Universal Boundary Entropies in Conformal Field Theory: A Quantum Monte Carlo Study

Recently, entropy corrections on nonorientable manifolds such as the Klein bottle are proposed as a universal characterization of critical systems with an emergent conformal field theory (CFT). We show that entropy correction on the Klein bottle can be interpreted as a boundary effect via transforming the Klein bottle into an orientable manifold with nonlocal boundary interactions. The interpretation reveals the conceptual connection of the Klein bottle entropy with the celebrated Affleck-Ludwig entropy in boundary CFT. We propose a generic scheme to extract these universal boundary entropies from quantum Monte Carlo calculation of partition function ratios in lattice models. Our numerical results on the Affleck-Ludwig entropy and Klein bottle entropy for the $q$-state quantum Potts chains with $q=2,3$ show excellent agreement with the CFT predictions. For the quantum Potts chain with $q=4$, the Klein bottle entropy slightly deviates from the CFT prediction, which is possibly due to marginally irrelevant terms in the low-energy effective theory.Comment: 10 pages, 4 figures. Published versio

Performance of multiple-input multiple-output wireless communications systems using distributed antennas

In this contribution we propose and investigate a multiple-input multiple-output (MIMO) wireless communications system, where multiple receive antennas are distributed in the area covered by a cellular cell and connected with the base-station (BS). We first analyze the total received power by the BS through the distributed antennas, when assuming that the mobile's signal is transmitted over lognormal shadowed Rayleigh fading channels. Then, the outage probability of the distributed antenna MIMO systems is investigated, when considering various antenna distribution patterns. Furthermore, space-time coding at the mobile transmitter is considered for enhancing the outage performance of the distributed antenna MIMO system. Our study and simulation results show that the outage performance of a distributed antenna MIMO system can be significantly improved, when either increasing the number of distributed receive antennas or increasing the number of mobile transmit antennas

Mutual productivity spillovers between foreign and local firms in China

The existing literature treats advanced technology sourcing as the only cause of reverse productivity spillovers from local to foreign firms and implies that mutual spillovers between foreign and local firms can only happen in the developed world. This paper argues that the diffusion of indigenous technology and local knowledge helps the productivity enhancement of multinationals, so that there can be mutual spillovers even in a developing country. The results from a large-sample firm-level econometric analysis and a comparative case study of seven companies in Chinese manufacturing support this new argument, as mutual spillovers are identified between local Chinese firms and overseas Chinese or OECD-invested firms

High-capacity quantum secure direct communication based on quantum hyperdense coding with hyperentanglement

We present a quantum hyperdense coding protocol with hyperentanglement in polarization and spatial-mode degrees of freedom of photons first and then give the details for a quantum secure direct communication (QSDC) protocol based on this quantum hyperdense coding protocol. This QSDC protocol has the advantage of having a higher capacity than the quantum communication protocols with a qubit system. Compared with the QSDC protocol based on superdense coding with $d$-dimensional systems, this QSDC protocol is more feasible as the preparation of a high-dimension quantum system is more difficult than that of a two-level quantum system at present.Comment: 5 pages, 2 figur

Giant tunnel magnetoresistance with a single magnetic phase-transition electrode

Magnetic phase transition tunnel magnetoresistance (MPT-TMR) effect with a single magnetic electrode has been investigated by first-principles calculations. The calculations show that the MPT-TMR of FeRh/MgO/Cu tunnel junction can be as high as hundreds of percent when the magnetic structure of FeRh changes from G-type antiferromagnetic (GAFM) to ferromagnetic order. This new type of MPT-TMR may be superior to the tunnel anisotropic magnetoresistance because of its huge magneto-resistance effect and similar structural simplicity. The main mechanism for the giant MPT-TMR can be attributed to the formation of interface resonant states at GAFM-FeRh/MgO interface. A direct FeRh/MgO interface is found to be necessary for achieving high MPT-TMR experimentally. Moreover, we find the FeRh/MgO interface with FeRh in ferromagnetic phase has nearly full spin-polarization due to the negligible majority transmission and significantly different Fermi surface of two spin channels. Thus, it may act as a highly efficient and tunable spin-injector. In addition, electric field driven MPT of FeRh-based hetero-magnetic nanostructures can be utilized to design various energy efficient tunnel junction structures and the corresponding lower power consumption devices. Our results will stimulate further experimental investigations of MPT-TMR and other fascinating phenomenon of FeRh-based tunnel junctions that may be promising in antiferromagnetic spintronics