arXiv subjects

Naoki Yamamoto

Publications and source records attributed to Naoki Yamamoto.

At least 181 records · Page 10Linked to original sources

arXiv2015

Chiral Langevin theory for non-Abelian plasmas

Charged plasmas with chirality imbalance are unstable and tend to reduce the imbalance. This chiral plasma instability is, however, not captured in (anomalous) hydrodynamics for high-temperature non-Abelian plasmas. We derive a Langevin-type classical effective theory with anomalous parity-violating effects for non-Abelian plasmas that describes the chiral plasma instability at the magnetic scale. We show that the time scale of the instability is of order $[g^4 T \ln(1/g)]^{-1}$ at weak coupling.

hep-th↗

arXiv2014

Coherent versus measurement feedback: Linear systems theory for quantum information

To control a quantum system via feedback, we generally have two options in choosing control scheme. One is the coherent feedback, which feeds the output field of the system, through a fully quantum device, back to manipulate the system without involving any measurement process. The other one is the measurement-based feedback, which measures the output field and performs a real-time manipulation on the system based on the measurement results. Both schemes have advantages/disadvantages, depending on the system and the control goal, hence their comparison in several situation is important. This paper considers a general open linear quantum system with the following specific control goals; back-action evasion (BAE), generation of a quantum non-demolished (QND) variable, and generation of a decoherence-free subsystem (DFS), all of which have important roles in quantum information science. Then some no-go theorems are proven, clarifying that those goals cannot be achieved by any measurement-based feedback control. On the other hand it is shown that, for each control goal, there exists a coherent feedback controller accomplishing the task. The key idea to obtain all the results is system theoretic characterizations of BAE, QND, and DFS in terms of controllability and observability properties or transfer functions of linear systems, which are consistent with their standard definitions.

quant-ph↗

arXiv2014

Zero-dynamics principle for perfect quantum memory in linear networks

In this paper, we study a general linear networked system that contains a tunable memory subsystem; that is, it is decoupled from an optical field for state transportation during the storage process, while it couples to the field during the writing or reading process. The input is given by a single photon state or a coherent state in a pulsed light field. We then completely and explicitly characterize the condition required on the pulse shape achieving the perfect state transfer from the light field to the memory subsystem. The key idea to obtain this result is the use of zero-dynamics principle, which in our case means that, for perfect state transfer, the output field during the writing process must be a vacuum. A useful interpretation of the result in terms of the transfer function is also given. Moreover, a four-nodes network composed of atomic ensembles is studied as an example, demonstrating how the input field state is transferred to the memory subsystem and how the input pulse shape to be engineered for perfect memory looks like.

quant-ph↗

arXiv2014

Preparation of Pure Gaussian States via Cascaded Quantum Systems

This paper provides an alternative approach to the problem of preparing pure Gaussian states in a linear quantum system. It is shown that any pure Gaussian state can be generated by a cascade of one-dimensional open quantum harmonic oscillators, without any direct interaction Hamiltonians between these oscillators. This is physically advantageous from an experimental point of view. An example on the preparation of two-mode squeezed states is given to illustrate the theory.

quant-ph↗

arXiv2014

Energy dissipation of electrons at a p-type GaAs(110) surface

Electron injection from the tip of a scanning tunneling microscope into a p-type GaAs(110) surface have been used to induce luminescence in the bulk. Atomically-resolved photon maps revealed significant reduction of luminescence intensity at surface states localized near Ga atoms. Quantitative analysis based on the first principles calculation and a rate equation approach was performed to describe overall energy dissipation processes of the incident tunneling electrons. Our study shows that the recombination processes in the bulk electronic states are suppressed by the fast electron scattering at the surface, and the electrons dominantly undergo non-radiative recombination through the surface states.

cond-mat.mes-hall↗

arXiv2014

New critical point for QCD in a magnetic field

We provide a general argument for the possible existence of a new critical point associated with a deconfinement phase transition in QCD at finite temperature $T$ and in a magnetic field $B$ with zero chemical potential. This is the first example of a QCD critical point in a physical external parameter region that can be studied using lattice QCD simulations without suffering from a sign problem.

hep-ph↗

arXiv2014

Decoherence-free linear quantum subsystems

This paper provides a general theory for characterizing and constructing a decoherence-free (DF) subsystem for an infinite dimensional linear open quantum system. The main idea is that, based on the Heisenberg picture of the dynamics rather than the commonly-taken Schrodinger picture, the notions of controllability and observability in control theory are employed to characterize a DF subsystem. A particularly useful result is a general if and only if condition for a linear system to have a DF component; this condition is used to demonstrate how to actually construct a DF dynamics in some specific examples. It is also shown that, as in the finite dimensional case, we are able to do coherent manipulation and preservation of a state of a DF subsystem.

quant-ph↗

arXiv2014

Banks-Casher-type relations for complex Dirac spectra

For theories with a sign problem there is no analog of the Banks-Casher relation. This is true in particular for QCD at nonzero quark chemical potential. However, for QCD-like theories without a sign problem the Banks-Casher relation can be extended to the case of complex Dirac eigenvalues. We derive such extensions for the zero-temperature, high-density limits of two-color QCD, QCD at nonzero isospin chemical potential, and adjoint QCD. In all three cases the density of the complex Dirac eigenvalues at the origin is proportional to the BCS gap squared.

hep-lat↗

arXiv2014

Structure identification and state initialization of spin networks with limited access

For reliable and consistent quantum information processing carried out on a quantum network, the network structure must be fully known and a desired initial state must be accurately prepared on it. In this paper, for a class of spin networks with its single node only accessible, we provide two continuous-measurement-based methods to achieve the above requirements; the first one identifies the unknown network structure with high probability, based on continuous-time Bayesian update of the graph structure; the second one is, with the use of adaptive measurement technique, able to deterministically drive any mixed state to a spin coherent state for network initialization.

quant-ph↗

arXiv2014

Magnetars and the Chiral Plasma Instabilities

We propose a possible new mechanism for a strong and stable magnetic field of compact stars due to an instability in the presence of a chirality imbalance of electrons---the chiral plasma instability. A large chirality imbalance of electrons inevitably occurs associated with the parity-violating weak process during core collapse of supernovae. We estimate the maximal magnetic field due to this instability to be of order 10^{18} G at the core. This mechanism naturally generates a large magnetic helicity from the chiral asymmetry, which ensures the stability of the large magnetic field.

astro-ph.HE↗

arXiv2013

Information amplification via postselection: A parameter estimation perspective

It is known that weak measurement can significantly amplify the mean of measurement results, sometimes out of the range limited in usual quantum measurement. This fact, as actively demonstrated recently in both theory and experiment, implies the possibility to estimate a very small parameter using the weak measurement technique. But does the weak measurement really bring about the increase of "information" for parameter estimation? This paper clarifies that, in a general situation, the answer is NO; more precisely, the weak measurement cannot further decrease the lower bound of the estimation error, i.e. the so-called Cramér-Rao bound, which is proportional to the inverse of the quantum Fisher information.

quant-ph↗

arXiv2013

Chiral Plasma Instabilities

We study the collective modes in relativistic electromagnetic or quark-gluon plasmas with an asymmetry between left- and right-handed chiral fermions, based on the recently formulated kinetic theory with Berry curvature corrections. We find that there exists an unstable mode, signaling the presence of a plasma instability. We argue the fate of this "chiral plasma instability" including the effect of collisions, and briefly discuss its relevance in heavy ion collisions and compact stars.

nucl-th↗

arXiv2013

Kinetic theory with Berry curvature from quantum field theories

A kinetic theory can be modified to incorporate triangle anomalies and the chiral magnetic effect by taking into account the Berry curvature flux through the Fermi surface. We show how such a kinetic theory can be derived from underlying quantum field theories. Using the new kinetic theory, we also compute the parity-odd correlation function that is found to be identical to the result in the perturbation theory in the next-to-leading order hard dense loop approximation.

hep-th↗

arXiv2013

Banks-Casher-type relation for the BCS gap at high density

We derive a new Banks-Casher-type relation which relates the density of complex Dirac eigenvalues at the origin to the BCS gap of quarks at high density. Our relation is applicable to QCD and QCD-like theories without a sign problem, such as two-color QCD and adjoint QCD with baryon chemical potential, and QCD with isospin chemical potential. It provides us with a method to measure the BCS gap through the Dirac spectrum on the lattice.

hep-lat↗

arXiv2013

Deterministic generation of Gaussian pure state in quasi-local dissipative system

This paper shows that an arbitrary Gaussian pure state can be deterministically generated in a dissipative open system that has quasi-local interactions between the subsystems and couples to surrounding environment in a local manner. A quasi-local interaction, which means that the interaction occurs among only a few subsystems, is a crucial requirement for practical engineering of a dissipative system. The key idea is that, first, an auxiliary system having local interaction with environment is prepared, and then that auxiliary system is coupled to the underlying target system via a set of two-body Hamiltonians in such a way that a desired pure state is generated. Moreover, we show that, even with a simple single-mode auxiliary system, deterministic generation of an arbitrary approximate Gaussian cluster state is possible, by devising an appropriate switching scheme. Lastly, we discuss in a specific example how much a dissipation-induced pure Gaussian state can be perturbed by decoherence and parameter error.

quant-ph↗

arXiv2012

Singular values of the Dirac operator at nonzero density

At nonzero density the eigenvalues of the Dirac operator move into the complex plane, while its singular values remain real and nonnegative. In QCD-like theories, the singular-value spectrum carries information on the diquark (or pionic) condensate. We have constructed low-energy effective theories in different density regimes and derived a number of exact results for the Dirac singular values, including Banks-Casher-type relations for the diquark (or pionic) condensate, Smilga-Stern-type relations for the slope of the singular-value density, and Leutwyler-Smilga-type sum rules for the inverse singular values. We also present a rigorous index theorem for non-Hermitian Dirac operators.

hep-lat↗

arXiv2012

Some exact results on the QCD critical point

We show, in a model-independent manner, that the QCD critical point can appear only inside the pion condensation phase of the phase-quenched QCD as long as the contribution of flavor-disconnected diagrams is negligible. The sign problem is known to be maximally severe in this region, implying that the QCD critical point is reachable by the present lattice QCD techniques only if there is an enhancement of the flavor-disconnected contribution at finite baryon chemical potential.

hep-lat↗

arXiv2012

Berry Curvature, Triangle Anomalies, and the Chiral Magnetic Effect in Fermi Liquids

In a three-dimensional Fermi liquid, quasiparticles near the Fermi surface may possess a Berry curvature. We show that if the Berry curvature has a nonvanishing flux through the Fermi surface, the particle number associated with this Fermi surface has a triangle anomaly in external electromagnetic fields. We show how Landau's Fermi liquid theory should be modified to take into account the Berry curvature. We show that the "chiral magnetic effect" also emerges from the Berry curvature flux.

cond-mat.mes-hall↗