Yuhang Ren

Creation of high-quality long-distance entanglement with flexible resources

We present a quantum repeater protocol that generates the elementary segments of entangled photons through the communication of qubus in coherent states. The input photons at the repeater stations can be in arbitrary states to save the local state preparation time for the operations. The flexibility of the scheme accelerates the generation of the elementary segments (close to the exact Bell states) to a high rate for practical quantum communications. The entanglement connection to long distances is simplified and sped up, possibly realizing an entangled pair of high quality within the time in the order of that for classical communication between two far-away locations.

Schottky solar cells based on CsSnI3 thin-films

We describe a Schottky solar cell based on the perovskite semiconductor CsSnI3 thin-film. The cell consists of a simple layer structure of indium-tin-oxide/CsSnI3/Au/Ti on glass substrate. The measured power conversion efficiency is 0.9%, which is limited by the series and shunt resistance. The influence of light intensity on open-circuit voltage and short-circuit current supports the Schottky solar cell model. Additionally, the spectrally resolved short-circuit current was measured, confirming the unintentionally doped CsSnI3 is of p-type characteristics. The CsSnI3 thin-film was synthesized by alternately depositing layers of SnCl2 and CsI on glass substrate followed by a thermal annealing process.

Energy barrier at the N719-dye/CsSnI3 interface for photogenerated holes in dye-sensitized solar cells

This report is to address the question if black γ-polymorph of cesium tin tri-iodide (B-γ-CsSnI3) can be used as a solid-state hole-transport material in the conventional DSSCs with the N719 dye to replace the liquid electrolyte as reported by I. Chung et al. on Nature 485, 486, (2012). Here we demonstrate rigorously that B-γ-CsSnI3 is not energetically possible to collect photogenerated holes because of the large energy barrier at the interface of N719/B-γ-CsSnI3. Therefore, it cannot serve as a hole-transporter for the conventional DSSCs although it is a good hole-conducting material. A solution-based method was employed to synthesize the B-γ-CsSnI3 polycrystalline thin-films used for this work. These thin-films were then characterized by X-ray diffraction, Hall measurements, optical reflection, and photoluminescence (PL). Particularly, spatially resolved PL intensity images were taken after B-γ-CsSnI3 was incorporated in the DSSC structure to insure the material integrity. The means of ultraviolet photoemission spectroscopy (UPS) was used to reveal why B-γ-CsSnI3 could not act as the substitute of liquid electrolyte in the conventional DSSCs. For the completeness, other two related compounds, one is the yellow polymorph of CsSnI3 and other is Cs2SnI6 with tetravalent tin instead of double-valent tin in CsSnI3 were also investigated by UPS.

Processing multiphoton states through operation on a single photon: Methods and applications

Multiphoton states are widely applied in quantum information technology. By the methods presented in this paper, the structure of a multiphoton state in the form of multiple single-photon qubit products can be mapped to a single-photon qudit, which could also be in a separable product with other photons. This makes possible the manipulation of such multiphoton states by processing single-photon states. The optical realization of unknown qubit discrimination [B. He, J. A. Bergou, and Y.-H. Ren, Phys. Rev. A 76, 032301 (2007)] is simplified with the transformation methods. Another application is the construction of quantum logic gates, where the inverse transformations back to the input state spaces are also necessary. We especially show that the modified setups to implement the transformations can realize the deterministic multicontrol gates (including Toffoli gate) operating directly on the products of single-photon qubits.

Light-induced magnetic precession in (Ga,Mn)As slabs: Hybrid standing-wave Damon-Eshbach modes

Coherent oscillations associated with spin precessions were observed in ultrafast optical experiments on ferromagnetic (Ga,Mn)As films. Using a complete theoretical description of the processes by which light couples to the magnetization, values for the anisotropy constants and the spin stiffness were unambiguously determined from the data. Estimates for the hole-Mn exchange coupling are significantly larger than those previously reported. Results also reveal an important negative contribution to the energy due to surface anisotropy leading to excitations that are a mixture of bulk waves and surface modes.

Universal entangler with photon pairs in arbitrary states

We propose a setup that transforms a photon pair in arbitrary rank-four mixed state, which could also be unknown, to a Bell state. The setup involves two linear optical circuits processing the individual photons and a parity gate working with weak cross-Kerr nonlinearity. By the photon number resolving detection on one of the output quantum bus or communication beams, the setup will realize a near deterministic transformation to a Bell state for every entangling attempt. With the simple threshold detectors, on the other hand, the system can still reach a considerable success probability of 50% per try. The decoherence effect caused by photon absorption losses in the operation is also discussed.

Generation and detection of coherent longitudinal acoustic phonons in the La0.67Sr0.33MnO3 thin films by femtosecond light pulses

The authors report on an anomalous first-to-zero sound crossover in the colossal magnetoresistance compound La0.67Sr0.33MnO3 (LSMO) using ultrafast time-resolved optical techniques. Two-color pump-probe setup was employed to record the photoinduced reflectance oscillations due to excitations of longitudinal acoustic phonons. By changing the sample thickness and probe wavelength, they determine the longitudinal sound velocities of both the thin film and substrate . The values of the sound velocity in LSMO at gigahertz frequencies are more than 20% larger than those from ultrasound experiments in the megahertz regime near TC. The results show the possible existence of dynamic polaron correlations in LSMO.

Universal discriminator for completely unknown optical qubits

We propose an experimental setup that is capable of unambiguously discriminating any pair of linearly independent single photon polarization qubits, about which we do not have any knowledge except that an extra pair of these unknown states are provided as the reference. This setup, which is constructed with optical controlled-NOT (CNOT) gates, weak cross Kerr nonlinearities, Bell state analyzers, and other linear optical elements, transforms the unknown triple photon input states to the corresponding single photon states to be deterministically processed by a linear optics circuit. The optimal discrimination of the unknown states is achieved by this setup.

Determination of magnetic anisotropies, interlayer coupling, and magnetization relaxation in FeCoB/Cr/FeCoB

We studied magnetic anisotropic properties, interlayer coupling, and spin wave relaxation in ten periods of CoFeB/Cr/CoFeB films grown on seed layers of Cu with a Co:Fe:B composition ratio of 2:2:1. The measurements were taken in samples with 50 A layers of CoFeB using the ferromagnetic resonance technique. The thickness of the Cr interlayers was varied from 4 to 40 A for understanding the mechanisms of interlayer coupling. We investigated the magnetic anisotropy parameters by rotating the sample with respect to the microwave magnetic field from in plane to perpendicular to the plane. We identify both the acoustic branch and the optical branch in the spin wave resonance spectra. The effective interlayer coupling constant and the out-of-plane anisotropy show an oscillatory change, while the uniaxial in-plane anisotropy increases monotonically with increasing the thickness of the spacing layers. Moreover, we show that the spin wave relaxation can be optimized by adjusting the interlayer exchange interactions.

Electric field modulation of surface anisotropy and magneto-dynamics in multiferroic heterostructures

The effects of an electric field on the magneto-dynamics of a FeGaB/PZN-PT (lead zinc niobate-lead titanate) multiferroic heterostructure were investigated. It was found that with the increase of electric field applied on the PZN-PT layer, the magnetoelectric coupling is increased, and the magnetization of the FeGaB film changes gradually from an unpinned to a pinned boundary condition at the FeGaB/PZN-PT interface. This change leads to the observation of the first-order standing spin wave mode and the varied resonance intensity ratio between the standing mode and the uniform mode from 1 to 5% in the FMR spectra. The E-field induced surface anisotropy change confirms the strain mediated magnetoelectric mechanism through the interface between multiferroic heterostructures.