1
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Roh Y, Chae M, Oh J, Kim W, Ryu MY, Seo M, Jeong J. Ultrastrong Coupling Enhancement with Squeezed Mode Volume in Terahertz Nanoslots. NANO LETTERS 2023; 23:7086-7091. [PMID: 37471630 DOI: 10.1021/acs.nanolett.3c01913] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 07/22/2023]
Abstract
Metallic nanogaps have emerged as a versatile platform for realizing ultrastrong coupling in terahertz frequencies. Increasing the coupling strength generally involved reducing the gap width to minimize the mode volume, which presents challenges in fabrication and efficient material coupling. Here, we propose employing terahertz nanoslots, which can efficiently squeeze the mode volume in an extra dimension alongside the gap width. Our experiments using 500 nm wide nanoslots integrated with an organic-inorganic hybrid perovskite demonstrate ultrastrong phonon-photon coupling with a record-high Rabi splitting of 48% of the original resonance (Ω = 0.48ω0), despite having a gap width 5 times larger than previously reported structures with Ω = 0.45ω0. Mechanisms underlying this effective light--matter coupling are investigated with simulations using coupled mode theory. Moreover, bulk polariton analyses reveal that our results account for 68% of the theoretical maximum Rabi splitting, with the potential to reach 82% through further optimization of the nanoslots.
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Affiliation(s)
- Yeeun Roh
- Sensor System Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea
| | - Minjun Chae
- Department of Physics and Institute of Quantum Convergence Technology, Kangwon National University, Gangwon 24341, Republic of Korea
| | - Jaewon Oh
- Department of Physics and Institute of Quantum Convergence Technology, Kangwon National University, Gangwon 24341, Republic of Korea
| | - Woochul Kim
- Sensor System Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea
| | - Mee-Yi Ryu
- Department of Physics and Institute of Quantum Convergence Technology, Kangwon National University, Gangwon 24341, Republic of Korea
| | - Minah Seo
- Sensor System Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea
- KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Republic of Korea
| | - Jeeyoon Jeong
- Department of Physics and Institute of Quantum Convergence Technology, Kangwon National University, Gangwon 24341, Republic of Korea
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2
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Ginis V, Benea-Chelmus IC, Lu J, Piccardo M, Capasso F. Resonators with tailored optical path by cascaded-mode conversions. Nat Commun 2023; 14:495. [PMID: 36717562 PMCID: PMC9886910 DOI: 10.1038/s41467-023-35956-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2022] [Accepted: 01/10/2023] [Indexed: 01/31/2023] Open
Abstract
Optical resonators enable the generation, manipulation, and storage of electromagnetic waves. The physics underlying their operation is determined by the interference of electromagnetic waves, giving rise to the resonance spectrum. This mechanism causes the limitations and trade-offs of resonator design, such as the fixed relationship between free spectral range, modal linewidth, and the resonator's refractive index and size. Here, we introduce a new class of optical resonators, generating resonances by designing the optical path through transverse mode coupling in a cascaded process created by mode-converting mirrors. The generalized round-trip phase condition leads to resonator characteristics that are markedly different from Fabry-Perot resonators and can be tailored over a wide range. We confirm the existence of these modes experimentally in an integrated waveguide cavity with mode converters coupling transverse modes into one supermode. We also demonstrate a transverse mode-independent transmission and show that its engineered spectral properties agree with theoretical predictions.
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Affiliation(s)
- Vincent Ginis
- grid.38142.3c000000041936754XHarvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 USA ,grid.8767.e0000 0001 2290 8069Data Lab / Applied Physics, Vrije Universiteit Brussel, 1050 Brussel, Belgium
| | - Ileana-Cristina Benea-Chelmus
- grid.38142.3c000000041936754XHarvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 USA ,grid.5333.60000000121839049Hybrid Photonics Laboratory, École Polytechnique Fédérale de Lausanne, Lausanne, CH-1015 Switzerland
| | - Jinsheng Lu
- grid.38142.3c000000041936754XHarvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 USA
| | - Marco Piccardo
- grid.38142.3c000000041936754XHarvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 USA ,grid.25786.3e0000 0004 1764 2907Center for Nano Science and Technology, Istituto Italiano di Tecnologia, Milan, 20133 Italy
| | - Federico Capasso
- grid.38142.3c000000041936754XHarvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 USA
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3
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Macrì V, Mercurio A, Nori F, Savasta S, Sánchez Muñoz C. Spontaneous Scattering of Raman Photons from Cavity-QED Systems in the Ultrastrong Coupling Regime. PHYSICAL REVIEW LETTERS 2022; 129:273602. [PMID: 36638299 DOI: 10.1103/physrevlett.129.273602] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/25/2022] [Accepted: 11/09/2022] [Indexed: 06/17/2023]
Abstract
We show that spontaneous Raman scattering of incident radiation can be observed in cavity-QED systems without external enhancement or coupling to any vibrational degree of freedom. Raman scattering processes can be evidenced as resonances in the emission spectrum, which become clearly visible as the cavity-QED system approaches the ultrastrong coupling regime. We provide a quantum mechanical description of the effect, and show that ultrastrong light-matter coupling is a necessary condition for the observation of Raman scattering. This effect, and its strong sensitivity to the system parameters, opens new avenues for the characterization of cavity QED setups and the generation of quantum states of light.
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Affiliation(s)
- Vincenzo Macrì
- Theoretical Quantum Physics Laboratory, Cluster for Pioneering Research, RIKEN, Wakoshi, Saitama 351-0198, Japan
| | - Alberto Mercurio
- Dipartimento di Scienze Matematiche e Informatiche, Scienze Fisiche e Scienze della Terra, Università di Messina, I-98166 Messina, Italy
| | - Franco Nori
- Theoretical Quantum Physics Laboratory, Cluster for Pioneering Research, RIKEN, Wakoshi, Saitama 351-0198, Japan
- Quantum Computing Center, RIKEN, Wakoshi, Saitama 351-0198, Japan
- Physics Department, The University of Michigan, Ann Arbor, Michigan 48109-1040, USA
| | - Salvatore Savasta
- Dipartimento di Scienze Matematiche e Informatiche, Scienze Fisiche e Scienze della Terra, Università di Messina, I-98166 Messina, Italy
| | - Carlos Sánchez Muñoz
- Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain
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4
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Zhao T, Wu SP, Yang GQ, Huang GM, Li GX. Ultranarrow spectral line of the radiation in double qubit-cavity ultrastrong coupling system. OPTICS EXPRESS 2021; 29:23939-23952. [PMID: 34614648 DOI: 10.1364/oe.432358] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/25/2021] [Accepted: 07/05/2021] [Indexed: 06/13/2023]
Abstract
The ultrastrongly coupling (USC) system has very important research significance in quantum simulation and quantum computing. In this paper, the ultranarrow spectrum of a circuit QED system with two qubits ultrastrongly coupled to a single-mode cavity is studied. In the regime of USC, the JC model breaks down and the counter-rotating terms in the quantum Rabi Hamiltonian leads to the level anti-crossing in the energy spectrum. Choosing a single-photon driving field at the point of avoided-level crossing, we can get an equivalent four-level dressed state model, in which the dissipation of the two intermediate states is only related to the qubits decay. Due to the electron shelving of these two metastable states, a narrow peak appears in the cavity emission spectrum. Furthermore, we find that the physical origin for the spectral narrowing is the vacuum-induced quantum interference between two transition pathways. And this interference effect couples the slowly decaying incoherent components of the density matrix into the equations of the sidebands. This result provides a possibility for the study of quantum interference effect in the USC system.
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5
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Garziano L, Ridolfo A, Miranowicz A, Falci G, Savasta S, Nori F. Atoms in separated resonators can jointly absorb a single photon. Sci Rep 2020; 10:21660. [PMID: 33303819 PMCID: PMC7729986 DOI: 10.1038/s41598-020-78299-x] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2020] [Accepted: 11/18/2020] [Indexed: 11/28/2022] Open
Abstract
The coherent nonlinear process where a single photon simultaneously excites two or more two-level systems (qubits) in a single-mode resonator has recently been theoretically predicted. Here we explore the case where the two qubits are placed in different resonators in an array of two or three weakly coupled resonators. Investigating different setups and excitation schemes, we show that this process can still occur with a probability approaching one under specific conditions. The obtained results provide interesting insights into subtle causality issues underlying the simultaneous excitation processes of qubits placed in different resonators.
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Affiliation(s)
- Luigi Garziano
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako, Saitama, 351-0198, Japan
| | - Alessandro Ridolfo
- Dipartimento di Fisica e Astronomia, Università di Catania, 95123, Catania, Italy.,INFN Sezione Catania, Catania, Italy
| | - Adam Miranowicz
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako, Saitama, 351-0198, Japan.,Faculty of Physics, Adam Mickiewicz University, 61-614, Poznan, Poland
| | - Giuseppe Falci
- Dipartimento di Fisica e Astronomia, Università di Catania, 95123, Catania, Italy.,CNR-IMM Catania e INFN Sezione Catania, Catania, Italy
| | | | - Franco Nori
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako, Saitama, 351-0198, Japan.,Department of Physics, University of Michigan, Ann Arbor, MI, 48109-1040, USA
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6
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Garbe L, Wade P, Minganti F, Shammah N, Felicetti S, Nori F. Dissipation-induced bistability in the two-photon Dicke model. Sci Rep 2020; 10:13408. [PMID: 32770061 PMCID: PMC7414202 DOI: 10.1038/s41598-020-69704-6] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2019] [Accepted: 07/13/2020] [Indexed: 11/09/2022] Open
Abstract
The Dicke model is a paradigmatic quantum-optical model describing the interaction of a collection of two-level systems with a single bosonic mode. Effective implementations of this model made it possible to observe the emergence of superradiance, i.e., cooperative phenomena arising from the collective nature of light-matter interactions. Via reservoir engineering and analogue quantum simulation techniques, current experimental platforms allow us not only to implement the Dicke model but also to design more exotic interactions, such as the two-photon Dicke model. In the Hamiltonian case, this model presents an interesting phase diagram characterized by two quantum criticalities: a superradiant phase transition and a spectral collapse, that is, the coalescence of discrete energy levels into a continuous band. Here, we investigate the effects of both qubit and photon dissipation on the phase transition and on the instability induced by the spectral collapse. Using a mean-field decoupling approximation, we analytically obtain the steady-state expectation values of the observables signaling a symmetry breaking, identifying a first-order phase transition from the normal to the superradiant phase. Our stability analysis unveils a very rich phase diagram, which features stable, bistable, and unstable phases depending on the dissipation rate.
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Affiliation(s)
- Louis Garbe
- Laboratoire Matériaux et Phénomènes Quantiques, Sorbonne Paris Cité, CNRS UMR 7162, Université de Paris, 75013, Paris, France.
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama, 351-0198, Japan.
| | - Peregrine Wade
- Laboratoire Matériaux et Phénomènes Quantiques, Sorbonne Paris Cité, CNRS UMR 7162, Université de Paris, 75013, Paris, France
| | - Fabrizio Minganti
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama, 351-0198, Japan
| | - Nathan Shammah
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama, 351-0198, Japan
- Unitary Fund, 340 S Lemon Ave. 7770, Walnut, CA, 91789, USA
| | - Simone Felicetti
- Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049, Madrid, Spain
- Istituto di Fotonica e Nanotecnologie, Consiglio Nazionale delle Ricerche (IFN-CNR), Piazza Leonardo da Vinci 32, 20133, Milano, Italy
| | - Franco Nori
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama, 351-0198, Japan
- Physics Department, The University of Michigan, Ann Arbor, MI, 48109-1040, USA
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7
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Projecting an ultra-strongly-coupled system in a non-energy-eigenbasis with a driven nonlinear resonator. Sci Rep 2020; 10:1751. [PMID: 32019941 PMCID: PMC7000414 DOI: 10.1038/s41598-019-56866-1] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2019] [Accepted: 12/02/2019] [Indexed: 11/22/2022] Open
Abstract
We explore the problem of projecting the ground-state of an ultra-strong-coupled circuit-QED system into a non-energy-eigenstate. As a measurement apparatus we consider a nonlinear driven resonator. We find that the post-measurement state of the nonlinear resonator exhibits a large correlation with the post-measurement state of the ultra-strongly coupled system even when the coupling between measurement device and system is much smaller than the energy scales of the system itself. While the projection is imperfect, we argue that because of the strong nonlinear response of the resonator it works in a practical regime where a linear measurement apparatus would fail.
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8
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Buchholz F, Theophilou I, Nielsen SEB, Ruggenthaler M, Rubio A. Reduced Density-Matrix Approach to Strong Matter-Photon Interaction. ACS PHOTONICS 2019; 6:2694-2711. [PMID: 31788499 PMCID: PMC6875895 DOI: 10.1021/acsphotonics.9b00648] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/30/2019] [Indexed: 05/04/2023]
Abstract
We present a first-principles approach to electronic many-body systems strongly coupled to cavity modes in terms of matter-photon one-body reduced density matrices. The theory is fundamentally nonperturbative and thus captures not only the effects of correlated electronic systems but accounts also for strong interactions between matter and photon degrees of freedom. We do so by introducing a higher-dimensional auxiliary system that maps the coupled fermion-boson system to a dressed fermionic problem. This reformulation allows us to overcome many fundamental challenges of density-matrix theory in the context of coupled fermion-boson systems and we can employ conventional reduced density-matrix functional theory developed for purely fermionic systems. We provide results for one-dimensional model systems in real space and show that simple density-matrix approximations are accurate from the weak to the deep-strong coupling regime. This justifies the application of our method to systems that are too complex for exact calculations and we present first results, which show that the influence of the photon field depends sensitively on the details of the electronic structure.
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Affiliation(s)
- Florian Buchholz
- Theory
Department, Max Planck Institute for the
Structure and Dynamics of Matter - Luruper Chaussee 149, 22761 Hamburg, Germany
- E-mail:
| | - Iris Theophilou
- Theory
Department, Max Planck Institute for the
Structure and Dynamics of Matter - Luruper Chaussee 149, 22761 Hamburg, Germany
- E-mail:
| | - Soeren E. B. Nielsen
- Theory
Department, Max Planck Institute for the
Structure and Dynamics of Matter - Luruper Chaussee 149, 22761 Hamburg, Germany
| | - Michael Ruggenthaler
- Theory
Department, Max Planck Institute for the
Structure and Dynamics of Matter - Luruper Chaussee 149, 22761 Hamburg, Germany
- E-mail:
| | - Angel Rubio
- Theory
Department, Max Planck Institute for the
Structure and Dynamics of Matter - Luruper Chaussee 149, 22761 Hamburg, Germany
- Center
for Computational Quantum Physics (CCQ), Flatiron Institute, 162 Fifth Avenue, New York, New York 10010, United
States
- E-mail:
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9
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Parity-Assisted Generation of Nonclassical States of Light in Circuit Quantum Electrodynamics. Symmetry (Basel) 2019. [DOI: 10.3390/sym11030372] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
We propose a method to generate nonclassical states of light in multimode microwave cavities. Our approach considers two-photon processes that take place in a system composed of N extended cavities and an ultrastrongly coupled light–matter system. Under specific resonance conditions, our method generates, in a deterministic manner, product states of uncorrelated photon pairs, Bell states, and W states in different modes on the extended cavities. Furthermore, the numerical simulations show that the generation scheme exhibits a collective effect which decreases the generation time in the same proportion as the number of extended cavity increases. Moreover, the entanglement encoded in the photonic states can be transferred towards ancillary two-level systems to generate genuine multipartite entanglement. Finally, we discuss the feasibility of our proposal in circuit quantum electrodynamics. This proposal could be of interest in the context of quantum random number generator, due to the quadratic scaling of the output state.
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10
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Di Stefano O, Settineri A, Macrì V, Ridolfo A, Stassi R, Kockum AF, Savasta S, Nori F. Interaction of Mechanical Oscillators Mediated by the Exchange of Virtual Photon Pairs. PHYSICAL REVIEW LETTERS 2019; 122:030402. [PMID: 30735421 DOI: 10.1103/physrevlett.122.030402] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/01/2017] [Indexed: 06/09/2023]
Abstract
Two close parallel mirrors attract due to a small force (Casimir effect) originating from the quantum vacuum fluctuations of the electromagnetic field. These vacuum fluctuations can also induce motional forces exerted upon one mirror when the other one moves. Here, we consider an optomechanical system consisting of two vibrating mirrors constituting an optical resonator. We find that motional forces can determine noticeable coupling rates between the two spatially separated vibrating mirrors. We show that, by tuning the two mechanical oscillators into resonance, energy is exchanged between them at the quantum level. This coherent motional coupling is enabled by the exchange of virtual photon pairs, originating from the dynamical Casimir effect. The process proposed here shows that the electromagnetic quantum vacuum is able to transfer mechanical energy somewhat like an ordinary fluid. We show that this system can also operate as a mechanical parametric down-converter even at very weak excitations. These results demonstrate that vacuum-induced motional forces open up new possibilities for the development of optomechanical quantum technologies.
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Affiliation(s)
- Omar Di Stefano
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama 351-0198, Japan
| | - Alessio Settineri
- Dipartimento di Scienze Matematiche e Informatiche, Scienze Fisiche e Scienze della Terra, Università di Messina, I-98166 Messina, Italy
| | - Vincenzo Macrì
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama 351-0198, Japan
| | - Alessandro Ridolfo
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama 351-0198, Japan
- Dipartimento di Fisica e Astronomia, Universitá di Catania, I-95123 Catania, Italy
| | - Roberto Stassi
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama 351-0198, Japan
| | - Anton Frisk Kockum
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama 351-0198, Japan
- Wallenberg Centre for Quantum Technology, Department of Microtechnology and Nanoscience, Chalmers University of Technology, 412 96 Gothenburg, Sweden
| | - Salvatore Savasta
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama 351-0198, Japan
- Dipartimento di Scienze Matematiche e Informatiche, Scienze Fisiche e Scienze della Terra, Università di Messina, I-98166 Messina, Italy
| | - Franco Nori
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama 351-0198, Japan
- Physics Department, The University of Michigan, Ann Arbor, Michigan 48109-1040, USA
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11
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Di Stefano O, Kockum AF, Ridolfo A, Savasta S, Nori F. Photodetection probability in quantum systems with arbitrarily strong light-matter interaction. Sci Rep 2018; 8:17825. [PMID: 30546126 PMCID: PMC6292927 DOI: 10.1038/s41598-018-36056-1] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/26/2018] [Accepted: 11/14/2018] [Indexed: 12/02/2022] Open
Abstract
Cavity-QED systems have recently reached a regime where the light-matter interaction strength amounts to a non-negligible fraction of the resonance frequencies of the bare subsystems. In this regime, it is known that the usual normal-order correlation functions for the cavity-photon operators fail to describe both the rate and the statistics of emitted photons. Following Glauber’s original approach, we derive a simple and general quantum theory of photodetection, valid for arbitrary light-matter interaction strengths. Our derivation uses Fermi’s golden rule, together with an expansion of system operators in the eigenbasis of the interacting light-matter system, to arrive at the correct photodetection probabilities. We consider both narrow- and wide-band photodetectors. Our description is also valid for point-like detectors placed inside the optical cavity. As an application, we propose a gedanken experiment confirming the virtual nature of the bare excitations that enrich the ground state of the quantum Rabi model.
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Affiliation(s)
- Omar Di Stefano
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama, 351-0198, Japan
| | - Anton Frisk Kockum
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama, 351-0198, Japan.,Wallenberg Centre for Quantum Technology, Department of Microtechnology and Nanoscience, Chalmers University of Technology, 412 96, Gothenburg, Sweden
| | - Alessandro Ridolfo
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama, 351-0198, Japan
| | - Salvatore Savasta
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama, 351-0198, Japan. .,MIFT - Dipartimento di Scienze Matematiche e Informatiche Scienze Fisiche e Scienze della Terra, Università di Messina, I-98166, Messina, Italy.
| | - Franco Nori
- Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research, Wako-shi, Saitama, 351-0198, Japan.,Physics Department, The University of Michigan, Ann Arbor, Michigan, 48109-1040, USA
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