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Physical Review A

Quantum phase transition and interference trapping of populations in a coupled-resonator waveguide

Lei Qiao, Ya‐Ju Song, Chang-Pu Sun

Published Jul 15, 2019• 25 citations
Publisher

Abstract

We study the energy structure and dynamics of a V-type three-level emitter embedded in a one-dimensional waveguide with model dispersion. Due to the presence of two different couplings between the upper levels and the waveguide modes, we find that the bound states in this system are quite different from the case of a two-level emitter. The energy structure is dependent on the relative position of the emitter's transition frequencies from the band with finite width. We obtain the phase diagrams through numerical analysis. The dynamics of spontaneous emission of the system is calculated exactly in the general case by means of the Green's function approach. The emitted photon is characterized by two components: a localized part and a traveling part, and the average frequency of the traveling part is different from that for resonant scattering. Through the quantum interference between the emitter's two transitions, it is found that the total excitation number of the localized photon and emitter can be greatly enhanced in some case in the spontaneous-emission process.

DOI
10.1103/physreva.100.013825

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We study the energy structure and dynamics of a V-type three-level emitter embedded in a one-dimensional waveguide with model dispersion. Due to the presence of two different couplings between the upper levels and the waveguide modes, we find that the bound states in this system are quite different from the case of a two-level emitter. The energy structure is dependent on the relative position of the emitter's transition frequencies from the band with finite width. We obtain the phase diagrams through numerical analysis. The dynamics of spontaneous emission of the system is calculated exactly in the general case by means of the Green's function approach. The emitted photon is characterized by two components: a localized part and a traveling part, and the average frequency of the traveling part is different from that for resonant scattering. Through the quantum interference between the emitter's two transitions, it is found that the total excitation number of the localized photon and emitter can be greatly enhanced in some case in the spontaneous-emission process.
The spontaneous emission from a three-level atom embedded in a photonic band gap structure is studied. Interference between two transitions leads to quasiperiodic oscillations of population between the two upper levels with large amplitudes. The spontaneous emission of the atom is characterized by three components in the radiated field: a localized part, a traveling pulse, and a (1/t)3(1/\sqrt{t}{)}^{3}(1/t​)3 decaying part. An analytical expression for the localization distance of the localized field is obtained. The energy velocity for the traveling pulse could be close to zero. By selecting an appropriate initial superposition state, a large amount of population trapping can be achieved.
We study the energy structure and dynamics of a two-level emitter (2LE) locally coupled to a semi-infinite one-dimensional (1D) coupled-resonator array (CRA). The energy spectrum in the single-excitation subspace features a continuous band with scattering states, discrete levels with bound states, and a quantum phase transition characterized by the change of the number of bound states. The number of bound states is revealed by the behavior of the excited-state population at long times with quantum beat, residual oscillation, a constant with either non-zero or zero.
We investigate the spontaneous emission from a V three-level atom embedded in two-band isotropic photonic crystals. The dipoles of the two transitions from the two upper levels to the lower level are parallel. Due to the quantum interference between the two transitions and the existence of two bands, the populations in the upper levels display some novel properties, such as anti-trapping and continuous oscillation, which differ from that of a two-level atom (with two bands) and also differ from that if only one band (for three-level atom) is considered. The spontaneous emission field is composed of two parts: localized field and travelling field. The localized field is composed of one or two localized modes, and the travelling field is composed of no, one, two or three propagating modes depending on different conditions. The conditions for different combinations of localized modes and propagating modes are discussed.
Spontaneous emission from a V-type three-level atom in a photonic crystal is investigated. Quantum interference between the two atomic transitions affects the constructor of dressed states, and leads to an interesting behavior of the populations in the two upper levels: antitrapping, periodic oscillation, and no population inversion. Those properties depend strongly on the relative position of the upper levels from the forbidden gap and the initial state of the atom, and differ from that of a two-level atom in a photonic crystal. The emitted field, which is composed of localized mode(s) and propagating mode(s), is also studied. Quantum interference can enhance or reduce the energy of the localized field.
The spontaneous emission from a V-type three-level atom embedded in an isotropic photonic crystal with dynamic photonic band edge is studied. We consider the situation where the atom interacts with all possible radiation modes, and calculate numerically the evolution of atomic population without using Markov approximation. The calculation method can be used in related researches. In the present paper, we mainly discuss the effects of modulation parameters and the quantum interference on spontaneous emission when the band edge is modulated with step function or triangle function. We hope that the results can contribute to the applications in the dynamic photonic crystal environment in controlling the spontaneous emission via the quantum interference. The results show that in the step-modulated situation, the number of the photon-atom bound dressed states after the modulation has happened depends on atomic transition frequencies and the band edge frequency at that time, and is identical to the one in the unmodulated situation with the same parameters. The long-time evolution of the atomic population is affected by the time when the modulation happens. Depending on the system initial state, after the modulation has happened, the quantum interference can weaken the probability amplitude components corresponding to the photon-atom bound dressed states, and cause the upper-level population to decay quickly from a great value to a value near zero; or on the contrary, it can strengthen the bound dressed states, and make the upper levels retain a high population. In the modulated situation with trigonometric functions, after long enough time, the total upper-level population presents a decaying quasi-periodic oscillation behaviour. And the evolution of the total upper-level population tends to synchronize with the modulation, so the frequency of the quasi-periodic oscillation is approximately equal to the modulation frequency. But, the quantum interference can destroy the synchronization under some conditions. The decay rate of the total upper-level population is affected by the modulation frequency, and also by the initial state of the system and the angle between two dipole moment because of the quantum interference.
We study the properties of atom-photon bound states and single-photon cooperative dynamics in a waveguide system which consists of a finite-bandwidth channel with model dispersion and an ensemble of two-level atoms whose size is ignorable when compared with the lattice constant. The bound states are formed by all atoms and a localized photonic excitation. We find that the effect of atomic collection is equivalent to the case of one atom by rescaling the coupling strength with the square root of the atom number, as far as the eigenenergy equation is concerned. Besides, it is found there is a quantum phase transition when more than one type of atom are present. The characteristic lengths and wave functions are analyzed near the phase transition point. The exact analytical results for the cooperative dynamics at the single excitation level are obtained and we point out the dark state in this system leads to a universal population trapping in the time evolution process. This type of trapping obeys a simple law that is only associated with the atom number. A direct conclusion that results from the trapping law is that the single-photon cooperative emission is suppressed when the number of atoms is large enough.