セミナー 2020年

Toyohiro Tsurumaru, Mitsubishi Electric

[Quantum-Physics Seminar] Leftover hashing from quantum error correction: Unifying the two approaches to the security proof of quantum key distribution (in Japanese)

Seminar room, Kenkyu honkan 3F
We show that the Mayers-Shor-Preskill approach and Renner’s approach to proving the security of quantum key distribution (QKD) are essentially the same. We begin our analysis by considering a special case of QKD called privacy amplification (PA). PA itself is an important building block of cryptography, both classical and quantum. The standard theoretical tool used for its security proof is called the leftover hashing lemma (LHL). We present a direct connection between the LHL and the coding theorem of a certain quantum error correction code. Then we apply this result to proving the equivalence between the two approaches to proving the security of QKD.
T. Tsurumaru, “Leftover Hashing From Quantum Error Correction: Unifying the Two Approaches to the Security Proof of Quantum Key Distribution”, in IEEE Transactions on Information Theory, vol. 66, no. 6, pp. 3465-3484, June 2020, (arXiv:1809.05479).

Toru Kojo, Central China Normal University

[QCD theory seminar] Hard- and Soft- Deconfinement from nuclear to quark matter

Online (Zoom)
We propose a novel concept of hard and soft realizations of deconfinement from nuclear to quark matter. Hard Deconfinement takes place when bulk thermodynamics is dominated by the core properties. The energy density and mechanical pressure in a nucleon, which are related to the gravitational form factor in scattering experiments, are found to be consistent with high density constraints known from neutron star phenomenology. Meanwhile Soft Deconfinement is driven by quark exchanges at intermediate distance and begins before Hard Deconfinement happens. To describe this phenomenon we use a model of quantum percolation, and discuss a quantum mechanical problem of quarks hopping among baryons. We describe delocalization of quark wavefunctions as well as the Anderson localization.
Finally we discuss how the quark Fermi sea is developed as nuclear matter transforms into quark matter, and conjecture a scenario leading to a momentum shell model in Quarkyonic Matter.

Yuichiro Tada, Nagoya University

Manifestly covariant theory of stochastic inflation

Online (Zoom) https://kds.kek.jp/event/35914/
The stochastic approach to inflation can non-perturbatively treat the superhorizon fluctuations as the classical random noise to go beyond the standard perturbative QFT approach. I first review how this approach enables us to calculate the observable curvature perturbations in a non-perturbative way and how it can be implemented in a numerical program. I then describe the covariance of the stochastic theory and the discretization scheme of the stochastic noise, which are important both theoretically and practically. Mathematically the stochastic noise can be discretized in an arbitrary manner, but we reveal the physically motivated discretization, which ensures the covariance of the stochastic theory on the inflatons’ target manifold.

Koji Hashimoto, Department of Physics, Osaka University

Building bulk from Wilson loops

Online (Zoom), indico page, slides (kek.jp only)
We provide formulas for holographically building a bulk metric from given expectation values of rectangular Wilson loops. As a corollary, we prove that any confining quark potential necessarily leads to the existence of a bulk IR bottom.

Shi Pi, Kavli IPMU, U. of Tokyo

Some Properties of the Stochastic Induced Gravitational Wave Background

Online (Zoom) https://kds.kek.jp/indico/event/35730/
Our universe is fulfilled by stochastic background of gravitational waves with a large range of frequencies, which may have various astrophysical/cosmological origins in the early universe. As our universe is transparent to gravitational wave, it is a fossil recording the information of its generation and how our universe evolves. In this talk I will briefly review the stochastic background of the gravitational waves from the nonlinear coupling of the scalar and tensor perturbations, as well as their connection to the primordial black holes. I will also talk about the properties of the spectrum shape of such waves: infrared scaling and the peak width.

岡隆史, 東京大学物性研究所/マックスプランク複雑物理学研究所


Yuji Okawa, The University of Tokyo

Nonperturbative definition of closed string theory via open string field theory

Online (Zoom), indico page, slides (kek.jp only)
In typical examples of the AdS/CFT correspondence, the world-sheet theory with holes in the presence of D-branes is assumed to be equivalent in a low-energy limit to a world-sheet theory without holes for a different background such as $AdS_5 ¥times S^5$. In the case of the bosonic string, we claim under the assumption of this equivalence that open string field theory on $N$ coincident D-branes can be used to provide a nonperturbative definition of closed string theory based on the fact that the $1/N$ expansion of correlation functions of gauge-invariant operators reproduces the world-sheet theory with holes where the moduli space of Riemann surfaces is precisely covered.

Yuji Chinone, RESCEU, U of Tokyo

The results and achievements of the POLARBEAR experiment in the 2010s and its future in the 2020s

Online (Zoom) https://kds.kek.jp/indico/event/35385/
POLARBEAR is a ground-based experiment designed to measure polarization of the Cosmic Microwave Background (CMB) at 150 GHz at an elevation of 5,190m in the Atacama Desert in Chile. Our main science goals are for searching for the B-mode signal created by primordial gravitational waves (PGWs) seen at degree scales, as well as for characterizing the B-mode signal from gravitational lensing with high angular resolution seen at sub-degree scales. POLARBEAR was deployed in 2011 and started science observations in 2012 focusing on small patches of the sky (deep survey). In 2014, we started to observe a large patch of the sky focusing on search for PGWs (wide survey). We have published a series of results from the deep survey, including the first/update measurement of a non-zero B-mode auto-power spectrum at sub-degree scales induced by gravitational lensing, constraint of cosmic birefringence and primordial magnetic fields, and demonstration of delensing. We also have achieved the measurement of cross-correlations of gravitational lensing between our CMB data and optical surveys by the Subaru Hyper Suprime-Cam. With the wide survey, we have reported the measurement of large angular scale B-mode signals induced by the Galactic dust foreground and successfully put an upper limit on PGWs. In this talk, I summarize the science results and achievements of the POLARBEAR experiment. I also show the status of Simons Array, which is an upgraded experiment from POLARBEAR and consists of three new receivers. Finally I describe future CMB projects which Japanese institutes have been contributing to.

Masahiro Isaka, Hosei University

Low-lying level structure of p-sd shell Lambda hypernuclei with antisymmetrized molecular dynamics

Online (Zoom), indico page, slides (kek.jp only)
We focus on low-lying level structure such as ΛΛN spin-spin and spin-orbit splittings in excitation spectra of p-sd shell Lambda hypernuclei. One of the major goals of hypernuclear physics is to understand properties of the baryon-baryon interaction including hyperon(s). Another important issue is to clarify dynamics of baryon many-body systems. In the last decades, by the developments of the experimental studies, the low-lying level structure of light ΛΛ hypernuclei has been revealed precisely. It is expected that crucial information on both properties of ΛΛN interaction and hypernuclear structure from the low-lying level structure. In this study, we use the antisymmetrized molecular dynamics with the ΛΛN G-matrix interaction derived from the baryon-baryon interaction model ESC. It is found that the ground-state spin-parity is systematically reproduced in the p-shell ΛΛ hypernuclei by tuning the ΛΛN spin-spin and spin-orbit interactions so as to reproduce the experimental data of 4ΛΛ4H, 7ΛΛ7Li, and 9ΛΛ9Be. Furthermore, it is found that the energy spacing of the ground-state doublet in 19ΛΛ19F is successfully reproduced using the same ΛΛN interaction. We also focus on the excitation energies and energy shifts of the excited doublets by the addition of a ΛΛ particle. In this talk, we will discuss how the spin-dependent properties of the ΛΛN interactions and structure of the core nuclei affect the low-lying spectra of the ΛΛ hypernucle

Masaki Yamada, Tohoku University

Unified Origin of Axion and Monopole Dark Matter, and Solution to the Domain-wall Problem

Online (Zoom) https://kds.kek.jp/indico/event/35279/
A ‘t Hooft-Polyakov monopole in a dark sector is a viable dark-matter candidate as it is stable due to its topological charge. If the spontaneous symmetry breaking proceeds via the first order phase transition, monopoles can be produced from the collisions of expanding bubbles. In the case with a Coleman-Weinberg potential, a monopole with mass of O(10^{10}) GeV can explain the observed DM density. Interestingly, this is within a viable window for the PQ-symmetry breaking. Motivated by the coincidence of the energy scales, we have pursued a possibility of unifying the PQ symmetry breaking and the production of the monopole DM. Even simple models lead to interesting phenomena in cosmological history, like the Witten effect on the axion in the presence of the monopoles, the Lazarides-Shafi mechanism to avoid the domain wall problem, and the formation of Alice strings. I will explain these effects in a KSVZ-type axion model as well as a DFSZ-type axion model and will show how the cosmological domain wall problem of the axion models is avoided by these effects.

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