arXiv subjects

Y. -C. Chen

Publications and source records attributed to Y. -C. Chen.

27 records · Page 2Linked to original sources

arXiv2016

Cooperative single-photon subradiant states

We propose a set of subradiant states which can be prepared and detected in a one-dimensional optical lattice. We find that the decay rates are highly dependent on the spatial phases imprinted on the atomic chain, which gives systematic investigations of the subradiance in the fluorescence experiments. The time evolution of these states can have long decay time where up to hundred milliseconds of lifetime is predicted for one hundred atoms. They can also show decayed Rabi-like oscillations with a beating frequency determined by the difference of cooperative Lamb shift in the subspace. Experimental requirements are also discussed for practical implementation of the subradiant states. Our proposal provides a novel scheme for quantum storage of photons in arrays of two-level atoms through the preparation and detection of subradiant states, which offer opportunities for quantum many-body state preparation and quantum information processing in optical lattices.

quant-ph↗

arXiv2015

Spectral Shaping of Cascade Emissions from Multiplexed Cold Atomic Ensembles

We investigate the spectral properties of the biphoton state from the cascade emissions of cold atomic ensembles, which composes of a telecommunication photon (signal) followed by an infrared one (idler) via four-wave mixing.\ With adiabatic conditions for Gaussian driving pulses of width $τ$, the spectrum of the biphoton state has the form of a Gaussian that conserves signal and idler photon energies within $\hbar/τ$ modulated by a Lorentzian with a superradiant linewidth.\ Multiplexing the atomic ensembles with frequency-shifted cascade emissions, we may manipulate and shape the spectrum of the biphoton state.\ The entropy of entanglement is derived from Schmidt decomposition, which can be larger if we multiplex the atomic ensembles in a way that conserves signal and idler photon central energies.\ The eigenvalues in Schmidt bases are degenerate in pairs for symmetric spectral shaping in which the mode probability densities show interference patterns.\ We also demonstrate the excess entropy of entanglement that comes from continuous frequency space, which scales up the total entropy.\ The scheme of multiplexed cascade-emitted biphoton state provides multimode structures that are useful in long-distance quantum communication and multimode quantum information processing.

quant-ph↗

arXiv2014

Low-energy trions in graphene quantum dots

We investigate, within the envelope function approximation, the low-energy states of trions in graphene quantum dots. The presence of valley pseudospin in graphene as an electron degree of freedom apart from spin adds convolution to the interplay between exchange symmetry and the electron-electron interaction in the trion, leading to new states of trions as well as a low energy trion spectrum different from those in semiconductors. Due to the involvement of valley pseudospin, it is found that the low-energy spectrum is nearly degenerate and consists of states all characterized by having an antisymmetric (pseudospin) (spin) component in the wave function, with the spin (pseudospin) part being either singlet (triplet) or triplet (singlet), as opposed to the spectrum in a semiconductor whose ground state is known to be nondegenerate and always a spin singlet in the case of X- trions. We investigate trions in the various regimes determined by the competition between quantum confinement and electron-electron interaction, both analytically and numerically. The numerical work is performed within a variational method accounting for electron mass discontinuity across the QD edge. The result for electron-hole correlation in the trion is presented. Effects of varying quantum dot size and confinement potential strength on the trion binding energy are discussed.

cond-mat.mes-hall↗

arXiv2013

The VOI-Based Valleytronics

We discuss the valley-orbit interaction (VOI) and the concept of VOI based valleytronics. Potential of such valleytronics is illustrated, with graphene as an example material, in several frontier applications comprising FETs, quantum computing, and quantum communications. Two important devices are discussed as examples, namely, 1) valley pair qubits in coupled graphene quantum dots, to build quantum networks consisting of graphene and photons, and 2) valleybased FETs consisting of graphene quantum wires (channels) and armchair graphene nanoribbons (sources and drains), to build graphene-based, low-power FET circuits. This demonstration makes the VOI-based valleytronics an attractive R & D direction in the area of electronics.

cond-mat.mes-hall↗

arXiv2013

Quantum Manipulation of Valleys in Bilayer Graphene: Theory and Applications

Quantum manipulation of valleys in bilayer graphene is investigated. We establish an effective Schrodinger model, and identify two key mechanisms for valley manipulation - band structure warping and generalized valley-orbit interaction. Specifically, we implement valley qubits / FETs in bilayer graphene, as prospective quantum devices to build valley-based quantum / classical information processing.

cond-mat.mes-hall↗

arXiv2012

Valley-based FETs in graphene

An analogue of the Datta-Das spin FET is investigated, which is all-graphene and based on the valley degree of freedom of electrons / holes. The "valley FET" envisioned consists of a quantum wire of gapped graphene (channel) sandwiched between two armchair graphene nanoribbons (source and drain), with the following correspondence to the spin FET: valley (K and K') \leftrightarrow spin (up and down), armchair graphene nanoribbons \leftrightarrow ferromagnetic electrodes, graphene quantum wire \leftrightarrow semiconductor quantum wire, valley-orbit interaction \leftrightarrow Rashba spin-orbit interaction. The device works as follows. The source (drain) injects (detects) carriers in a specific valley polarization. A gate electric field is applied to the channel and modulates the valley polarization of carriers due to the valley-orbit interaction, thus controlling the amount of current collected at the drain. The valley FET is characterized by: i) smooth interfaces between electrodes and the channel, ii) strong valley-orbit interaction for electrical control of drain current, and iii) vanishing interband valley-flip scattering. By its analogy to the spin FET, the valley FET provides a potential framework to develop low-power FETs for graphene-based nanoelectronics.

cond-mat.mes-hall↗

arXiv2012

Field-induced long-lived supermolecules

We demonstrate that the long-lived bound states (super-molecules) can exist in the dilute limit when we tune the shape of effective potential between polar molecules by an external microwave field. Binding energies, average sizes, and phase diagrams for both s-orbital (bosons) and p-orbital (fermions) dimers are studied, together with bosonic trimer states. We explicitly show that the non- adiabatic transition rate can be easily tuned small for such ground state super-molecules, so that the system can be stable from collapse even near the associated potential resonance. Our results, therefore, suggest a feasible cold molecule system to investigate both novel few-body and many-body physics (for example, the p-wave BCS-BEC crossover for fermions and the paired condensate for bosons) that can not be easily accessed in single species atomic gases.

cond-mat.quant-gas↗

arXiv2004

Role of heating and current-induced forces in the stability of atomic wires

We investigate the role of local heating and forces on ions in the stability of current-carrying aluminum wires. We find that heating increases with wire length due to a red shift of the frequency spectrum. Nevertheless, the local temperature of the wire is relatively low for a wide range of biases provided good thermal contact exists between the wire and the bulk electrodes. On the contrary, current-induced forces increase substantially as a function of bias and reach bond-breaking values at about 1 V. These results suggest that local heating promotes low-bias instabilities if dissipation into the bulk electrodes is not efficient, while current-induced forces are mainly responsible for the wire break-up at large biases. We compare these results to experimental observations.

cond-mat.mes-hall↗

arXiv1996

Phase String Effect in the t-J Model: General Theory

We reexamine the problem of a hole moving in an antiferromagnetic spin background and find that the injected hole will always pick up a sequence of nontrivial phases from the spin degrees of freedom. Previously unnoticed, such a string-like phase originates from the hidden Marshall signs which are scrambled by the hopping of the hole. We can rigorously show that this phase string is non-repairable at low energy and give a general proof that the spectral weight Z must vanish at the ground-state energy due to the phase string effect. Thus, the quasiparticle description fails here and the quantum interference effect of the phase string dramatically affects the long-distance behavior of the injected hole. We introduce a so-called phase-string formulation of the t-J model for a general number of holes in which the phase string effect can be explicitly tracked. As an example, by applying this new mathematical formulation in one dimension, we reproduce the well-known Luttinger-liquid behaviors of the asymptotic single-electron Green's function and the spin-spin correlation function. We can also use the present phase string theory to justify previously developed spin-charge separation theory in two dimensions, which offers a systematic explanation for the transport and magnetic anomalies in the high-T_c cuprates.

cond-mat.str-el↗