Gao-Xiang Li

We demonstrate a simple technique to lock the frequency of a laser to an atomic transition with a new technique of subnatural linewidth spectroscopy. The coupled rubidium system exhibits an enhanced absorption spectrum of the probe beam with a subnatural linewidth. Compared with other techniques with an additional laser, our scheme only utilizes a single laser, which will be locked. The observed spectroscopic linewidth is down to about 3.5 MHz. The laser frequency fluctuation is about 226 kHz after locking on. The demonstrated scheme offers a convenient and low-cost technique for laser frequency stabilization and related precision measurements.

In a coherently-driven nanocavity QED system, the one-photon blockade via quantum interference is investigated by the modified Lindblad master equation and without using the secular approximation as well. Based on the dressed bases of the Rabi Hamiltonian, a modified Lindblad master equation is obtained, which is valid for any arbitrary degree of the qubit-cavity interaction. It is found that the damping coefficients are very sensitive to interaction strength between the qubit and the cavity mode. How to enhance the one-photon blockade by using the quantum interference effect is discussed with the generalized second-order correlation function and the second-order perturbation in the five-state truncation of the Hilbert space. It is found that, under suitable pump or detection conditions, a strong one-photon blockade can be realized by completely eliminating the two-photon emission. Moreover, even for a strong cavity damping rate, there exhibits a large number of cavity photons by utilizing the quantum interference mechanism.

We examine conditions under which an open quantum system composed of a driven degenerated parametric oscillator cavity and a driven two-level atom coupled to a waveguide could decay to a pure dark state rather than the expected mixed state. The calculations are carried out analytically in a low dimensional Hilbert space truncated at the double-excitation states of the combined system. The validity of the truncation is confirmed by the exact numerical analysis. It is found that one way to produce the pure state is to chirally couple the cavity and the atom to the waveguide. Another way to produce the pure state is to drive the cavity and the atom with unequal detunings. In both cases, if the driving fields are weak, the produced state is a coherent superposition of only the single-excitation and ground states of the combined system. In addition, we have found a direct correspondence between the generation of the dark state and the photon blockade effect. In other words, the generation of the dark state acts as a blockade to the number of photons so that only a single photon can be present in the cavity. We investigate the normalized second-order correlation function of the cavity field and find that the conditions under which the correlation function vanishes coincide with the conditions for the creation of the pure dark state. This system is, therefore, suggested as an alternative scheme for the generation of single-photon states.

We examine conditions under which an open quantum system composed of a driven degenerated parametric oscillator cavity and a driven two-level atom coupled to a waveguide could decay to a pure dark state rather than the expected mixed state. The calculations are carried out analytically in a low-dimensional Hilbert space truncated at the double-excitation states of the combined system. The validity of the truncation is confirmed by the exact numerical analysis. It is found that one way to produce the pure state is to chirally couple the cavity and the atom to the waveguide. Another way to produce the pure state is to drive the cavity and the atom with unequal detunings. In both cases, if the driving fields are weak, the produced state is a coherent superposition of only the single-excitation and ground states of the combined system. In addition, we have found a direct correspondence between the generation of the dark state and the photon blockade effect. In other words, the generation of the dark state acts as a blockade to the number of photons so that only a single photon can be present in the cavity. We investigate the normalized second-order correlation function of the cavity field and find that the conditions under which the correlation function vanishes coincide with the conditions for the creation of the pure dark state. This system is, therefore, suggested as an alternative scheme for the generation of single-photon states.

We study the dynamics of two two-level atoms embedded near to the interface of paired meta-material slabs, one of negative permeability and the other of negative permittivity. This combination generates a strong surface plasmon field at the interface between the meta-materials. It is found that the symmetric and antisymmetric modes of the two-atom system couple to the plasmonic field with different Rabi frequencies. Including the Ohmic losses of the materials we find that the Rabi frequencies exhibit threshold behaviour which distinguish between the non-Markovian (memory preserving) and Markovian (memoryless) regimes of the evolution. Moreover, it is found that significantly different dynamics occur for the resonant and an off-resonant couplings of the plasmon field to the atoms. In the case of the resonant coupling, the field does not appear as a dissipative reservoir to the atoms. We adopt the image method and show that the dynamics of the two atoms coupled to the plasmon field are analogous to the dynamics of a four-atom system in a rectangular configuration. A large and long living entanglement mediated by the plasmonic field in both Markovian and non-Markovian regimes of the evolution is predicted. We also show that a simultaneous Markovian and non-Markovian regime of the evolution may occur in which the memory effects exist over a finite evolution time. In the case of an off-resonant coupling of the atoms to the plasmon field, the atoms interact with each other by exchanging virtual photons which results in the dynamics corresponding to those of two atoms coupled to a common reservoir. In addition, the entanglement is significantly enhanced.

We demonstrate how an acoustic phonon bath when coupled to a quantum dot with the help of a bichromatic laser field may effectively form a quantum squeezed reservoir. This approach allows to achieve an arbitrary degree of squeezing of the effective reservoir and it incorporates the properties of the reservoir into two parameters, which can be controlled by varying the ratio of the Rabi frequencies of the bichromatic field. It is found that for unequal Rabi frequencies, the effective reservoir may appear as a quantum squeezed field of ordinary or inverted harmonic oscillators. When the Rabi frequencies are equal the effective reservoir appears as a perfectly squeezed field in which the decay of one of the polarization quadratures of the quantum dot dipole moment is inhibited. The decay of the quantum dot to a stationary state which depends on the initial coherence is predicted. This unusual result is shown to be a consequence of a quantum-nondemolition type coupling of the quantum dot to the engineered squeezed reservoir. The effect of the initial coherence on the steady-state dressed-state population distribution and the fluorescence spectrum is discussed in details. The complete polarization of the dressed state population and asymmetric spectra composed of only a single Rabi sideband peak are obtained under strictly resonant excitation.

We demonstrate how an acoustic phonon bath when coupled to a quantum dot with the help of a bichromatic laser field may effectively form a quantum squeezed reservoir. This approach allows one to achieve an arbitrary degree of squeezing of the effective reservoir and it incorporates the properties of the reservoir into two parameters, which can be controlled by varying the ratio of the Rabi frequencies of the bichromatic field. It is found that for unequal Rabi frequencies, the effective reservoir may appear as a quantum squeezed field of ordinary or inverted harmonic oscillators. When the Rabi frequencies are equal the effective reservoir appears as a perfectly squeezed field in which the decay of one of the polarization quadratures of the quantum dot dipole moment is inhibited. The decay of the quantum dot to a stationary state which depends on the initial coherence is predicted. This unusual result is shown to be a consequence of a quantum-nondemolition-type coupling of the quantum dot to the engineered squeezed reservoir. The effect of the initial coherence on the steady-state dressed-state population distribution and the fluorescence spectrum is discussed in detail. The complete polarization of the dressed state population and asymmetric spectra composed of only a single Rabi sideband peak are obtained under strictly resonant excitation.

We study the dynamics of two two-level atoms embedded near to the interface of paired meta-material slabs, one of negative permeability and the other of negative permittivity. The interface behaves as a plasmonic waveguide composed of surface-plasmon polariton modes. It is found that significantly different dynamics occur for the resonant and an off-resonant couplings of the plasma field to the atoms. In the case of the resonant coupling, the plasma field does not appear as a dissipative reservoir to the atoms. We adopt the image method and show that the dynamics of the two atoms are completely equivalent to those of a four-atom system. Moreover, two threshold coupling strengths exist, one corresponding to the strength of coupling of the plasma field to the symmetric and the other the antisymmetric mode of the two-atom system. The thresholds distinguish between the non-Markovian (memory preserving) and Markovian (memoryless) regimes of the evolutions that different time scales of the evolution of the memory effects and entanglement can be observed. The Markovian regime is characterized by exponentially decaying whereas the non-Markovian regime by sinusoidally oscillating contributions to the evolution of the probability amplitudes. The solutions predict a large and long living entanglement mediated by the plasma field in both Markovian and non-Markovian regimes of the evolution. We also show that a simultaneous Markovian and non-Markovian regime of the evolution may occur in which the memory effects exist over a finite evolution time. In the case of an off-resonant coupling of the atoms to the plasma field, the atoms interact with each other by exchanging virtual photons which results in the dynamics corresponding to those of two atoms coupled to a common reservoir. In addition, the entanglement is significantly enhanced under the off-resonant coupling.

We propose a rapid ground-state optomechanical cooling scheme in a hybrid system, where a two-level quantum dot (QD) is placed in a single-mode cavity and a nanomechanical resonator (NMR) is also coupled to the cavity via radiation pressure. The cavity is driven by a weak laser field while the QD is driven by another weak laser field. Due to the quantum destructive interference arisen from different transition channels induced by simultaneously driving the QD-cavity system in terms of the two different lasers, two-photon absorption for the cavity field can be effectively eliminated by performing an optimal quantum interference condition. Furthermore, it is demonstrated that the QD-cavity system can be unbalancedly prepared in two single-polariton states with different eigenenergies. If the frequency of the NMR is tuned to be resonant with transition between two single-polariton states, it is found that a fast ground-state cooling for the NMR can also be achieved, even when the QD-cavity system is originally in the moderate-coupling regime. Thus the present ground-state cooling scheme for the NMR may be realized with currently available experimental technology.

We consider a chain of six coupled single-mode cavities, each containing an ensemble of four-level atoms, and demonstrate how to prepare stationary six-mode cluster states of the atomic ensembles. We show that a six-mode cluster state can be created in a single step by driving the ensembles with laser pulses of suitably chosen Rabi frequencies and phases. Introducing linear superpositions of the collective bosonic modes of the atomic ensembles, we show that the superposition modes can independently evolve to a desired stationary state with the help of the cavity damping. We illustrate this method by examining three examples of the creation of linear, hexagon, and double square cluster states.