Yong-Chun Liu

Spin squeezing in a generalized one-axis twisting model

We investigate the dependence of spin squeezing on the polar angle of the initial coherent spin state |θ0, 0 in a generalized one-axis twisting model, where the detuning δ is taken into account. We show explicitly that regardless of δ and 0, previous results of the ideal one-axis twisting are recovered as long as θ0=π/2. For a small departure of θ0 from π/2, however, the achievable variance (V −)min ~N2/3, which is larger than the ideal case N1/3. We also find that the maximal squeezing time tmin scales as N−5/6. Analytic expressions of (V−)min and tmin are presented and they agree with numerical simulations.

Strongly enhanced light-matter interaction in a hybrid photonic-plasmonic resonator

State Key Lab for Mesoscopic Physics, Department of Physics, Peking University, P. R. China(Dated: June 13, 2012)We propose a hybrid photonic-plasmonic resonant structure which consists of a metal nanoparticle(MNP) and a whispering gallery mode (WGM) microcavity. It is found that the hybrid mode enablesa strong interaction between the light and matter, and the single-atom cooperativity is enhanced bymore than two orders of magnitude compared to that in a bare WGM microcavity. This remarkableimprovement originates from two aspects: (1) the MNP offers a highly enhanced local field in thevicinity of an emitter, and (2), surprisingly, the high-Q property of WGMs can be maintained inthe presence of the MNP. Thus the present system has great advantages over a single microcavityor a single MNP, and holds great potential in quantum optics, nonlinear optics and highly sensitivebiosening.

Experimental observation of Fano resonance in a single whispering-gallery microresonator

We experimentally observe Fano resonance in a single silica toroidal microresonator, in which two whispering-gallery modes (WGMs) are excited simultaneously through a fiber taper. By adjusting the fiber-cavity coupling strength and the polarization of incident light, the Fano-like resonance line shape can be engineered and further convert to the electromagnetically induced transparency (EIT) like line shape. Our theoretical analysis reveals that both the Fano and EIT resonances originate from an indirect-coupling of two originally orthogonal WGMs, which is mediated by the common fiber taper waveguide. The sharp Fano line shape holds great potential in optical switching and sensitivity-enhanced biochemical sensing.

Experimental controlling of Fano resonance in indirectly coupled whispering-gallery microresonators

We experimentally studied the transmission spectrum of a coupled resonator structure in which a low-Q microdisk and a high-Q microtoroid indirectly interact with each other mediated by a fiber taper. Asymmetric Fano resonances were observed and could be controlled to change periodically by adjusting the distance between the two microresonators. It is revealed that the Fano resonance originates from the coupling of the two modes belonging to the two microresonators. The observed period of distance change is around 8 μm, which shows good agreement with the theoretical prediction by the beat of multiple propagating modes in the fiber taper.

Dynamic dissipative cooling of a mechanical resonator in strong coupling optomechanics.

Cooling of mesoscopic mechanical resonators represents a primary concern in cavity optomechanics. In this Letter, in the strong optomechanical coupling regime, we propose to dynamically control the cavity dissipation, which is able to significantly accelerate the cooling process while strongly suppressing the heating noise. Furthermore, the dynamic control is capable of overcoming quantum backaction and reducing the cooling limit by several orders of magnitude. The dynamic dissipation control provides new insights for tailoring the optomechanical interaction and offers the prospect of exploring mesoscopic quantum physics.

Spin squeezing: transforming one-axis twisting into two-axis twisting.

Squeezed spin states possess unique quantum correlation or entanglement and are significantly promising for advancing quantum information processing and quantum metrology. In recent back-to-back publications [C. Gross et al., Nature (London) 464, 1165 (2010) and Max F. Riedel et al., Nature (London) 464, 1170 (2010)], reduced spin fluctuations are observed leading to spin squeezing at -8.2 and -2.5 dB, respectively, in two-component atomic condensates exhibiting one-axis-twisting interactions. The noise reduction limit for the one-axis twisting scales as ∝1/N(2/3), which for a condensate with N∼10(3) atoms is about 100 times below the standard quantum limit. We present a scheme using repeated Rabi pulses capable of transforming the one-axis-twisting spin squeezing into the two-axis-twisting type, leading to Heisenberg limited noise reduction ∝1/N or an extra tenfold improvement for N∼10(3).

Polarization-dependent detection of cylinder nanoparticles with mode splitting in a high-Q whispering-gallery microresonator

Using the single-scatterer-induced coupling mechanism of a pair of counterpropagating high-Q whispering-gallery modes (WGMs), we investigate the highly sensitive detection of single nonspherical nanoparticles. The nonspherical particle may produce distinct frequency splitting and additional damping for TE and TM WGMs. This polarization-dependent effect allows for studying the orientation of single biomolecule, molecule-molecule interaction on the microcavity surface, and possibly distinguishing different inner configurations of similar biomolecules.

Electromagnetically induced transparency in optical microcavities

Abstract Electromagnetically induced transparency (EIT) is a quantum interference effect arising from different transition pathways of optical fields. Within the transparency window, both absorption and dispersion properties strongly change, which results in extensive applications such as slow light and optical storage. Due to the ultrahigh quality factors, massive production on a chip and convenient all-optical control, optical microcavities provide an ideal platform for realizing EIT. Here we review the principle and recent development of EIT in optical microcavities. We focus on the following three situations. First, for a coupled-cavity system, all-optical EIT appears when the optical modes in different cavities couple to each other. Second, in a single microcavity, all-optical EIT is created when interference happens between two optical modes. Moreover, the mechanical oscillation of the microcavity leads to optomechanically induced transparency. Then the applications of EIT effect in microcavity systems are discussed, including light delay and storage, sensing, and field enhancement. A summary is then given in the final part of the paper.

Coherent Polariton Dynamics in Coupled Highly Dissipative Cavities

Coherent light-matter interaction at the single photon and electronic qubit level promises the remarkable potential for nonclassical information processing. Against the efforts of improving the figure of merit of the cavities, here we demonstrate strong anharmonicity in the polariton dressed states via dark state resonances in a highly dissipative cavity. It is shown that vacuum Rabi oscillation occurs for a single quantum emitter inside a cavity even with bosonic decay-to-interaction rate ratio exceeding

Optimal limits of cavity optomechanical cooling in the strong-coupling regime

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