Showing papers by "Jinwu Ye published in 2009"

Super-radiance, Berry phase, Photon phase diffusion and Number squeezed state in the $ U(1) $ Dicke (Tavis-Cummings) model

Abstract: Recently, strong coupling regimes of superconducting qubits or quantum dots inside a micro-wave circuit cavity and BEC atoms inside an optical cavity were achieved experimentally. The strong coupling regimes in these systems were described by the Dicke model. Here, we solve the Dicke model by a $ 1/N $ expansion. In the normal state, we find a $ \sqrt{N} $ behavior of the collective Rabi splitting. In the superradiant phase, we identify an important Berry phase term which has dramatic effects on both the ground state and the excitation spectra of the strongly interacting system. The single photon excitation spectrum has a low energy quantum phase diffusion mode in {\sl imaginary time} with a large spectral weight and also a high energy optical mode with a low spectral weight. The photons are in a number squeezed state which may have wide applications in high sensitive measurements and quantum information processing. Comparisons with exact diagonization studies are made. Possible experimental schemes to realize the superradiant phase are briefly discussed.

Angle-Resolved Photoluminescence Spectrum of the Exciton Condensate in Electron-Hole Semiconductor Bilayers

Abstract: The electron-hole semiconductor bilayer system is one of the most promising systems to search for exciton superfluid. The exciton superfluid is metastable and will eventually decay through emitting photons. Here we show that the angle-resolved photon spectrum, momentum distribution curve, energy distribution curve, and quasiparticle excitation spectrum in the exciton superfluid show many unique and unusual features not shared by any other atomic or condensed matter systems. Observing all these salient features in the photoluminescence experiments can provide convincing evidence for exciton superfluid in electron-hole semiconductor bilayers. We also comment on relevant experimental data in both exciton and exciton-polariton systems and also suggest possible future experiments to test our theoretical predictions.

Photon condensation and its phase diffusion

Abstract: Recently, several experiments successfully achieved the strong coupling of a BEC of $ N \sim 10^{5} $ $ ^{87}Rb $ atoms to the photons inside an ultrahigh-finesse optical cavity. The strong coupling regime was also achieved with artificial atoms such as superconducting qubits inside micro-wave circuit cavity. These systems are described by the Dicke model where a single mode of photons coupled to an assembly of $ N $ atoms with the same coupling strength $ g $. In this paper, by solving the Dicke model by $ 1/ N $ expansion, we show that at the thermodynamic limit $ N = \infty $, one can achieve the Bose-Einstein Condensation (BEC) of the cavity photons at sufficiently strong coupling $ g $. At a finite $ N $, we identify an emergent quantum phase diffusion mode inside the BEC phase and also work out many remarkable experimental consequences of this mode such as its low frequency, photon number squeezing properties and photon statistics. The photons from the BEC phase are in a number squeezed state with much enhanced signal/noise ratio which may have wide applications in the field of high resolution, high sensitive measurement and also in quantum information processing. The effects of dissipations due to leaking photons out of the cavity are also briefly discussed. The difference with a Laser beam is stressed. Several experimental schemes to realize the BEC of the photons and detect the phase diffusion mode are proposed.

Angle resolved photon spectrum and quasiparticle excitation spectrum in an exciton superfluid

Abstract: There have been extensive experimental search for possible exciton superfluid in semiconductor electron-hole bilayer systems below liquid Helium temperature. However, exciton superfluid are meta-stable and will eventually decay through emitting photons. Here we show that the light emitted from the excitonic superfluid has unique and unusual features not shared by any other atomic or condensed matter systems. We evaluate angle resolved photon spectrum, momentum distribution curve, energy distribution curve and quasiparticle excitation spectrum in the exciton superfluid and comment on relevant experimental data in both exciton and exciton-polariton systems.