Tight bound on coherent-state-based entanglement generation over lossy channels

The first stage of the hybrid quantum repeaters is entanglement generation based on transmission of pulses in coherent states over a lossy channel. Protocols to make entanglement with only one type of error are favorable for rendering subsequent entanglement distillation efficient. Here we provide the tight upper bound on performances of these protocols that is determined only by the channel loss. In addition, we show that this bound is achievable by utilizing a proposed protocol [arXiv:0811.3100] composed of a simple combination of linear optical elements and photon-number-resolving detectors.Comment: 12 pages, 3 figure

Quantum repeaters and computation by a single module

We present a protocol of remote nondestructive parity measurement (RNPM) on a pair of quantum memories. The protocol works as a single module for key operations such as entanglement generation, Bell measurement, parity check measurement, and an elementary gate for extending one-dimensional cluster states. The RNPM protocol is achieved by a simple combination of devices such as lasers, optical fibers, beam splitters, and photon detectors. Despite its simplicity, a quantum repeater composed of RNPM protocols is shown to have a communication time that scales sub-exponentially with the channel length, and it can be further equipped with entanglement distillation. With a reduction in the internal losses, the RNPM protocol can also be used for generating cluster states toward measurement-based quantum communication.Comment: 7 pages, 4 figure

Probabilistic cloning with supplementary information

We consider probabilistic cloning of a state chosen from a mutually nonorthogonal set of pure states, with the help of a party holding supplementary information in the form of pure states. When the number of states is 2, we show that the best efficiency of producing m copies is always achieved by a two-step protocol in which the helping party first attempts to produce m-1 copies from the supplementary state, and if it fails, then the original state is used to produce m copies. On the other hand, when the number of states exceeds two, the best efficiency is not always achieved by such a protocol. We give examples in which the best efficiency is not achieved even if we allow any amount of one-way classical communication from the helping party.Comment: 6 pages, no figure

Toward Mandatory Environmental Disclosure for Capital Markets: Discussion and Empirical Evidence from Japan

Increasing demand for environmental disclosure by capital markets hasamplified discussions calling for mandatory environmental disclosure. Towardmandating, capabilities and shortcomings of current voluntary environmentalreporting must be clarified from an investor’s perspective. The authors carriedout extensive empirical research in Japan, one of the most advanced countriesin the world in terms of environmental reporting. Global information vendorsare currently practicing a promising approach that enables investors to utiliseenvironmental data disclosed voluntarily. Relying on their approach, the authorsdeveloped an environmental database that covers 185 companies in the NikkeiIndex. This paper uses the information in this database to discuss the capabilitiesand shortcomings of voluntary environmental disclosure. The databaseenables investors to undertake simple analysis, but blanks and discrepanciesin boundaries damage the comparability and reliability of the data. Thesefundamental shortcomings stem largely from the adverse incentive: the morea company is engaged in environmental reporting, the worse it could appearin the database we developed. In contrast to the fact that an “invisible hand”could increase the number of sustainability reports published voluntarily, theadverse incentive faced by companies indicates that policy-making in the area ofenvironmental disclosure may become more vital in the future

Optimal supplier of single-error-type entanglement via coherent-state transmission

Compared with entanglement with multiple types of noise, entanglement including only one type of error is a favorable fundamental resource not only for quantum communication but also for distributed quantum computation. We consider protocol that presents single-error-type entanglement for distant qubits via coherent-state transmission over a lossy channel. This protocol is regarded as a subroutine to serve entanglement for larger protocol to yield a final output, such as ebits or pbits. In this paper, we provide a subroutine protocol which achieves the global optimal for typical jointly convex yield functions monotonically non-decreasing with respect to the singlet fraction, such as an arbitrary convex function of a singlet fraction and two-way distillable entanglement/key. Entanglement generation based on remote non-destructive parity measurement protocol [K. Azuma, H. Takeda, M. Koashi, and N. Imoto, Phys. Rev. A 85, 062309 (2012)] is identified as such an optimal subroutine.Comment: 12 pages, 6 figure