Bright squeezed light in the kilohertz frequency band

Bright squeezed light, exhibiting sub-shot-noise quantum noise combined with significant optical power, is essential for enhanced sensitivity in quantum metrology and precision measurement. In the deep application within the field, the squeezing must be extended to kHz–MHz bandwidth with milliwatt optical power. However, it remains a longstanding challenge to achieve this goal with conventional technologies, since they suffer from low-frequency technical noise and vacuum noise coupling, which caps the squeezing property in the generation process. Nonclassical active stabilization can compensate this drawback, but must make a trade-off between the feedback gain-bandwidth and optical power.

In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Yaohui Zheng from State Key Laboratory of Quantum Optics Technologies and Devices, Institute of Opto-Electronics, Shanxi University, China, have developed a novelty nonclassical hybrid passive–active power stabilization technique to break the limit. By integrating the broadband passive noise suppression technique based on second-harmonic-generation (SHG), the team created a dual-stage stabilization architecture to reduces the technical noise from −122 dB/Hz to −165 dB/Hz in kHz band, which is 9 dB below the shot noise limit of 1 mW optical power. More significantly, the hybrid stabilization architecture extends the feedback bandwidth from 50 kHz to MHz range. Notably, the passively stabilized laser beam is extracted from the residual fundamental wave reflected from the SHG cavity, which is a part of the squeezed vacuum generation unit. Therefore, no extra laser power is needed to produce the bright squeezed light, which is benefit to increase the utilization efficiency of the laser power and raise the integration level of our bright squeezed light source.

Taking advantage of this approach, they demonstrated a milliwatt-level −5.5 dB bright amplitude squeezed light across kHz–MHz band, which shows excellent agreement with theoretical prediction by a newly developed comprehensive model. The milliwatt-level bright amplitude squeezed light among kHz-MHz frequency range with almost no squeezing strength degeneration, which supplies a new methodology to enhance the low frequency performance of this type of bright quantum source. It can support various kinds of successive metrological tasks with a kilohertz bright squeezing. These scientists summarize the operational principle of their quantum source:

“The authors report the generation of a bright amplitude-squeezed light source in the kilohertz frequency band. By integrating two-stage noise suppression architecture including a passive noise filter and an active noise stabilization, the overall performance of the output quantum light is far beyond the state of art. The quantum noise is reduced to -5.5 dB across the kilohertz-to-megahertz frequency range. In addition, the output power reaches milliwatt level, which is one order of magnitude higher than previous reports. The results are interesting and potentially useful in quantum optic applications.”

“The authors addressed a major concern of mine: the technical novelty. It makes a convincing case for the novelty of the hybrid approach of combining a passive and active stabilization technique. Furthermore, the authors outline how their method differs from passive interference of a pre-stabilized beam and a squeezed vacuum state. Concerning the broader relevance of the work the authors provide a more measured and realistic outlook, which elevates the scientific objectivity.” they added.  

“Their efforts significantly enhance the completeness and readability of the manuscript. In particular, the new version effectively clarifies the contributions of this work, highlighting its key breakthroughs in principle and technology, as well as the significantly improved indicators and applications presented.” the scientists praise.

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