiewicz K., Friend M., Fujii Y., Fukuda Y., Galinski G., Gao J., Garde C., Garfagnini A., Garode S., Gialanella L., Giganti C., Gomez-Cadenas J.J., Gonin M., González-Nuevo J., Gorin A., Gornea R., Gousy-Leblanc V. Gramegna F. Grassi M. Grella G. Guigue M. Gumplinger P. Hadley D.R. Harada M., Hartfiel B., Hartz M., Hassani S., Hastings N.C., Hayato Y., Hernando-Morata J.A., Herrero V., Hill J., Hiraide K., Hirota S., Holin A., Horiuchi S., Hoshina K., Hultqvist K., Iacob F., Ichikawa A.K., Idrissi Ibnsalih W., Iijima T., Ikeda M., Inomoto M., Inoue K., Insler J., Ioannisian A., Ishida T., Ishidoshiro K., Ishino H., Ishitsuka M., Ito H., Ito S., Itow Y., Iwamoto K., Izmaylov A., Izumi N., Izumiyama S., Jakkapu M., Jamieson B., Jang H.I., Jang J.S., Jenkins S.J., Jeon S.H., Jiang M., Jo H.S., Jonsson P., Joo K.K., Kajita T., Kakuno H., Kameda J., Kano Y., Kalaczynski P., Karlen D., Kasperek J., Kataoka Y., Kato A., Katori T., Kazarian N., Kearns E., Khabibullin M., Khotjantsev A., Kikawa T., Kekic M., Kim J.H., Kim J.Y., Kim S.B., Kim S.Y., King S., Kinoshita T., Kisiel Jan, Klekotko A., Kobayashi T., Koch L., Koga M., Koerich L., Kolev N., Konaka A., Kormos L.L., Koshio Y., Korzenev A., Kotsar Y., Kouzakov K.A., Kowalik K.L., Kravchuk L., Kryukov A.P., Kudenko Y., Kumita T., Kurjata R., Kutter T., Kuze M., Kwak K., La Commara M., Labarga L., Lagoda J., Lamers James J., Lamoureux M., Laveder M., Lavitola L., Lawe M., Learned J.G., Lee J., Leitner R., Lezaun V., Lim I.T., Lindner T., Litchfield R.P., Long K.R., Longhin A., Loverre P., Lu X., Ludovici L., Maekawa Y., Magaletti L., Magar K., Mahn K., Makida Y., Malek M., Malinský M., Marchi T., Maret L., Mariani C., Marinelli A., Martens K., Marti L., Martin J.F. Martin D., Marzec J., Matsubara T., Matsumoto R., Matsuno S., Matusiak M., Mazzucato E., McCarthy M., McCauley N., McElwee J., McGrew C., Mefodiev A., Medhi A., Mehta P., Mellet L., Menjo H., Mermod P., Metelko C., Mezzetto M., Migenda J., Migliozzi P., Mijakowski P., Miki S., Miller E.W., Minakata H., Minamino A., Mine S., Mineev O., Mitra A., Miura M., Moharana R., Mollo C.M., Mondal T., Mongelli M., Monrabal F., Moon D.H., Moon C.S., Mora F.J., Moriyama S., Mueller Th.A., Munteanu L., Murase K., Nagao Y., Nakadaira T., Nakagiri K., Nakahata M., Nakai S., Nakajima Y., Nakamura K., Nakamura KI., Nakamura H., Nakano Y., Nakaya T., Nakayama S., Nakayoshi K., Nascimento Machado L., Naseby C.E.R., Navarro-Garcia B., Needham M., Nicholls T., Niewczas K., Nishimura Y., Noah E., Nova F., Nugent J.C., Nunokawa H., Obrebski W., Ochoa-Ricoux J.P., O’Connor E., Ogawa N., Ogitsu T., Ohta K., Okamoto K., O’Keeffe H.M., Okumura K., Onishchuk Y., Orozco-Luna F., Oshlianskyi A., Ospina N., Ostrowski M., O’Sullivan E., O’Sullivan L., Ovsiannikova T., Oyama Y., Ozaki H., Pac M.Y., Paganini P., Palladino V., Paolone V., Pari M., Parsa S., Pasternak J., Pastore C., Pastuszak G., Patel D.A., Pavin M., Payne D., Peña-Garay C., Pidcott C., Pinzon Guerra E., Playfer S., Pointon B.W., Popov A., Popov B., Porwit Kamil, Posiadala-Zezula M., Poutissou J.M., Pozimski J., Pronost G., Prouse N.W., Przewlocki P., Quilain B., Quiroga A.A., Radicioni E., Radics B., Rajda P.J., Renner J., Rescigno M., Retiere F., Ricciardi G., Riccio C., Richards B., Rondio E., Rose H.J., Roskovec B., Roth S., Rott C., Rountree S.D., Rubbia A., Ruggeri A.C., Ruggles C., Russo S., Rychter A., Ryu D., Sakashita K., Samani S., Sánchez F., Sánchez M.L., Sanchez M.C., Sano S., Santos J.D., Santucci G., Sarmah P., Sashima I., Sato K., Scott M., Seiya Y., Sekiguchi T., Sekiya H., Seo J.W., Seo S.H., Sgalaberna D., Shaikhiev A., Shan Z., Shaykina A., Shimizu I., Shin C.D., Shinoki M., Shiozawa M., Sinnis G., Skrobova N., Skwarczynski K., Smy M.B., Sobczyk J., Sobel H.W., Soler F. J. P., Sonoda Y., Spina R., Spisso B., Spradlin B., Stankevich K.L., Stawarz L., Stellacci S.M., Stopa K., Studenikin A.I., Suárez Gómez S.L., Suganuma T., Suvorov S., Suwa Y., Suzuki A.T., Suzuki S.Y., Suzuki Y., Svirida D., Svoboda R., Taani M., Tada M., Takeda A., Takemoto Y., Takenaka A., Taketa A., Takeuchi Y., Takhistov V., Tanaka H., Tanaka H.A., Tanaka H.I., Tanaka M., Tashiro T., Thiesse M., Thompson L.F., Toledo J., Tomatani-Sánchez A.K., Tortone G., Tsui K.M., Tsukamoto T., Tzanov M., Uchida Y., Vagins M.R., Valder S., Valentino V., Vasseur G., Vijayvargi A., Vilela C., Vinning W. G. S., Vivolo D., Vladisavljevic T., Vogelaar R.B., Vyalkov M.M., Wachala T., Walker J., Wark D., Wascko M.O., Wendell R.A., Wilkes R.J., Wilking M.J., Wilson M.R., Wronka S., Xia J., Xie Z., Xin T., Yamaguchi Y., Yamamoto K., Yanagisawa C., Yano T., Yen S., Yershov N., Yeum D.N., Yokoyama M., Yonenaga M., Yoo J., Yu I., Yu M., Zakrzewski T., Zaldivar B., Zalipska J., Zaremba K., Zarnecki G., Ziembicki M., Zietara K., Zito M., Zsoldos S., Core-collapse supernovae are among the most magnificent events in the observable universe. They produce many of the chemical elements necessary for life to exist and their remnants-neutron stars and black holes-are interesting astrophysical objects in their own right. However, despite millennia of observations and almost a century of astrophysical study, the explosion mechanism of core-collapse supernovae is not yet well understood. Hyper-Kamiokande is a next-generation neutrino detector that will be able to observe the neutrino flux from the next galactic core-collapse supernova in unprecedented detail. We focus on the first 500 ms of the neutrino burst, corresponding to the accretion phase, and use a newly-developed, high-precision supernova event generator to simulate Hyper-Kamiokandeʼs response to five different supernova models. We show that Hyper-Kamiokande will be able to distinguish between these models with high accuracy for a supernova at a distance of up to 100 kpc. Once the next galactic supernova happens, this ability will be a powerful tool for guiding simulations toward a precise reproduction of the explosion mechanism observed in nature.

We propose a method based on deep reinforcement learning that efficiently prepares a quantum many-body pure state in thermal or prethermal equilibrium. The main physical intuition underlying the method is that the information on the equilibrium states can be efficiently encoded/extracted by focusing on only a few local observables, relying on the typicality of equilibrium states. Instead of resorting to the expensive preparation protocol that adopts global features such as the quantum state fidelity, we show that the equilibrium states can be efficiently prepared only by learning the expectation values of local observables. We demonstrate our method by preparing two illustrative examples: Gibbs ensembles in non-integrable systems and generalized Gibbs ensembles in integrable systems. Pure states prepared solely from local observables are numerically shown to successfully encode the macroscopic properties of the equilibrium states. Furthermore, we find that the preparation errors, with respect to the system size, decay exponentially for Gibbs ensembles and polynomially for generalized Gibbs ensembles, which are in agreement with the finite-size fluctuation within thermodynamic ensembles. Our method paves a path toward studying the thermodynamic and statistical properties of quantum many-body systems in quantum hardware., Comment: 18 pages, 15 figures;Appendix F-I added, some additional descriptions, script & fugures improved, results unchanged

The Super-Kamiokande detector can be used to search for neutrinos in time coincidence with gravitational waves detected by the LIGO-Virgo Collaboration (LVC). Both low-energy ($7-100$ MeV) and high-energy ($0.1-10^5$ GeV) samples were analyzed in order to cover a very wide neutrino spectrum. Follow-ups of 36 (out of 39) gravitational waves reported in the GWTC-2 catalog were examined; no significant excess above the background was observed, with 10 (24) observed neutrinos compared with 4.8 (25.0) expected events in the high-energy (low-energy) samples. A statistical approach was used to compute the significance of potential coincidences. For each observation, p-values were estimated using neutrino direction and LVC sky map ; the most significant event (GW190602_175927) is associated with a post-trial p-value of $7.8\%$ ($1.4\sigma$). Additionally, flux limits were computed independently for each sample and by combining the samples. The energy emitted as neutrinos by the identified gravitational wave sources was constrained, both for given flavors and for all-flavors assuming equipartition between the different flavors, independently for each trigger and by combining sources of the same nature., Comment: 16 pages, 5 figures. v2: adding corrections from The Astrophysical Journal review

K. Abe, N. Akhlaq, R. Akutsu, A. Ali, C. Alt, C. Andreopoulos, M. Antonova, S. Aoki, T. Arihara, Y. Asada, Y. Ashida, E. T. Atkin, Y. Awataguchi, G. J. Barker, G. Barr, D. Barrow, M. Batkiewicz-Kwasniak, A. Beloshapkin, F. Bench, V. Berardi, L. Berns, S. Bhadra, A. Blanchet, A. Blondel, S. Bolognesi, T. Bonus, B. Bourguille, S. B. Boyd, A. Bravar, D. Bravo Berguno, C. Bronner, S. Bron, M. Buizza Avanzini, S. Cao, S. L. Cartwright, M. G. Catanesi, A. Cervera, J. Chakrani, D. Cherdack, G. Christodoulou, M. Cicerchia, J. Coleman, G. Collazuol, L. Cook, D. Coplowe, A. Cudd, G. De Rosa, T. Dealtry, C. C. Delogu, S. R. Dennis, C. Densham, A. Dergacheva, F. Di Lodovico, S. Dolan, D. Douqa, T. A. Doyle, J. Dumarchez, P. Dunne, A. Eguchi, L. Eklund, S. Emery-Schrenk, A. Ereditato, A. J. Finch, G. Fiorillo, C. Francois, M. Friend, Y. Fujii, R. Fukuda, Y. Fukuda, K. Fusshoeller, C. Giganti, M. Gonin, E. A. G. Goodman, A. Gorin, M. Grassi, M. Guigue, D. R. Hadley, P. Hamacher-Baumann, D. A. Harris, M. Hartz, T. Hasegawa, S. Hassani, N. C. Hastings, Y. Hayato, A. Hiramoto, M. Hogan, N. T. Hong Van, T. Honjo, F. Iacob, A. K. Ichikawa, M. Ikeda, T. Ishida, M. Ishitsuka, K. Iwamoto, A. Izmaylov, N. Izumi, M. Jakkapu, B. Jamieson, S. J. Jenkins, C. Jesus-Valls, J. J. Jiang, P. Jonsson, C. K. Jung, P. B. Jurj, M. Kabirnezhad, H. Kakuno, J. Kameda, S. P. Kasetti, Y. Kataoka, Y. Katayama, T. Katori, E. Kearns, M. Khabibullin, A. Khotjantsev, T. Kikawa, H. Kikutani, S. King, T. Kobata, T. Kobayashi, L. Koch, A. Konaka, L. L. Kormos, Y. Koshio, A. Kostin, K. Kowalik, Y. Kudenko, S. Kuribayashi, R. Kurjata, T. Kutter, M. Kuze, L. Labarga, J. Lagoda, M. Lamoureux, D. Last, M. Laveder, M. Lawe, S.-K. Lin, R. P. Litchfield, S. L. Liu, A. Longhin, L. Ludovici, X. Lu, T. Lux, L. N. Machado, L. Magaletti, K. Mahn, M. Malek, S. Manly, L. Maret, A. D. Marino, L. Marti-Magro, T. Maruyama, T. Matsubara, K. Matsushita, C. Mauger, A. Maurel, K. Mavrokoridis, E. Mazzucato, N. McCauley, J. McElwee, K. S. McFarland, C. McGrew, A. Mefodiev, G. D. Megias, L. Mellet, M. Mezzetto, A. Minamino, O. Mineev, S. Mine, M. Miura, L. Molina Bueno, S. Moriyama, Th. A. Mueller, D. Munford, L. Munteanu, Y. Nagai, T. Nakadaira, M. Nakahata, Y. Nakajima, A. Nakamura, H. Nakamura, K. Nakamura, Y. Nakano, S. Nakayama, T. Nakaya, K. Nakayoshi, C. E. R. Naseby, T. V. Ngoc, V. Q. Nguyen, K. Niewczas, Y. Nishimura,K. Nishizaki, E. Noah, T. S. Nonnenmacher, F. Nova, J. Nowak, J. C. Nugent, H. M. O’Keeffe, L. O’Sullivan, T. Odagawa, T. Ogawa, R. Okada, K. Okumura, T. Okusawa, R. A. Owen, Y. Oyama,,† V. Palladino, V. Paolone, M. Pari, W. C. Parker, J. Parlone, S. Parsa, J. Pasternak, M. Pavin, D. Payne, G. C. Penn, D. Pershey, L. Pickering, C. Pidcott, G. Pintaudi, C. Pistillo, B. Popov, M. Posiadala-Zezula, A. Pritchard, B. Quilain, T. Radermacher, E. Radicioni, B. Radics, P. N. Ratoff, M. Reh, C. Riccio, E. Rondio, S. Roth, A. Rubbia, A. C. Ruggeri, C. A. Ruggles, A. Rychter, L. S. M. Lakshmi, K. Sakashita, F. Sanchez, G. Santucci, C. M. Schloesser, K. Scholberg, M. Scott , Y. Seiya, T. Sekiguchi, H. Sekiya, D. Sgalaberna, A. Shaikhiev, A. Shaykina, M. Shiozawa, W. Shorrock, A. Shvartsman, K. Skwarczynski, M. Smy, J. T. Sobczyk, H. Sobel, F. J. P. Soler, Y. Sonoda, R. Spina, S. Suvorov, A. Suzuki, S. Y. Suzuki, Y. Suzuki, A. A. Sztuc, M. Tada,, M. Tajima, A. Takeda, Y. Takeuchi, H. K. Tanaka, Y. Tanihara, M. Tani, N. Teshima, N. Thamm, L. F. Thompson, W. Toki, C. Touramanis, T. Towstego, K. M. Tsui, T. Tsukamoto, M. Tzanov, Y. Uchida, M. Vagins, S. Valder, D. Vargas, G. Vasseur, C. Vilela, W. G. S. Vinning, T. Vladisavljevic, T. Wachala, J. Walker, J. G. Walsh, Y. Wang, L. Wan, D. Wark, M. O. Wascko, A. Weber, R. Wendell, M. J. Wilking, C. Wilkinson, J. R. Wilson, K. Wood, C. Wret, J. Xia, Y.-h. Xu, K. Yamamoto, C. Yanagisawa, G. Yang, T. Yano, K. Yasutome, N. Yershov, M. Yokoyama, T. Yoshida, Y. Yoshimoto, M. Yu, R. Zaki, A. Zalewska, J. Zalipska, K. Zaremba, G. Zarnecki, M. Ziembicki, M. Zito, S. Zsoldos

Power and efficiency of heat engines are two conflicting objectives, and a tight efficiency bound is expected to give insights on the fundamental properties of the power-efficiency tradeoff. Here we derive an upper bound on the efficiency of steady-state heat engines, which incorporates higher-order fluctuations of the power. In a prototypical model of nonlinear nanostructured thermoelectrics, we show that the obtained bound is tighter than a well-established efficiency bound based on the thermodynamic uncertainty relation, demonstrating that the higher-order terms have rich information about the thermodynamic efficiency in the nonlinear regime. In particular, we find that the higher-order bound is exactly achieved if the tight coupling condition is satisfied. The obtained bound gives a consistent prediction with the observation that nonlinearity enhances the power-efficiency tradeoff, and would also be useful for various nanoscale engines., Comment: 10 pages, 6 figures

The quest to identify the best heat engine has been at the center of science and technology. Considerable studies have so far revealed the potentials of nanoscale thermal machines to yield an enhanced thermodynamic efficiency in noninteracting regimes. However, the full benefit of many-body interactions is yet to be investigated; identifying the optimal interaction is a hard problem due to combinatorial explosion of the search space, which makes brute-force searches infeasible. We tackle this problem with developing a framework for reinforcement learning of network topology in interacting thermal systems. We find that the maximum possible values of the figure of merit and the power factor can be significantly enhanced by electron-electron interactions under nondegenerate single-electron levels with which, in the absence of interactions, the thermoelectric performance is quite low in general. This allows for an alternative strategy to design the best heat engines by optimizing interactions instead of single-electron levels. The versatility of the developed framework allows one to identify full potential of a broad range of nanoscale systems in terms of multiple objectives. While the thermodynamic power and efficiency of nanoscale heat engines in noninteracting regimes has been well-explored, revealing effect of many-body interactions remains a challenge. Here, the authors develop a reinforcement learning framework to achieve optimal power and efficiency in nanoengines where two-body interactions among elementary components are nonnegligible.

We report measurements of the flux-integrated ν̅μ and ν̅μ + νμ charged-current cross-sections on water and hydrocarbon targets using the T2K anti-neutrino beam with a mean beam energy of 0.86 GeV. The signal is defined as the (anti-)neutrino charged-current interaction with one induced $\mu^\pm$ and no detected charged pion or proton. These measurements are performed using a new WAGASCI module recently added to the T2K setup in combination with the INGRID Proton Module. The phase space of muons is restricted to the high-detection efficiency region, $p_{\mu}>400~{\rm MeV}/c$ and $\theta_{\mu}200~{\rm MeV}/c$, $\theta_{\pi}600~{\rm MeV}/c$, $\theta_{\rm p}

A review is given on the foundations and applications of non-Hermitian classical and quantum physics. First, key theorems and central concepts in non-Hermitian linear algebra, including Jordan normal form, biorthogonality, exceptional points, pseudo-Hermiticity and parity-time symmetry, are delineated in a pedagogical and mathematically coherent manner. Building on these, we provide an overview of how diverse classical systems, ranging from photonics, mechanics, electrical circuits, acoustics to active matter, can be used to simulate non-Hermitian wave physics. In particular, we discuss rich and unique phenomena found therein, such as unidirectional invisibility, enhanced sensitivity, topological energy transfer, coherent perfect absorption, single-mode lasing, and robust biological transport. We then explain in detail how non-Hermitian operators emerge as an effective description of open quantum systems on the basis of the Feshbach projection approach and the quantum trajectory approach. We discuss their applications to physical systems relevant to a variety of fields, including atomic, molecular and optical physics, mesoscopic physics, and nuclear physics with emphasis on prominent phenomena/subjects in quantum regimes, such as quantum resonances, superradiance, continuous quantum Zeno effect, quantum critical phenomena, Dirac spectra in quantum chromodynamics, and nonunitary conformal field theories. Finally, we introduce the notion of band topology in complex spectra of non-Hermitian systems and present their classifications by providing the proof, firstly given by this review in a complete manner, as well as a number of instructive examples. Other topics related to non-Hermitian physics, including nonreciprocal transport, speed limits, nonunitary quantum walk, are also reviewed., Review article commissioned by Advances in Physics - 191 pages, 49 figures, 8 tables

Band topology has been studied as a design principle of realizing robust boundary modes. Here, by exploring non-Hermitian topology, we propose a three-dimensional topological laser that amplifies surface modes. The topological surface laser is protected by nontrivial topology around branchpoint singularities known as exceptional points. In contrast to two-dimensional topological lasers, the proposed three-dimensional setup can realize topological boundary modes without judicious gain at the edge or symmetry protection, which are thus robust against a broad range of disorders. We also propose a possible optical setup to experimentally realize the topological surface laser. Our results provide a general guiding principle to construct non-Hermitian topological devices in three-dimensional systems., Comment: 12 pages, 12 figures

We have searched for proton decay via p→e+π0 and p→μ+π0 modes with the enlarged fiducial volume data of Super-Kamiokande from April 1996 to May 2018, which corresponds to 450 kton·years exposure. We have accumulated about 25% more livetime and enlarged the fiducial volume of the Super-Kamiokande detector from 22.5 kton to 27.2 kton for this analysis, so that 144 kton·years of data, including 78 kton·years of additional fiducial volume data, has been newly analyzed. No candidates have been found for p→e+π0 and one candidate remains for p→μ+π0 in the conventional 22.5 kton fiducial volume and it is consistent with the atmospheric neutrino background prediction. We set lower limits on the partial lifetime for each of these modes: τ/B(p→e+π0)>2.4×1034 years and τ/B(p→μ+π0)>1.6×1034 years at 90% confidence level.

The light-matter interaction can be utilized to qualitatively alter physical properties of materials. Recent theoretical and experimental studies have explored this possibility of controlling matter by light based on driving many-body systems via strong classical electromagnetic radiation, leading to a time-dependent Hamiltonian for electronic or lattice degrees of freedom. To avoid inevitable heating, pump-probe setups with ultrashort laser pulses have so far been used to study transient light-induced modifications in materials. Here, we pursue yet another direction of controlling quantum matter by modifying quantum fluctuations of its electromagnetic environment. In contrast to earlier proposals on light-enhanced electron-electron interactions, we consider a dipolar quantum many-body system embedded in a cavity composed of metal mirrors, and formulate a theoretical framework to manipulate its equilibrium properties on the basis of quantum light-matter interaction. We analyze hybridization of different types of the fundamental excitations, including dipolar phonons, cavity photons, and plasmons in metal mirrors, arising from the cavity confinement in the regime of strong light-matter interaction. This hybridization qualitatively alters the nature of the collective excitations and can be used to selectively control energy-level structures in a wide range of platforms. Most notably, in quantum paraelectrics, we show that the cavity-induced softening of infrared optical phonons enhances the ferroelectric phase in comparison with the bulk materials. Our findings suggest an intriguing possibility of inducing a superradiant-type transition via the light-matter coupling without external pumping. We also discuss possible applications of the cavity-induced modifications in collective excitations to molecular materials and excitonic devices., Comment: 30 pages, 14 figures, to appear in PRX

We thank the J-PARC staff for superb accelerator performance. We thank the CERN NA61/SHINE Collaboration for providing valuable particle production data. We acknowledge the support of MEXT, Japan; NSERC (Grant No. SAPPJ-2014-00031), the NRC and CFI, Canada; the CEA and CNRS/IN2P3, France; the DFG, Germany; the INFN, Italy; the National Science Centre and Ministry of Science and Higher Education, Poland; the RSF (Grant No. 19-12-00325) and the Ministry of Science and Higher Education, Russia; MINECO and ERDF funds, Spain; the SNSF and SERI, Switzerland; the STFC, UK; and the DOE, USA. We also thank CERN for the UA1/NOMAD magnet, DESY for the HERA-B magnet mover system, NII for SINET4, the WestGrid and SciNet consortia in Compute Canada, and GridPP in the United Kingdom. In addition, participation of individual researchers and institutions has been further supported by funds from the ERC (FP7), “la Caixa” Foundation (ID 100010434, fellowship code LCF/BQ/IN17/11620050), the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Sklodowska- Curie Grants Agreement No. 713673 and No. 754496, and H2020 Grant No. RISE-GA822070-JENNIFER2 2020 and No. RISE-GA872549-SK2HK; the JSPS, Japan; the Royal Society, UK; French ANR Grant No. ANR-19- CE31-0001; and the DOE Early Career programme, USA.

A quantum system that strongly correlates with an external world is ubiquitous in nature. There, one has to deal with the strong entanglement between the system and the environment. Ideally, this can be achieved by explicitly taking into account all the degrees of freedom of the environment rather than eliminating them as done in the master-equation approach. In this chapter, we move on to studies of in- and out-of-equilibrium physics in such a strongly correlated open quantum system by focusing on its most fundamental paradigm, that is, a quantum impurity. We develop a versatile and efficient theoretical approach to study ground-state and out-of-equilibrium properties of generic quantum spin-impurity systems. In particular, we introduce a new canonical transformation that can completely disentangle the localized spin and the environmental degrees of freedom. After introducing our general variational formalism for a fermionic environment, we benchmark our approach by comparing it with other numerical and analytical results in both in- and out-of-equilibrium regimes. We also reveal new types of nonequilibrium dynamics such as long-time crossover dynamics mimicking nonmonotonic renormalization group flows, which has been difficult to study in other methods. We propose a possible experiment to test the predicted dynamics by using quantum gas microscopy. We also generalize our approach to a bosonic environment and apply it to study a novel type of strongly correlated systems realized in the state-of-the-art experiments of Rydberg molecules, which have been otherwise challenging to analyze in previous theoretical approaches.

Quantum light-matter systems at strong coupling are notoriously challenging to analyze due to the need to include states with many excitations in every coupled mode. We propose a nonperturbative approach to analyze light-matter correlations at all interaction strengths. The key element of our approach is a unitary transformation that achieves asymptotic decoupling of light and matter degrees of freedom in the limit where light-matter interaction becomes the dominant energy scale. In the transformed frame, truncation of the matter/photon Hilbert space is increasingly well-justified at larger coupling, enabling one to systematically derive low-energy effective models, such as tight-binding Hamiltonians. We demonstrate the versatility of our approach by applying it to concrete models relevant to electrons in crystal potential and electric dipoles interacting with a cavity mode. A generalization to the case of spatially varying electromagnetic modes is also discussed., Comment: 7+6 pages, 3+2 figures, to appear in PRL

The seminal theoretical works of Berezinskii, Kosterlitz, and Thouless presented a new paradigm for phase transitions in condensed matter that are driven by topological excitations. These transitions have been extensively studied in the context of two-dimensional XY models -- coupled compasses -- and have generated interest in the context of quantum simulation. Here, we use a circuit quantum-electrodynamics architecture to study the critical behavior of engineered XY models through their dynamical response. In particular, we examine not only the unfrustrated case but also the fully-frustrated case which leads to enhanced degeneracy associated with the spin rotational [U$(1)$] and discrete chiral ($Z_2$) symmetries. The nature of the transition in the frustrated case has posed a challenge for theoretical studies while direct experimental probes remain elusive. Here we identify the transition temperatures for both the unfrustrated and fully-frustrated XY models by probing a Josephson junction array close to equilibrium using weak microwave excitations and measuring the temperature dependence of the effective damping obtained from the complex reflection coefficient. We argue that our probing technique is primarily sensitive to the dynamics of the U$(1)$ part., Comment: 10 pages, 7 figures

We investigate entanglement phase transitions from volume-law to area-law entanglement in a quantum many-body state under continuous position measurement on the basis of the quantum trajectory approach. We find the signatures of the transitions as peak structures in the mutual information as a function of measurement strength, as previously reported for random unitary circuits with projective measurements. At the transition points, the entanglement entropy scales logarithmically and various physical quantities scale algebraically, implying emergent conformal criticality, for both integrable and nonintegrable one-dimensional interacting Hamiltonians; however, such transitions have been argued to be absent in noninteracting regimes in some previous studies. With the aid of $U(1)$ symmetry in our model, the measurement-induced criticality exhibits a spectral signature resembling a Tomonaga-Luttinger liquid theory from symmetry-resolved entanglement. These intriguing critical phenomena are unique to steady-state regimes of the conditional dynamics at the single-trajectory level, and are absent in the unconditional dynamics obeying the Lindblad master equation, in which the system ends up with the featureless, infinite-temperature mixed state. We also propose a possible experimental setup to test the predicted entanglement transition based on the subsystem particle-number fluctuations. This quantity should readily be measured by the current techniques of quantum gas microscopy and is in practice easier to obtain than the entanglement entropy itself., Comment: 15 pages, 14 figures, v3: This version includes corrections to the results obtained by an erroneous numerical algorithm in the previous versions, which caused quantitative changes to the numerical results

We review a general theory to describe the nonunitary evolution of quantum systems under measurement, which is the main subject of the first part of this Thesis. Quantum measurement theory provides us with a theoretical framework to discuss how a quantum system exhibits an unavoidable change due to a measurement process. In particular, theory of continuous measurements gives a unified description to study the nonunitary dynamics of quantum systems subject to weak and frequent repeated measurements. In Sect. 2.1, we formulate a quantum measurement process based on an indirect measurement model and review its mathematical property. In Sect. 2.2, we apply it to formulate a theory of continuous observations, in which measurements are performed continuously in time.

In this Chapter, we study yet another fundamental paradigm of a quantum impurity, a mobile spinless particle interacting with a many-particle environment. The ultimate building block of such a system is the formation of a polarization cloud of collective excitations around an impurity, which is known as a polaron cloud. Yet, ever since the original paper by Landau and Pekar [1], it has long remained a fundamental challenge to measure it in an unambiguous manner due to the elusive nature of the polaron cloud. We here present a novel platform using ultracold atoms to overcome the obstacle and allow one to directly probe the polaron-cloud formation in real time. We reveal the emergence of rich nonequilibrium dynamics of the polaron cloud in the strong-coupling regime that is not readily attainable in solid-state materials. To our knowledge, our work suggests the first concrete possibility for a direct real-time measurement of the polaron-cloud formation.

Synchronization of coupled oscillators is a ubiquitous phenomenon found throughout nature. Its robust realization is crucial to our understanding of various nonlinear systems, ranging from biological functions to electrical engineering. On another front, in condensed matter physics, topology is utilized to realize robust properties like topological edge modes, as demonstrated by celebrated topological insulators. Here, we integrate these two research avenues and propose a nonlinear topological phenomenon, namely topological synchronization, where only the edge oscillators synchronize while the bulk ones exhibit chaotic dynamics. We analyze concrete prototypical models to demonstrate the presence of positive Lyapunov exponents and Lyapunov vectors localized along the edge. As a unique characteristic of topology in nonlinear systems, we find that unconventional extra topological boundary modes appear at emerging effective boundaries. Furthermore, our proposal shows promise for spatially controlling synchronization, such as on-demand pattern designing and defect detection. The topological synchronization can ubiquitously appear in topological nonlinear oscillators and thus can provide a guiding principle to realize synchronization in a robust, geometrical, and flexible way., Comment: 24 pages, 26 figures

Quantum critical phenomena originate from collective behavior of strongly correlated particles and lie at the heart of universal low-energy properties in many-body systems. The strong correlation between quantum particles is particularly prominent in a low-dimensional system. In the first part of this Chapter, we identify what types of measurements are relevant to one-dimensional low-energy properties and address how they qualitatively alter the underlying quantum critical behavior. In the second part, we study how the measurement backaction influences on quantum phase transitions in higher dimensions by focusing on the Bose-Hubbard model. For all the theoretical considerations, we discuss possible experimental implementations in ultracold atomic gases.

We review the background of the results presented in this thesis, which studies the fundamental aspects of many-body physics in quantum systems open to an external world. Firstly, we review recent remarkable experimental developments of observing and manipulating quantum matter at the single-quantum level. We discuss how they motivate the studies of elucidating the influence of an external observer on quantum many-body phenomena beyond the conventional paradigm of closed systems. Secondly, we review the physics of open quantum systems strongly correlated with an external environment and motivate the need of unveiling the entanglement between the system and the environment to understand genuine many-body effects such as the Kondo effect. Finally, we provide a short summary of each Chapter.

We report our realization of a parity-time (PT) symmetric non-Hermitian many-body system using cold atoms with dissipation. After developing a theoretical framework on PT-symmetric many-body systems using ultracold atoms in an optical lattice with controlled dissipation, we describe our experimental setup utilizing one-body atom loss as dissipation with special emphasis on calibration of important system parameters. We discuss loss dynamics observed experimentally., Comment: 13 pages, 8 figures

We present three general frameworks to describe the dynamics of quantum many-particle systems under continuous observation. In Sect. 4.1, we formulate the dynamics conditioned on the number of quantum jump events, which we term as the full-counting dynamics. We apply it to an exactly solvable model of noninteracting fermions and analyze its out-of-equilibrium dynamics after the quench. We find nonlocal and chiral propagation of correlations beyond the Lieb-Robinson bound. The unique features originate from the non-Hermiticity of the continuously monitored dynamics and do not appear in the corresponding closed systems or the ensemble-averaged dissipative dynamics. In Sect. 4.2, we formulate the thermalization and heating dynamics in generic many-body systems under measurements. Employing the eigenstate thermalization hypothesis, we show that a generic (nonintegrable) many-body system will thermalize at a single-trajectory level under continuous observation. We provide numerical evidence of our findings by studying specific nonintegrable models that are relevant to state-of-the-art experimental setups in ultracold gases. In Sect. 4.3, we formulate the diffusive dynamics under a minimally destructive spatial observation. We derive the many-body stochastic Schrodinger equation for indistinguishable particles under continuous position measurement. We show that the measurement indistinguishability of particles results in complete suppression of relative positional decoherence, leading to persistent correlations in transport dynamics under measurement. We apply the theory of minimally destructive spatial observation to a setup of ultracold atoms in an optical lattice. In Sect. 4.4, we discuss possible experimental realizations of our theoretical studies presented in this chapter. Finally, we conclude this chapter with an outlook in Sect. 4.5.

We consider dynamics of a Rydberg impurity in a cloud of ultracold bosonic atoms in which the Rydberg electron can undergo spin-changing collisions with surrounding atoms. This system realizes a new type of the quantum impurity problem that compounds essential features of the Kondo model, the Bose polaron, and the central spin model. To capture the interplay of the Rydberg-electron spin dynamics and the orbital motion of atoms, we employ a new variational method that combines an impurity-decoupling transformation with a Gaussian ansatz for the bath particles. We find several unexpected features of this model that are not present in traditional impurity problems, including interaction-induced renormalization of the absorption spectrum that eludes simple explanations from molecular bound states, and long-lasting oscillations of the Rydberg-electron spin. We discuss generalizations of our analysis to other systems in atomic physics and quantum chemistry, where an electron excitation of high orbital quantum number interacts with a spinful quantum bath., Comment: 6 pages, 5 figures. See also Phys. Rev. A 100, 043618 (2019) [arXiv:1905.09615]

We generalize a standard benchmark of reinforcement learning, the classical cartpole balancing problem, to the quantum regime by stabilizing a particle in an unstable potential through measurement and feedback. We use state-of-the-art deep reinforcement learning to stabilize a quantum cartpole and find that our deep learning approach performs comparably to or better than other strategies in standard control theory. Our approach also applies to measurement-feedback cooling of quantum oscillators, showing the applicability of deep learning to general continuous-space quantum control., 5+4 pages, 2+2 figures, 2+2 tables, 5 videos at an external link

Active systems exhibit spontaneous flows induced by self-propulsion of microscopic constituents and can reach a nonequilibrium steady state without an external drive. Constructing the analogy between the quantum anomalous Hall insulators and active matter with spontaneous flows, we show that topologically protected sound modes can arise in a steady-state active system in continuum space. We point out that the net vorticity of the steady-state flow, which acts as a counterpart of the gauge field in condensed-matter settings, must vanish under realistic conditions for active systems. The quantum anomalous Hall effect thus provides design principles for realizing topological metamaterials. We propose and analyze the concrete minimal model and numerically calculate its band structure and eigenvectors, demonstrating the emergence of nonzero bulk topological invariants with the corresponding edge sound modes. This new type of topological active systems can potentially expand possibilities for their experimental realizations and may have broad applications to practical active metamaterials. Possible realization of non-Hermitian topological phenomena in active systems is also discussed., Comment: 6+5 pages, 4+4 figures, to appear in PRL, see also supplementary movie published with the manuscript

Topological materials exhibit edge-localized scattering-free modes protected by their nontrivial bulk topology through the bulk-edge correspondence in Hermitian systems. While topological phenomena have recently been much investigated in non-Hermitian systems with dissipations and injections, the fundamental principle of their edge modes has not fully been established. Here, we reveal that, in non-Hermitian systems, robust gapless edge modes can ubiquitously appear owing to a mechanism that is distinct from bulk topology, thus indicating the breakdown of the bulk-edge correspondence. The robustness of these edge modes originates from yet another topological structure accompanying the branchpoint singularity around an exceptional point, at which eigenvectors coalesce and the Hamiltonian becomes nondiagonalizable. Their characteristic complex eigenenergy spectra are applicable to realize lasing wave packets that propagate along the edge of the sample. We numerically confirm the emergence and the robustness of the proposed edge modes in the prototypical lattice models. Furthermore, we show that these edge modes appear in a model of chiral active matter based on the hydrodynamic description, demonstrating that active matter can exhibit an inherently non-Hermitian topological feature. The proposed general mechanism would serve as an alternative designing principle to realize scattering-free edge current in non-Hermitian devices, going beyond the existing frameworks of non-Hermitian topological phases., Topological phenomena appear in non-Hermitian systems but the fundamental principles of the edge modes remain less understood. Here, Sone et al. report robust gapless edge modes due to topological structure around an exceptional point rather than bulk-edge correspondence.

Contrary to the conventional wisdom in Hermitian systems, a continuous quantum phase transition between gapped phases is shown to occur without closing the energy gap $\Delta$ in non-Hermitian quantum many-body systems. Here, the relevant length scale $\xi \simeq v_{\rm LR}/\Delta$ diverges because of the breakdown of the Lieb-Robinson bound on the velocity (i.e., unboundedness of $v_{\rm LR}$) rather than vanishing of the energy gap $\Delta$. The susceptibility to a change in the system parameter exhibits a singularity due to nonorthogonality of eigenstates. As an illustrative example, we present an exactly solvable model by generalizing Kitaev's toric-code model to a non-Hermitian regime., Comment: 7 + 5 pages, 1 + 2 figures

The T2K experiment measures muon neutrino disappearance and electron neutrino appearance in accelerator-produced neutrino and antineutrino beams. With an exposure of $14.7(7.6)\times 10^{20}$ protons on target in neutrino (antineutrino) mode, 89 $\nu_e$ candidates and 7 anti-$\nu_e$ candidates were observed while 67.5 and 9.0 are expected for $\delta_{CP}=0$ and normal mass ordering. The obtained $2\sigma$ confidence interval for the $CP$ violating phase, $\delta_{CP}$, does not include the $CP$-conserving cases ($\delta_{CP}=0,\pi$). The best-fit values of other parameters are $\sin^2\theta_{23} = 0.526^{+0.032}_{-0.036}$ and $\Delta m^2_{32}=2.463\pm0.065\times10^{-3} \mathrm{eV}^2/c^4$.

Recent experimental advances in controlling dissipation have brought about unprecedented flexibility in engineering non-Hermitian Hamiltonians in open classical and quantum systems. A particular interest centers on the topological properties of non-Hermitian systems, which exhibit unique phases with no Hermitian counterparts. However, no systematic understanding in analogy with the periodic table of topological insulators and superconductors has been achieved. In this paper, we develop a coherent framework of topological phases of non-Hermitian systems. After elucidating the physical meaning and the mathematical definition of non-Hermitian topological phases, we start with one-dimensional lattices, which exhibit topological phases with no Hermitian counterparts and are found to be characterized by an integer topological winding number even with no symmetry constraint, reminiscent of the quantum Hall insulator in Hermitian systems. A system with a nonzero winding number, which is experimentally measurable from the wave-packet dynamics, is shown to be robust against disorder, a phenomenon observed in the Hatano-Nelson model with asymmetric hopping amplitudes. We also unveil a novel bulk-edge correspondence that features an infinite number of (quasi-)edge modes. We then apply the K-theory to systematically classify all the non-Hermitian topological phases in the Altland-Zirnbauer classes in all dimensions. The obtained periodic table unifies time-reversal and particle-hole symmetries, leading to highly nontrivial predictions such as the absence of non-Hermitian topological phases in two dimensions. We provide concrete examples for all the nontrivial non-Hermitian AZ classes in zero and one dimensions. In particular, we identify a Z2 topological index for arbitrary quantum channels. Our work lays the cornerstone for a unified understanding of the role of topology in non-Hermitian systems., 31 pages, 18 figures, 2 tables, to appear in Physical Review X