Characterization of quantum squeezing generated from the phase-sensitive and phase-insensitive amplifiers in the ultra-low average input photon number regime
We give the general expressions of intensity-difference squeezing (IDS) generated from two types of optical parametric amplifiers [i.e. phase-sensitive amplifier (PSA) and phase-insensitive amplifier (PIA)] based on the four-wave mixing process, which clearly shows the IDS transition between the ultra-low average input photon number regime and the ultra-high average input photon number regime. We find that both the IDS of the PSA and the IDS of the PIA get enhanced with the decrease of the average input photon number especially in the ultra-low average input photon number regime. This result is substantially different from the result in the ultra-high average input photon number regime where the IDS does not vary with the average input photon number. Moreover, under the same intensity gain, we find that the optimal IDS of the PSA is better than the IDS of the PIA in the ultra-low average input photon number regime. Our theoretical work predicts the presence of strong quantum correlation in the ultra-low average input photon number regime, which may have potential applications for probing photon-sensitive biological samples.
We give the general expressions of intensity-difference squeezing (IDS) generated from two types of optical parametric amplifiers [i.e. phase-sensitive amplifier (PSA) and phase-insensitive amplifier (PIA)] based on the four-wave mixing process, which clearly shows the IDS transition between the ultra-low average input photon number regime and the ultra-high average input photon number regime. We find that both the IDS of the PSA and the IDS of the PIA get enhanced with the decrease of the average input photon number especially in the ultra-low average input photon number regime. This result is substantially different from the result in the ultra-high average input photon number regime where the IDS does not vary with the average input photon number. Moreover, under the same intensity gain, we find that the optimal IDS of the PSA is better than the IDS of the PIA in the ultra-low average input photon number regime. Our theoretical work predicts the presence of strong quantum correlation in the ultra-low average input photon number regime, which may have potential applications for probing photon-sensitive biological samples.
We experimentally demonstrate a method for realizing quantum squeezing enhancement which is induced by the interference in a two-beam phase-sensitive amplifier (PSA) based on a four-wave mixing process. Compared to the normal phase-insensitive amplifier with an intensity-difference squeezing (IDS) of 8.97±0.24 dB or 8.76±0.26 dB, the IDS of our two-beam PSA is enhanced to 10.13±0.21 dB under the same experimental situation. Furthermore, we study how various parameters influence the quantum squeezing enhancement of the PSA. These results clearly show that the physical mechanism inducing the IDS enhancement of the two-beam PSA is its intrinsic interference nature. Our results may find potential applications in improving the fidelity of quantum information processing and the precision of quantum metrology.
We experimentally realize the enhancement of six-beam quantum squeezing by utilizing a six-beam phase-sensitive amplifier (PSA) based on cascaded four-wave mixing processes. Compared to the intensity-difference squeezing (IDS) of about 5.03 or 5.09 dB generated by a six-beam phase-insensitive amplifier (PIA), the six-beam IDS generated by the six-beam PSA is enhanced to about 6.64 dB. The intrinsic interference of six-beam PSA induces this enhancement. In addition, we investigate the dependence of IDS generated by the six-beam PSA and PIAs on the powers of two pump beams. The results show that the IDS of six-beam PSA is always better than that of six-beam PIAs. Our scheme may find potential applications in high-precision multi-parameter quantum metrology.
Phase-sensitive amplifiers (PSAs) have been widely studied in fiber amplifiers, with remarkable recent advances. They have also been implemented in an SU(1,1) interferometer. In this paper, we study an experimental scheme for the implementation of a two-mode PSA based on a four-wave mixing process in rubidium vapor. With the process seeded by coherent probe and conjugate beams, quantum correlation including intensity difference/sum squeezing and quadrature entanglement between the output probe and conjugate fields are theoretically analyzed. Compared to previous related research, several new and interesting results are reported here. The maximal degree of intensity difference squeezing can be enhanced by nearly 3 dB compared to a phase-insensitive amplifier with the same gain. It is also possible to generate intensity sum squeezing between the probe and conjugate fields by choosing the specific phase of the input beams. Moreover, quadrature entanglement between the probe and conjugate beams, which can be manipulated by the phase of the input beams, is predicted. Our scheme may find a variety of applications in quantum metrology and quantum information processing owing to its ability of quantum squeezing and entanglement manipulation.
This dissertation is a summary of investigations on the optical properties of a quantum-noise-limited phase-sensitive amplifier (PSA). The PSA is implemented using four-wave mixing in hot Rb 85 vapor based on a double-Lambda atomic scheme. We experimentally demonstrate the ability of a PSA to pre-amplify quantum correlations in twin light beams produced by a phase-insensitive amplifier (PIA) before degradation due to loss and detector inefficiency. By including a PSA before loss, one is able to preserve the correlations as well as the two-mode squeezing level. We compare the results to simulations employing a simple quantum-mechanical model and find a good agreement. We have demonstrated that the cross-correlation between the two modes of a bipartite entangled state can be advanced by propagation through a PIA acting as a fast-light medium. The extra noise added by the PIA has been speculated to be the mechanism that limits the advance of entanglement, preventing the mutual information from traveling superluminally. As an extension of this phase-insensitive, gain-assisted, anomalous dispersion investigation, we explore the advance and delay of information transmitted through the PSA. We start with a two-mode squeezed state created by the PIA and measure the mutual information shared by the correlated quadratures. We then pass one of these two modes through a PSA and investigate the shift of the mutual information as a function of the PSA phase. In the case of a PSA, it is well known that no extra noise will be added to the quadrature with the correct input phase (e.g., the quadrature with the maximal amplification or the maximal deamplification). We find that there is no dispersion-like behavior at these two phases, however, the peak of mutual information could either be delayed or advanced at any other phase. We also observe an almost identical behavior when we input an amplitude modulated signal to the PSA. We are able to explain the physics of this ``fast-and-slow-light'' type of behavior utilizing a model assuming imbalanced gain on the positive and negative side bands. We obtain a good agreement between the experimental results and the theoretical simulations.
The intensity-difference squeezed state is an important concept in quantum optics, which is not only of great significance for fundamental research in quantum physics, but also an important quantum resource in the fields of quantum communication, quantum computing, and quantum precision measurement. The optical parametric amplifier based on atomic four-wave mixing is one of the most effective means to achieve intensity-difference squeezed light. However, due to the absorption loss of atomic vapor in the light field, the output squeezing still needs improving. By feeding the non-classical optical field from the optical parametric amplifier back to the input port, the quantum characteristics of its output optical field can be enhanced. However, the intensity-difference squeezing enhancement from a phase-insensitive amplifier is experimentally realized based on coherent feedback control. The intensity-difference squeezing enhancement of the phase-sensitive amplifier has not been discussed. In this work, a two-port coherent feedback-controlled phase-sensitive amplifier is analyzed theoretically. The dependence of the intensity-difference squeezing, respectively, on the feedback intensity, the intensity gain of the optical parametric amplifier, and the losses of the system are investigated. For the ideal case in which the losses of the system are ignored, infinite squeezing can be achieved by adjusting the strength and phase of feedback. Considering the actual atomic absorption losses, squeezing enhancement can also be achieved over a wide range of intensity gains within a certain feedback intensity range. In addition, the squeezing enhancement is quite efficient for the medium intensity gain range. The intensity-difference squeezing enhancement strongly depends on the absorption loss of atomic vapor. The smaller the absorption loss, the more significant the squeezing enhancement effect is. Furthermore, the experimental feasibility of this scheme is also considered in detail. Our research can provide useful references for achieving high-quality non classical light fields in experiment, which may find applications in quantum information processing and quantum precise measurement.
Nonlinear interferometers offer a robust platform for quantum sensing beyond the standard quantum limit. Specifically, the generation of high-quality intensity-difference-squeezed (IDS) states provides a distinct advantage by circumventing the need for complex phase-coherent local oscillators. However, practical imperfections such as asymmetric inter-stage transmission and unbalanced detection losses severely degrade the generation of these states. In this work, we analyze IDS state generation within fiber nonlinear interferometers utilizing cascaded parametric amplifiers. Using a single-mode theoretical framework and experimental demonstration, we derive an optimal electronic gain ratio for differential detection to compensate for asymmetric detection-induced noise mismatches, extending achievable squeezing into the high-gain regime. We further analyze system tolerance to inter-stage losses and path delays. Experiments demonstrate that while higher initial parametric gain improves quantum correlations, it amplifies intensity noise and accelerates gain saturation, resulting in optimal squeezing at balanced low-gain configurations. These findings establish a robust framework for the design and optimization of all-fiber quantum sensors.