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arXiv:1903.02355 · Journal of Physics B Atomic Molecular and Optical Physics

Suppressing deleterious effects of spontaneous emission in creating bound states in cold atom continuum

Somnath Naskar, Dibyendu Sardar, Bimalendu Deb, G. S. Agarwal

Published Oct 17, 2019• 4 citations
arXivHTMLPDF

Abstract

Abstract In a previous paper [Deb and Agarwal 2014 Phys. Rev. A 90 063417], it was theoretically shown that, magneto-optical manipulation of low energy scattering resonances and atom-molecule transitions could lead to the formation of a bound state in continuum (BIC), provided there is no spontaneous emission. We find that even an exceedingly small spontaneous decay from exited molecular states can spoil the BIC. In this paper, we show how to circumvent the detrimental effect of spontaneous emission by making use of vacuum-induced coherence (VIC) which results in the cancellation or suppression of spontaneous emission. VIC occurs due to the destructive interference between two spontaneous decay pathways. An essential condition for VIC is the non-orthogonality of two transition dipole moments associated with the radiative decays form two excited states. Furthermore, the interference between two decay pathways requires that the spacing between the two decaying states must be comparable to or smaller than the square root of the product of the two spontaneous linewidths. We demonstrate that these conditions can be fulfilled by optically manipulating appropriately chosen molecular excited states, opening a promising prospect for the experimental realization of BIC of cold atoms.

DOI
10.1088/1361-6455/ab4ef2

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Abstract In a previous paper [Deb and Agarwal 2014 Phys. Rev. A 90 063417], it was theoretically shown that, magneto-optical manipulation of low energy scattering resonances and atom-molecule transitions could lead to the formation of a bound state in continuum (BIC), provided there is no spontaneous emission. We find that even an exceedingly small spontaneous decay from exited molecular states can spoil the BIC. In this paper, we show how to circumvent the detrimental effect of spontaneous emission by making use of vacuum-induced coherence (VIC) which results in the cancellation or suppression of spontaneous emission. VIC occurs due to the destructive interference between two spontaneous decay pathways. An essential condition for VIC is the non-orthogonality of two transition dipole moments associated with the radiative decays form two excited states. Furthermore, the interference between two decay pathways requires that the spacing between the two decaying states must be comparable to or smaller than the square root of the product of the two spontaneous linewidths. We demonstrate that these conditions can be fulfilled by optically manipulating appropriately chosen molecular excited states, opening a promising prospect for the experimental realization of BIC of cold atoms.
The earliest studies on the subject of spontaneous emission from an excited atom via interaction with the vacuum modes of the radiation field go back to the early days of quantum mechanics. In view of the fundamental nature of the vacuum fluctuations, it was long thought that spontaneous emission could not be controlled. However, in 1946 Purcell noted that the spontaneous emission rate of the atom inside a resonant cavity can be substantially enhanced (or suppressed) over its free-space value. This is due to the dramatic change of the density of modes of the vacuum field in a cavity. The enhancement and suppression of spontaneous emission in resonant and off-resonant cavities were observed experimentally in the 1980s. It was also realized in the 1950s and 1960s that coherent driving fields can dramatically change the characteristics of the spontaneous emission. The Autler-Townes doublet and Mollow three-peak spectrum are the examples of such behavior. A dramatic narrowing of the spectral peaks were discovered in driven multilevel atomic systems in the early 1990s. The role of atomic coherence in quenching spontaneous emission fluctuations in the context of laser physics was studied in the 1980s, via the correlated spontaneous emission laser (CEL). In this case atomic coherence results in the correlation of two spontaneous emission noise events so that the spontaneous emission quantum noise is eliminated in the relative linewidth. Later, it was “discovered” that the spontaneous emission can be suppressed through quantum interference between decay channels. Properly stated, this was rediscovered since it was first discovered and explained by Lamb in the 1940s. If the dipoles of two transitions from one upper level to two lower levels are parallel, the spontaneous emission in an ordinary vacuum can be cancelled. In a nonuniform vacuum, e.g., a cavity, the quenching of spontaneous emission can be achieved, even for the orthogonal dipoles.
Abstract We theoretically investigate how population can be trapped in the closely spaced excited levels in presence of vacuum-induced coherence (VIC). We employ delayed pulses to transfer population from a meta-stable state to the excited states. Subsequently, spontaneous emission from these excited states builds coherence between them. This coherence can be probed by using chirping, which leads to the decoupling of the excited states from the ground state thereby ensuring population transfer via delayed pulses. Our results indicate that the existence of VIC leads to the generation of a mixed state in the excited state manifold, where trapping of the population occurs even in the presence of large decay. This trapping may be realized in molecular systems and can be interpreted as a sensitive probe of VIC. We present suitable numerical analysis to support our results.
We present an alternative route to realize coherent perfect absorption (CPA) and subnatural narrowing by vacuum-induced coupling (VIC) between excited levels of an atomic ensemble in an optical nanofiber (ONF) cavity quantum electrodynamics (CQED) system. By employing the VIC arising from quantum interference between the spontaneous emission pathways from the upper excited doublet to one common ground state in the three-level VVV-type atoms, a very narrow absorption resonance can be observed in the generated dark-state spectral window owing to quantum destructive interference between the spontaneous emission pathways. The spectroscopic output feature exhibits a reduced linewidth, which is much narrower than both the empty-cavity linewidth and the atomic transition linewidth. As a check, the approximate analytical results for the ONF cavity-output power and linewidth of interest agree well with the numerical simulations. Furthermore, we find that the CPA can be used to probe the existence of the VIC effect. By tuning the relative phase between the two input light fields appropriately, the conversion between the CPA and coherent perfect transmission (CPT) can be realized in both the linear and nonlinear excitation regimes, i.e., the relative phase can switch the ONF CQED system from a perfect photon absorber to a nearly perfect photon transmitter. Also, we analyze the optical bistability of the ONF CQED system and its phase dependence in the nonlinear excitation regime. Such interference control of the CPA-to-CPT (or vice versa) scheme proposed here may have potential applications in optical logical devices for optical computing and quantum information processing.
We present an experimentally viable approach for suppressing the spontaneous emission in several atomic and molecular systems. We have shown [E. Frishman and M. Shapiro, Phys. Rev. Lett. 87, 253001 (2001)] that a coherent excitation of overlapping resonances followed by the application of infrequent interruptions can result in complete suppression of spontaneous emission. In the present work, the overlapping resonances result from splitting a single resonance with an external (spectator) field. An optimized superposition of the two components is formed by a shaped pulse excitation from the ground state.
Abstract The entanglement dynamics of a pair of three-level V-type atoms decaying spontaneously in a common vacuum reservoir is investigated. Under the condition that the decaying transitions in the atoms have parallel dipole moments, the effect of coherences induced by spontaneous emission is considered in the atomic dynamics. We show that vacuum-induced coherence (VIC) and collective effects in atomic decay play a significant role in the creation of entanglement. By using negativity as a measure of entanglement, we study the time evolution of entanglement for initial separable and generalized Dicke states as well as maximally entangled qutrit states. We show that the effects of VIC enhance the production of entanglement from initial separable states of the atoms. We also show that the entanglement can be protected in steady-state for atoms evolving from initial entangled states. The amount of entanglement that can be preserved is more in the presence of VIC than in its absence.