Zhen-Biao Yang

Distributed coherent manipulation of qutrits by virtual excitation processes

We propose a scheme for the deterministic coherent manipulation of two atomic qutrits, trapped in separate cavities coupled through a short optical fibre or optical resonator. We study such a system in the regime of dispersive atom-field interactions, where the dynamics of atoms, cavities and fibre operates through virtual population of both the atomic excited states and photonic states in the cavities and fibre. We show that the resulting effective dynamics allows for the creation of robust qutrit entanglement, and thoroughly investigate the influence of imperfections and dissipation, due to atomic spontaneous emission and photon leakage, on the entanglement of the two-qutrit state.

Faithful teleportation of an unknown atomic state and a cavity field entangled state without Bell-state measurement

An alternative scheme is proposed for teleportation of an unknown atomic state and an entanglement of zero- and one-photon states. The scheme is based on the resonant interaction of a two-mode cavity field with a ∧-type three-level atom. In contrast with the previously proposed schemes, the present scheme is ascendant, since the fidelity is 1.0 in principle similarly without the Bell-state measurement. And the scheme is experimentally feasible based on the current cavity QED technique.

Preparation of two-qubit steady entanglement through driving a single qubit

Inspired by a recent paper [J. Phys. B 47, 055502 (2014)], we propose a simplified scheme to generate and stabilize a Bell state of two qubits coupled to a resonator. In the scheme only one qubit is needed to be driven by external classical fields, and the entanglement dynamics is independent of the phases of these fields and insensitive to their amplitude fluctuations. This is a distinct advantage as compared with the previous ones that require each qubit to be addressed by well-controlled classical fields. Numerical simulation shows that the steady singlet state with high fidelity can be obtained with currently available techniques in circuit quantum electrodynamics.

Electromagnetically induced transparency with controlled van der Waals interaction

We study the electromagnetically induced transparency (EIT) effect with two individually addressed four-level Rydberg atoms subjected to the interatomic van der Waals interaction. We derive an effectively atomic Raman transition model where two ladders of the usual Rydberg EIT setting terminating at the same upper Rydberg level of long radiative lifetime are turned into a Rydberg EIT

Ground state of the asymmetric Rabi model in the ultrastrong coupling regime

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Rydberg-interaction gates via adiabatic passage and phase control of driving fields

setup via two-photon transitions, leaving the middle levels of each ladder largely detuned from the coupling and probe laser beams. It can hence overcome the limits of applications for EIT with atoms of the ladder-type level configuration involving a strongly decaying intermediate state by inducing coherence between two ground states. By probing one of the atoms, we observe four doublets of absorption induced by the Autler-Townes splitting and the van der Waals interaction. In particular, we find that the location of the EIT center remains unchanged compared to the interatomic-interaction-free case, which demonstrates that the interference among the multiple transition channels is basically destructive. The EIT with controlled Rydberg-Rydberg interaction among a few atoms provides a versatile tool for engineering the propagation dynamics of light.

Enhancement of entanglement in distant micromechanical mirrors using parametric interactions

We study the ground states of the single- and two-qubit asymmetric Rabi models, in which the qubit–oscillator coupling strengths for the counterrotating-wave and corotating-wave interactions are unequal. We take the transformation method to obtain the approximately analytical ground states for both models and numerically verify its validity for a wide range of parameters under the near-resonance condition. We find that the ground-state energy in either the single- or two-qubit asymmetric Rabi model has an approximately quadratic dependence on the coupling strengths stemming from different contributions of the counterrotating-wave and corotating-wave interactions. For both models, we show that the ground-state energy is mainly contributed by the counterrotating-wave interaction. Interestingly, for the two-qubit asymmetric Rabi model, we find that, with the increase in the coupling strength in the counterrotating-wave or corotating-wave interaction, the two-qubit entanglement first reaches its maximum and then drops to zero. Furthermore, the maximum of the two-qubit entanglement in the two-qubit asymmetric Rabi model can be much larger than that in the two-qubit symmetric Rabi model.

Two-photon absorption and emission by Rydberg atoms in coupled cavities

In this paperwe propose two theoretical schemes for implementation of quantum phase gates by engineering the phase-sensitive dark state of two atoms subjected to Rydberg-Rydberg interaction. Combining the conventional adiabatic techniques and currently developed approaches of phase control, a feasible proposal for implementation of a geometric phase gate is presented, where the conditional phase shift (Berry phase) is achieved by adiabatically and cyclically changing the parameters of the driving fields. Here we find that the geometric phase acquired is related to the way how the relative phase is modulated. In the second scheme, the system Hamiltonian is adiabatically changed in a noncyclic manner, so that the acquired conditional phase is not a Berry phase. A detailed analysis of the experimental feasibility and the effect of decoherence is also given. The proposed schemes provide new perspectives for adiabatic manipulation of interacting Rydberg systems with tailored phase modulation.

Ground state of three qubits coupled to a harmonic oscillator with ultrastrong coupling

Abstract We theoretically investigate the stability of a two cascaded cavity optomechanical system with optical parametric amplifiers (OPAs) inside the two coupled cavities, and study the steady-state entanglement between two distant mechanical resonators. We show that the parameter regime where the system is unstable without OPAs, such as relatively high laser intensity and blue detuning, can be exploited to build the steady-state mechanical entanglement by modulating the parametric gain. The application of OPAs is helpful to preserve the mechanical entanglement suffered from the dissipation at some finite temperature. The scheme provides an alternative way for improving and engineering the quantum entanglement of two distant mechanical oscillators. Graphical abstract

Quantum teleportation and computation with Rydberg atoms in an optical lattice

We study the dynamics of a system composed of two coupled cavities, each containing a single Rydberg atom. The interplay between Rydberg-Rydberg interaction and photon hopping enables the transition of the atoms from the collective ground state to the double Rydberg excitation state by individually interacting with the hybrid cavity modes and suppressing the up conversion process between them. The atomic transition is accompanied by the two-photon absorption and emission of the hybrid modes. Since the energy level structure of the atom-cavity system is photon number dependent, there is only a pair of states being in the two-photon resonance. Therefore, the system can act as a quantum nonlinear absorption filter through the nonclassical quantum process, converting coherent light field into a non-classical state. Meanwhile, the vacuum field in the cavity inspires the Rydberg atoms to simultaneously emit two photons into the hybrid mode, resulting in obvious emission enhancement of the mode.