Magnetoelectric effects and valley controlled spin quantum gates in transition metal dichalcogenide bilayers

In monolayer group-VI transition metal dichalcogenides (TMDC), charge carriers have spin and valley degrees of freedom, both associated with magnetic moments. On the other hand, the layer degree of freedom in multilayers is associated with electrical polarization. Here, we show that TMDC bilayers offer an unprecedented platform to realize a strong coupling between the spin, layer pseudospin, and valley degrees of freedom of holes. Such coupling not only gives rise to the spin Hall effect and spin circular dichroism in inversion symmetric bilayer, but also leads to a variety of magnetoelectric effects permitting quantum manipulation of these electronic degrees of freedom. Oscillating electric and magnetic fields can both drive the hole spin resonance where the two fields have valley-dependent interference, making possible a prototype interplay between the spin and valley as information carriers for potential valley-spintronic applications. We show how to realize quantum gates on the spin qubit controlled by the valley bit.

Protecting dissipative quantum state preparation via dynamical decoupling

We show that dissipative quantum state preparation processes can be protected against qubit dephasing by interlacing the state preparation control with dynamical decoupling (DD) control consisting of a sequence of short $pi$-pulses. The inhomogeneous broadening can be suppressed to second order of the pulse interval, and the protection efficiency is nearly independent of the pulse sequence but determined by the average interval between pulses. The DD protection is numerically tested and found to be efficient against inhomogeneous dephasing on two exemplary dissipative state preparation schemes that use collective pumping to realize many-body singlets and linear cluster states respectively. Numerical simulation also shows that the state preparation can be efficiently protected by $pi$-pulses with completely random arrival time. Our results make possible the application of these state preparation schemes in inhomogeneously broadened systems. DD protection of state preparation against dynamical noises is also discussed using the example of Gaussian noise with a semiclasscial description.

Adiabatic Creation of Atomic Squeezing in Dark States vs. Decoherences

We study the multipartite correlations of the multi-atom dark states, which are characterized by the atomic squeezing beyond the pairwise entanglement. It is shown that, in the photon storage process with atomic ensemble via electromagnetically induced transparency (EIT) mechanism, the atomic squeezing and the pairwise entanglement can be created by adiabatically manipulating the Rabi frequency of the classical light field on the atomic ensemble. We also consider the sudden death for the atomic squeezing and the pairwise entanglement under various decoherence channels. An optimal time for generating the greatest atomic squeezing and pairwise entanglement is obtained by studying in details the competition between the adiabatic creation of quantum correlation in the atomic ensemble and the decoherence that we describe with three typical decoherence channels.

Effective Hamiltonian approach to the Kerr nonlinearity in an optomechanical system

Using the Born-Oppenheimer approximation, we derive an effective Hamiltonian for an optomechanical system that leads to a nonlinear Kerr effect in the systems vacuum. The oscillating mirror at one edge of the optomechanical system induces a squeezing effect in the intensity spectrum of the cavity field. A near-resonant laser field is applied at the other edge to drive the cavity field, in order to enhance the Kerr effect. We also propose a quantum-nondemolition-measurement setup to monitor a system with two cavities separated by a common oscillating mirror, based on our effective Hamiltonian approach.

Topological Peierls Transitions in M{o}bius Molecular Devices

We study the topological properties of Peierls transitions in a monovalent M{o}bius ladder. Along the transverse and longitudinal directions of the ladder, there exist plenty Peierls phases corresponding to various dimerization patterns. Resulted from a special modulation, namely, staggered modulation along the longitudinal direction, the ladder system in the insulator phase behaves as a ``topological insulator, which possesses charged solitons as the gapless edge states existing in the gap. Such solitary states promise the dispersionless propagation along the longitudinal direction of the ladder system. Intrinsically, these non-trivial edges states originates from the Peierls phases boundary, which arises from the non-trivial $mathbb{Z}^{2}$ topological configuration.

Controlling Quasibound States in 1D Continuum Through Electromagnetic Induced Transparency Mechanism

We study the coherent scattering process of a single photon confined in an one-dimensional (1D) coupled cavity-array, where a $Lambda$-type three-level atom is placed inside one of the cavities in the array and behaves as a functional quantum node (FQN). We show that, through the electromagnetic induced transparency (EIT) mechanism, the $Lambda$-type FQN bears complete control over the reflection and transmission of the incident photon along the cavity-array. We also demonstrate the emergence of a quasibound state of the single photon inside a secondary cavity constructed by two distant FQNs as two end mirrors, from which we are motivated to design an all-optical single photon storage device of quantum coherence.