No abstract

The suitable band structure is vital for perovskite solar cells, which greatly affect the high photoelectric conversion efficiency. Cation substitution is an effective approach to tune the electric structure, carrier concentration, and optical absorption of hybrid lead iodine perovskites. In this work, the electronic structures and optical properties of cation (Bi, Sn, and TI) doped tetragonal formamidinium lead iodine CH(NH2)2PbI3 (FAPbI3) are studied by first-principles calculations. For comparison, the cation-doped tetragonal methylammonium lead iodine CH3NH3PbI3 (MAPbI3) are also considered. The calculated formation energies reveal that the Sn atom is easier to dope in the tetragonal MAPbI3/FAPbI3 structure due to the small formation energy of about 0.3 eV. Besides, the band gap of Sn-doped MAPbI3/FAPbI3 is 1.30/1.40 eV, which is considerably smaller than the un-doped tetragonal MAPbI3/FAPbI3. More importantly, compare with the un-doped tetragonal MAPbI3/FAPbI3, the Sn-doped MAPbI3 and FAPbI3 have the larger optical absorption coefficient and theoretical maximum efficiency, especially for Sn-doped FAPbI3. The lower formation energy, suitable band gap and outstanding optical absorption of the Sn-doped FAPbI3 make it promising candidates for high-efficient perovskite cells.

We present a scheme for asymmetric quantum information splitting, where a sender distributes asymmetrically a qubit to distant agents in a network. The asymmetric distribution leads to that the agents have different powers to reconstruct the sender's qubit. In other words, the authorities of the agents for getting the quantum secret are hierarchized. The scheme does not need the agents to get together and make nonlocal operations. Our scheme can also be modified to implement controlled teleportation against uncooperation of part of supervisors.

The electronic structures and photocatalytic properties of bismuth oxyhalide bilayers (BiOX1/BiOX2, X1 and X2 are Cl, Br, I) are studied by density functional theory. Briefly, their compositionally tunable bandgaps range from 1.85 to 3.41 eV, suitable for sun-light absorption, and all bilayers have band-alignments good for photocatalytic water-splitting. Among them, heterogeneous BiOBr/BiOI bilayer is the best as it has the smallest bandgap. More importantly, photo-excitation of BiOBr/BiOI leads to electron supply to the conduction band minimum with localized states belonging mainly to bismuth of BiOBr where the H+/H2 half-reaction of water-splitting can be sustained. Meanwhile, holes generated by such photo-excitation are mainly derived from the iodine states of BiOI in the valence band maximum; thus, the O2/H2O half-reaction of water splitting is facilitated on BiOI. Detailed band-structure analysis also indicates that this intriguing spatial separation of photo-generated electron-hole pairs and the two half-reactions of water splitting are good for a wide photo-excitation spectrum from 2–5 eV; as such, BiOBr/BiOI bilayer can be an efficient photocatalyst for water-splitting, particularly with further optimization of its optical absorptivity.

In order to design the high-performance spintronics, it is rather critical to develop new materials, which can easily regulate the magnetism of nanostructures.

The cross-Kerr nonlinearity (XKNL) effect can induce efficient photon interactions in principle with which photonic multiqubit gates can be performed using far fewer physical resources than linear optical schemes. Unfortunately, it is extremely challenging to generate giant cross-Kerr nonlinearities. In recent years, much effort has been made to perform multiqubit gates via weak XKNLs. However, the required nonlinearity strengths are still difficult to achieve in the experiment. We here propose an XKNL-based scheme for realizing a twophoton polarization-parity gate, a universal two-qubit gate, in which the required strength of the nonlinearity could be orders of magnitude weaker than those required for previous schemes. The scheme utilizes a ring cavity fed by a coherent state as a quantum information bus which interacts with a path mode of the two polarized photons (qubits). The XKNL effect makes the bus pick up a phase shift dependent on the photon number of the path mode. Even when the potential phase shifts are very small they can be effectively measured using photon-number resolving detectors, which accounts for the fact that our scheme can work in the regime of tiny XKNL. The measurement outcome reveals the parity (even parity or odd parity) of the two polarization qubits.

No abstract

We propose a scheme for multiparty hierarchical quantum-information splitting (QIS) with a multipartite entangled state, where a boss distributes a secret quantum state to two grades of agents asymmetrically. The agents who belong to different grades have different authorities for recovering boss's secret. Except for boss's Bell-state measurement, no nonlocal operation is involved. The presented scheme is also shown to be secure against eavesdropping. Such a hierarchical QIS is expected to find useful applications in the field of modern multipartite quantum cryptography. A fundamental ingredient for implementation of quantum technologies is the ability to faithfully transmit quantum states among quantum mechanical systems which are even far apart. Quantum-information splitting (QIS, also be referred to as quantum-secret sharing or quantum-state sharing in the literature), first introduced by Hillery, Bužek, and Berthiaume (HBB) [1], is a typical way for quantum state transfer, in which a secret quantum state is distributed by quantum teleportation [2] from a boss to more than one agents so that any one of them can recover the state with assistance of the others. QIS is a generalization of classical-secret sharing to quantum scenario. Classical-secret sharing is one of the most important information-theoretically secure cryptographic protocols and is germane to online auctions, electronic voting, shared electronic banking, cooperative activation of bombs, and so on. Also, QIS has extensive applications in quantum-information science, such as creating joint checking accounts containing quantum money [3], secure distributed quantum computation [4,5], and so on. KeywordsIn the original HBB QIS proposal with the quantum channel being a three-qubit Greenberger-Horne-Zeilinger (GHZ) state [6], the collaboration of two agents is implemented by means of classical communication about their single-particle measurement outcomes. This idea can be directly generalized to the case of N agents by using an (N + 1)-particle GHZ state, or by the way of Ref. [7]. These schemes are (N , N )-threshold schemes where all the N agents need collaborating in order to recover the secret state. Soon after, Cleve, Gottesman, and Lo (CGL) [8] proposed another type of QIS scheme with the idea of quantum error-correcting codes. The CGL scheme is a (K, N )-threshold scheme where K ([N/2] < K ≤ N ) of N agents can extract the quantum information of the original secret state by cooperation. The CGL QIS scheme, however, needs the cooperated agents to make nonlocal operations on their particles. That is, the K cooperated agents need to transmit their K particles to one laboratory and perform a collective operation (decoding operation) on them. In the last decade, both of the above two QIS ideas have triggered significant research activity (see, e.g., [9][10][11][12][13][14][15][16][17][18][19]), and some schemes have already been experimentally realized [20,21].A more general QIS scheme should involve the asymmetry between the powers of the differen...