Quantum Optics and Entanglement

Chen Wei, LIU Yan-Hong, ZHAO Lu-Jie

Quantum entanglement is an important quantum resource.Entanglement of quantum nodes is the foundation for constructing large-scale quantum networks consisting of quantum nodes and quantum channels.In practical applications of quantum information， the ineluctable optical transmission loss， phase diffusion noise， and mixing with the thermal states lead to decoherence and disruption of entangled states， which limit the communication distance and affect the quality of entanglement between quantum nodes.Entanglement distillation， which can extract a small portion highly entangled states from a quantity of weakly entangled states， in addition to overcoming this detrimental problem.By preparing entangled optical mode twice and interfering with atomic ensembles， the establishment of entanglement between atomic ensembles and entanglement distillation based on quantum memory and balanced homodyne detection （BHD） can be achieved.The first-prepared entangled optical fields are mapped into two atomic ensembles via electromagnetically-induced-transparency （EIT） quantum memory to establish entanglement between atomic ensembles.Subsequently， the second-prepared entangled optical fields are then distributed to these two entangled atomic ensembles through quantum channels to implement interference.The transmitted optical fields from two atomic ensembles are detected by BHDs， the detection results below the triggered threshold determine the success or failure.During the distribution process， the entangled optical fields prepared twice passed through optical choppers， which are divided into three different zones according to the requirements， and each zone is glued with different optical components.Zone I does not have any optics， zone Ⅱ has an optical attenuator， and zone Ⅲ is used to block the optical beams.The second-prepared entangled optical fields interfere with these two entangled atomic ensembles through zone Ⅱ， and the transmittance of the optical attenuator directly affects the effect of entanglement distillation.In this paper， we theoretically analyze the effect of different transmittance of optical attenuator on the effect of entanglement distillation， and find the corresponding quantum writing efficiency when the entanglement distillation achieves the best effect.As the transmittance increases， the effect of entanglement distillation is better； when the transmittance is certain， there exists an optimal writing efficiency which makes the entanglement between atomic ensembles optimal.As the transmittance increases gradually， the optimal writing efficiency becomes smaller， which is easier to realize in the experiment.Additionally， the correlation variaces before and after the entanglement distillation of atomic spin waves with other physical parameters are analyzed， which provides a basis for the selection of physical parameters in the entanglement distillation scheme， and lays a foundation for construction of large-scale quantum networks.

Andrzej Jamiołkowski

Yaoyao Zhou, Jianfeng Tian, Zhihui Yan +1 more

Entanglement source with high entanglement degree is the guarantee for accomplishing the quantum information transmission and process with higher fidelity. Continuous variable Einstein-Podolsky-Rosen (EPR) entangled optical field with quantum correlation of amplitude and phase quadrature is a basic and important quantum resource in the quantum information science area, which can be obtained by a non-degenerate optical parametric amplifier (NOPA) operated below the threshold pump power. Because of the limitation of the imperfect performance of optical components in optical cavity, we should find efficient methods of implementing quantum manipulation to improve the entanglement degree of the entangled state of light. Connecting NOPA1 and NOPA2 in series, the entangled state of light output from the NOPA1 can be manipulated by NOPA2, and the entanglement degree can be enhanced under certain conditions. To improve the entanglement degree to a greater extent, the structure of the NOPA1 is chosen as a half-monolithic standing-wave optical resonator with the triple resonance of the pump and two subharmonic modes. The NOPA1 is able to output the entangled optical fields with an entanglement degree of 8.4 dB, which is the highest entanglement generated by a single device so far. The structure of the NOPA2 can be chosen as a standing-wave optical cavity or a four-mirror ring optical cavity. According to the different structures of the NOPA2, we theoretically design two kinds of optical systems with two cascaded cavities and compare the effects of the two optical systems on the continuous variable EPR entanglement cascaded enhancement in detail. Based on the above contrastive analysis, when the entanglement degree of the input optical fields is 8.4 dB and the transmissivity of the output coupler is lower, the structure of a four-mirror ring optical cavity for NOPA2 cannot enhance the entanglement degree, so the optical system including NOPA2 with standing wave cavity structure and the optical isolator with high transmission efficiency is appropriate. When the transmissivity of the output coupler and transmission efficiency of the optical isolator are higher, the structure of the NOPA2 should be chosen as a standing-wave optical cavity, otherwise the structure of the NOPA2 should be chosen as a four-mirror ring optical cavity. We also theoretically analyze the dependence of the correlation degree of output optical fields on physical parameters. The results show that under the conditions of higher input and output coupling efficiency, higher transmission efficiency and lower intro-cavity loss, the entangled state of light with higher entanglement degree can be obtained experimentally. This provides the reference for obtaining entangled optical fields with higher entanglement degree in the future.

Shabnam Abutalebi, A. Asghari Nejad, Hassan Randjbar Askari +1 more

We theoretically investigate entanglement in a hybrid quantum system comprising two optical cavities coupled to a shared microwave cavity via optomechanical interactions. Our results demonstrate that increasing the microwave input power and frequency in the allowed range enhances entanglement between the microwave–optical and optical–optical modes. We also show by change frequency of the one optical cavity in the communication domain that we still have entanglement. This ability to generate robust microwave–optical photon entanglement shows promise for various quantum technologies. Optical photons could enable secure quantum communication in optical fibers, while microwave photons allow transmission to satellites. Overall, this hybrid cavity optomechanical system offers prospects as an efficient source of entangled photon pairs, vital for implementing long-distance quantum communication networks.

Yanhua H. Shih