SEMINAR

近藤(近畿大学)、Le(近畿大学)、箱嶋(産総研)、久木田(上海大学)、中原(上海大学)

各トークは30分と10分程度の議論　
（１）Controllable open system: Theory and Realization
近藤、近畿大学

（２）Quantum estimation of the magnetic field with twisting and turning
Le、近畿大学

（３）Single spin detection with entangled states
箱嶋、産総研、ナノエレクトロニクス研究部門、エレクトロインフォマティクス グループ


（４）Information protection in open quantum systems
久木田、上海大学数学科

（５）Majorana representation and some of its applications in quantum information processing
中原、上海大学数学科

連絡先：理学科物理学コース、近藤

==============================概要====================================
Title: Controllable open system: Theory and Realization
Yasushi Kondo, Kindai University

Abstract: An open quantum system is now attracting much attention because quantum
device such as quantum computers and quantum sensors is an emerging technology.
Here, we present a model of the open system that shows either time-homogeneous
Markovian relaxations or non-Markovian relaxations depending on its parameters
that we can control. This model is fully described with the master equation
that is analytically solvable. More importantly, this model can be easily realized
with molecules in isotropic liquids and measured with NMR techniques.

=====================================================================
Title: Quantum estimation of the magnetic field with twisting and turning
Le Bin Ho, Kindai University

Abstract: Quantum estimation theory is important for scientific research and technology
applications. Attaining and minimizing an ultimate bound imposed by fundamental laws of
quantum mechanics are some of its demanded tasks. In this work, we propose a protocol based
on one axis twisting (OAT) and twist-and-turn (TNT) for multiphase estimation. We first discuss
the saturation of the ultimate bound. We then show that the sensitivity reaches the Heisenberg
scale and can be minimized by using OAT and TNT methods.

=====================================================================
Title: Single spin detection with entangled states
Hideaki Hakoshima, AIST, Nanoelectronics Research Institute, Electroinformatics Group

Abstract: One of the most important objectives in quantum metrology is to detect
single (electron or nuclear) spin efficiently. However, a long measurement time is
required because of the weak magnetic fields from the single spin, and therefore
the efficient detection of the single spin has not been achieved yet.
Here, we propose the single spin detection using entangled states such as
the GHZ states, which are known as powerful probes for the detection of
the weak global (homogeneous) magnetic fields. In contrast to the global
magnetic fields, it is not clear whether they also improve the sensitivity
of the inhomogeneous magnetic field from the single spin.
We found that the entanglement considerably increases the signal-to-noise
ratio even under the effect of realistic (non-Markovian) noise.

=====================================================================
Title: Information protection in open quantum systems
Shingo Kukita, Department of Mathematics, Shanghai University

Abstract: Estimation of parameters is an important task in many fields. The quantum
Fisher information (QFI) provides an upper bound of precision of parameter estimation.
When we consider an open quantum system, noises caused by environments tend
to decrease the QFI in many cases. Thus, it is an important problem how we protect
the QFI from the noises. In this talk, I will discuss how non-Markovianity affects
the time dependence of the QFI in a specific model. Also, some possibilities
to protect the QFI will be discussed.

=====================================================================
Title: Majorana representation and some of its applications in quantum information processing
Mikio Nakahara, Department of Mathematics, Shanghai University

Abstract: A qubit state is an element of CP^1, which can be identified as a sphere S^2.
Thus a qubit state can be visualized as a unit vector in R^3 (the Bloch vector). The
Majorana representation is a nice way to "visualize" a normalized complex vector in a
higher-dimensional vector space. A higher-dimensional vector space is isomorphic
to a symmetric combination of the 2-d representation space of SU(2) and we take
advantage of this fact to represent a d-dimensional normalized vector as a symmetric
combination of (d-1) Bloch vectors, which is called the Majorana representation of
the complex vector and each Bloch vector is called the Majorana vector in this context.
In my talk, I introduce our recent work on the inner product of two complex vectors
written in terms of the Majorana vectors. This is interesting from combinatorial point
of view. I also introduce application of the Majorana representations to find SIC-POVM
(A symmetric, informationally complete, positive operator-valued measure) in d=2 and d=3.
Its application to visualization of topological excitations in BEC with higher hyperspin
state will be also introduced if time permits.

6月19日（水）
31号館3階シミュレーション実験室　From 14:30 ~