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Optics Letters

Enhancement of entanglement using cascaded four-wave mixing processes

Jun Xin, Jian Feng Qi, Jietai Jing

Published Jan 11, 2017• 29 citations
Publisher

Abstract

A maximal joint quadrature squeezing of -6.8±0.4 dB is experimentally obtained by a scheme of cascaded four-wave mixing (FWM) processes, which gives strong proof about the inseparability or entanglement between output of the twin beams from the system. Here joint quadrature is the difference between the two quadratures of the twin beam output from the cascaded FWM processes. This result is enhanced by about 3.1 dB, compared with the one of the single FWM process. We also study the gain dependence of the entanglement enhancement in this cascaded system. Theoretical predictions with the considerations of the losses in the experiment are also studied, and a similar trend in the low-gain regime can be found between the experimental results and the theoretical predictions. The scheme of cascaded FWM processes, which can be used to improve or even manipulate the degree of the entanglement between the output fields from the single FWM process, may find its applications in the continuous-variable quantum communication protocols.

DOI
10.1364/ol.42.000366

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A maximal joint quadrature squeezing of -6.8±0.4 dB is experimentally obtained by a scheme of cascaded four-wave mixing (FWM) processes, which gives strong proof about the inseparability or entanglement between output of the twin beams from the system. Here joint quadrature is the difference between the two quadratures of the twin beam output from the cascaded FWM processes. This result is enhanced by about 3.1 dB, compared with the one of the single FWM process. We also study the gain dependence of the entanglement enhancement in this cascaded system. Theoretical predictions with the considerations of the losses in the experiment are also studied, and a similar trend in the low-gain regime can be found between the experimental results and the theoretical predictions. The scheme of cascaded FWM processes, which can be used to improve or even manipulate the degree of the entanglement between the output fields from the single FWM process, may find its applications in the continuous-variable quantum communication protocols.
Multipartite quantum correlation is important for both fundamental science and quantum information processing. Its enhancement is critical to improve the performance of physical systems. Therefore, it is of great practical significance to study the enhancement of multipartite quantum correlation. Here, we theoretically investigate a scheme for enhancing quadripartite quantum correlation by utilizing a phase-sensitive cascaded four-wave mixing (CFWM) process. We find that the intensity-difference squeezing (IDS) among the four output beams generated by the phase-insensitive CFWM process can be largely enhanced by introducing the phase-sensitive CFWM process. We also find that our phase-sensitive CFWM process can be used to generate intensity-sum squeezing (ISS), which cannot be generated by the phase-insensitive CFWM process. Moreover, we find that the maximum squeezing value of ISS is equal to that of IDS when the intensity gains of the three four-wave mixing processes are equal in the phase-sensitive cascaded scheme. In the end, we discuss the effects of the losses and the phase fluctuations on the squeezing values of our phase-sensitive CFWM process and the corresponding phase-insensitive CFWM process with the same intensity gains. It can be seen that the squeezing enhancement of our phase-sensitive CFWM process holds also in presence of the losses and the phase fluctuations. Our results pave the way for experimental implementation and may find applications in quantum metrology and quantum communication.
Theoretical studies and experimental implementations of quantum correlation are the important contents of continuous variables quantum optics and quantum information science. There are various systems for the study of quantum correlation. Here, we study an experimental scheme for generating three quantum correlated beams based on phase-sensitive cascaded four-wave-mixing (FWM) processes in rubidium vapor. Quantum correlation including intensity difference or sum squeezing, two other combinatorial squeezing, and quantum entanglement among the three output light fields are theoretically analyzed in this paper. Also, the comparison of the quantum correlations have been made between the phase-sensitive cascaded FWM processes and the phase-insensitive cascaded FWM processes. By changing the phases and intensities of the input beams, it is interesting to find that the maximum degrees of various combinatorial squeezing are equal when the two FWM processes share a common intensity gain. When the common intensity gain of the two FWM processes changes, the maximum degrees of different combinatorial squeezing will be synchronously controlled. At last we discuss the genuine tripartite entanglement and steering in our phase-sensitive cascaded scheme, and compare them with the cases of the phase-insensitive cascaded scheme.
Quantum squeezed states, exhibiting noise reduction below the shot-noise limit, are important resources in quantum optics. The degree of quantum squeezing directly influences the performance of physical systems, making the achievement of higher squeezing levels essential. Here, by utilizing the phase-sensitive cascaded four-wave mixing (PSCFWM) processes, we experimentally demonstrate the enhancement of three-beam quantum squeezing. The intensity-difference squeezing among the three output beams from PSCFWM processes is about 8.2 dB, which is enhanced by about 1.3 dB compared with that generated by phase-insensitive cascaded four-wave mixing processes. Our scheme provides an effective way for enhancing multi-beam quantum squeezing and holds promise for applications in multimode quantum correlated imaging and multiparameter quantum precision measurement.
We show that the scheme of cascaded four-wave-mixing (FWM) processes can be a promising alternative method for generating quadripartite entanglement. This scheme utilizes the strongly entangled twin beams produced by a FWM process as seed beams for another two FWM processes. To present the entanglement properties of this scheme, we verify that the generated state is a H\mathcal{H}H-graph state by using the graphical calculus for Gaussian pure states at first. Then we proceed with investigating the possibility of this scheme for generating quadripartite entanglement by applying the positivity under partial transposition criterion. It is verified that quadripartite entanglement exists for the whole gain region in this scheme. In addition, the possibilities of generating bipartite and tripartite entanglement are studied as well to reveal the entanglement structure. Our results pave the way to experimentally generate quadripartite entanglement based on cascaded FWM processes.
We theoretically calculate and experimentally investigate the performance of the pairwise correlations from quadruple quantum correlated beams based on the cascaded four-wave mixing (FWM) processes. The experimental results have similar trends to their theoretical predictions. By properly changing the intensity gains of the cascaded FWM processes, we can gradually convert some pairs of classical correlated beams into quantum correlated beams. Our results may find potential applications in the manipulation of quantum correlated beams generated from the cascaded FWM processes.
Abstract We report the quantum correlated triple beams via cascaded four-wave mixing (CFWM) amplified in single hot atomic vapor. Experimentally, we show that strong quantum correlation of three light beams, among them any two’s quantum correlation is characterized by the maximum value of intensity-difference squeezing (IDS) about −7.8 ± 0.3 dB. We found there is IDS between two idler beams, because two pairs of Einstein–Podolsky–Rosen injections potentially exist in our system. Besides, CFWM can emit three-mode beams at three different frequencies, in which these beams can be well separated in the spatial domain. Moreover, much difference with other methods, the injecting probe field can manipulate the gain and IDS of output three-mode light beams, which is resulting from competition relationship between cascaded two four-wave mixing processes. More interestingly, Autler–Townes splitting of gain peaks of output signals due to dressing effect of pumping fields, will lead to the evolution of measured two- and three-mode IDS from single-mode to multi-mode at frequency domain. This result will provide a multimode quantum resource which can potentially realize multimode entanglement and quantum networks.