Changchen Chen

Bio: Changchen Chen is an academic researcher from Massachusetts Institute of Technology. The author has contributed to research in topics: Materials science & Ion. The author has an hindex of 20, co-authored 44 publications receiving 2248 citations. Previous affiliations of Changchen Chen include The Institute of Optics & University of Rochester.

Magnetic excitations in pure, lightly doped, and weakly metallic La2CuO4.

Abstract: We report a comprehensive neutron-scattering study of the evolution of the magnetic excitations in La[sub 2[minus][ital x]]Sr[sub [ital x]]CuO[sub 4] for 0[le][ital x][le]0.04. We first present accurate measurements of the magnetic correlation length and the sublattice magnetization of a carrier-free La[sub 2]CuO[sub 4] crystal and analyze these in the context of recent theoretical predictions. We then systematically investigate the influence of different dopants on the magnetism: Our measurements indicate that static vacancies in the La[sub 2]Cu[sub 1[minus][ital y]]Zn[sub [ital y]]O[sub 4] system affect the magnetic correlations in a similar manner as electrons in Pr[sub 2[minus][ital x]]Ce[sub [ital x]]CuO[sub 4]. The magnetic correlation length is much more rapidly suppressed as a function of [ital x] in La[sub 2[minus][ital x]]Sr[sub [ital x]]CuO[sub 4], and for [ital x][le]0.04 we find that it obeys the empirical relation [xi][sup [minus]1]([ital x],[ital T])=[xi][sup [minus]1]([ital x],0)+[xi][sup [minus]1](0,[ital T]), where [xi](0,[ital T]) is the measured correlation length of the carrier-free sample. We also report an extensive set of measurements of the dynamical magnetic response function of a crystal of composition La[sub 1.96]Sr[sub 0.04]CuO[sub 4] for excitation energies 0.75[le][omega][le]45 meV and temperatures 1.5[le][ital T][le]500 K.

Quantum transport simulations in a programmable nanophotonic processor

Abstract: Environmental noise and disorder play critical roles in quantum particle and wave transport in complex media, including solid-state and biological systems. While separately both effects are known to reduce transport, recent work predicts that in a limited region of parameter space, noise-induced dephasing can counteract localization effects, leading to enhanced quantum transport. Photonic integrated circuits are promising platforms for studying such effects, with a central goal of developing large systems providing low-loss, high-fidelity control over all parameters of the transport problem. Here, we fully map the role of disorder in quantum transport using a nanophotonic processor: a mesh of 88 generalized beamsplitters programmable on microsecond timescales. Over 64,400 experiments we observe distinct transport regimes, including environment-assisted quantum transport and the ‘quantum Goldilocks’ regime in statically disordered discrete-time systems. Low-loss and high-fidelity programmable transformations make this nanophotonic processor a promising platform for many-boson quantum simulation experiments. A large-scale, low-loss and phase-stable programmable nanophotonic processor is fabricated to explore quantum transport phenomena. The signature of environment-assisted quantum transport in discrete-time systems is observed for the first time.

Antisymmetric exchange and its influence on the magnetic structure and conductivity of La 2 Cu O 4

Abstract: Measurements of the magnetic moment of antiferromagnetic ${\mathrm{La}}_{2}$Cu${\mathrm{O}}_{4}$ at high fields reveal a new phase boundary originating from a previously undetected canting of the ${\mathrm{Cu}}^{2+}$ spins out of the Cu${\mathrm{O}}_{2}$ planes. This canting, together with the exponential temperature dependence of the two-dimensional correlation length, accounts quantitatively for the susceptibility peak at the N\'eel temperature. Enhancement of the conductivity in the ferromagnetic phase demonstrates a strong connection between the magnetism and charge transport.

Rapid Generation of Light Beams Carrying Orbital Angular Momentum

Abstract: We report a technique for encoding both amplitude and phase variations onto a laser beam using a single digital micro-mirror device (DMD). Using this technique, we generate Laguerre-Gaussian and vortex orbital-angular-momentum (OAM) modes, along with modes in a set that is mutually unbiased with respect to the OAM basis. Additionally, we have demonstrated rapid switching among the generated modes at a speed of 4 kHz, which is much faster than the speed regularly achieved by spatial light modulators (SLMs). The dynamic control of both phase and amplitude of a laser beam is an enabling technology for classical communication and quantum key distribution (QKD) systems that employ spatial mode encoding.

Soft-phonon behavior and transport in single-crystal La 2 CuO 4

Abstract: Using a flux technique we have grown sizable single crystals of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$. With x rays and neutrons we have studied both the static and dynamic aspects of the tetragonal to orthorhombic structural phase transition; classic soft-phonon behavior is observed at the ((1/2, 1) / 2 , 0) zone boundary involving rotations of ${\mathrm{CuO}}_{6}$ octahedra. The pure and lightly-doped single crystals show hopping conductivity, ln\ensuremath{\rho}\ensuremath{\sim}(${T}_{0}$/T${)}^{1/4}$, indicating that the electronic states at the Fermi energy are localized.

Principles Of Optics Electromagnetic Theory Of Propagation Interference And Diffraction Of Light

Abstract: Thank you for reading principles of optics electromagnetic theory of propagation interference and diffraction of light. As you may know, people have search hundreds times for their favorite novels like this principles of optics electromagnetic theory of propagation interference and diffraction of light, but end up in harmful downloads. Rather than enjoying a good book with a cup of coffee in the afternoon, instead they are facing with some infectious bugs inside their computer.

Localization: theory and experiment

Abstract: The transport properties of disordered solids have been the subject of much work since at least the 1950s, but with a new burst of activity during the 1980s which has survived up to the present day. There have been numerous reviews of a more or less specialized nature. The present review aims to fill the niche for a non-specialized review of this very active area of research. The basic concepts behind the theory are introduced with more detailed sections covering experimental results, one-dimensional localization, scaling theory, weak localization, magnetic field effects and fluctuations.

Optical communications using orbital angular momentum beams

Abstract: Orbital angular momentum (OAM), which describes the “phase twist” (helical phase pattern) of light beams, has recently gained interest due to its potential applications in many diverse areas. Particularly promising is the use of OAM for optical communications since: (i) coaxially propagating OAM beams with different azimuthal OAM states are mutually orthogonal, (ii) inter-beam crosstalk can be minimized, and (iii) the beams can be efficiently multiplexed and demultiplexed. As a result, multiple OAM states could be used as different carriers for multiplexing and transmitting multiple data streams, thereby potentially increasing the system capacity. In this paper, we review recent progress in OAM beam generation/detection, multiplexing/demultiplexing, and its potential applications in different scenarios including free-space optical communications, fiber-optic communications, and RF communications. Technical challenges and perspectives of OAM beams are also discussed.

Electronic structure of the high-temperature oxide superconductors

Abstract: Since the discovery of superconductivity above 30 K by Bednorz and M\"uller in the La copper oxide system, the critical temperature has been raised to 90 K in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and to 110 and 125 K in Bi-based and Tl-based copper oxides, respectively. In the two years since this Nobel-prize-winning discovery, a large number of electronic structure calculations have been carried out as a first step in understanding the electronic properties of these materials. In this paper these calculations (mostly of the density-functional type) are gathered and reviewed, and their results are compared with the relevant experimental data. The picture that emerges is one in which the important electronic states are dominated by the copper $d$ and oxygen $p$ orbitals, with strong hybridization between them. Photon, electron, and positron spectroscopies provide important information about the electronic states, and comparison with electronic structure calculations indicates that, while many features can be interpreted in terms of existing calculations, self-energy corrections ("correlations") are important for a more detailed understanding. The antiferromagnetism that occurs in some regions of the phase diagram poses a particularly challenging problem for any detailed theory. The study of structural stability, lattice dynamics, and electron-phonon coupling in the copper oxides is also discussed. Finally, a brief review is given of the attempts so far to identify interaction constants appropriate for a model Hamiltonian treatment of many-body interactions in these materials.

Variational Quantum Algorithms

Abstract: Applications such as simulating complicated quantum systems or solving large-scale linear algebra problems are very challenging for classical computers due to the extremely high computational cost. Quantum computers promise a solution, although fault-tolerant quantum computers will likely not be available in the near future. Current quantum devices have serious constraints, including limited numbers of qubits and noise processes that limit circuit depth. Variational Quantum Algorithms (VQAs), which use a classical optimizer to train a parametrized quantum circuit, have emerged as a leading strategy to address these constraints. VQAs have now been proposed for essentially all applications that researchers have envisioned for quantum computers, and they appear to the best hope for obtaining quantum advantage. Nevertheless, challenges remain including the trainability, accuracy, and efficiency of VQAs. Here we overview the field of VQAs, discuss strategies to overcome their challenges, and highlight the exciting prospects for using them to obtain quantum advantage.