- Akio Hosoya (Tokyo Institute of Technology)
- "Shall we do thought experiments?"
- Norihiro Iizuka (Department of Physics, Osaka University)
- "Krylov complexity in the IP matrix model"
- Nobuyuki Imoto (The University of Tokyo)
- "Time and space issues that appeared in my quantum information research"
** [Abstract] ** ** [pdf] **
**Abstract**

I have been working on quantum information ever since I
published a paper [PRA32, 2285 (1985)] proposing a quantum non-
demolition measurement via the optical Kerr effect, and since then I
have often been troubled by the treatment of time and space. This is
because in the field of quantum computing people uses Hamiltonians, a
world where space has a common time everywhere and follows the time
evolution of quantum components. Quantum communication, on the other
hand, is based on propagation equations and coupled mode equations. Here,
as in the time evolution representation, variables are shown as
operators wearing hats, but what we are dealing with is the spatial
evolution of temporal modes. These two different worlds are simply
connected and intermingled in real devices and experiments. Most
researchers vary their methodologies and variables to successfully
connect different worlds, which makes me want to think more. I have
thought about this problem in parallel with my main research and have
written papers about it, but for some problems I have not found good
ideas. So today I would like to present some of them and ask for your
wisdom.

- Akihiro Ishibashi (Department of Physics, Kindai University)
- "Quantum improved black holes and thermodynamics"
** [Abstract] **
**Abstract**

We study quantum mechanically corrected black holes
within the framework of asymptotic safe quantum gravity. The Newton coupling in
this formulation depends on an energy scale, which must be identified with some
length scale in order to study physical consequences of quantum corrected black
holes. The main issue in such a “scale” identification is how to “quantum-improve”
black hole so that the resultant quantum black hole (i) exhibits consistency
with the laws of black hole thermodynamics, and at the same time, (ii) resolves
the singularity inside the corresponding classical geometry. For the static
black hole case, it is rather straightforward to achieve these two issues, but
for the rotating case (whose classical counterpart is the Kerr black hole), no
satisfactory results have been so far made. We propose that physically sensible
scale identifications, which satisfy both properties (i) and (ii), should be made
in terms of the horizon area function and the black hole mass. We also discuss that
closed timelike curves inside the Cauchy horizon of the classical Kerr geometry
can be resolved by quantum improvement.

- Tomoyuki Morimae (Yukawa Institute for Theoretical Physics, Kyoto University)
- "Quantum cryptography without one-way functions"
** [Abstract] **
**Abstract**

In classical cryptography, one-way functions is the most fundamental assumptions. We show that it is not necessarily true in quantum cryptography. We also mention some relations to complexity theory and blackhole physics.

- Tatsuma Nishioka (Department of Physics, Osaka university)
- "CFTs from QEC"
** [Abstract] **
**Abstract**

We construct a discrete subset of Narain CFTs from quantum stabilizer codes with qudit (including qubit) systems over finite fields and finite rings. Our construction exploits three important relations. The first relation is between qudit stabilizer codes and classical codes. The second is between classical codes and Lorentzian lattices. The third is between Lorentzian lattices and Narain CFTs. In particular, we study qudit Calderbank-Shor-Steane (CSS) codes as a special class of qudit stabilizer codes and the ensembles of the Narain code CFTs constructed from CSS codes. We also investigate the gauging of a
Z2 symmetry in Narain CFTs and establish a connection between Z2 gauging procedures and modifications of the momentum lattice by vectors characterizing the Z2 symmetry. We exploit our construction to search for fermionic CFTs with supersymmetry by focusing on quantum stabilizer codes of the Calderbank-Shor-Steane type, and derive simple criteria for the theories to be supersymmetric. We provide several examples of fermionic CFTs that meet the criteria, and find quantum codes that realize N = 4 supersymmetry.

- Kouichi Okunishi (Department of Physics, Niigata University)
- "A statistical mechanics approach to holographic renormalization group"
** [Abstract] **
**Abstract**

We discuss a holographic aspect of the Bethe lattice Ising model, a classical model of the phase transition in statistical mechanics. We analytically formulate a holographic RG for the model and derive the scaling dimensions associated with boundary spins. We also reveal its connection to the p-adic AdS/CFT.

- Tadashi Takayanagi (Yukawa Institute for Theoretical Physics, Kyoto University)
- "Aspects of Holographic Pseudo Entropy"
** [Abstract] ** ** [pdf] **
**Abstract**

Pseudo entropy is a generalization of entanglement entropy such that it depends on both an initial and final state, aiming at post-selection processes. It has an explicit and simple gravity dual via the AdS/CFT, namely the minimal surface area in a Euclidean time-dependent AdS background. This quantity can also be used as a quantum order parameter which distinguishes two different quantum phases. Holographic pseudo entropy naturally arises in the context of the dS/CFT and suggests that a time coordinate emerges from the imaginary part of pseudo entropy, as the space coordinate does from the entanglement entropy in AdS/CFT. We will also introduce a new quantity called SVD entropy as a modification of pseudo entropy which has a clearer quantum information theoretic meaning.

- Tetsuya Shiromizu/Norihiro Tanahashi (Graduate School of Mathematics, Nagoya University/Deraprtment of Physics)
- "Krylov complexity and chaos in quantum mechanics"
** [Abstract] ** ** [pdf] **
**Abstract**

Recently, Krylov complexity was proposed as a measure of complexity and
chaoticity of quantum systems. We consider the stadium billiard as a typical example of
the quantum mechanical system obtained by quantizing a classically chaotic system, and
numerically evaluate Krylov complexity for operators and states. Despite no exponential
growth of the Krylov complexity, we find a clear correlation between variances of Lanczos
coefficients and classical Lyapunov exponents, and also a correlation with the statistical
distribution of adjacent spacings of the quantum energy levels. This shows that the variances
of Lanczos coefficients can be a measure of quantum chaos. The universality of the
result is supported by our similar analysis of Sinai billiards. This study provides a firm
bridge between Krylov complexity and classical/quantum chaos.

- Masaki Tezuka (Department of Physics, Kyoto University)
- "Quantum error correction in spin chains and related models"
** [pdf] **
- Go Yusa (Department of Physics, Tohoku University)
- "Quantum cosmology experiments in quantum Hall systems"
** [Abstract] **
**Abstract**

We will report on the current experimental and theoretical progress on the quantum universe created by quantum Hall systems