- Date
- October 20th (Friday), 2017

- Time
- 3:00 pm - 5:00 pm

(The site visit is scheduled after the presentation.)

- Venue
- Frontier Research Laboratory of RIKEN Wako campus, 3F meeting room

http://www.riken.jp/en/access/wako-map/

(C41 building on the map.)

【JPN】http://www.riken.jp/access/wako-map/#anchor1

【EN】http://www.riken.jp/en/access/wako-map/

Dr. Akiko Masaki (Yunoki Group)

Owning to newly developed various numerical methods, the ground state properties of two-dimensional quantum lattice models such as two-dimensional quantum Heisenberg spin systems have been theoretically understood in the last decade. On the other hand, excitation dynamics are still not well understood even in simple models such as S=1/2 square lattice antiferromagnetic Heisenberg models (SLAHM), because it is not easy to calculate dynamic quantities despite that the numerical methods have drastically advanced recently. Recent experiments have observed that the magnon excitation spectrum of the wave number k = (π, 0) becomes much broader than that of k = (π/2, π/2), and the peak position also shifts to the lower energy side. Especially, recent neutron scattering experiments with metal-organic compound Cu(DCOO)2・4D2O (CFTD) which can be well described by S=1/2 SLAHM have suggested that the source of incoherent excitation at wave number k = (π, 0) is spinon excitation.

Very recently, a new numerical analytic continuation method that is the hybrid of the stochastic optimization method and consistent constraints (SOCC) method has been proposed [2]. This method allows us to evaluate the spectrum without any biases. In this study, we have calculated the excitation spectra of S=1/2 SLAHM along high symmetric wave numbers k using QMC+SOCC methods [3]. We have successfully obtained spectra that quantitatively reproduce the neutron scattering experiments for CFTD. Our results show that the peak at k = (π, 0) is broad and asymmetric while the sharp peak at k = (π/2, π/2) is consistent with the presence of symmetric well-defined magnon excitation. Moreover, we have carefully checked lower and higher energy excitations. We have found that a delta function peak corresponding to a single magnon around k = (π, 0) is absent with a large amount of continuum and high energy excitations in the longitudinal channel show a high energy threshold that is independent from momenta.

[1]B. Dalla Piazza et al., Nat. Phys. (2015).

[2] O. Goulko et al., Phys. Rev. B 95, 014102 (2017).

[3] A. Masaki-Kato, A. S. Mishchenko, T. Shirakawa, S. Yunoki and N. Nagaosa, unpublished.

Dr. Andrey S. Mishchenko (Nagaosa Group)

High Tc cuprates exhibit strong effects of electron–phonon interaction (EPI) at low hole-doping evidenced in ARPES [1,2,3], static and pump-probe optical conductivity [4,5], Raman scattering [6], etc. However, the EPI effects are suppressed with doping [3,4]. We check a hypothesis that this is due to screening of the EPI by Fermi statistics of carriers which is the consequence of the Migdal theorem stating that high order Feynman diagrams are suppressed by factor ωph/εF, where εF and ωph is the Fermi energy and phonon frequency, respectively. We use recently developed exact bold-line diagrammatic Monte Carlo technique [7] which circumvents famous many-Fermion sign problem in Monte Carlo. In particular, we study spin-polarized fermions in Holstein model which mimics t-J-Holstein model because two holes on one site cannot be created at the same time. We take the strong coupling limit λ=1.07 when mass renormalization m*=4m0 is considerable. We find (Fig.1(a)) that (i) filling of carriers really suppresses manifestations of EPI like it is in high Tc materials; (ii) diagrams higher than 2-nd order are suppressed at εF/ωph >2.5; (iii) 10-th (14-th) order diagrams become important only at εF/ωph < 1.5 (<0.5). We highlight properties of electron-phonon coupling in various doping regimes (Fig. 1(b)).

Fig. 1. (a) Dependence of the mass renormalization on the carrier filling and on the Feynman diagram order; (b) dependence of the energy renormalization on the Feynman diagram order for dense, dilute, and intermediate density polaron gas.

References

[1] A.S. Mishchenko and N. Nagaosa, Phys. Rev. Lett. 93, 036402 (2004).

[2] V. Cataudella et al, Phys. Rev. Lett. 99 226402 (2007).

[3] A.S. Mishchenko et al, EPL 95, 57007 (2011).

[4] A.S. Mishchenko et al, Phys. Rev. Lett. 100, 166401 (2008).

[5] F. Novelli et al, Nat. Comm. 5:5112 (2014).

[6] D. Farina et al, to be published.

[7] A. S. Mishchenko, N. Nagaosa and N. Prokof’ev, Phys. Rev. Lett. 113, 166402 (2014).

- Date
- September 15th (Friday), 2017

- Time
- 3:00 pm - 5:00 pm

(The site visit is scheduled after the presentation.)

- Venue
- Laser Science Laboratory of RIKEN Wako campus, Meeting room 123/1F

http://www.riken.jp/en/access/wako-map/

(The Laser Science Laboratory is C32 building on the map.)

【JPN】http://www.riken.jp/access/wako-map/#anchor1

【EN】http://www.riken.jp/en/access/wako-map/

Dr. Takeshi Fukuhara

Since optical lattice systems are well separated from uncontrolled environments, they provide an ideal platform to study non-equilibrium quantum dynamics of many-body systems. Furthermore, the time scale of their dynamics is sufficiently slow that we can apply a variety of manipulation and observation, which are difficult to realize for ultrafast phenomena in strongly correlated materials. In this presentation, I will discuss possibilities of a quantum simulator with optical lattice systems for non-equilibrium phenomena, such as photo-induced phase transitions. I also report recent and ongoing activities of our experiments with bosonic atoms towards this goal: realization of a quantum gas and optimization of experimental parameters with machine learning.

Dr. Yuto Ashida

Realizations of quantum gas microscopy have offered novel possibilities to measure quantum many-body systems at the single-particle level [1]. Further developments could allow us to perform a continuous monitoring of the many-body system [2,3]. In the first part of the talk, we discuss the influence of the measurement backaction on quantum criticality due to continuous observation. By analyzing the effective non-Hermitian Hamiltonian, we show that the measurement backaction can trigger (i) the bifurcation of critical exponents in TomonagaLuttinger liquid [4] and (ii) a new universality class of critical phenomena that has no counterpart in closed systems [5]. In the second part, we discuss the propagation of correlations under the measurement backaction [6]. Analyzing the density matrix including a fixed number of quantum jumps, we find that there appear supersonic modes propagating beyond the generalized Lieb-Robinson bound. We discuss possible experimental realizations in ultracold atoms using controlled loss of atoms [7,8] together with quantum gas microscopy technique.

[1] W. S. Bakr et al., Nature 462, 74-77 (2009).

[2] Y. A. Patil et al., PRL 115, 140402 (2015).

[3] YA and M. Ueda, PRL 115, 095301 (2015).

[4] YA, S. Furukawa, and M. Ueda, PRA 94, 053615 (2016).

[5] YA, S. Furukawa, and M. Ueda, Nat. Commun. 8, 15791 (2017).

[6] YA and M. Ueda, submitted.

[7] H. P. Luschen et al., PRX 7, 011034 (2017).

[8] T. Tomita et al., arXiv:1608.05061 (2017).

- Date
- July 21st (Friday), 2017

- Time
- 3:00 pm - 5:00 pm

(The site visit is scheduled after the presentation.)

Dr. Zhihui Peng (Prof. Tsai's group)

We study a superconducting artificial atom strongly coupled to a coplanar waveguide resonator and simultaneously to a transmission line. A vacuum induced Aulter-Townes splitting is observed in the reflection spectrum of the three-level atom via the transmission line when the transition between two excited states is resonant with the resonator. By varying an amplitude of the driving field applied to the resonator, we observe quantum-to-classical transition of the Aulter-Townes splitting. An anomalous wide band resonance fluorescence is also observed by driving the resonator when the transition between the two lowest levels of atom is resonant with the resonator. It is because the coupling strength between the atom and resonator is sufficiently large

and the variation in transition frequencies as a function of the photon number in the system widens the fluorescent sidebands.

Dr. Yu Zhou (Prof. Tsai's group)

To achieve boson sampling, a problem which has been proven to be intractable for classical computers, the microwave regime approach using superconducting circuits is promising for a largescale and compact implementation when compared with its original candidate--quantum optical networks. Among the required building blocks for boson sampling in microwave regime, the highquality microwave single photon source is the most important and essential one. In this talk, I will mainly focus on our recent work on tunable microwave single photon source based on transmon qubits, which is more simple for the scalability and with better decoherence performance. I will also talk about our control system based on PXIe technology, which is easy to scale. Finally, I will briefly describe our near-term goal of integration of single photon source and single photon detector, which should be another essential demonstration towards the microwave regime approach of boson sampling.

- Date
- June 30th (Friday), 2017

- Time
- 3:00 pm - 5:00 pm

- Venue
- Main Research Bldg. of RIKEN Wako campus,

Meeting room # 435 & 437 (The main research bldg. is C01.)

【JPN】http://www.riken.jp/access/wako-map

【EN】http://www.riken.jp/en/access/wako-map/

Dr. F. Nori

I will present a brief overview of QuTiP. The basics are covered in the QuTiP web site and also here:

• J.R. Johansson, P.D. Nation, F. Nori

QuTiP 2: A Python framework for the dynamics of open quantum systems

Comp. Phys. Comm. 184, 1234 (2013). [PDF] [Link] [arXiv]

• J.R. Johansson, P.D. Nation, F. Nori

QuTiP: An open-source Python framework for the dynamics of open quantum systems

Comp. Phys. Comm. 183, 1760-1772 (2012). [PDF] [Link] [arXiv]

Dr. Mauro Cirio (Dr. Nori's group)

Recent developments in the field of cavity quantum electrodynamics showed the possibility to engineer the interaction strength between light and matter to a wide range of physical regimes. These regimes can be characterized by different ground-state properties. For example, when the coupling strength becomes a sensible fraction of the characteristic frequency of the system in the absence of interaction (ultra-strong coupling regime), the ground state contains virtual light-matter excitations. In our work, we theoretically analyzed different ways to probe the virtual excitations present in the ground state of light-matter systems in the ultra-strong coupling regime:

-) Ground State Electroluminescence [1]. Electroluminescence, the emission of light in the presence of an electric current, provides information on the allowed electronic transitions of a given system. We showed that, together with the usual electroluminescence, systems in the ultrastrong light-matter coupling regime emit a uniquely quantum radiation when a flow of current is driven through them.

-) Amplified opto-mechanical transduction of virtual radiation pressure [2]. Here we describe how, utilizing a time-dependent opto-mechanical interaction, a mechanical probe can provide an amplified measurement of the virtual photons dressing the quantum ground state of an ultra-strongly-coupled light-matter system.

-) Ground State Electroluminescence in systems with flat Electronic-Bands [3]. Here we show the existence of ground-state electroluminescence emission channels in systems characterized by electronic flatband models. Specifically, we generalize the single-electron model in [1] to describe systems with many non-interacting electrons.

[1] M. Cirio, S. De Liberato, N. Lambert, F. Nori, Phys. Rev. Lett. 116, 113601 (2016).

[2] M. Cirio, K. Debnath, N. Lambert, F. Nori, Phys. Rev. Lett. 2017 (in press), see arXiv:1612.02953.

[3] M. Cirio, et al. (in preparation).

The above works benefited greatly from the use of the QuTiP software to model open quantum systems.

- Date
- June 6th (Tuesday), 2017

- Time
- 3:00 pm - 5:00 pm

- Venue
- RIKEN Wako Campus, Laser Science Laboratory,

Meeting room #123 on the 1st floor

【JPN】http://www.riken.jp/access/wako-map

【EN】http://www.riken.jp/en/access/wako-map/

(The Laser Science Laboratory is C32 building on the map.)

" Future prospects in topological QC"

- Date
- April 28th (Friday), 2017

- Time
- 3:00 pm - 5:00 pm

- Venue
- JST Tokyo Headquarters (K's Gobancho), Meeting room A/9F

http://www.jst.go.jp/EN/location/index.html

"The possibility of room temperature superconductivity"

- Date
- January 27th (Friday), 2017

- Time
- 3:00 pm - 5:00 pm

- Venue
- RIKEN Wako Campus, Laser Science Laboratory,

Meeting room #123 on the 1st floor

【JPN】http://www.riken.jp/access/wako-map

【EN】http://www.riken.jp/en/access/wako-map/

(The Laser Science Laboratory is C32 building on the map.)

Seigo Tarucha

Quantum Functional System Research Group, RIKEN Center for Emergent Matter Science

and

Department of Applied Physics, Graduate School of Engineering, The University of Tokyo

We have been working on quantum dynamics of open quantum systems to explore non-equilibrium quantum statistics and fractional quantum statistics with non-abelian anyons. The first will provide information about non-equilibrium dynamics of correlated quantum systems coupled to environment, and the second will be helpful to explore new principles of quantum simulation with anyons. I will first outline our approaches to these topics and then explain progresses in study on non-trivial dynamics of quantum spin systems coupled to spin bath, and search for Majorana fermion using two different systems of nanowire-superconductor junctions and topological insulator-superconductor junctions.

Michael Fraser

Quantum Functional System Research Group, RIKEN Center for Emergent Matter Science

Anyons, and in particular, non-Abelian anyons, are long sought-after quasi-particles with fractional exchange statistics, having promising applications in topological forms of quantum computation and memory ^{[1]}, and quantum simulation ^{[2]}. Anyons most famously occur when low temperature, two-dimensional electrons are subjected to strong magnetic fields leading to Landau level formation, and in particularly clean systems, to the fractional quantum Hall state.

Rapid rotation of a system of two-dimensional bosons also analogously leads to the formation of Landau levels and is predicted to transition into a fractional quantum Hall state ^{[3]}. I explore the use of exciton-polaritons ^{[4,5]}, hybrid light-matter, bosonic quasi-particles in semiconductors, as a suitable candidate to observe the bosonic fractional quantum Hall effect with the potential to realise a robust and optically controllable source of non-Abelian anyonic excitations.

[1] C. Nayak, S. H. Simon, A. Stern, M. Freedman and S. Das Sarma “Non-Abelian anyons and topological quantum computation” Rev. Mod. Phys. 80, 1083 (2008).

[2] G. K. Brennen and J. K. Pachos "Why should anyone care about computing with anyons?" Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 464, 2089 (2008).

[3] N. R. Cooper “Rapidly Rotating Atomic Gases” Advances in Physics 57, 539 (2008).

[4] H. Deng, H. Haug, and Y. Yamamoto “Exciton-polariton Bose-Einstein condensation” Rev. Mod. Phys. 82, 1489 (2010).

[5] I. Carusotto and C. Ciuti “Quantum fluids of light” Rev. Mod. Phys. 85, 299 (2013).

Takashi Nakajima

Quantum Functional System Research Group, RIKEN Center for Emergent Matter Science

Steering coherent quantum states in the noisy environment is of great interest for applications of quantum mechanics. It is therefore important to investigate the dynamics of the environment and characterize its influence to the quantum states. As a good example of such systems, we study the real-time fluctuation of the nuclear spin environment by using an electron spin in a quantum dot as a sensitive magnetic probe. We find the non-ergodic and non-Markovian characteristic of the fluctuation in short time scales, where the effective spin coherence is improved significantly ^{[1]}.

We will discuss the application of the improved coherence to quantum functions with an adaptive control technique.

[1] M. R. Delbecq et al., Phys. Rev. Lett. 116 046802 (2016).

- Date
- October 28th (Friday), 2016

- Time
- 3:00 pm - 5:00 pm

- Venue
- JST Tokyo Headquarters (K's Gobancho, ② on the access map)

Meeting room A on the 9F

http://www.jst.go.jp/EN/location/index.html

Hideo Aoki

Department of Physics, University of Tokyo, Hongo, Tokyo 113-0033, Japan

and

Electronics and Photonics Research Institute, Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan

Flat-band systems can be an interesting avenue for designing superconductivity due to their highly entangled wavefunctions. I shall describe repulsively interacting fermions on the diamond chain, a simplest 1D flat-band lattice as studied with the density matrix renormalisation group (DMRG), one of the matrix-product methods. A sign of long-tailed pair correlation is detected ^{[1]}. Intriguingly, the superconducting phase sits right next to a topological insulator phase. Indeed, flat-band systems are suggested to be a good place for realising topological systems ^{[2]}, and exploring possible connections between superconductivity and topological properties will be an interesting future problem.

[1] K. Kobayashi, M. Okumura, S. Yamada, M. Machida and H. Aoki, arXiv:1608.00125.

[2] See, e.g., M. G. Yamada, T. Soejima, N. Tsuji, D. Hirai, M. Dinca and H. Aoki, Phys. Rev. B 94, 081102(R) (2016).

Naoki Kawashima

Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan

Tensor network states (TNS) are quantum states that can be represented by a network of contracted tensors. Recently, it was found that the TNS is an efficiently compressed representation of many quantum states of physical interest and may be a cure to the notorious negative sign problem. Some application has already shown that the TNS is useful in dealing with frustrated spin systems and fermion problems. In addition, real-space renormalization group transformation can be implemented on the TNS in a much more systematic way than the conventional Migdal-Kadanoff (MK) approximation. From the renormalized tensor, we can not only obtain more accurate estimates of relevant critical exponents, but also deduce various other critical parameters, such as irrelevant scaling fields and fusion coefficients among scaling operators, which are inaccessible in the MK approximation.

- Date
- September 23th (Friday), 2016

- Time
- 3:00 pm - 5:00 pm

- Venue
- Room No.401 on the 4th

Graduate School of Science Bldg.5

Graduate School of Science, Kyoto University,

Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto

http://yagura.scphys.kyoto-u.ac.jp/access-e.html

towards room-temperature superconductivity

Yoshiro Takahashi

Department of Physics, Graduate School of Science, Kyoto University

yitk(at)scphys.kyoto-u.ac.jp ※Please replace "(at)" with "@".

In this short presentation, I will talk on possibilities of a quantum simulator using ultracold atoms in an optical lattice towards the realization of room temperature superconductivity. Our research in this ImPACT project focuses on a novel lattice configuration, called a bi-layer lattice, first proposed by Kuroki et al. ^{[1]} and also studied in ^{[2]} which predict Tc higher than that for a 2D square lattice corresponding to Cu-based superconductors. This bi-layer lattice is interesting because it shows a disconnected Fermi surface similar to Fe-based superconductors. I also briefly discuss the possible realization of a low enough temperature which is an issue for cold atom quantum simulator.

[1] K. Kuroki, T. Kimura, and R. Arita, Phys. Rev. B 66, 184508 (2002).

[2] T. A. Maier and D. J. Scalapino, Phys. Rev. B 84, 180513(R) (2011).

Kazuhiko Kuroki

Department of Physics, Graduate School of Science, Osaka University

kuroki(at)phys.sci.osaka-u.ac.jp ※Please replace "(at)" with "@".

Purely electronic mechanisms of Cooper pairing are attractive from the viewpoint of high temperature superconductivity in the sense that they involve only the electrons, which typically have an energy scale of ~eV. On the other hand, repulsive pairing interaction arising from such mechanisms gives rise to sign reversing superconducting gap. This often results in gap nodes intersecting the Fermi surface, which suppresses the superconducting transition temperature (Tc). In this talk, I will show that the Fermi surface consists of disconnected pieces in certain lattice structures with dimerization, where the sign reversal of the superconducting gap can take place without nodes intersecting the Fermi surface. Many body calculations for the Hubbard model on these lattice structures show that the superconducting transition temperature can be as high as 0.1t, where t is the nearest neighbor hopping integral [1-3]. This is much higher than the Tc (＜0.03t) of the Hubbard model on a square lattice, which is considered as a model for the high Tc cuprate superconductors.

[1] K. Kuroki and R. Arita, Phys. Rev. B 64, 024501 (2001).

[2] K. Kuroki, T. Kimura, and R. Arita, Phys. Rev. B 66, 184508 (2002).

[3] T. A. Maier and D. J. Scalapino, Phys. Rev. B 84, 180513(R) (2011).

Yosuke Takasu

Department of Physics, Graduate School of Science, Kyoto University

takasu(at)scphys.kyoto-u.ac.jp ※Please replace "(at)" with "@".

To provide important insights for room temperature superconductivity is one of the important goals for quantum simulation research. Kuroki et al theoretically showed that transition temperature in bilayer system is higher than that in a monolayer system ^{[1]}, which also attracted much interest from the view points of Fe-based superconductivity, although it is in under active discussion. We planned quantum simulation of this novel superconductivity by using fermionic 173 Yb atoms loaded in a bilayer optical lattice system. In this talk, I report recent progress of the experiments, such as building of a bilayer optical lattice system, superfluid-Mott insulator transition of bosons, and observation of spin singlet-triplet oscillation of two-component fermions.

[1] K. Kuroki, T. Kimura, and R. Arita, Phys. Rev. B 66, 184508 (2002).

- Date
- August 26th (Friday), 2016

- Time
- 3:00 pm - 5:00 pm

- Venue
- JST Tokyo Headquarters (K's Gobancho, ② on the access map)

Meeting room A on the 9F

http://www.jst.go.jp/EN/location/index.html

Naoto Nagaosa^{1,2 }

1.RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan

2.Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan

nagaosa(at)riken.jp ※Please replace "(at)" with "@".

Electron-phonon interaction gives the fundamental for the superconductivity, which mediate the attractive interaction between the electrons. There are many experimental tools to detect this interaction such as optical spectra, angle-resolved photoemission spectroscopy (ARPES), and neutron scattering. Theoretically, exact diagonalization of finite clusters, quantum Monte Carlo simulation, variational methods etc. have been used to analyze this problem, but the exact solutions are rarely available. Diagrammatic quantum Monte Carlo methods combined with the stochastic analytic continuation method opened a powerful way to calculate the physical properties of interacting electron-phonon systems without any approximations. First, I will review the results obtained for the polaron problem, i.e., a particle interacting with the phonon bath. Second, the extension of the method to the many fermion case will be discussed with the focus on the adiabatic-diabatic crossover as the Fermi energy increases. And finally, the non-equilibrium phenomena under light irradiation will be discussed. This works have been done in collaboration with Andrey S. Mishchenko, Nikolay Prokof'ev.

beyond Adiabatic Regime

Makoto Yamaguchi

RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan

makoto.yamaguchi(at)riken.jp ※Please replace "(at)" with "@".

Quantum annealing is one of the promising schemes to solve optimization problems. In this scheme, the system is initially prepared in its ground state that is easily realizable, and then, the Hamiltonian is gradually deformed to satisfy the adiabatic theorem. Hence, the ground state is maintained throughout the evolution and finally provides the solution of the problem to be solved, at least in principle. However, in practice, it is known that dissipative nature of the system as well as non-adiabatic transitions play significant roles on the computational performance. Nevertheless, there is no general approach to theoretically address the non-adiabatic regime with dissipation, for example, by using the Markovian quantum master equation (QME).

In this talk, I will give a general formulation of the Markovian QME to go well beyond the adiabatic regime, by introducing a temporal change timescale ¥tau_A for the time-dependent system Hamiltonian. It is further found that the framework can be well justified even if ¥tau_A is faster than the decay timescale of the bath correlation function. An application to the dissipative Landau-Zener model demonstrates this general result. The findings are applicable to a wide range of fields as well as the quantum annealing, providing a basis for quantum control beyond the adiabatic regime.

and possible spin liquid in 2D Hubbard models

Seiji Yunoki^{1,2 }

1.RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan

2.RIKEN Advanced Institute for Computational Science (AICS), Kobe, Hyogo 650-0046, Japan

yunoki(at)riken.jp ※Please replace "(at)" with "@".

The metal-insulator transition has been a subject of intense research since Nevil Mott has first proposed that the electron correlation can turn a metallic state into an insulating one. Here, we consider first electrons with massless Dirac-like dispersion on 2D bipartite lattices and perform numerically exact quantum Monte Carlo simulations that allow us to explore the quantum critical behavior of the metal-insulator transition [1,2]. We find that the transition is continuous without an intermediate phase [1] and determine the quantum criticality for the corresponding universality class, which is described in the continuous limit by the Gross-Neveu model [2]. We also discuss a fluctuation-driven scenario that the metal-insulator transition in the Dirac electrons is triggered by the vanishing of the quasiparticle weight but not the Dirac Fermi velocity [2]. Second, we explore the ground state phase diagram of the half-filled Hubbard model on the triangular lattice. Our 2D density-matrix renormalization group calculations reveal two discontinuities in the double occupancy with increasing U/t, indicating that there are three phases separated by first order transitions. The intermediate state, between the metallic and Neel ordered states, displays a paramagnetic nature which resembles gapless spin liquid [3].

[1] S. Sorella, Y. Otsuka, and S. Yunoki, Sci. Rep. 2, 992 (2012).

[2] Y. Otsuka, S. Yunoki, and S. Sorella, Phys. Rev. X 6, 011029 (2016).

[3] T. Shirakawa, T. Tohyama, J. Kokalj, S. Sota, and S. Yunoki, arXiv:1606.06814 (2016).

- Date
- July 29th (Friday), 2016

- Time
- 3:00 pm - 5:00 pm

- Venue
- JST Tokyo Headquarters (K's Gobancho, ② on the access map)

Meeting room A on the 9F

http://www.jst.go.jp/EN/location/index.html

Hiroki Ikegami, CEMS RIKEN

Understanding and engineering of quantum many-body systems is a big challenge in quantum physics and quantum information processing. Studies in artificial quantum many-body systems with well-controlled parameters should play a central role for that purpose. One of the promising platforms for realizing an artificial quantum many-body system is a Josephson junction array (JJA). It consists of an array of superconducting islands connected by small Josephson junctions, where various kinds of classical and quantum Hamiltonians, such as Bose-Hubbard and XY Hamiltonians, can be implemented depending on parameters of the system. Here, we present our recent progress of experimental study of JJAs using the circuit quantum electrodynamics (cQED) architecture. In contrast to the conventional transport measurements in JJAs, the cQED approach has some advantages: The system is weakly perturbed by the microwave excitation and properties of not only ground state but excited states are investigated at single photon level. In this talk, we particularly focus on our investigations of JJAs in a magnetic field, where the Bose-Hubbard-type system of quantum vortices is realized.

J. M. Taylor, JQI/QuICS/NIST

I will discuss theoretical approaches for realizing nontrivial phases of matter using microwave-domain devices. Specific emphasis will be placed upon using photons as the constituent particles of the simulator, with the theoretical development of non-interacting and interacting theories, as well as consideration of nominal equilibrium states of such systems and their connection to standard thermodynamic ensembles.

- Date
- June 24th (Friday) 2016

- Time
- 3:00 pm - 5:00 pm

- Venue
- Room No.115 on the 1st

Graduate School of Science Bldg.5

Graduate School of Science, Kyoto University,

Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto

http://yagura.scphys.kyoto-u.ac.jp/access-e.html

Jaw-Shen Tsai, Riken & TUS

We proposed to consider doing quantum simulation based on boson sampling. Boson sampling is a short to medium term application of quantum technology that has been proven to be classically difficult to solve. Recent results from the Harvard group has illustrated a possible application for Boson sampling in quantum chemistry in simulating the vibrational spectra of certain large molecules. In this talk, I will describe the overall circuit, and the key components necessary for the circuit. The key components required are:

(a) Demonstration of an on demand single photon source with a good fidelity

(b) Demonstration of coupling a single photon into a superconducting resonator with a good fidelity.

(c) Demonstration of deterministic readout of the superconducting resonator with a good fidelity

(d) Coupling of two independent microwave resonators and demonstration of a hong-ou-mandel quantum interference experiment between two independently generated microwave photons with a good visibility.

(e) Theoretical development of a complete, finite temperature, boson sampling algorithm to model the vibrational spectra of complex molecules (between 10- 100 vibrational modes)

(f) Benchmarking of the boson sampling protocol under experimentally relevant error models, specific to superconducting designs

(g) Implementation of a microwave boson sampler that demonstrates a sampling protocol beyond 5 simulated modes.

The items (a)[1], (b)[2] were accomplished to a certain level, and item (d) was accomplished concerning the coherent coupling of two cavities [3]. These results will be discussed in the talk.

[1] Peng, et al, Nature Communication, accepted

[2] Inomata, et al, Nature Communication, submitted

[3] Peng, et al, PRL 2016

boson sampling quantum computer

Simon Devitt, Riken

Quantum simulation has for a long time been a very ill defined term. Initially thought of as a stepping stone to a fully fault-tolerant quantum computer that can implement computations such as factoring, it has become clear that simulations as envisaged by Feynman will be even more resource intensive than applications such as factoring. While more stepping stone applications, mostly related to condensed matter systems, such as building a bose-hubbard simulator might be of interest to many researchers, it is unlikely to be a commercially viable application that would warrant hundreds of millions of dollars in development.

It is common estimated that 30% of worldwide supercomputing time is dedicated to quantum chemistry. While it is relatively unknown what fraction is devotes to what particular problem, it is clear that chemistry applications for quantum computers could be the most lucrative pathway forward. Recently, the Harvard group of Aspuru-Guzik proposed a method for simulating the vibrational spectra of small to mid sized molecules using what is known as a boson sampling quantum computer. In this talk we will review what a boson sampling system is, why it has attracted a lot of attention from the quantum computing community and how quantum chemistry calculations can be implemented on such a machine. We will finish the talk with a particular implementation in superconducting systems that is designed to mimic a linear optical quantum computer that is the platform of choice for boson sampling.

- Date
- May 27th (Friday), 2016

- Time
- 3:00 pm - 5:00 pm

- Venue
- JST Tokyo Headquarters (K's Gobancho, ② on the access map)

Meeting room A on the 9F

http://www.jst.go.jp/EN/location/index.html

Neill Lambert, Christian Flindt Franco Nori

We study the photon emission from a voltage-biased double quantum dot coupled to a microwave cavity [1,2], an example of so-called hybrid circuit-QED. We find that the resulting photonic statistics can exhibit a dynamic bistability, which we validated by showing that the distribution describing these statistics has the shape of a tilted ellipse. The switching rates which describe the bistability can be extracted from the electrical current and the shot noise in the quantum dots, and used to predict this elliptic form of the photonic distribution. We also explore how the state of the cavity can be manipulated and made "more coherent" with techniques from feedback control. As part of this project, we have developed a suite of numerical tools to calculate counting statistics of generic open-quantum-systems, which we are including in the QuTip library.

In addition, we show how multiple dots coupled to a common microwave cavity can exhibit photon-mediated electronic transport [2]. We develop a perturbative method to explore, order by order, how correlated transport statistics depends on the coupling to the common cavity, which acts like a "coupler" of distant dots.

[1] N. Lambert, F. Nori, and C. Flindt, Physical Review Letters 115, 216803 (2015).

[2] N. Lambert, C. Flindt, and F. Nori, Euro. Phys. Lett. 103, 17005 (2013)

exact solvers for strongly correlatedsystems

Franco Nori, Neill Lambert

QuTiP [1,2] is open-source software for simulating the dynamics of open quantum systems. QuTiP aims to provide user-friendly and efficient numerical simulations of a wide variety of Hamiltonians, including those with arbitrary timedependence, commonly found in a wide range of physics applications such as quantum optics, trapped ions, superconducting circuits, and quantum nanomechanical resonators.

As part of the further development of QuTip we are developing efficient numerical implementations of exact system-bath solvers, to describe the dynamics of open quantum systems strongly coupled to non-Markovian environments. We have focused on the Hierarchy method of Tanimura and Kubo, and used it as both benchmark for other methods [3] and in as part of a study on the measurement of non-Markovianity [4]. More recently we have extended this method to model environments with arbitrary spectral densities. Our future plans include (i) extending this method to apply to time-dependent Hamiltonians, for use in quantum-control theory, and (ii) generalization to Fermionic baths, to study strongly correlated electronic systems.
[1] J. R. Johansson, P. D. Nation, and F. Nori, Comp. Phys. Commun. 183, 1760 (2012).

[2] J. R. Johansson, P. D. Nation, and F. Nori, Comp. Phys. Commun. 184, 1234 (2013).

[3] H-B. Chen, N. Lambert, Y.-C. Cheng, Y.-N. Chen, F. Nori, Scientific Reports 5, (2015).

[4] S. L. Chen, N. Lambert, C. M. Li, A. Miranowicz, Y. N. Chen, F. Nori, Physical Review Letters 116 (2), 020503, (2016).

- Date
- April 22nd (Friday), 2016

- Time
- 3:00 pm - 5:00 pm

- Venue
- JST Tokyo Headquarters (K's Gobancho, ② on the access map)

Meeting room A on the 9F

http://www.jst.go.jp/EN/location/index.html

- Theme
- "Combinatorial optimization by quantum mechanics"

- Prof. Hidetoshi Nishimori (Tokyo Institute of Technology)

Yoshihisa Yamamoto

- Date
- February 2nd (Tuesday) 2016

- Time
- 2:00 pm - 5:00 pm

- Venue
- JST Tokyo Headquarters (K's Gobancho, ② on the access map)

Meeting room A on the 9F

http://www.jst.go.jp/EN/location/index.html

- Theme
- "How can we realize room-temperature superconductors?"

The pseudogap explained.

Prof. Hiroshi Kamimura (Tokyo University of Science, ImPACT Advisor)

Since Bednorz and Müller discovered high temperature superconductivity in copper oxides, thirty years have passed, but a consensus on its origin has not yet been obtained. However, paying attention to the experimental fact that copper oxides are Jahn-Teller materials, we have recently found that the Fermi surface of La_{2-x}Sr_{x}CuO_{4} consists of Fermi arcs in the nodal regions and Fermi pockets in the antinodal regions in the momentum space. Here Fermi arcs are due to the itinerant hole of mainly O 2p character that forms a spin-singlet (SS) state with a localized spin, which forms an antiferromagnetic (AF) order when undoped but is perturbed by the doped hole, which changes the valency from O^{2-} to O^{1-} and thus weakens the superexchange coupling between the localized spins. Thus, an SS quasiparticle emerges and itinerates by the interplay of quantum entanglement and tunneling in the non-AF region. This quasiparticle yields a new concept of the Fermi arc, which is formed on a mobility edge separating the Anderson-localized and extended states in the momentum space. Another particle yielding the Fermi pocket emerges owing to the anti-Jahn-Teller (JT) type lattice distortion. We name this novel particle the KS (Kamimura-Suwa) particle, and it creates a new state which consists of alternating spin-singlet and spin-triplet states coexisting with the AF order, causing d-wave superconductivity in the phonon mechanism. The competition between Fermi pockets and arcs in the momentum space causes the outstanding temperature and doping evolutions of the Fermi surface structure, thereby clarifying an unusual behavior of the Fermi surface that has been ascribed to the pseudogap. This novel picture is derived from a theoretical model, called Kamimura-Suwa (K-S) model^{1, 2}, and the predicted Fermi surface and a new phase diagram are consistent with recent photoemission experimental results^{3, 4}. Finally we show that the electron-phonon interaction in copper oxides is unusually strong due to the anti-JT effect.

References:

1. H. Kamimura and Y. Suwa, J. Phys. Soc. Jpn. 62, 3368-3371 (1993).

2. H. Kamimura, T. Hamada, and H. Ushio, Phys. Rev. B 66, 054504 (2002).

3. T. Yoshida, et al. Phys. Rev. B 74, 224510 (2006).

4. I.M. Vishik, et al. Proc. Natl. Acad. Sci. USA 109, 18332-18337 (2012).

(Coworkers: Jaw-Shen Tsai, Osamu Sugino, Kunio Ishida, and Hideki Ushio)

Dr. Ryotaro Arita (First-Principles Materials Science Research Team / Riken CEMS)

First, I will review three state-of-the-art schemes to evaluate Tc from first principles: (1) SCDFT (2) ab initio Migdal-Eliashberg (3) DFT+Many body. Then I will focus on (1) and (2), and show our recent results on H2S/H3S under high pressures. Finally I will discuss what we learn from calculations for MgB2, doped diamond, and sulfur hydrides.

Prof. Hideo Aoki (The University of Tokyo)

I shall discuss which calculational bottlenecks should be overcome for breakthrough in theoretical materials design and exploration of mechanisms for superconductivity with higher Tc's, from a viewpoint of quantum simulations. For fermion systems such as the Hubbard model, the sign problem is the most formidable barrier, while retardation effects require heavy computations for phonon-coupled systems such as the Holstein model. If one wants to propose a mechanism that goes beyond the usual boson (phonon, spin fluctuation, etc) exchange mechanisms, the sign problem can become even more serious. Hence these points are envisaged to be a key for pursuing a route to higher-Tc superconductivity.

- Date
- October 30 (Fri.) 2015

- Time
- 3:00 pm - 5:00 pm

- Venue
- Room No.115 on the 1st

Graduate School of Science Bldg.5

Graduate School of Science, Kyoto University,

http://yagura.scphys.kyoto-u.ac.jp/access-e.html

Dr. Yuta Murakami (Department of Physics, The University of Tokyo )

Recent developments in laser techniques make it possible to access the amplitude Higgs mode in superconductors^{ [1]} and photo-induced superconductivity ^{[2]}. These experiments show possibilities to manipulate superconductivity through non-equilibrium physics, and perhaps it might be possible to achieve superconductivity with very high Tc out of equilibrium. In order to search for these potentials, it is important to obtain better understanding of properties of non-equilibrium superconductors.

In this talk, we present recent results from our study on non-equilibrium dynamics in strongly-coupled phonon-mediated superconductors with the dynamical mean-field theory. From the analysis of weak excitation, we have revealed properties of the amplitude mode beyond the BCS regime and also found a new mode. For systems far out of equilibrium, we have analyzed relaxation dynamics of the superconducting phase. There, we find an additional critical point other than the thermal one, which separates regimes with qualitatively different relaxation processes.

These works are collaborations with N. Tsuji, P. Werner and H. Aoki.

[1] R. Matsunaga et al., PRL. 111, 057002 (2013); R. Matsunaga et al., Science 345, 1145 (2014).

[2] D. Fausti. et al., Science 331, 189 (2011).

Dr. Zhirong Lin (Nakamura group/Riken)

Searching the ground state of the Ising Hamiltonian could map to combinatorial optimization problems, many of which are characterized as NP-hard problems. Recently, a programmable network of degenerate optical parametric oscillators was proposed to create an Ising machine capable of finding the ground state of the Ising Hamiltonian. In this talk, I will discuss a single parametric oscillator based on superconducting circuits, named Josephson parametric oscillator. The device consists of a superconducting coplanar waveguide resonator with a SQUID termination and a pump line inductively coupled to the SQUID loop. It is operated at the pump power above the oscillation threshold. We have demonstrated that Josephson parametric oscillator can work as a sensitive microwave phase demodulator. By employing the demodulation capability, we developed a new qubit readout scheme, which offer fast, high-fidelity, and non-destructive readout. In addition, we study the dynamics of the parametric oscillator near the threshold, in which the quantum fluctuations induce transitions between two phase states. Finally, I will discuss prospects for coupling Josephson parametric oscillators.

- Date
- October 6, 2015

- Time
- 3:00 pm - 5:00 pm

- Venue
- Meeting room on the 1F of Residential Bldg.

JST Tokyo Headquarters (Science Plaza)

Dr. Takeshi Fukuhara (RIKEN)

For the development of new materials, a better understanding of quantum many-body systems is important. Key concepts in such systems arequantum phase transition and non-equilibrium quantum dynamics, which are also expected to play crucial roles in improvement of superconducting transition temperatures. In the ImPACT project, I plan to try these challenges with experiments of ultracold atoms in optical lattices. One example is an ultrafast electron dynamics triggered by injecting a hole in the antiferromagnetic state. This kind of dynamics might be related to the high-Tc superconductors realized in underdoped region, where the holes are doped into the antiferromagnetic ordering states. In this talk, I will present some of my targets, and explain plans to approach them.

Dr. Giles Allison (RIKEN/Tarucha group)

Quantum electrodynamics is the interaction between light and matter at its most elementary level, i.e. between single quanta of photons and atoms. The field was first developed by enclosing atoms and photons within an optical cavity, cavity quantum electrodynamics (CQED), and then further by implementing the architecture on chip, so-called circuit electrodynamics (cQED) [1].
Here we show how a hybrid cQED architecture may act as a model system for investigating non-equilibrium physics in condensed matter. The hybrid architecture scheme consists of a microwave cavity coupled to a pair of spatially separated SiGe double quantum dots that may be driven out of equilibrium [2]. We show that key parameters such as inter-dot and dot-lead coupling can be used to control and manipulate non-locality of the system. The electron-photon coupling scheme is analogous to the electron-phonon coupling prevalent in other condensed matter problems.
Finally, we conclude with prospects for coupling the photon mode to the electron spin within the quantum dots.

[1] A. Wallraff et al., Nature 431, 162-167 (2004); A. Blais et al., Phys. Rev. A 64, 062320 (2004).

[2] L.D. Contreras-Pulido et al., New Journal of Physics 15, 095008 (2013); N. Lambert et al., Europhys. Lett. 103, 17005 (2013);

C. Bergenfeldt et al., Phys. Rev. B 87, 195427 (2013).

- Date
- August 28, 2015

- Time
- 3:00 pm - 5:00 pm

- Venue
- Osaka University, Toyonaka Campus

H601 seminar room on the 6F of Building H, School of Science

Dr. Yosuke Takasu (Takahashi group)

BCS superfluidity states of fermionic atoms as realized experimentally in 2003 and various quantum simulations of superconductivity in solid states are theoretically proposed. We planned quantum simulation of superconductivity by using of fermionic 171 Yb atoms in the 1S0 ground sate and the metastable 3P2 state. In this talk, after I briefly report our recent progress towards BCS superfluidity of fermionic Yb atoms including quick review of BCS superfluidity, I discuss prospects for possible application of the phenomena to high-Tc superconductivity in solid state.

Dr. Kenta Takeda (Tarucha group)

Non-equilibrium physics is one of the important key phrases towards the realization of room-temperature superconductors in this ImPACT project.
In this talk we will present our recent experimental approaches to study such non-equilibrium physics in an electron spin system. Electron spins confined within semiconductor quantum dots can be prepared and measured precisely using well-known electron spin resonance and Pauli spin blockade techniques. Additionally, the electron spins can be coupled to electron or nuclear spin baths which are typical open and/or non-equilibrium systems in semiconductor. By controlling the environments using pulse and/or feedback controls [1], we will explore the physics of non-equilibrium dynamics of the electron spins in quantum dots.

[1] H. Bluhm et al., PRL 105, 216803 (2010)

- Date
- July 31, 2015

- Time
- 3:00 pm - 5:00 pm

- Venue
- JST

Dr. Motoaki Bamba(Ogawa group)

In the superconducting circuits, the interaction between artificial atoms and microwave in a resonator can be in the ultrastrong regime, where we can no longer apply the rotating-wave approximation (RWA) onto the light-matter interaction. The ultrastrong interaction can also be realized effectively in cold atoms driven coherently by laser light. The ultrastrong interaction brings out some remarkable phenomena (e.g., superradiant phase transition [1], laser [2], etc.) that are qualitatively different from the conventional regimes.

Further, since the free energy (or the energy of the ground state) is shifted due to the lack of the RWA [3], the Tc of the superconductivity may be improved by the ultrastrong interaction in the superconducting circuits in the thermal equilibrium.

[1] M. Bamba and T. Ogawa, Phys. Rev. A 90, 063825 (2014)

[2] M. Bamba and T. Ogawa, arXiv:1410.3912 [quant-ph]

[3] A. Canaguier-Durand, et al., Angew. Chem. 125, 10727 (2013)

Dr. Shuta Nakajima (Takahashi group)

Quantum annealing is one of the leading candidates of practical quantum information processing. The quantum annealing, however, can be regarded not only as a method for searching the ground state of a Hamiltonian encoding a given problem but as a method for achieving the ground state of a physically interesting Hamiltonian. We propose an experiment for realizing magnetic ordered structures in an optical lattice using quantum annealing scheme.

We use two magnetic sublevels of ^3P_2 state of ytterbium atoms as effective spin states and introduce the Raman coupling bewteen these spin states, which can be regarded as a transverse field term added to the original Hamiltonian. The magnetic ordered structure arising from magnetic dipole-dipole interactions of ^3P_2 states could be realized by reducing the Raman coupling adiabatically. We will report the experimental setup and the progress of the experiment.

- Date
- June 26, 2015

- Time
- 15:00～17:00

- Venue
- Kyoto University

Kunihiro Inomata

In order to realize quantum simulations using superconducting circuits, artificial quantum systems and high-fidelity readout/control circuits are required. We have implemented a single microwave photon detector using an artificial $\Lambda$ atom as the high-fidelity readout circuits. This detector is applicable to a photon based quantum simulation.

Michael Fraser

The Bose-Einstein condensate (BEC) of exciton-polaritons in semiconductor microcavities is a macroscopically coherent bosonic state with the novel feature of tunable hybridisation of the properties of a semiconductor optical laser and dilute-gas atomic BEC. The emerging techniques for easy manipulation of this system via optical or lithographic means enables the creation of controlled dynamic flow and confinement to arbitrary potentials, and the further possibility of achieving strongly correlated and topologically non-trivial states of polaritons. In this seminar I will present an overview of the polariton condensate system and the latest techniques for manipulation and confinement alongside the theoretical proposal for, and progress in, experiments towards the observation of bosonic fractional quantum Hall-like states and the engineering of quantum simulators for highly-correlated states found in condensed matter and high energy physics.

- Date
- May 29, 2015

- Time
- 15:00～17:00

- Venue
- JST

Anubhav Vardhan

Shintaro Taie

- Date
- May 8, 2015

- Time
- 15:00～17:00

- Venue
- Osaka University

Andrey Mishchenko

Pierre-Marie Billangeon

- Date
- March 20, 2015

- Time
- 15:00～17:00

- Venue
- JST

Special talk by Prof. Hiroshi Kamimura

Tatsuro Yuge

Jun Kobayashi

- Date
- February 27, 2015

- Time
- 15:00～17:00

- Venue
- Kyoto University

Makoto Yamaguchi

Zhihui Peng

- Date
- January 30, 2015

- Time
- 15:00～17:00

- Venue
- JST

Prof. Naoto Nagaosa

Prof. Yoshiro Takahashi