Abstract A higher degree of entanglement of entangled sources in quantum communications is always desired and can be achieved with high nonlinear gains. The dressing of atomic coherence and optical pumping can be considered as effective parameters to obtain such entangled sources. In this regard, we have experimentally studied doubly dressed effects in a small range of frequency, and optical pumping and de-pumping effects over a wide frequency range in multi-level atomic system in order to enhance the nonlinear gain. The multi-mode phenomenon that results from the multi-peak of Autler–Townes splitting can be observed at the large beam size of the pinhole of the pump field. Theoretical calculations fit well with the experimentally measured results. The dressing of atomic coherence and optical pumping lead to the enhancement of nonlinear gain up to 8.2 when the original nonlinear gain is 4.7.
Abstract A higher degree of entanglement of entangled sources in quantum communications is always desired and can be achieved with high nonlinear gains. The dressing of atomic coherence and optical pumping can be considered as effective parameters to obtain such entangled sources. In this regard, we have experimentally studied doubly dressed effects in a small range of frequency, and optical pumping and de-pumping effects over a wide frequency range in multi-level atomic system in order to enhance the nonlinear gain. The multi-mode phenomenon that results from the multi-peak of Autler–Townes splitting can be observed at the large beam size of the pinhole of the pump field. Theoretical calculations fit well with the experimentally measured results. The dressing of atomic coherence and optical pumping lead to the enhancement of nonlinear gain up to 8.2 when the original nonlinear gain is 4.7.
Abstract We report the control of the dressed state in three-mode optical entanglement in an N-type four-level system using two cascaded parametric amplification four-wave mixing steps inside an atom-like optical cavity and injected by thermal optical fields: a coherent field and the Einstein–Podolsky–Rosen field. First, cavity dressing results in Aulter–Towns-like (AT-like peak) anti-squeezing. And the three-cavity dressing interaction induces multi-AT-like peaks. Meanwhile, the cavity enhanced nonlinear gain results in one squeezing dip polariton. In addition, the coupling of three enhanced nonlinear gains brings about multi-dip squeezing. Second, as two nonlinear coefficients change, the nonlinear phase makes the squeezing profile (in phase, amplification) turn into an anti-squeezing profile due to nonlinear loss (out of phase, de-amplification). Finally, the squeezing profile can also be modified by field dressing (AT splitting, enhancement and suppression), and the squeezing degree is affected by injection fields. Thus, by judging the squeezing of three nonseparability criteria, we get three-mode optical entanglement.
Abstract We demonstrate the influence of dressed states on two-mode optical entanglement in a double Λ-type energy level atomic ensemble of parametric amplification four-wave mixing (PA-FWM) processes. By injecting a coherent or Einstein–Podolsky–Rosen field into PA-FWM channels, we investigate the corresponding entanglement. The quantum entanglement may be enhanced or suppressed via a bright state or a dark state. In free space, the two-mode entanglement is determined by nonlinear gain, which can be manipulated by field dressing in an atomic ensemble (i.e. Autler–Towns splitting, dressed enhancement/suppression of entanglement). However, in a ring cavity, the cavity dressing brings about the AT-like splitting of entanglement. Such an entanglement profile may be modified by field dressing through vacuum Rabi splitting, vacuum-induced enhancement and suppression of entanglement.
With a forward cone emitting from the strong pump laser in a thermal rubidium atomic vapor, we investigate the non-degenerate parametrically amplified four-wave mixing (PA-FWM) process with dressing effects in a three-level "double-Λ" configuration both theoretically and experimentally. By seeding a weak probe field into the Stokes or anti-Stokes channel of the FWM, the gain processes are generated in the bright twin beams which are called conjugate and probe beams, respectively. However, the strong dressing effect of the pump beam will dramatically affect the gain factors both in the probe and conjugate channels, and can inevitably impose an influence on the quantum effects such as entangled degree and the quantum noise reduction between the two channels. We systematically investigate the intensity evolution of the dressed gain processes by manipulating the atomic density, the Rabi frequency and the frequency detuning. Such dressing effects are also visually evidenced by the observation of Autler-Townes splitting of the gain peaks. The investigation can contribute to the development of quantum information processing and quantum communications.
Focusing on the frequency division multiplexing technology in the applications of large scale optical communication, the double-channel optical gain spectrum with two frequency bands is studied in this paper. The double-channel gain spectrum, named probe channel and four wave mixing channel, comes from a co-propagating degenerate four wave mixing in a hot atomic ensemble. The intention is to divide the gain spectrum into several sub frequency bands through dressed four wave mixing. When a dressed field is exerted on one transition that shares the common excited state with the degenerate four wave mixing, the excited state can experience dressed splitting. It opens two transition paths for the degenerate four wave mixing simultaneously. Because of quantum interference between the two paths, the degenerate four wave mixing are suppressed at two-photon resonance. Consequently, Autler-Townes splitting appears in the gain spectrum, i.e. spectrum is changed from single frequency band into two “M”-type bands. In this paper, the nonlinear density matrix element describing the degenerate (dressed) four wave mixing is solved through perturbation theory, and then the gain spectrum in Doppler broadening atomic medium is plotted, and its Autler-Townes splitting is analyzed by using the dressed-state theory. It shows that the Autler-Townes splitting depends on both the Rabi frequency and single photon detuning of the dressed field. Relevant experiment is performed in cesium vapor at 60 ℃, a pair of high-gain optical spectra with two frequency bands for both double channels is successfully obtained. Moreover, the Autler-Townes splitting as a function of the dressed field intensity and single photon detuning are studied quantitatively. The experimental results accord well with the theoretical predictions. Compared with the degenerate four wave mixing, the atom-field coupled system is changed from an original open two-level into a closed Λ three-level due to the external dressed field, which greatly improves the atomic population on the coherent ground state via optical pumping, and therefore enhancing the gain significantly. This work is important for the field of atom-based optical communication. It provides an optional way of conveying multi-frequency information to the two parallel channels as well as improving the gain of four wave mixing.
Abstract We report a theoretical study into the two- and three-mode entanglement inside an atom-like optical cavity. A five-level system is considered and the influence of the multi-dressed parametric amplification four-wave mixing (PA-FWM) process on the quantum correlation of fluctuation spectra is researched. Three-mode entanglement is determined by the coupling of two nonlinear gains; one of enhanced gains via the dressing state plays a dominant role in controlling and optimizing the profile of three-mode entanglement via vacuum Rabi splitting, enhancement/suppression of entanglement as well as two-mode. Specifically, increasing the quantity of dressing fields may result in the single-channel entanglement turning into nonlocal multichannel (multiple anti-crossing behaviors). Moreover, these entanglement channels can be squashed via the lateral squeezing effect of the cavity. Such multichannel entanglement has potential applications in nonlocal quantum imaging and quantum key distribution.
The frequency and spatial multimode of correlated fields are seen as essential resources for future multimode entanglement and quantum networks. Here, we present multimode properties from the process of parametric amplified cascaded four-wave mixing through a rubidium atomic ensemble under dressing and Kerr nonlinear effects. The experimental results complement the theoretical simulations. Three frequency modes and 34 spatial spots were experimentally observed due to the double-dressing and Kerr effect of the two pump fields. Further, it is verified that changing the pump field frequency has a larger line shift over pump field power and the angle between the two pump fields, which can be realized as a multicontact switch. Through a dressing effect, interestingly the number of coherent channels also increases by the order of dressing, indicating a large possibility of frequency and spatial multimode. These results will have important applications in quantum communications.