We demonstrate the enhanced quantum noise reduction of the output field from an optical parametric amplifier (OPA) by employing the cascaded optical parametric amplifiers (COPA), in which the output field of one OPA is the input field of the subsequent one. The output field noise formulas of the cascaded degenerate and nondegenerate OPAs (CDOPA and CNOPA) are obtained. The analytical results show that a CDOPA and a CNOPA can generate squeezing beams and entangled beams with a wider bandwidth under a low pump parameter, respectively. The cavity output efficiency and the intracavity loss are the main factors for improving the degrees of squeezing and entanglement. Our work will provide improved schemes for designing COPAs to efficiently produce high-quality squeezed and entangled states, which have potential applications in quantum-information processing.
We demonstrate the enhanced quantum noise reduction of the output field from an optical parametric amplifier (OPA) by employing the cascaded optical parametric amplifiers (COPA), in which the output field of one OPA is the input field of the subsequent one. The output field noise formulas of the cascaded degenerate and nondegenerate OPAs (CDOPA and CNOPA) are obtained. The analytical results show that a CDOPA and a CNOPA can generate squeezing beams and entangled beams with a wider bandwidth under a low pump parameter, respectively. The cavity output efficiency and the intracavity loss are the main factors for improving the degrees of squeezing and entanglement. Our work will provide improved schemes for designing COPAs to efficiently produce high-quality squeezed and entangled states, which have potential applications in quantum-information processing.
The squeezing level of light on atomic resonance produced by a degenerate optical parametric amplifier (DOPA) is now limited by blue or violet pump-light-induced loss, and it is difficult to improve the squeezing by simply reducing the loss of DOPA. In this study, two promising schemes, namely, the cascaded DOPA (C-DOPA) and the coherent feedback-controlled DOPA (CFC-DOPA), are theoretically discussed. For a ideal case, the CFC-DOPA can realize infinite squeezing output but requires almost infinite accuracy in phase locking. For a practical case with feasible physical parameters of realistic systems, the performance of squeezing enhancement for the C-DOPA and CFC-DOPA is compared. The C-DOPA shows advantages in the megahertz frequency range, while the CFC-DOPA shows advantages in the low-frequency range. At a general analysis frequency of 2 MHz, C-DOPA shows better robustness against the loss of the system. As a price, it requires a smaller instability tolerance for phase locking. With the C-DOPA, the squeezing on Rb atomic resonance can be pushed from − 5 d B to − 7 d B . Our analytical results can provide a valuable reference for designing experimental devices to efficiently produce high-quality squeezed light on atomic resonance, which has potential applications in quantum information processing and quantum metrology.
We present a cascaded system consisting of three nondegenerate optical parametric amplifiers (NOPAs) for the generation and the enhancement of quantum entanglement of continuous variables. The entanglement of optical fields produced by the first NOPA is successively enhanced by the second and the third NOPAs from $\ensuremath{-}5.3$ to $\ensuremath{-}8.1$ dB below the quantum noise limit. The dependence of the enhanced entanglement on the physical parameters of the NOPAs and the reachable entanglement limitation for a given cascaded NOPA system are calculated. The calculation results are in good agreement with the experimental measurements.
Optical parametric amplifiers (OPAs) are promising to overcome the wavelength coverage and noise limitations in conventional optical amplifiers based on rare-earth doping and semiconductor gain. However, the high power requirement remains a major obstacle to the widespread use of OPAs. Integrated OPAs can in principle improve the pump efficiency with tight mode confinement; however, challenges associated with propagation loss, limited nonlinearity, and susceptibility to nanoscale fabrication imperfections prevent them from competing with conventional bulk and fiber-based OPAs. Here, we demonstrate a highly efficient integrated OPAs with continuous-wave net gain. The pump efficiency is improved by over one order of magnitude. Phase-sensitive gain of 23.5 dB is demonstrated, significantly exceeding previous integrated OPAs, using only 110 mW pump power and no cavity enhancement. This is achieved with parametric down-conversion in thin-film lithium niobate waveguides using the adapted poling technique to maintain the coherence of nonlinear interactions. Moreover, the high parametric gain exceeds fibre-chip-fibre losses, leading to appreciable net gain up to 10 dB. The 3 dB bandwidth is approximately 120 nm, covering telecommunication S-, C-, and Lbands. Quantum-limited noise performance is confirmed through the measurement of output field fluctuation below the classical limit. We further demonstrate that signalto-noise ratio in noisy optical communications can be increased by leveraging this efficient integrated OPA. Our work marks a significant step towards ideal optical amplifiers with strong amplification, high efficiency, quantum-limited noise, large bandwidth, and continuous-wave operation, unlocking new possibilities for next-generation photonic information processing systems.
The squeezed light field in the band of alkali metal atomic transitions is an important quantum resource in the field of quantum information and precision measurement. The wavelengths of atomic transition lines (760–860 nm) are relatively short. Limited by the gray-tracking effect of nonlinear crystals, the squeezing degree of the squeezed light in this band generated by the optical parametric amplifiers is low. Now, the squeezing is about 3–5 dB. Considering the problems in the experimental generation of the squeezed light at the wavelengths of atomic transitions, the variation law of quantum noise of the light field output from the single optical parametric amplifier with its physical parameters is studied theoretically, and the optimal physical parameters are obtained. To further improve the squeezing in the band of alkali metal atomic transitions, the cascaded optical parametric amplifiers are considered. Based on the basic theory of the optical parametric amplifiers, the theoretical model of the cascaded optical parametric amplifiers is constructed, in which the optical loss and phase noise of the cascaded optical loops are considered. Based on this, the quantum noise characteristics of the light field output from the cascaded system versus the optical loss and phase noise are analyzed at the frequencies of 2 MHz and 100 kHz, respectively. It is found that for the squeezing at 2 MHz, cascading 2 to 3 optical parametric amplifiers can significantly improve the squeezing under the premise of the low optical path loss and phase noise; for the squeezing in the low-frequency band, the enhancement of the squeezing for the cascaded system is quite weak. Under the current experimental parameters, the squeezing at 2 MHz of the squeezed light on rubidium resonance can be improved from –5 dB to –7 dB by cascading another DOPA. For the squeezing at low frequency band, the cascaded system proves to be useless, and the efforts should be made to reduce the technique noise in the low frequency band. Furthermore, the quantum limit and spectral characteristics of the squeezed light field output from the cascaded system are further explored. This study can provide reference and guidance for the improvement in the squeezing degree of the band of atomic transitions.
Abstract We propose a method to generate an entangled squeezed state using an optical parametric amplifier (OPA) and a beam splitter (BS). This device can manipulate the output quantum light field by modulating two parameters: the intensity of the OPA and the transmittance of the optical BS. For the output quantum state associated with a symmetric BS, there is no quadrature squeezing. Moreover, neither photon correlation nor entanglement are observed. However, in terms of sum squeezing, the quantum state corresponding to the symmetric BS is the strongest. Therefore, an asymmetric BS and a highly squeezing OPA are the best choices for generating the quantum entangled squeezed states.
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.