arXiv subjects

Li Gao

Publications and source records attributed to Li Gao.

97 records · Page 6Linked to original sources

arXiv2017

Capacity Estimates via comparison with TRO channels

A ternary ring of operators (TRO) in finite dimensions is a diagonal sum of spaces of rectangular matrices. TRO as operator space corresponds to quantum channels that are diagonal sums of partial traces, which we call TRO channels. TRO channels admits simple, single-letter capacity formula. Using operator space and complex interpolation techniques, we give perturbative capacities estimates for a wider class of quantum channels by comparison to TRO channels. Our estimates applies mainly for quantum and private capacity and also strong converse rates. The examples includes random unitary from group representations which in general are non-degradable channels.

quant-ph↗

arXiv2017

Heralded Channel Holevo Superadditivity Bounds from Entanglement Monogamy

We show that for a particular class of quantum channels, which we call heralded channels, monogamy of squashed entanglement limits the superadditivity of Holevo capacity. Heralded channels provide a means to understand the quantum erasure channel composed with an arbitrary other quantum channel, as well as common situations in experimental quantum information that involve frequent loss of qubits or failure of trials. We also show how entanglement monogamy applies to non-classicality in quantum games, and we consider how faithful, monogamous entanglement measures may bound other entanglement-dependent quantities in many-party scenarios.

quant-ph↗

arXiv2017

Capacity Bounds via Operator Space Methods

Quantum capacity, as the ultimate transmission rate of quantum communication, is characterized by regularized coherent information. In this work, we reformulate approximations of the quantum capacity by operator space norms and give both upper and lower bounds on quantum capacity, and potential quantum capacity using interpolation techniques. We identify a situation in which nice classes of channels satisfy a "comparison property" on entropy, coherent information and capacities. The paradigms for our estimates are so-called conditional expectations. These generally non-degradable channels admit a strongly additive expression for $Q^{(1)}$. We also identify conditions on channels showing that the "hashing bound" is optimal for the cb-entropy. These two estimates combined give upper and lower bounds on quantum capacity on our "nice" classes of channels, which differ only up to a factor 2, independent of the dimension. The estimates are discussed for certain classes of channels, including group channels, Pauli channels and other high-dimensional channels.

quant-ph↗

arXiv2016

Continuous perturbations of noncommutative Euclidean spaces and tori

We prove a noncompact version of Haagerup and Rørdam's result about continuous paths of the rotation $C^*$-algebras. It gives a continuous Moyal deformation of Euclidean plane. Moveover, the construction is generalized to noncommutative Euclidean spaces of dimension $d\ge 2$. As a corollary, we obtain Lip$^{\frac12}$ continuous maps for the generators of noncommutative $d$-tori.

math.OA↗

arXiv2012

A p-type Heusler compound: Growth, structure, and properties of epitaxial thin NiYBi films on MgO(100)

Epitaxial semiconducting NiYBi thin films were directly prepared on MgO(100) substrates by magnetron sputtering. The intensity ratio of the (200) and (400) diffraction peaks, I(200)/I(400) = 2.93, was close to the theoretical value (3.03). The electronic structure of NiYBi was calculated using WIEN2k and a narrow indirect band gap of width 210 meV was found. The valence band spectra of the films obtained by linear dichroism in hard X-ray photoelectron spectroscopy exhibit clear structures that are in good agreement with the calculated band structure of NiYBi.

cond-mat.mtrl-sci↗

arXiv2012

Electronic and crystalline structures of zero band-gap PdLuBi thin films grown epitaxially on MgO(100)

Thin films of the proposed topological insulator PdLuBi - a Heusler compound with the C1b structure - were prepared on Ta-Mo-buffered MgO(100) substrates by co-sputtering from PdBi2 and Lu targets. Epitaxial growth of high-quality PdLuBi films was confirmed by X-ray spectrometry and reflection high-energy electron diffraction. The root-mean-square roughness of the films was as low as 1.45 nm, although the films were deposited at high temperature. The film composition is close to the ideal stoichiometric ratio. The valence band spectra of the PdLuBi films, observed by hard X-ray photoelectron spectroscopy, correspond perfectly to the ab-initio-calculated density of states.

cond-mat.mtrl-sci↗

arXiv2009

Thermal-magnetic noise measurement of spin-torque effects on ferromagnetic resonance in MgO-based magnetic tunnel junctions

Thermal-magnetic noise at ferromagnetic resonance (T-FMR) can be used to measure magnetic perpendicular anisotropy of nanoscale magnetic tunnel junctions (MTJs). For this purpose, T-FMR measurements were conducted with an external magnetic field up to 14 kOe applied perpendicular to the film surface of MgO-based MTJs under a dc bias. The observed frequency-field relationship suggests that a 20 A CoFeB free layer has an effective demagnetization field much smaller than the intrinsic bulk value of CoFeB, with 4PiMeff = (6.1 +/- 0.3) kOe. This value is consistent with the saturation field obtained from magnetometry measurements on extended films of the same CoFeB thickness. In-plane T-FMR on the other hand shows less consistent results for the effective demagnetization field, presumably due to excitations of more complex modes. These experiments suggest that the perpendicular T-FMR is preferred for quantitative magnetic characterization of nanoscale MTJs.

cond-mat.mes-hall↗