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1. Broadband quantum enhancement of the LIGO detectors with frequency-dependent squeezing

   Ganapathy, D.; Jia, W.; Nakano, M.; Xu, V.; Dwyer, S.E.; Mullavey, A.; McCuller, L. (October 2023, Physical review X)

   Quantum noise imposes a fundamental limitation on the sensitivity of interferometric gravitational-wave detectors like LIGO, manifesting as shot noise and quantum radiation pressure noise. Here we present the first realization of frequency-dependent squeezing in full-scale gravitational-wave detectors, resulting in the reduction of both shot noise and quantum radiation pressure noise, with broadband detector enhancement from tens of Hz to several kHz. In the LIGO Hanford detector, squeezing reduced the detector noise amplitude by a factor of 1.6 (4.0 dB) near 1 kHz, while in the Livingston detector, the noise reduction was a factor of 1.9 (5.8dB). These improvements directly impact LIGO’s scientific output for high-frequency sources (e.g., binary neutron star post-merger physics). The improved low-frequency sensitivity, which boosted the detector range by 15–18 % with respect to no squeezing, corresponds to an increase in astrophysical detection rate of up to 65%. Frequency-dependent squeezing was enabled by the addition of a 300-meter long filter cavity to each detector as part of the LIGO A+ upgrade. 

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2. Squeezed dual-comb spectroscopy

   https://doi.org/10.1126/science.ads6292  

   Herman, Daniel I; Walsh, Mathieu; Kreider, Molly Kate; Lordi, Noah; Tsao, Eugene J; Lind, Alexander J; Heyrich, Matthew; Combes, Joshua; Genest, Jérôme; Diddams, Scott A (February 2025, Science)

   Optical frequency combs have enabled distinct advantages in broadband, high-resolution spectroscopy and precision interferometry. However, quantum mechanics ultimately limits the metrological precision achievable with laser frequency combs. Quantum squeezing has led to substantial measurement improvements with continuous wave lasers, but experiments demonstrating metrological advantage with squeezed combs are less developed. Using the Kerr effect in nonlinear optical fiber, a 1-gigahertz frequency comb centered at 1560 nanometers is amplitude-squeezed by >3 decibels (dB) over a 2.5-terahertz bandwidth. Dual-comb interferometry yields mode-resolved spectroscopy of hydrogen sulfide gas with a signal-to-noise ratio nearly 3 dB beyond the shot-noise limit. The quantum noise reduction leads to a twofold quantum speedup in the determination of gas concentration, with implications for high-speed measurements of multiple species in dynamic chemical environments. 

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3. Trade-offs between unitary and measurement induced spin squeezing in cavity QED

   https://doi.org/10.1103/PhysRevResearch.6.L032037  

   Barberena, Diego; Chu, Anjun; Thompson, James K; Rey, Ana Maria (August 2024, Physical Review Research)

   We study the combined effects of measurements and unitary evolution on the preparation of spin squeezing in an ensemble of atoms interacting with a single electromagnetic field mode inside a cavity. We derive simple criteria that determine the conditions at which measurement based entanglement generation overperforms unitary protocols. We include all relevant sources of decoherence and study both their effect on the optimal spin squeezing and the overall size of the measurement noise, which limits the dynamical range of quantum-enhanced phase measurements. Our conclusions are relevant for state-of-the-art atomic clocks that aim to operate below the standard quantum limit. Published by the American Physical Society2024 

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4. Strong nanophotonic quantum squeezing exceeding 3.5  dB in a foundry-compatible Kerr microresonator

   https://doi.org/10.1364/OPTICA.542968  

   Shen, Yichen; Hsieh, Ping-Yen; Sridhar, Sashank Kaushik; Feldman, Samantha; Chang, You-Chia; Smith, Thomas A; Dutt, Avik (February 2025, Optica)

   Squeezed light, with its quantum noise reduction capabilities, has emerged as a powerful resource in quantum information processing and precision metrology. To reach noise reduction levels such that a quantum advantage is achieved, off-chip squeezers are typically used. The development of on-chip squeezed light sources, particularly in nanophotonic platforms, has been challenging. We report 3.7±0.2dB of directly detected nanophotonic quantum squeezing using foundry-fabricated silicon nitride (Si₃N₄) microrings with an inferred squeezing level of 10.2 dB on-chip. The squeezing level is robust across multiple devices and pump detunings, and is consistent with the overcoupling degree without noticeable degradation from excess classical noise. We also offer insights to mitigate thermally induced excess noise, which typically degrades squeezing, by using small-radius rings with a larger free spectral range (450 GHz) and consequently lower parametric oscillation thresholds. Our results demonstrate that Si₃N₄is a viable platform for strong quantum noise reduction in a CMOS-compatible, scalable architecture. 

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5. Entanglement-enhanced optomechanical sensor array with application to dark matter searches

   https://doi.org/10.1038/s42005-023-01357-z  

   Brady, Anthony J.; Chen, Xin; Xia, Yi; Manley, Jack; Dey Chowdhury, Mitul; Xiao, Kewen; Liu, Zhen; Harnik, Roni; Wilson, Dalziel J.; Zhang, Zheshen; et al (September 2023, Communications Physics)

   Abstract Squeezed light has long been used to enhance the precision of a single optomechanical sensor. An emerging set of proposals seeks to use arrays of optomechanical sensors to detect weak distributed forces, for applications ranging from gravity-based subterranean imaging to dark matter searches; however, a detailed investigation into the quantum-enhancement of this approach remains outstanding. Here, we propose an array of entanglement-enhanced optomechanical sensors to improve the broadband sensitivity of distributed force sensing. By coherently operating the optomechanical sensor array and distributing squeezing to entangle the optical fields, the array of sensors has a scaling advantage over independent sensors (i.e.,$$\sqrt{M}\to M$$ $\sqrt{M}\to M$, whereMis the number of sensors) due to coherence as well as joint noise suppression due to multi-partite entanglement. As an illustration, we consider entanglement-enhancement of an optomechanical accelerometer array to search for dark matter, and elucidate the challenge of realizing a quantum advantage in this context. 

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