Quantifying quantum coherence via nonreal Kirkwood-Dirac quasiprobability

Kirkwood-Dirac (KD) quasiprobability is a quantum analog of phase space probability of classical statistical mechanics, allowing negative or/and nonreal values. It gives an informationally complete representation of a quantum state. Recent works have revealed the important roles played by the KD quasiprobability in the broad fields of quantum science and quantum technology. In the present work, we use the KD quasiprobability to access the quantum coherence in a quantum state. We show that the $l_1$-norm of the imaginary part of the KD quasiprobability over an incoherent reference basis and a second basis, maximized over all possible choices of the latter, can be used to quantify quantum coherence, satisfying certain desirable properties. It is upper bounded by the quantum uncertainty, i.e., the quantum standard deviation, of the incoherent basis in the state. It gives a lower bound to the $l_1$-norm quantum coherence, and for a single qubit, they are identical. We discuss the measurement of the KD coherence based on the measurement of the KD quasiprobability and an optimization procedure in hybrid quantum-classical schemes, and suggest statistical interpretations. We also discuss its relevance in the physics of linear response regime.Comment: 23 pages, no figure

Quantum coherence as asymmetry from complex weak values

Quantum coherence as an asymmetry relative to a translation group generated by a Hermitian operator, is a necessary resource for the quantum parameter estimation. On the other hand, the sensitivity of the parameter estimation is known to be related to the imaginary part of the weak value of the Hermitian operator generating the unitary imprinting of the parameter being estimated. This naturally suggests a question if one can use the imaginary part of the weak value to characterize the coherence as asymmetry. In this work, we show that the average absolute imaginary part of the weak value of the generator of the translation group, maximized over all possible projective measurement bases, can be used to quantify the coherence as asymmetry relative to the translation group, satisfying certain desirable requirements. We argue that the quantifier of coherence so defined, called TC (translationally-covariant) w-coherence, can be obtained experimentally using a hybrid quantum-classical circuit via the estimation of weak value combined with a classical optimization procedure. We obtain upper bounds of the TC w-coherence in terms of the quantum standard deviation, quantum Fisher information, and the imaginary part of the Kirkwood-Dirac quasiprobability. We further obtain a lower bound and derive a relation between the TC w-coherences relative to two generators of translation group taking a form analogous to the Kennard-Weyl-Robertson uncertainty relation.Comment: 31 pages, no figure

Anisotropic Li diffusion in pristine and defective ZnO

We study the Li interstitial diffusion in pristine and defective ZnO bulk by means of first-principles density functional theory (DFT) coupled with Nudged Elastic Band (NEB) calculations. We consider three types of point defects, i.e., oxygen vacancy (Ovac), Zn vacancy (Znvac) and ZnO vacancy pair (ZnOvac-pair) and investigate their individual effect on the energy barrier of Li interstitial diffusion. Our results predict that Ovac and Znvac lower the Li diffusion energy barrier as compared to the pristine ZnO case. However, we further find that Li interstitial on the other hand may possibly be trapped inside the Znvac subsequently forming the LiZn substitutional type of defect. The similar behavior also observed for Li interstitial in the vicinity of Zn-Ovac_pair though with less change of Li diffusion barriers as compared to the other two cases. Our results indicate that among the three considered defects only Ovac shows possible enhancement of the kinetics of Li diffusion inside ZnO bulk

Graphite as A Hydrogen Storage in Fuel Cell System: Computational Material Study for Renewable Energy

The Hydrogen storage based-graphite materials have been investigated theoretically via Density Functional Theory (DFT) approach. The native graphite was compared to the modified graphite, namely the intercalation graphite (GICs, graphite intercalated compounds). Here the GICs was intercalated by alkali metals (Li, Na and K). The electronic structures, energetics and atomic orbital contributions of hydrogen-graphite system, GICs, and hydrogen-GICs were studied by calculation approach of gradient corrected PBE (Perdew-Burke-Ernzerhof) for recovery of exchange-correlation energy. The calculation was supported by using basis set of the plane waves whereas the computation of electron-core by using Ultrasoft Vanderbilt pseudopotential. The computational calculation provides four main studies i.e. molecular geometry relaxation, determination of electronic bands structure of energy, energy state density (DOS) and atomic orbital contribution by charge density differences.Keywords: Density Functional Theory, hydrogen gas, graphite intercalated materia

PERMUKAAN ENERGI POTENSIAL HIDROGEN PADA SISTEM GRAFIT TERINTERKALASI INVESTIGASI TEORI FUNGSI KERAPATAN

Permukaan energi potensial untuk sistem hidrogen pada permukaan grafit terinterkalasi alkali (Li, Na dan K) telah  diteliti,  secara  teoritis.  Model  struktural,  sifat  energik,  dan  elektronik  dari  hidrogen  pada  sistem alkali/grafit dihitung melalui teori fungsi kerapatan (DFT) dengan menggunakan pendekatan koreksi gradien Perdew-Burke-Ernzerhof  (PBE).  Perhitungan  dilakukan  dengan  menggunakan  basis  set  plane-wave,  dan interaksi  elektron-inti  dijelaskan  menggunakan  pendekatan  pseudopotential.  Pada  langkah  pertama, permukaan energi potensial diperoleh dengan cara menghitung energi berbagai posisi molekul hidrogen pada permukaan  grafit,  yaitu  di  atas  atom  karbon  (top),  di  atas  ikatan  C=C  (bridge),  dan  di  atas  ring  (hollow). Diperoleh hasil bahwa molekul hidrogen yang paling stabil adalah pada posisi top, dengan energi sebesar 3,2 eV pada jarak 0,019 A. Selanjutnya semua perhitungan dilakukan pada jarak 3,2 A dari permukaan grafit. Pada langkah berikutnya, permukaan energi potensial atom alkali (Li, Na, dan K) pada permukaan grafit memberi hasil bahwa atom alkali paling stabil pada posisi hollow dengan jarak antara Li, Na, dan K pada permukaan grafit adalah 1,7 A, 2,3 A, dan 2,6 A dengan energi minimum -1,37 eV, -0,66 eV dan -0,96 eV, secara berurutan. Dari  data  kerapatan  muatan  menunjukkan  bahwa  terjadi  peningkatan  transfer  muatan  elektron  dari  atom alkali  terhadap  orbital  elektron  pi  grafit.  Pada  langkah  terakhir,  permukaan  energi  potensial  minimum diperoleh pada variasi posisi hidrogen molekul pada sistem grafit terinterkalasi atom alkali, dan menunjukkan model permukaan energi potensial seperti yang ditunjukkan pada gambar di bawah. Diperoleh bahwa terdapat enam  titik  energi  potensial  terendah  yang  dapat  ditempati  oleh  enem  molekul  hidrogen  yaitu  pada  posisi bridge dari sistem ini. Jarak antara keenam molekul hidrogen dengan atom Li, Na, dan atom K adalah 2,6 A, 2,7 A,  dan  2,8  A,  dengan  energi  minimum  -0,082  eV,  eV  dan  -0,071  -0,079  eV,  secara  berurutan.  Hal  ini menunjukkan  bahwa  kehadiran  atom  Li  memberikan  nilai  kapasitas  yang  lebih  tinggi  dibandingkan  dengan atom  Na  dan  K.  Hasil  ini  mendukung  dan  menjelaskan  secara  kualitatif  adanya  peningkatan  kapasitas penyimpanan hidrogen dalam sistem alkali-grafit.   kata kunci : Teori fungsi kerapatan, hidrogen, grafit terinterkalasi atom alkal

Nanoteknologi terapan : konversi dari hasil penelitian menjadi produk

Judul asli : Applied nanotechnology The conversion of research result to product