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We discuss both the spectrum and the dynamics of cavity QED in the presence of dissipation beyond the standard perturbative treatment of losses. Using the dynamical polaron ansatz and matrix-product state simulations, we discuss the case where both light-matter g coupling and system-bath interaction are in the ultra-strong-coupling regime. We provide a critical g for the onset of Rabi oscillations. Besides, we demonstrate that the qubit is dressed by the cavity and dissipation. Such a dressing governs the dynamics and, thus, it can be measured. Finally, we sketch an implementation for our theoretical ideas within circuit QED technology. ; We acknowledge support by the Spanish Ministerio de Ciencia, Innovación y Universidades within Project No. MAT2017-88358-C3-1-R and No. FIS2015-70856-P. The Aragón Government project Q-MAD and CAM PRICYT Research Network QUITEMAD+ S2013/ICE-2801. EU-QUANTERA projectSUMO is also acknowledged. ; Peer reviewed

Coherent exchange between photons and different matter excitations (like qubits, acoustic surface waves or spins) allows for the entanglement of light and matter and provides a toolbox for performing fundamental quantum physics. On top of that, coherent exchange is a basic ingredient in the majority of quantum information processors. In this work, we develop the theory for coupling between magnetic textures (vortices and skyrmions) stabilized in ferromagnetic nanodiscs and microwave photons generated in a superconducting circuit. Within this theory we show that this hybrid system serves for performing broadband spectroscopy of the magnetic textures. We also discuss the possibility of reaching the strong coupling regime between these texture excitations and a single photon residing in a microwave superconducting cavity. ; We acknowledge support by the Spanish Ministerio de Ciencia, Innovación y Universidades, within projects MAT2015-73914- JIN, MAT2015-64083-R, and MAT2017-88358-C3-1-R, the Aragón Government project Q-MAD, and EU-QUANTERA project SUMO. ; Peer reviewed

It has been shown elsewhere that two spatially separated atoms can jointly absorb one photon, whose frequency is equal to the sum of the transition frequencies of the two atoms. We describe this process in the presence of an ensemble of many two-level atoms and show that it can be used to generate spin squeezing and entanglement. This resonant collective process allows us to create a sizable squeezing already at the single-photon limit. It represents a way to generate many-body spin-spin interactions, yielding a two-axis twisting-like interaction among the spins, which is very efficient for the generation of spin squeezing. We perform explicit calculations for ensembles of magnetic molecules coupled to a superconducting coplanar cavities. This system represents an attractive on-chip architecture for the realization of improved sensing. ; D.Z. acknowledges RIKEN for its hospitality and the support by the Spanish Ministerio de Ciencia, Innovaciòn y Universidades within Project No. MAT2017-88358-C3-1-R, the Aragòn Government Project Q-MAD, EU-QUANTERA Project SUMO, and the Fundaciòn BBVA. F.N. is supported in part by: NTT Research, Army Research Office (ARO) (Grant No. W911NF-18-1-0358), Japan Science and Technology Agency (JST) (via the Q-LEAP program, and the CREST Grant No. JPMJCR1676), Japan Society for the Promotion of Science (JSPS) (JSPS-RFBR Grant No. 17-52-50023, and JSPS-FWO Grant No. VS.059.18N), and the Grant No. FQXiIAF19-06 from the Foundational Questions Institute Fund (FQXi), a donor advised fund of the Silicon Valley Community Foundation. S.S. acknowledges the US Army Research Office (ARO) (Grant No. W911NF-19-1-0065). ; Peer reviewed

El pdf del artículo es la versión pre-print: arxiv:1003.2376.-- et al. ; In circuit quantum electrodynamics (QED), where superconducting artificial atoms are coupled to on-chip cavities, the exploration of fundamental quantum physics in the strong-coupling regime has greatly evolved. In this regime, an atom and a cavity can exchange a photon frequently before coherence is lost. Nevertheless, all experiments so far are well described by the renowned Jaynes-Cummings model. Here, we report on the first experimental realization of a circuit QED system operating in the ultrastrong-coupling limit, where the atom-cavity coupling rate g reaches a considerable fraction of the cavity transition frequency ‰r. Furthermore, we present direct evidence for the breakdown of the Jaynes-Cummings model. We reach remarkable normalized coupling rates g/ ‰r of up to 12% by enhancing the inductive coupling of a flux qubit to a transmission line resonator. Our circuit extends the toolbox of quantum optics on a chip towards exciting explorations of ultrastrong light-matter interaction. © 2010 Macmillan Publishers Limited. All rights reserved. ; We acknowledge financial support by the Deutsche Forschungsgemeinschaft through SFB 631 and the German Excellence Initiative through NIM. E.S. acknowledges financial support from UPV/EHU Grant GIU07/40, Ministerio de Ciencia e Innovación FIS2009-12773-C02-01, Basque Government Grant IT472-10, European Projects EuroSQIP and SOLID. D.Z. acknowledges financial support from FIS2008-01240 and FIS2009-13364-C02-0 (MICINN). ; Peer Reviewed

We introduce the use of entanglement entropy as a tool for studying the amount of information shared between the nodes of quantum complex networks. By considering the ground state of a network of coupled quantum harmonic oscillators, we compute the information that each node has on the rest of the system. We show that the nodes storing the largest amount of information are not the ones with the highest connectivity, but those with intermediate connectivity, thus breaking down the usual hierarchical picture of classical networks. We show both numerically and analytically that the mutual information characterizes the network topology. As a by-product, our results point out that the amount of information available for an external node connecting to a quantum network allows one to determine the network topology. © 2013 American Physical Society. ; We acknowledge Diego Blas for discussions and support from the Spanish DGICYT under Projects No. FIS2011-14539-E (EXPLORA program), No. FIS2011-23526, and No. FIS2011-25167, and by the Aragón (Grupo FENOL) and Balearic Governments. F.G. acknowledges the CSIC postdoctoral JAE program and the TIQS (FIS2011-23526) project. J.G.G. is supported by MINECO through the Ramón y Cajal program. ; Peer Reviewed

We study an interferometric method for measuring the statistics of work performed on a driven quantum system, which has been put forward recently (Dorner et al 2013 Phys. Rev. Lett. 110 230601, Mazzola et al 2013 Phys. Rev. Lett. 110 230602). This method allows the replacement of two projective measurements of the energy of the driven system with qubit tomography of an ancilla that is appropriately coupled to it. We highlight that this method could be employed to obtain the work statistics of closed as well as open driven system, even in the strongly dissipative regime. We then illustrate an implementation of the method in a circuit QED setup, which allows one to experimentally obtain the work statistics of a parametrically driven harmonic oscillator. Our implementation is an extension of the original method, in which two ancilla-qubits are employed and the work statistics are retrieved through two-qubit state tomography. Our simulations demonstrate the experimental feasibility. © IOP Publishing and Deutsche Physikalische Gesellschaft. ; This work was supported by the German Excellence Initiative 'Nanosystems Initiative Munich (NIM)' (MC, RB, PH), the Collaborative Research Center SFB 631 (RB, PH), the Spanish Government grants MAT2011-24331 (SK) and FIS2011-25167 (DZ), the DGA project FENOL (DZ), the EU project PROMISCE (DZ) and the COST Action MP1209 (MC, PH). ; Peer Reviewed

We study different architectures for a photonic crystal in the microwave regime based on superconducting transmission lines interrupted by Josephson junctions, both in one and two dimensions. A study of the scattering properties of a single junction in the line shows that the junction behaves as a perfect mirror when the photon frequency matches the Josephson plasma frequency. We generalize our calculations to periodic arrangements of junctions, demonstrating that they can be used for tunable band engineering, forming what we call a quantum circuit crystal. Two applications are discussed in detail. In a two-dimensional structure we demonstrate the phenomenon of negative refraction. We finish by studying the creation of stationary entanglement between two superconducting qubits interacting through a disordered media. © 2012 American Physical Society. ; This work was supported by Spanish Governement projects FIS2008-01240, FIS2009-10061, FIS2009-12773-C02-01, and FIS2011-25167 cofinanced by FEDER funds; CAM research consortium QUITEMAD; Basque Government Grants No. IT472-10, and No. UPV/EHU UFI 11/55; and PROMISCE, SOLID, and CCQED European projects. ; Peer Reviewed

The interaction between quantized electromagnetic fields in cavities and natural or artificial atoms has played a crucial role in developing our understanding of light-matter interactions and quantum technologies. Recently, new regimes beyond the weak and strong light-matter coupling of cavity-QED have been explored in several settings, wherein the light-matter coupling rate becomes comparable to (ultrastrong coupling) or even exceeds (deep-strong coupling) the photon frequency. These ultrastrong coupling regimes can give rise to new physical effects and applications, and they challenge our understanding of cavity QED; fundamental issues like the proper definition of subsystems, their quantum measurements, the structure of light-matter ground states, and the analysis of time-dependent interactions are subject to gauge ambiguities that lead to even qualitatively distinct predictions. The resolution of these ambiguities is important for understanding and designing next-generation quantum devices that can operate in extreme coupling regimes. Here we discuss and provide solutions to these ambiguities by adopting an approach based on operational procedures involving measurements on the individual light and matter components of the interacting system. ; A.S., D.Z., and S.H. acknowledge RIKEN for its hospitality, D.Z. acknowledges the support by the Spanish Ministerio de Ciencia, Innovaciòn y Universidades within project MAT2017-88358-C3-1-R, the Aragòn Government project Q-MAD, EU-QUANTERA project SUMO and the Fundaciòn BBVA. F.N. is supported in part by: Nippon Telegraph and Telephone Corporation (NTT) Research, the Japan Science and Technology Agency (JST) [via the Quantum Leap Flagship Program (Q-LEAP) program, the Moonshot R&D Grant No. JPMJMS2061, and the Centers of Research Excellence in Science and Technology (CREST) Grant No. JPMJCR1676], the Japan Society for the Promotion of Science (JSPS) [via the Grants-in-Aid for Scientific Research (KAKENHI) Grant No. JP20H00134 and the JSPS RFBR Grant No. JPJSBP120194828], the Army Research Office (ARO) (Grant No. W911NF-18-1-0358), the Asian Office of Aerospace Research and Development (AOARD) (via Grant No. FA2386-20-1-4069), and the Foundational Questions Institute Fund (FQXi) via Grant No. FQXi-IAF19-06. S.H. acknowledges funding from the Canadian Foundation for Innovation, and the Natural Sciences and Engineering Research Council of Canada. S.S. acknowledges the Army Research Office (ARO) (Grant No. W911NF-19-1-0065). ; Peer reviewed

We prove that for a combined system of classical and quantum particles, it is possible to write a dynamics for the classical particles that incorporates in a natural way the Boltzmann equilibrium population for the quantum subsystem. In addition, these molecular dynamics do not need to assume that the electrons immediately follow the nuclear motion (in contrast to any adiabatic approach), and do not present problems in the presence of crossing points between different potential energy surfaces (conical intersections or spin-crossings). A practical application of this molecular dynamics to study the effect of temperature in molecular systems presenting (nearly) degenerate states – such as the avoided crossing in the ring-closure process of ozone – is presented. ; We acknowledge support by the Spanish MICINN (FIS2007- 65702-C02-01, FIS2009-13364-C02-01, FIS2008-01240 and ACI-Promociona ACI2009-1036), by "Grupos de Excelencia del Gobierno de Aragón" (E24/3) by "Grupos Consolidados UPV/EHU del Gobierno Vasco" (IT-319-07), by the CSIC (200980I064) by the European Union through e-I3, and by the ETSF project (Contract Number 211956).

El pdf del artículo es la versión pre-print: arXiv:0812.2801 ; We present in detail the recently derived ab initio molecular dynamics (AIMD) formalism [Alonso et al. Phys. Rev. Lett. 2008, 101, 096403], which due to its numerical properties, is ideal for simulating the dynamics of systems containing thousands of atoms. A major drawback of traditional AIMD methods is the necessity to enforce the orthogonalization of the wave functions, which can become the bottleneck for very large systems. Alternatively, one can handle the electron−ion dynamics within the Ehrenfest scheme where no explicit orthogonalization is necessary, however the time step is too small for practical applications. Here we preserve the desirable properties of Ehrenfest in a new scheme that allows for a considerable increase of the time step while keeping the system close to the Born−Oppenheimer surface. We show that the automatically enforced orthogonalization is of fundamental importance for large systems because not only it improves the scaling of the approach with the system size but it also allows for an additional very efficient parallelization level. In this work, we provide the formal details of the new method, describe its implementation, and present some applications to some test systems. Comparisons with the widely used Car−Parrinello molecular dynamics method are made, showing that the new approach is advantageous above a certain number of atoms in the system. The method is not tied to a particular wave function representation, making it suitable for inclusion in any AIMD software package. ; This work has been supported by the research projects DGA (Aragón Government, Spain) E24/3, and MEC (Spain). P.E. is supported by a MEC/MICINN (Spain) postdoctoral contract. X.A and Á.R. acknowledge funding by the Spanish MEC (FIS2007-65702-C02-01), "Grupos Consolidados UPV/EHU del Gobierno Vasco" (IT-319-07), and the European Community through NoE Nanoquanta (NMP4-CT-2004- 500198), e-I3 ETSF (INFRA-2007-1.2.2: Grant Agreement Number 211956), NANO-ERA Chemistry, DNA-NANODEVICES (IST-2006-029192), and SANES (NMP4-CT-2006- 017310) projects. ; Peer reviewed

arXiv:1305.4844v1 ; The scattering through a Josephson junction (JJ) interrupting a superconducting line is revisited including power leakage. We also discuss how to make tunable and broadband resonant mirrors by concatenating junctions. As an application, we show how to construct cavities using these mirrors, thus connecting two research fields: JJ quantum metamaterials and coupled-cavity arrays. We finish by discussing the first nonlinear corrections to the scattering and their measurable effects. © 2013 IOP Publishing Ltd. ; This work was supported by Spanish government projects FIS2009-10061, and FIS2011-25167 conanced by FEDER funds. We thanks Aragon government support to group FENOL, CAM research consortium QUITEMAD and PROMISCE European project. ; Peer Reviewed

APS March Meeting, Boston, MA, March 4-8, 2019. -- https://www.aps.org/meetings/meeting.cfm?name=MAR19 ; Funds were provided by the Spanish MINECO (grants MAT2015-68204-R and Excellence Unit Maria de Maeztu MDM-2015- 0538) and the European Union (COST 15128 Molecular Spintronics and QUANTERA SUMO projects) ; Peer reviewed

Under the terms of the Creative Commons Attribution license.-- et al. ; We report on ultrastrong coupling between a superconducting flux qubit and a resonant mode of a system comprised of two superconducting coplanar stripline resonators coupled galvanically to the qubit. With a coupling strength as high as 17.5% of the mode frequency, exceeding that of previous circuit quantum electrodynamics experiments, we observe a pronounced Bloch-Siegert shift. The spectroscopic response of our multimode system reveals a clear breakdown of the Jaynes-Cummings approximation. In contrast to earlier experiments, the high coupling strength is achieved without making use of an additional inductance provided by a Josephson junction. ©2016 American Physical Society ; Thiswork is supported by the German Research Foundation through SFB 631 and FE 1564/1-1; Spanish MINECO FIS2012-36673-03-02, MAT2014-53432-C5-1-R, FIS2014- 55867-P and FIS2012-33022; CAM Research Network QUITEMAD+; UPV/EHU UFI 11/55, UPV/EHU PhD Grant, and Basque Government IT472-10; the Fondo Nacional de Desarrollo Cientifico y Tecnológico (FONDECYT, Chile) under Grant 1150653; the EU projects CCQED, PROMISCE, and SCALEQIT. We further acknowledge GEFENOL. ; Peer Reviewed

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al. ; We realize a device allowing for tunable and switchable coupling between two frequency-degenerate superconducting resonators mediated by an artificial atom. For the latter, we utilize a persistent current flux qubit. We characterize the tunable and switchable coupling in the frequency and time domains and find that the coupling between the relevant modes can be varied in a controlled way. Specifically, the coupling can be tuned by adjusting the flux through the qubit loop or by controlling the qubit population via a microwave drive. Our measurements allow us to find parameter regimes for optimal coupler performance and quantify the tunability range. ; This work is supported by the German Research Foundation through SFB 631, Spanish MINECO FIS2012-36673-C03-02; UPV/EHU UFI 11/55; Basque Government IT472-10; CCQED, PROMISCE, and SCALEQIT EU projects. B.P. acknowledges support from the STC Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319. ; Peer reviewed