Nondegenerate Optical Parametric Amplifier Research Articles

Effect of phase-sensitive manipulations on generation of low-frequency two-mode orthogonal squeezed vacuum states

<sec>Two-mode orthogonal squeezed vacuum states are an important quantum resource for quantum communication, quantum computing, quantum simulation, quantum precision measurement and sensing. It is essential to obtain stable two-mode orthogonal squeezed vacuum states in a low frequency range and compact configurations for practical applications, especially in quantum precision measurement and sensing. Two-mode orthogonal squeezed vacuum states are commonly produced via a subthreshold nondegenerate optical parametric amplifier (NOPA) in a continuous variable system. However, the subthreshold NOPA cavity is phase sensitive, so it is difficult to obtain stable squeezed vacuum states by using frequency-shift technology. Previous signal light injecting scheme relies on an injection of a weak light field into the subthreshold NOPA for phase sensitive manipulation. The injected signal light has the same frequency as the generated squeezed vacuum state. Thereby even the weakest injected signal light can introduce large amounts of excessive noise at low frequencies and the squeezing degree of two-mode squeezed vacuum states will be reduced or squeezing cannot be achieved.</sec><sec>In this paper, a single sideband frequency shifted light injecting scheme is proposed for phase sensitive manipulation of NOPA. The comparison between the single sideband frequency shifted light injecting scheme and the signal light injecting scheme for realization of phase sensitive manipulation of NOPA is conducted. The effects of the two schemes on the generation of the low-frequency two-mode orthogonal squeezed vacuum state light field are investigated experimentally . The experimental results show that in the signal light injecting scheme for phase sensitive manipulation, the squeezing degree of the two-mode orthogonal squeezed vacuum state continuously decreases until it disappears as the power of injected signal light increases. In the process of phase sensitive manipulation of NOPA by using the single sideband frequency shifted light injecting scheme, the squeezing degree of the two-mode orthogonal squeezed vacuum state does not change with the power of the injected frequency shifted light increasing. Stable phase sensitive manipulation is realized by injecting single sideband frequency shifted light into NOPA. The NOPA is operated in a phase sensitive amplification state for 30 min. Stable low-frequency two-mode orthogonal squeezed vacuum states are obtained. The (4.1 ± 0.1) dB amplitude orthogonal squeezed vacuum states and (4.0 ± 0.2) dB phase orthogonal squeezed vacuum states at a frequency of 200 kHz are generated stably, in a compact NOPA configuration.</sec>

Entanglement enhancement from single-port and two-port feedback optical parametric amplifiers

Coherent feedback control has been proved to be an effective approach in entanglement enhancement of the entangled states produced by a nondegenerate optical parametric amplifier (NOPA). However, the present research mainly focuses on a two-port feedback NOPA (TFPA). In this study, a scheme of single-port feedback NOPA (SFPA) is proposed and investigated. For the ideal case, both SFPA and TFPA have the potential to realize an infinite degree of entanglement. For the practical case with feasible physical parameters of realistic systems, the features of entanglement enhancement for two schemes are compared. The effect of entanglement enhancement for SFPA is relatively weaker than that of TFPA, but has a loose phase-locking accuracy requirement. Our results may provide useful references on the control of nonclassical light fields.

Simultaneous Preparation of Two Optical Cat States Based on a Nondegenerate Optical Parametric Amplifier

AbstractThe cat state, known as the superposition of coherent states, has broad applications in quantum computation and quantum metrology. Increasing the number of optical cat states is crucial to implement complex quantum information tasks based on them. Here, two optical cat states are prepared simultaneously based on a nondegenerate optical parametric amplifier. By subtracting one photon from each of two squeezed vacuum states, two odd cat states with orthogonal superposition direction in phase space are prepared simultaneously, which have similar fidelity of 60% and amplitude of 1.2. Compared with the traditional method to generate two odd optical cat states based on two degenerate optical parametric amplifiers, only one nondegenerate optical parametric amplifier is applied in this experiment, which saves half of the quantum resources of nonlinear cavities. The presented results make a step toward preparing the four‐component cat state, which has potential applications in fault‐tolerant quantum computation.

Compact source for quadripartite deterministically entangled optical fields

Since entangled multiple optical fields were identified as the building blocks of quantum networks, the quadripartite entangled optical fields have been produced by using four degenerate optical parametric amplifiers or two nondegenerate optical parametric amplifiers (NOPAs). However, realizing an efficient and compact source for multiple quantum users has remained an outstanding challenge, hindering their practical applications. Here, we proposed a compact and feasible scheme to deterministically entangle four spatially separated optical fields, employing only a single NOPA. Accordingly, two-sided output NOPA-based optical fields were coupled on a beam splitter network to form the quadripartite entangled state, causing the deterministic generation of both the Greenberger–Horne–Zeilinger (GHZ) and the linear cluster states in this compact entanglement source. We also obtained the optimal experimental parameters based on the simulation results, thereby providing a direct reference for experimental implementation. Our findings propose that the resultant GHZ and linear cluster states can be potentially applied in quantum-enhanced information science, specifically in quantum secret sharing, controlled quantum teleportation networks, and quantum-entangled atomic ensemble networks.

Entanglement enhancement from coherent feedback-controlled and cascaded nondegenerate optical parametric amplifiers

The entanglement enhancement of the entangled states from the nondegenerate optical parametric amplifier (NOPA) is an important issue in quantum information science and technology. The coherent feedback-controlled NOPA (CFC-NOPA) and cascaded NOPA (C-NOPA) are two promising schemes on entanglement enhancement. In this study, the features of the quantum entanglement of the entangled states produced by the two schemes is totally studied and compared based on feasible physical parameters of realistic systems. We demonstrate that the CFC-NOPA shows better superiority on entanglement enhancement and better robustness versus the losses of the system for low frequency range (below several hundred kHz). For the high frequency range (several MHz), the situation is inverse, and the C-NOPA is more advantageous. Our results can provide useful references for the design of high-quality entanglement resources.

Quantum state truncation using an optical parametric amplifier and a beam splitter

We present a scheme of quantum state truncation in the Fock basis (quantum scissors), based on the combined action of a nondegenerate optical parametric amplifier and a beamsplitter. Differently from previously proposed linear-optics-based quantum scissors devices, which depend on reliable Fock states sources, our scheme requires only readily available Gaussian states, such as coherent states inputs (vacuum state included). A truncated state is generated after performing photodetections in the global state. We find that, depending on which output ports each of the two photodetectors is positioned, different types of truncated states may be produced: i) states having a maximum Fock number of $N$, or ii) states having a minimum Fock number $N$. In order to illustrate our method, we discuss an example having as input states a coherent state in the beamsplitter and vacuum states in the amplifier, and show that the resulting truncated states display nonclassical properties, such as sub-Poissonian statistics and squeezing. We quantify the nonclassicality degree of the generated states using the Wigner-Yanase skew information measure. For complementarity, we discuss the efficiency of the protocol, e.g., generation probability as well as the effects of imperfections such as the detector's quantum efficiency and dark counts rate.

Nonlinear interferometer based on two-port feedback nondegenerate optical parametric amplification

In this paper, we propose a nonlinear interferometer based on two-port feedback nondegenerate optical parametric amplification (NITFPA), which employs two linear beam splitters to feed the two output beams of a nondegenerate optical parametric amplifier back to its input ports. By comparing the NITFPA with the SU(1,1) interferometer (SUI), which is another typical nonlinear interferometer, we demonstrate that the NITFPA has its unique advantages in the precision enhancement of the phase estimation. In addition, we demonstrate that the NITFPA is also advantageous over the SUI in the robustness against the losses. Such a NITFPA may find its potential applications in quantum metrology.

Entanglement Enhancement from a Two-Port Feedback Optical Parametric Amplifier

How to achieve the maximal entanglement, for given finite entanglement resources? In this paper, we propose a two-port feedback optical parametric amplifier (TFPA) consisting of a nondegenerate optical parametric amplifier (NOPA) and two linear beam splitters. The former is an entanglement resource, and the latter serve as feedback controllers that feed the two entangled beams generated from the NOPA back into its input ports. By manipulating the reflectivity of the beam splitters and the phase delays introduced by the feedback paths, an entanglement enhancement can be achieved by the TFPA. Furthermore, we demonstrate that such entanglement enhancement can happen at all phase points for the ideal case, and the maximal entanglement can be infinitely large. For the lossy case, our TFPA also has its unique advantages in the robustness of the losses of the system.

Quantum phase estimation with a stable squeezed state

Phase estimation of optical field is a vital measurement strategy that can be used to perform accurate measurements of various physical quantities. Phase estimation precision can be greatly enhanced by using the nonclassical state such as squeezed state and entangled state. Therefore, we addressed the generation of a stable squeezed state through locking the pump laser of non-degenerate optical parametric amplifier to a high finesse Fabry–Perot cavity based on an improved cascade Pound–Drever–Hall frequency stabilization strategy. Then the obtained squeezed state is used as the probe state of quantum phase estimation and its precision is enhanced from 9.3 E-5 (shot-noise-limit) to 4.2 E-5 at the optimal phase with a given average photon number.

Generation of continuous variable frequency comb entanglement based on nondegenerate optical parametric amplifier

Continuous variable (CV) quantum squeezed state and entangled state are important quantum resources, which have been widely used in quantum communication, quantum metrology and quantum computation. In recent years, people have paid much attention to the multi-mode optical parametric amplifier (OPO) process because the multi-mode non-classical light field is able to construct the multiplexing quantum information system for improving the working efficiency and channel capacity. As a special multi-mode optical field, optical frequency comb has been used in optical frequency measurement, atomic spectroscopy and frequency-division multiplex-based communication. Especially, there are a number of notable researches where quantum frequency combs are used, which exhibit multimode-entangled photon states. The quantum frequency combs provide a promising platform for quantum information technology based on time-bin-encoded qubits. In this paper, the entanglement characteristics of frequency comb in type II nondegenerate optical parametric amplifier (NOPA) below threshold are investigated experimentally. The bipartite entanglement with frequency comb structure between idle light (<inline-formula><tex-math id="M1">\begin{document}$\hat a_{{\rm{i}}, + n\varOmega }^{{\rm{out}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M1.png"/></alternatives></inline-formula>) and signal light(<inline-formula><tex-math id="M2">\begin{document}$\hat a_{{\rm{s}}, + n\varOmega }^{{\rm{out}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M2.png"/></alternatives></inline-formula>) is generated by the NOPA whose free spectral range (<i>Ω</i>) is 1.99 GHz operated in the de-amplification state and then analyzed by dual balanced homodyne detection system (BHD) with different values of frequency <inline-formula><tex-math id="M3">\begin{document}$\omega \pm n\varOmega $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M3.png"/></alternatives></inline-formula> (<i>n </i>= 0, 1, 2). The local light of BHD with frequency <inline-formula><tex-math id="M4">\begin{document}$\omega \pm n\varOmega $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M4.png"/></alternatives></inline-formula> is generated by the fiber intensity modulator and tailored by the mode cleaner. Here, we measure the correlation noise of side and frequency combs normalized to the shot noise limit relating to the phase of local oscillator beam, and we show the correlation noise of <inline-formula><tex-math id="M5">\begin{document}$\hat a_{\rm{i}}^{{\rm{out}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M5.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M6">\begin{document}$\hat a_{\rm{s}}^{{\rm{out}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M6.png"/></alternatives></inline-formula>, the correlation noise of <inline-formula><tex-math id="M7">\begin{document}$\hat a_{{\rm{i}}, + \varOmega }^{{\rm{out}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M7.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M8">\begin{document}$\hat a_{{\rm{s}}, - \varOmega }^{{\rm{out}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M8.png"/></alternatives></inline-formula>, the correlation noise of <inline-formula><tex-math id="M9">\begin{document}$\hat a_{{\rm{i}}, - \varOmega }^{{\rm{out}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M9.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M10">\begin{document}$\hat a_{{\rm{s}}, + \varOmega }^{{\rm{out}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M10.png"/></alternatives></inline-formula>, the correlation noise of <inline-formula><tex-math id="M11">\begin{document}$\hat a_{{\rm{i}}, + 2\varOmega }^{{\rm{out}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M11.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M12">\begin{document}$\hat a_{{\rm{s}}, - 2\varOmega }^{{\rm{out}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M12.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M12.png"/></alternatives></inline-formula> and the correlation noise of <inline-formula><tex-math id="M13">\begin{document}$\hat a_{{\rm{i}}, - 2\varOmega }^{{\rm{out}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M13.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M13.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M14">\begin{document}$\hat a_{{\rm{s}}, + 2\varOmega }^{{\rm{out}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M14.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20200107_M14.png"/></alternatives></inline-formula>. The experimental results show that the five pairs of entangled states with 4.5 dB entanglement are simultaneously produced by a type II OPO. Next, we can redesign NOPA to reduce its free spectral range and intracavity loss, and prepare local light with a high-order sideband frequency by fiber modulators with high bandwidth, it promises to obtain huge multiple bipartite entangled states. As a kind of extensible quantum information system, the frequency comb CV entanglement can be used to provide a necessary light source for realizing the experiment of frequency division multiplexing multi-channel teleportation, which lays a foundation for the future large-capacity quantum communication and network.

Long-term stable continuous variable entanglement generation in type-II non-degenerate optical parametric amplifier.

Long-term stable entanglement is a crucial aspect in the implementation of reliable quantum information processes. However, long-term continuous variable entanglement generation, especially in type-II non-degenerate optical parametric amplifier, has yet to be reported on. Here, we derive the relationship between entanglement and temperature fluctuations in the crystal of a type-II non-degenerate optical parametric amplifier, and propose a novel method for long-term stable entanglement generation by locking the temperature of the crystal. In the experiment, we obtain a 5.4 dB entanglement lasting two hours. The method holds promise in the generation of a truly usable above 10 dB entanglement and brings us closer to continuous-variable quantum information processing.

Generation of Gaussian-modulated entangled states for continuous variable quantum communication

We present and demonstrate a high-efficiency and compact scheme for generating Gaussian-modulated Einstein-Podolsky-Rosen (EPR) entangled optical fields by injecting a modulated signal field into a non-degenerate optical parametric amplifier (NOPA). We perform a quantum analysis of the scheme and derive the variance of the output signal (idler) mode and the quantum entanglement between the signal and idler modes from the NOPA. An experimental study is presented with different Gaussian modulation depths, showing that the modulation of the injected signal field successfully enlarges the distribution of the field quadratures of the EPR source in phase space and has a negligible effect on the entanglement quality. The experimental observations have good agreement with the theoretical analysis. Our scheme can be used readily in continuous variable quantum key distribution protocol.

Theoretical analysis of entanglement enhancement with two cascaded optical cavities

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.

Investigation of Four-Wave-Mixing Crosstalk in Phase-Sensitive Fiber Optical Parametric Amplifier

Four-wave-mixing (FWM) crosstalk in frequency nondegenerate phase-sensitive fiber optical parametric amplifiers (FOPAs) applied for wavelength-division-multiplexed (WDM) signals is investigated experimentally and theoretically. We analyze significant FWM interactions that can generate crosstalk components in the signal band. Approximate expressions are developed for the FWM crosstalk to conveniently estimate how it scales with the system parameters. The analytical results agree well with the experimental results and simulations. The results show that the FWM crosstalk grows dramatically with the increase of the system parameters, including input signal power per channel, signal gain, the length of nonlinear fiber, and signal channel number. We also compare the FWM crosstalk in the phase-sensitive FOPA and the phase-insensitive FOPA experimentally and explain the difference of their crosstalk behaviors through theoretical analysis and simulations. The results show that, due to the presence of idlers at the input, the phase-sensitive FOPA produces higher crosstalk than the phase-insensitive FOPA even when they have equivalent signal gains. It indicates that the crosstalk in the phase-sensitive FOPA is more detrimental so that careful design of the amplifier to suppress the impact of the FWM crosstalk is necessary. The analytical solutions obtained in this paper are useful in the design of PS-FOPAs for amplifying WDM signals.

Experimental investigation of transmission characteristics of continuous variable entangled state over optical fibers

Continuous variable (CV) quantum entanglement is an essential resource for quantum computation and communication protocols. The use of CV quantum entanglement at a telecommunication wavelength of 1.5m in combination with existing fiber telecommunication networks offers the possibility to implement long-distance quantum communication protocols like quantum key distribution (QKD) and applications such as quantum repeaters, quantum teleportation in the future. In spite of the fact that the optical power attenuation of light in a standard telecommunication fiber is lowest at a wavelength of 1.5m, the entangled states will interact with fiber channels and the disentanglement will occur. It is one of the important factors restricting the development of long distance quantum information. In this paper, CV entangled state at 1.5m telecommunication band is obtained by using a type-II periodically poled KTP (PPKTP) crystal inside a nondegenerate optical parametric amplifier (NOPA). A wedged PPKTP is used for implementing frequency-down-conversion of the pump field to generate the optically entangled state and achieving the dispersion compensation between the pump and the subharmonic waves. By controlling the temperature and the length of the PPKTP crystal, a triply resonant optical parametric oscillator with a threshold of 80 mW is realized. Einstein-PodolskyRosen (EPR)-entangled beams with quantum correlation of 8.3 dB for both the amplitude and phase quadratures are experimentally generated by using a single NOPA at a pump power of 40 mW and an injected signal power of 10 mW when the relative phase between the pump and injected signal is locked to . The generated entangled state is coupled into a single-mode optical fiber, and the transmission characteristics of the generated EPR entangled beams through standard single-mode fibers are investigated experimentally and theoretically. A fiber polarization controller is used to compensate for the polarization state variation induced by random fluctuations of birefringence of the single mode fiber when the light propagates along the fiber, and to keep the polarization of light linear at the fiber output. A 0.21 dB quantum entanglement could still be observed for the EPR-entangled beams transmitted through a 50-km-long single-mode fiber. The theoretical prediction considering the excess noise in fiber channel is in good agreement with the experimental result. The generated CV quantum entanglement is highly suitable for the required experiments, such as CV measurement-device-independence QKD based on standard fibers, owing to the fact that the tolerance of the excess noise in the quantum channel can be enhanced significantly with respect to a coherent state if EPR-entangled beams are used.

Generation of 8.3 dB continuous variable quantum entanglement at a telecommunication wavelength of 1550 nm

Continuous variable quantum entanglement at a telecommunication wavelength of 1550 nm is experimentally generated using a single nondegenerate optical parametric amplifier based on a type-II periodically poled KTiOPO4 crystal. The triply resonant of the nondegenerate optical parametric amplifier is adjusted by tuning the crystal temperature and tilting the orientation of the crystal in the optical cavity. Einstein–Podolsky–Rosen-entangled beams with quantum correlations of 8.3 dB for both the amplitude and phase quadratures are experimentally generated. This system can be used for continuous variable fibre-based quantum communication.

Optomechanical entanglement via non-degenerate parametric interactions

We present a scheme for the optomechanical entanglement between a micro-mechanical mirror and the field inside a bimodal cavity system using a non-degenerate optical parametric amplifier (NOPA). Our results show that the introduction of NOPA makes the entanglement stronger or more robust against the mean number of average thermal phonons and cavity decay. Interestingly, macroscopic entanglement depends upon the choice of the phase associated with classical field driving NOPA. We also consider the effects of input laser power on optomechanical entanglement.

Distribution of continuous variable quantum entanglement at a telecommunication wavelength over 20 km of optical fiber

The distribution of continuous variable (CV) Einstein-Podolsky-Rosen (EPR)-entangled beams at a telecommunication wavelength of 1550 nm over single-mode fibers is investigated. EPR-entangled beams with quantum entanglement of 8.3 dB are generated using a single nondegenerate optical parametric amplifier based on a type-II periodically poled KTiOPO4 crystal. When one beam of the generated EPR-entangled beams is distributed over 20 km of single-mode fiber, 1.02 dB quantum entanglement can still be measured. The degradation of CV quantum entanglement in a noisy fiber channel is theoretically analyzed considering the effect of depolarized guided acoustic wave Brillouin scattering in optical fibers. The theoretical prediction is in good agreement with the experimental results.

General Single-Mode Gaussian Operation with Two-Mode Entangled State**Supported by the National Natural Science Foundation of China under Grant Nos 61205115, 11474003 and 61675006, the Natural Science Foundation of Anhui Province under Grant Nos 1608085MF133 and 1408085MA19, the Foundation for the Young Talent of Anhui Province under Grant No gxyqZD2016065, and the Youth Foundation of Anhui University of Technology under Grant

Realizing the logic operations with small-scale states is pursued to improve the utilization of quantum resources and to simplify the experimental setup. We propose a scheme to realize a general single-mode Gaussian operation with a two-mode entangled state by utilizing only one nondegenerate optical parametric amplifier and by adjusting four angle parameters. The fidelity of the output mode can be optimized by changing one of the angle parameters. This scheme would be utilized as a basic efficient element in the future large-scale quantum computation.

Continuous variable quantum entanglement at 1.34 m

Continuous variable (CV) quantum entanglement is a fundamental resource of CV quantum communication and quantum computation. It is useful in a wide variety of applications, including quantum teleportation, quantum dense coding, quantum key distribution, and high-precision quantum measurement. In this paper, we generate CV quantum entanglement at a telecommunication wavelength of 1342 nm by using a nondegenerate optical parametric amplifier (NOPA) with a type-Ⅱ periodically poled KTiOPO4 (PPKTP) crystal. A home-made continuous-wave single-frequency dual-wavelength (671 nm and 1342 nm) Nd:YVO4/LiB3O5 laser is achieved with output powers of 1.5 W (671 nm) and 1.3 W (1342 nm). Then a mode cleaner (MC1) with a fineness of 400 and linewidth of 0.75 MHz and a mode cleaner MC2 with a fineness of 400 and linewidth of 0.75 MHz are used to filter the noises of laser at 1342 nm and 671 nm, respectively. By using MCs, the intensity noise of laser reaches a shot noise level (SNL) for analysis frequencies higher than 1.0 MHz, and the phase noise of laser reaches an SNL for analysis frequencies higher than 1.3 MHz. Utilizing this kind of low noise single-frequency 671 nm laser as a pump, a doubly-resonant optical parametric oscillator with a threshold of 325 mW is realised. When the low noise single-frequency 1342 nm laser is injected as a signal and the relative phase between the pump and injected signal is locked to , the NOPA is operated at deamplification. After optimizing the temperature of the type-Ⅱ PPKTP crystal and at a pump power of 260 mW, Einstein-Podolsky-Rosen (EPR)-entangled beams with quantum correlation of 3.0 dB for both the amplitude and phase quadratures are experimentally generated. The strength of EPR-entangled beams is relatively low. It is maybe due to the low nonlinear conversion efficiency and large absorption of the type-Ⅱ PPKTP crystal at 671 nm and 1342 nm. The generated CV quantum entanglement at 1.34 m has lower transmission loss and smaller phase diffusion effect in a silica fiber. The research contributes to a high quality quantum source for the CV quantum communication based on existing telecommunication fiber networks.