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Exceptional-point induced transparency and opacity

Adi Pick, Petra Ruth Kaprálová-Žďánská, Nimrod Moiseyev

Published Dec 4, 2018• 0 citations
OpenAlex

Abstract

We report on a new mechanism for electromagnetically induced transparency and opacity, which relies on the coalescence of two metastable states at an exceptional point (EP). By using two fields, a `pump,' which forms an EP, and a `probe,' which couples the EP-state to a third level, we show that small changes in the pump parameters lead to dramatic changes in the absorption of the probe; a sharp enhancement or complete inhibition. While most (if not all) of the effects associated with EPs were observed, so far, only in optics and microwave experiments, this work presents a feasible approach for measuring EP effects in atomic systems. We demonstrate these predictions for helium.

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We report on a new mechanism for electromagnetically induced transparency and opacity, which relies on the coalescence of two metastable states at an exceptional point (EP). By using two fields, a pump,' which forms an EP, and a probe,' which couples the EP-state to a third level, we show that small changes in the pump parameters lead to dramatic changes in the absorption of the probe; a sharp enhancement or complete inhibition. While most (if not all) of the effects associated with EPs were observed, so far, only in optics and microwave experiments, this work presents a feasible approach for measuring EP effects in atomic systems. We demonstrate these predictions for helium.
In this paper we reveal that the constructive interference of atomic coherence on absorption leads to electromagnetically induced absorption,which is in sharp contrast to the case in which destructive interference of atomic coherence gives rise to electromagnetically induced transparency. A four-level atomic system is employed in which three transitions are in N configuration and the middle transition is used as a probe transition. In the dressed states representation the probe transition is split into four transitions that are coupled to each other. The coupling originates from the spontaneous emission of the atomic medium and the spontaneous transfer from excited state coherence to ground state coherence. The former leads to couplings between all transitions with the same sign and to the absorption reduction. The latter results in couplings,of which t wo terms have positive sign and another two have negative sign. It is due to the couplings with positive and negative signs that significant modification of the contribution of dressed coherence to absorption occurs,i.e., absorption inhibition is changed to absorption enhancement.
We have investigated atomic coherence effects in the ladder scheme of the triplet transitions 2S-2P-3S of helium. A powerful dressing laser at 706 nm in resonance with the 2P-3S transition induces a transparency on the 2S-2P transition at 1083 nm probed by a weak laser. We have characterised this induced transparency also known as Electromagnetically Induced Transparency (EIT), demonstrating the possibility of reaching 100% of bleaching. Also, we have evidenced the presence of the two photon effect on the EIT process by studying its dependence on the mutual probe-dress polarisation angle. With such a simple level scheme a quantitative comparison with the theory has been made possible and many of the predicted effects have been investigated. Furthermore, polarisation of the dressing laser field has allowed for the first time the study of the electromagnetically induced birefringence which causes a change in the probe laser polarization. The experiments were performed not only in the clean environment of a metastable atomic beam but also in a weak radiofrequency discharge; this configuration could be suggestive for non inverted gain phenomena through incoherent population of excited levels.
We construct electromagnetically induced transparency (EIT) by dynamically coupling a superradiant state with a subradiant state. The superradiant and subradiant states with enhanced and inhibited decay rates act as the excited and metastable states in EIT, respectively. Their energy difference determined by the distance between the atoms can be measured by the EIT spectra, which renders this method useful in subwavelength metrology. The scheme can also be applied to many atoms in nuclear quantum optics, where the transparency point due to counter-rotating wave terms can be observed.
One of the most remarkable features that distinguish open systems from closed ones is the presence of exceptional points (EPs), where two or more eigenvectors of a non-Hermitian operator coalesce, accompanying the convergence of the corresponding eigenvalues. So far, EPs have been demonstrated on a number of platforms, ranging from classical optical systems to fully quantum-mechanical spin-boson models. In these demonstrations, the reservoir that induced the non-Hermiticity was treated as a Markovian one, without considering its memory effect. We here present the first experimental demonstration of non-Markovian quantum EPs, engineered by coupling a Josephson-junction-based qubit to a leaky electromagnetic resonator, which acts as a non-Markovian reservoir. We map out the spectrum of the extended Liouvillian superoperator by observing the quantum state evolution of the qubit and the pseudomode, in which the memory of the reservoir is encoded. We identify a twofold second-order EP and a third-order EP in the Liouvillian spectrum, which cannot be realized with a Markovian reservoir. Our results pave the way for experimental exploration of exotic phenomena associated with non-Markovian quantum EPs.
Exceptional points (EPs) in non-Hermitian systems have recently attracted wide interest and spawned intriguing prospects for enhanced sensing. However, EPs have not yet been realized in thermal atomic ensembles, which is one of the most important platforms for quantum sensing. Here we experimentally observe EPs in multilevel thermal atomic ensembles and realize enhanced sensing of the magnetic field for 1 order of magnitude. We take advantage of the rich energy levels of atoms and construct effective decays for selected energy levels by employing laser coupling with the excited state, yielding unbalanced decay rates for different energy levels, which finally results in the existence of EPs. Furthermore, we propose the optical polarization rotation measurement scheme to detect the splitting of the resonance peaks, which makes use of both the absorption and dispersion properties and shows an advantage with enhanced splitting compared with the conventional transmission measurement scheme. Additionally, in our system both the effective coupling strength and decay rates are flexibly adjustable, and thus the position of the EPs are tunable, which expands the measurement range. Our Letter not only provides a new controllable platform for studying EPs and non-Hermitian physics, but also provide new ideas for the design of EP-enhanced sensors and opens up realistic opportunities for practical applications in the high-precision sensing of magnetic field and other physical quantities.