Ai-Ping Fang

Electromagnetic chirality-induced negative refraction via atomic coherence

Electromagnetic chirality-induced negative refraction in a four-level atomic medium is investigated by initially preparing coherent states of the atom. We show that the negative refraction can be carried out with deeply depressing absorption and without simultaneously requiring both negative electric permittivity and magnetic permeability in an ideal situation where the two chirality coefficients have the same amplitude but the opposite phase.

Coherent frequency down-conversions and entanglement generation in a Sagnac interferometer

A waveguide loop coupled to two external line waveguides by a 50/50 beam splitter forms a Sagnac interferometer. We consider the situation where two Λ-type three-level emitters are symmetrically coupled to the loop of a Sagnac interferometer and a single photon is input through one end of the line waveguides. Since the incoming photon is always in a superposition of the clockwise and counterclockwise modes of the loop and the two emitters are positioned symmetrically with respect to the input port of photon, the processes of photon scattering at the two emitters are symmetric and coherent. When the separation of the emitters and the coupling strengths of the emitters with the waveguide loop take some special values, due to quantum interference, a frequency down-conversion can certainly happen at one of the two emitters during the photon scattering but one cannot know at which emitter the frequency down-conversion takes place. This indistinguishability of the coherent frequency down-conversion processes can result in the generation of the symmetric or antisymmetric two-qubit maximally entangled states of the emitters. In the present scheme, a single photon comes in and goes out of the waveguide loop, and no photon localization modes exists. The entangled states result from the coherent frequency down-conversion processes of the emitters. Thus, the resulting entangled states are stable if the two lower-lying states of the emitters have no decay. We also investigate the influence of the dissipation of the emitters and the finite bandwidth of an input photon wavepacket on the success probability of entanglement generation, and find that the present scheme is robust to these effects and feasible with current available technologies.

Two-mode squeezing generation in hybrid chains of superconducting resonators and nitrogen-vacancy-center ensembles

We consider a hybrid quantum system consisting of two independent chains of nearest-neighbor linearly interacting superconducting transmission-line resonators, each of which a nitrogen-vacancy-center ensemble (NVE) is magnetically coupled to. In addition, the first-beginning pair of the resonators is locally driven by a two-mode microwave squeezed bath. We show that in the steady state a series of NVE pairs in the up and down chains is in a two-mode spin squeezed state. In the low excitation limit, collective excitation modes of the up- and down- NVE pairs are in the two-mode squeezed state of the driven field, i.e., realizing a perfect squeezed state replication.

Phonon-assisted excitation energy transfer in photosynthetic systems*

The phonon-assisted process of energy transfer aiming at exploring the newly emerging frontier between biology and physics is an issue of central interest. This article shows the important role of the intramolecular vibrational modes for excitation energy transfer in the photosynthetic systems. Based on a dimer system consisting of a donor and an acceptor modeled by two two-level systems, in which one of them is coupled to a high-energy vibrational mode, we derive an effective Hamiltonian describing the vibration-assisted coherent energy transfer process in the polaron frame. The effective Hamiltonian reveals in the case that the vibrational mode dynamically matches the energy detuning between the donor and the acceptor, the original detuned energy transfer becomes resonant energy transfer. In addition, the population dynamics and coherence dynamics of the dimer system with and without vibration-assistance are investigated numerically. It is found that, the energy transfer efficiency and the transfer time depend heavily on the interaction strength of the donor and the high-energy vibrational mode, as well as the vibrational frequency. The numerical results also indicate that the initial state and dissipation rate of the vibrational mode have little influence on the dynamics of the dimer system. Results obtained in this article are not only helpful to understand the natural photosynthesis, but also offer an optimal design principle for artificial photosynthesis.

Detailed Photoisomerization Dynamics of a Green Fluorescent Protein Chromophore Based Molecular Switch

With density-functional-based nonadiabatic molecular dynamics simulations, trans-to-cis and cis-to-trans photoisomerizations of a green fluorescent protein chromophore based molecule 4-benzylidene-2-methyloxazol-5(4H)-one (BMH) induced by the excitation to its excited state were performed. We find a quantum yield of 32% for the trans-to-cis photoisomerization of BMH and a quantum yield of 33% for its cis-to-trans photoisomerization. For those simulations that did produce trans-to-cis isomerization, the average excited state lifetime of trans-BMH is about 1460 fs, which is much shorter than that of cis-BMH (3100 fs) in those simulations that did produce cis-to-trans isomerization. For both photoisomerization processes, rotation around the central C2=C3 bond is the dominant reaction mechanism. Deexcitation occurs at an avoided crossing near the / conical intersection, which is near the midpoint of the rotation.

REALIZATION OF FAST QUANTUM INFORMATION TRANSFER AND ENTANGLEMENT WITH SUPERCONDUCTING FLUX QUBITS COUPLED TO A RESONATOR

We propose a scheme of realization of fast quantum information transfer and two-qubit entangled state with two different superconducting flux qubits coupled to a single-mode resonator. In this approach, the two lowest levels of each Λ-type flux qubit serve as the logical states and a higher-energy intermediate level is used as the gate manipulation. This proposal does not require equal device parameters for the two flux qubits that is extremely difficult to achieve experimentally. In addition, neither a second-order detuning nor auxiliary qubits are required in this scheme. Moreover, there is no need to change the Rabi frequencies to satisfy the adiabatic passage. Therefore, the operation time can be shortened to 20–30 ns, which is faster by one order of magnitude compared to the schemes employing the second-order detuning technique.