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Physical Review A

Shot-noise-suppressing quantum nonlinearity in the vacuum-field-induced photon-photon interaction

Jiang-Shan Tang, Lei Tang, Zhixiang Li, Ya-Ping Ruan, Han Zhang, Keyu Xia

Published Dec 28, 2022• 0 citations
Publisher

Abstract

The shot noise in coherent control of a quantum system sets an ultimate precision limit in measurement and information processing. It is caused by the fundamental quantum fluctuation of the photon number of a coherent control field and thus is inevitable. By strongly coupling a two-level system to a quantum vacuum field, we propose an effective $N$-type four-level system. In such a configuration, the coupling of a conventional $N$-type system to a coherent control field is replaced by the quantum vacuum field. As a result, we can achieve a shot-noise-suppressing quantum nonlinearity, due to the vanishing fluctuation of photon number of the quantum vacuum field. The resulting photon blockade effect is only subject to the contribution from the probe field. Thus, it becomes shot-noise suppressing. Our work shows an advantage of a quantum vacuum protocol over a coherent-field protocol in manipulation of quantum systems and presents an opportunity to exploit the quantum vacuum field as a powerful tool in quantum information processing.

DOI
10.1103/physreva.106.062439

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The shot noise in coherent control of a quantum system sets an ultimate precision limit in measurement and information processing. It is caused by the fundamental quantum fluctuation of the photon number of a coherent control field and thus is inevitable. By strongly coupling a two-level system to a quantum vacuum field, we propose an effective NNN-type four-level system. In such a configuration, the coupling of a conventional NNN-type system to a coherent control field is replaced by the quantum vacuum field. As a result, we can achieve a shot-noise-suppressing quantum nonlinearity, due to the vanishing fluctuation of photon number of the quantum vacuum field. The resulting photon blockade effect is only subject to the contribution from the probe field. Thus, it becomes shot-noise suppressing. Our work shows an advantage of a quantum vacuum protocol over a coherent-field protocol in manipulation of quantum systems and presents an opportunity to exploit the quantum vacuum field as a powerful tool in quantum information processing.
When a four-level Y\mathsf{Y}Y-type atom with two highest nearly degenerate lying levels interacts with the same vacuum radiation field, its excited levels have vacuum-induced coherence due to the quantum interference between the spontaneous decay channels. We consider the effects of the vacuum-induced coherence on the electromagnetically induced transparency against single- and two-photon absorption in the atomic system. We find that the vacuum-induced coherence can lead to probe gain without incoherent pumping. On the other hand, the coherence can suppress the two-photon transparency, even in some cases where it can enhance the two-photon absorption. Another important result is the finding of the crucial role played by the relative phase between the probe and coupling fields: the two-photon transparency and the probe gain spectra can be modulated by this phase.
We show both theoretically and experimentally that fluctuations of a weak probe beam are increased above the coherent-state shot-noise level as the beam interacts with a strong degenerate pump beam in an atomic vapor. The origin of this noise amplification process is the two-beam-coupling process between the pump beam and the vacuum sidebands of the probe beam. The major contribution to this excess noise arises from the quantum fluctuations of the atomic medium that are induced by the pump beam. We believe that this mechanism can be important in preventing the reduction of noise below the quantum-noise limit in experiments that utilize atomic vapors.
The nonlinear quantum regime is crucial for implementing interesting quantum effects, which have wide applications in modern quantum science. Here we propose an effective method to reach the nonlinear quantum regime in a modulated optomechanical system (OMS), which is originally in the weak-coupling regime. The mechanical spring constant and optomechanical interaction are modulated periodically. This leads to the result that the resonant optomechanical interaction can be effectively enhanced into the single-photon strong-coupling regime by the modulation-induced mechanical parametric amplification. Moreover, the amplified phonon noise can be suppressed completely by introducing a squeezed vacuum reservoir, which ultimately leads to the realization of photon blockade in a weakly coupled OMS. The reached nonlinear quantum regime also allows us to engineer the nonclassical states (e.g., Schr\"odinger cat states) of the cavity field, which are robust against the phonon noise. This work offers an alternative approach to enhance the quantum nonlinearity of an OMS, which should expand the applications of cavity optomechanics in the quantum realm.
Quantum vacuum noise is very detrimental for photon propagation and optical quantum memory, which particularly occurs in optically thick atomic media working under the condition of electromagnetically induced transparency (EIT) accompanied by a process of four-wave mixing (FWM). The suppression of quantum vacuum noise becomes now an imperative task for advancing the research and application of quantum memory. Here we present a scheme to suppress quantum vacuum noise and hence to improve the fidelity of photon propagation, by assuming that signal and idler laser fields are initially prepared in a two-mode squeezed vacuum state. With such a scheme, we show that the normal gain mode inherent in the EIT-based FWM system, which contributes optical gain to both the signal and idler fields, can be largely inhibited, giving rise to a significant suppression of the quantum vacuum noise along with their amplification and hence a large enhancement of the fidelity for photon propagation. The results reported here may have promising applications for high-fidelity quantum information processing and transformation, especially for improving the quality of multimode optical quantum memory based on EIT techniques.
The search for optical fields with quantum fluctuations less than that of a perfectly coherent signal has reopened questions of appropriate signatures of non-classicality. Squeezed light fields have phase dependent noise that for some phase angles can be substantially less than the normal vacuum fluctuations. Sub-Poissonian light has photon number fluctuations below those expected from naive "shot noise" considerations. In this talk, I will review the progress made in understanding non-classical light, and discuss measures of non-classicality.
This research focuses on advanced noise suppression technologies for high-precision measurement systems, particularly addressing the limitations of classical noise reducing approaches. The noise level of laser sources is a crucial factor that directly affects the measurement sensitivity in applications such as gravitational wave detection and biomedical imaging. Classical feedback control technologies are effective but often encounter a bottleneck resulting from the classical noise suppression limits. To cope with these challenges, a novel method integrating quantum squeezed light with classical feedback control systems to reduce intensity noise is proposed in this work. By employing an amplitude-squeezed light field, a quantum-enhanced feedback control model is developed, thereby theoretically examining the influence of both the feedback loop gain and the degree of squeezing on the noise suppression performance. The results show that the injection of squeezed light significantly reduces the intensity noise, approaching the shot noise limit (SNL), thereby improving the system sensitivity beyond the classical noise reduction boundaries. Specifically, –10 dB squeezed state injection into the feedback system yields an additional noise suppression of approximately 8.97 dB, exceeding what is achievable using classical feedback alone. This demonstrates that the potential of the proposed method can enhance measurement precision close to the quantum noise limits without increasing the laser power. The analysis highlights the asymmetric noise suppression effects between the inner feedback loop and outer feedback loop. Although the outer loop benefits significantly from the squeezed light injection and achieves noise levels that are unattainable by classical feedback methods, the inner loop shows relatively minor improvements. This asymmetry is attributed to the inherent characteristics of quantum squeezing and the limitations of the feedback loop design. Further investigation into the individual noise components reveals that the primary contributors to the intensity noise include input noise, photodetector noise, and beam splitter-induced vacuum fluctuations. The injection of squeezed light effectively mitigates these vacuum fluctuations, which are typically a major noise source in high-precision laser systems. Theoretical research results show that the use of squeezed light in feedback control systems can effectively enhance noise suppression, equivalent to a nine fold increase in detected optical power, without the physical drawbacks of increasing laser power such as thermal noise. In conclusion, this study provides a theoretical validation of combining quantum squeezed states with classical feedback control to exceed classical noise suppression limits. The integration of a –10 dB squeezed state demonstrates a significant noise reduction, showing that this hybrid approach could revolutionize noise management in precision measurement applications. The results pave the way for further exploring quantum-enhanced control technologies in fields such as gravitational wave detection, quantum communication, and advanced optical sensing, providing a pathway for improving sensitivity and noise suppression without increasing additional power requirements.