Effect of the relative control phase as well as the squeezed phase parameter on the optical switching process of the optical bi-multistable behaviour are investigated in detail for a four-level Y-type atomic system; in the presence of a control field (coherent + dc fields) with squeezed vacuum field and quantum interference from spontaneous emission. The results indicated that (a) the quantum interference assists to decrease the rate of switching via the relative phase of the control field and makes the switching on/off points more rapid in squeezed vacuum compared to a normal vacuum case, and (b) the phase switching effects between the output field and the relative control phase increases the switching on/off processes in normal vacuum more than the squeezed vacuum case. In addition, we have demonstrated the possibility of displaying quantum beats with a novel type of bi-multistable behaviour in the cooperative switching diagram.
Effect of the relative control phase as well as the squeezed phase parameter on the optical switching process of the optical bi-multistable behaviour are investigated in detail for a four-level Y-type atomic system; in the presence of a control field (coherent + dc fields) with squeezed vacuum field and quantum interference from spontaneous emission. The results indicated that (a) the quantum interference assists to decrease the rate of switching via the relative phase of the control field and makes the switching on/off points more rapid in squeezed vacuum compared to a normal vacuum case, and (b) the phase switching effects between the output field and the relative control phase increases the switching on/off processes in normal vacuum more than the squeezed vacuum case. In addition, we have demonstrated the possibility of displaying quantum beats with a novel type of bi-multistable behaviour in the cooperative switching diagram.
The effect of a squeezed vacuum on the optical bistable and multistable behaviour of a system of N three-level atoms is investigated, where the quantum interference between the atomic transition paths is considered. It is shown that the squeezed vacuum can profoundly affect the output field of this system. A strong squeezed vacuum can lead to multistable behaviour, even in the case where the cooperation parameter C is so small that the input–output relationship curve is monotone in the absence of the squeezed vacuum. The optical multistability is most significant when the quantum interference is perfect. The bistability and multistability can be controlled by the squeezing photon number and the strength of the two-photon correlation.
Abstract This work presents a theoretical study of optical bistability (OB) and multistability (OM) in a four-level N-type atomic system embedded in a unidirectional ring cavity, with particular attention to the role of quantum interference (QI) between decay channels. Going beyond earlier reports that focused on single-parameter or cavity-based control without considering quantum interference or multistability, we develop a unified framework investigating how the cooperative parameter C, probe and control field detunings (Δ₂, Δ₃), non-radiative decay rates (γ₀, γ₀₀), quantum interference (P), pump/control Rabi frequencies (Ω₁, Ω₂), and relative phase (φ) collectively govern OB and OM thresholds. Systematic analysis reveals that increasing C broadens hysteresis loops and raises thresholds, thereby enhancing switching stability, while quantum interference significantly lowers the OB threshold, enabling bistability at lower input intensities. Non-radiative decay rates γ₀ and γ₀₀, pump/control Rabi frequencies, and the relative phase φ further provide flexible switching between bistable and multistable regimes. Notably, we demonstrate that OB ↔ OM transitions can be induced through multiple independent pathways beyond phase control alone—including tuning of the cooperative parameter, non-radiative decay rates, and pump/control field intensities—extending the phase-driven switching mechanism established in earlier N-type studies to a broader, multi-parameter control framework. Furthermore, probe and control field detunings (Δ₂, Δ₃) enable dynamic tuning of the system's dispersive and absorptive properties, offering additional control over threshold behavior. These results identify practical routes for designing reconfigurable optical memory, quantum information processors, and all-optical switching devices with enhanced controllability.
We study the intensity–intensity correlation in a closely spaced doublet V-type three-level atom driven by an external coherent field and damped by a broadband squeezed vacuum. We are interested in the manner in which the atom emits photons under the combined effect of quantum interference and squeezed vacuum. For distinguishable photons and degenerate transitions, the squeezed field produces anticorrelation, whereas for nondegenerate transitions the effect of the squeezed field is the surpassing of the antibunching during certain time intervals. We find that bunching/antibunching behaviour and the symmetry of the two emission processes can be controlled by the relative phase of the squeezed vacuum to the coherent field. For particular conditions we show a bunching behaviour which turns to antibunching when the two-photon correlations in the squeezed vacuum are reduced to the classical level. Therefore, this transition must be attributed to the quantum nature of the squeezed vacuum. We also study the sideband correlations. We find that squeezed vacuum makes the sidebands evolve at different rates which are phase dependent.
Abstract In this study, we investigate the optical bistability (OB) and optical multistability (OM) phenomena within a V-type three-level atomic system interacting with two independent broadband squeezed baths confined in a unidirectional cavity. Our analysis reveals that the transition between OB and OM is influenced by the strength of the control field and the intensity of the squeezed vacuum. Notably, the impact of each of these factors on the transition exhibits distinct characteristics. Furthermore, we explore the effects of additional system parameters, such as the incoherent pumping rate and cooperation parameter. Our findings emphasize the crucial roles played by these parameters in shaping the observed results. Specifically, we observe that variations in the incoherent pumping rate and cooperation parameter contribute significantly to the system’s OB and OM behavior, further underscoring the multifaceted nature of the studied atomic system. This comprehensive investigation not only elucidates the intricate interplay between key parameters but also unveils the sensitivity of the system to variations in control field strength, intensity of the squeezed vacuum, incoherent pumping rate, and cooperation parameter.
The relaxation process of a single two-level atom driven by an intense resonant coherent field is studied in the presence of a broadband squeezed vacuum field. Generalized forms for the self-field operator and the field-dependent damping coefficients are derived. In the steady state, positive atomic inversion (~5%) is shown for some range of the phase of the squeezed vacuum field. The squeezing-induced enhanced and asymmetric coherence (i.e.?non-zero dispersive atomic polarization) induces profound asymmetry in the side-bands of the Mollow fluorescent spectrum and the absorptive?dispersive spectra near the Rabi side-band frequencies. The case of two and three cooperative atoms, instead of a single atom, shows (i)?positive atomic inversion in the steady state for larger interval of the squeeze phase parameter and more enhanced extrema for the atomic polarization components and (ii) the extra side peaks in the fluorescent spectrum become asymmetric and of dispersive-like profile. Dressed-state analysis in the single-atom case with the field-dependent decay process in the presence of the squeezed vacuum field is presented and shows that the inequality of the (field-dependent) decay rates of the dressed states leads to more positive inversion in the dressed states as compared with the normal vacuum case.
Quantum interference and coherence lead to many interesting phenomenon and applications in quantum optics. In this dissertation, we study the quantum coherent properties in the following systems and aspects. We first investigate the optical bistability in a combined cavity-cold atoms system. In such a system the atom-photon interaction provides an optical lattice to the atoms and affects the mechanical motion of the atoms while atoms induce a position dependent phase shift on the cavity field. This highly nonlinearity induces optical bistability of intra-cavity photon number with respect to the pumping light added along the cavity axis. We show that the presence of this bistability can be controlled by a second pumping field added perpendicular to the cavity axis. It is also found that the critical input intensity of switching from one branch of bistability to the other depends on the the way of the field being added. This behavior is similar to the anomalous switching of the dispersive optical bistability of atomic media in the cavity. We also study the effect of counter-rotating terms in the control of spontaneous emission. We make use of a unitary transformation method and investigate the effect of dynamic energy shifts on the spontaneous emission modification via quantum interference in a four-level atomic system. We show that the counter-rotating terms, which are normally neglected in the usual investigation of atomic systems, do produce a significant influence on the evolution of the atomic amplitudes and the emission spectrum. This effect of counter-rotating terms can be observed in the time scale of the decay rate even when the dipole moments of the two upper levels are orthogonal to each other. The effect of counter-rotating terms on the spontaneous emission in a 3-D anisotropic photonic crystal is also discussed. It is shown that the behavior of the emission is similar to the case making rotating wave approximation, i.e., the localized and propagating fields are also separated by two characteristic atomic transition frequencies. However, the two characteristic frequencies are shifted due to the full Lamb shift which is obtained with counter-rotating terms being included in the Hamiltonian. We also utilize the unitary transformation method to show how the Lamb shift in a multi-level atom can be controlled by a driving field. Finally, we propose a subwavelength atom localization scheme for an atom located in a standing-wave field. The strategy is based on the observation that the photon statistics of resonance fluorescence depends on the position-dependent Rabi frequency.