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arXiv:2409.09336 · Advanced Quantum Technologies

Frequency‐Dependent Squeezing via Einstein–Podolsky–Rosen Entanglement Based on Silicon Nitride Microring Resonators

Haodong Xu, Zijun Shu, Nianqin Li, Yang Shen, Bo Ji, Yongjun Yang, Tengfei Wu, Mingliang Long, Guangqiang He

Published Dec 31, 2024• 1 citation

Abstract

Abstract Considerable efforts have been devoted to augmenting the performance of displacement sensors constrained by quantum noise, particularly within high‐precision applications such as gravitational wave detection. Frequency‐dependent squeezing methodologies have adeptly exceeded the standard quantum limit in optomechanical force measurements, catalyzing profound advancements in the field. Concurrently, notable strides in integrated photonics have paved the way for the realization of integrated Kerr quantum frequency combs (QFCs). In this work, a sophisticated platform designed for the creation of Einstein–Podolsky–Rosen (EPR)‐entangled QFCs utilizing on‐chip silicon nitride microring resonators is presented. This platform facilitates an exhaustive analysis and optimization of entanglement performance, establishing a robust framework for noise mitigation. By incorporating the quantum dynamics of Kerr nonlinear microresonators, the system accommodates at least 12 continuous‐variable quantum modes, including 6 pairs of concurrently EPR‐entangled states. Moreover, through precise tuning of the detection angle of the idler mode, the signal mode transitions into a single‐mode squeezed state. Harnessing the frequency‐dependent nature of this detection angle enables the achievement of frequency‐dependent squeezing. A comparative analysis under different dispersion conditions is also presented.

DOI: 10.1002/qute.202400473

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Frequency-dependent squeezing (FDS) represents a well established way to address quantum noise (QN) in Gravitational-Wave (GW) Earth-based detectors, such as LIGO, Virgo, and KAGRA. This technique is realized with the use of an external detuned optical resonator, the filter cavity. The experiment we present here is a table-top prototype that will probe a cheaper, more compact and more flexible strategy for broadband QN reduction (Ma et al., 2017). This scheme is based on two-mode Einstein–Podolsky–Rosen (EPR) entangled squeezed light, and it works without any filter cavity. The EPR-entangled beams will propagate in a small-scale suspended interferometer with high-finesse arm-cavities. This experiment aims at validating the EPR conditional squeezing at audio frequencies, suited for GW detection, implementing also innovative optical techniques.

Squeezed light offers genuine quantum advantage in enhanced sensing and quantum computation; yet the level of squeezing or quantum noise reduction generated from nanophotonic chips has been limited. In addition to strong quantum noise reduction, key desiderata for such a nanophotonic squeezer include frequency agility or tunability over a broad frequency range, and simultaneous operation in many distinct, well-defined quantum modes (qumodes). Here we present a strongly overcoupled silicon nitride squeezer based on a below-threshold optical parametric amplifier (OPA) that produces directly detected squeezing of 5.6 dB

\pm

0.2 dB, surpassing previous demonstrations in both continuous-wave and pulsed regimes. We introduce a seed-assisted detection technique into such nanophotonic squeezers that reveals a quantum frequency comb (QFC) of 16 qumodes, with a separation of 11~THz between the furthest qumode pair, while maintaining a strong squeezing. Additionally, we report spectral tuning of a qumode comb pair over one free-spectral range of the OPA, thus bridging the spacing between the discrete modes of the QFC. Our results significantly advance both the generation and detection of nanophotonic squeezed light in a broadband and multimode platform, establishing a scalable, chip-integrated path for compact quantum sensors and continuous-variable quantum information processing systems.

Future gravitational-wave detectors will use frequency-dependent squeezed vacuum states to obtain broadband reduction of quantum noise. Quantum noise is one of the major limitations to the sensitivity of these detectors. Advanced LIGO+, Advanced Virgo+, and KAGRA plan to generate frequency-dependent squeezed states by coupling a frequency-independent squeezed light state with a filter cavity. An alternative technique is under consideration, based on conditional squeezing with quantum entanglement: Einstein–Podolsky–Rosen (EPR) squeezing. In the EPR scheme, two vacuum entangled states, the signal field at ω 0 and the idler field at ω 0 + Δ , must be spatially separated with an optical resonator and sent to two separate homodyne detectors. In this framework, we have designed and tested a solid Fabry–Perot etalon, to be used in an EPR table-top experiment prototype, thermally controlled without the use of a control probe optical beam. This device can also be used in optical experiments where the use of a bright beam to control an optical resonator is not possible, or where a simpler optical device is preferred.

Integrated nonlinear microring resonators are promising sources of bright twin-beams exhibiting intensity difference squeezing. To take advantage of the noise reduction of these quantum states, many applications require high squeezing levels. However, the attainable on-chip squeezing is constrained by the ratio of intrinsic to coupling loss, which is strongly related to design and fabrication limits. Here, we demonstrate a silicon nitride microring resonator engineered to enable a set of periodic strongly overcoupled resonances from which the estimated on-chip intensity difference squeezing is 11.8 dB, exceptionally high for an integrated device. Due to significant setup loss, we directly measure 1.4±0.2 dB of squeezing, corresponding to an on-chip level of 9.2±5.1 dB. In addition, the new, to the best of our knowledge, design inherently suppresses mode competition in the modes surrounding the twin-beams, enabling a constant high squeezing level up to moderate powers. These results represent a significant step toward the miniaturization of integrated devices whose performance benefits from surpassing the quantum noise limit using squeezed states, with particular relevance to sensing applications.