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1. Measurement of sub-fm/Hz ^1/2 displacement spectral densities in ultrahigh-Q single-crystal microcavities with hertz-level lasers

   https://doi.org/10.1364/PRJ.449782  

   Jang, Yoon-Soo ; Lim, Jinkang ; Wang, Wenting ; Kim, Seung-Woo ; Savchenkov, Anatoliy ; Matsko, Andrey B. ; Wong, Chee Wei ( April 2022 , Photonics Research)

   Tracing a resonance frequency of a high quality factor (Q) optical cavity facilitates subpicometer displacement measurements of the optical cavity via Pound–Drever–Hall (PDH) locking scheme, tightly synchronizing a laser frequency to the optical cavity. Here we present observations of subfemtometer displacements on a ultrahigh-Qsingle-crystal${\mathrm{MgF}}_2$whispering-gallery-mode microcavity by frequency synchronization between a 1 Hz cavity-stabilized laser and a resonance of the${\mathrm{MgF}}_2$cavity using PDH laser-cavity locking. We characterize not only the displacement spectral density of the microcavity with a sensitivity of$1.5\times {10}^{-16}\mathrm{m}/{\mathrm{Hz}}^{1/2}$over the Fourier offset frequency ranging from 15 mHz to 100 kHz but also a 1.77 nm displacement fluctuation of the microcavity over 4500 s. Such measurement capability not only supports the analysis of integrated thermodynamical and technical cavity noise but allows for minute displacement measurements using laser-cavity locking for ultraprecise positioning, metrology, and sensing.
2. Widely tunable cavity-enhanced frequency combs

   https://doi.org/10.1364/OL.389412  

   Silfies, Myles C. ; Kowzan, Grzegorz ; Chen, Yuning ; Lewis, Neomi ; Hou, Ryan ; Baehre, Robin ; Gross, Tobias ; Allison, Thomas K. ( April 2020 , Optics Letters)

   We describe the cavity enhancement of frequency combs over a wide tuning range of 450–700 nm ($><\#comment/>7900{\mathrm{c}\mathrm{m}}^{-<\#comment/>1}$), covering nearly the entire visible spectrum. Tunable visible frequency combs from a synchronously pumped optical parametric oscillator are coupled into a four-mirror, dispersion-managed cavity with a finesse of 600–1400. An intracavity absorption path length enhancement greater than 190 is obtained over the entire tuning range, while preserving intracavity spectral bandwidths capable of supporting sub-200 fs pulse durations. These tunable cavity-enhanced frequency combs can find many applications in nonlinear optics and spectroscopy.
3. Acoustic-resolution photoacoustic microscopy based on an optically transparent focused transducer with a high numerical aperture

   https://doi.org/10.1364/OL.423287  

   Fang, Cheng ; Zou, Jun ( July 2021 , Optics Letters)

   This Letter reports acoustic-resolution-photoacoustic microscopy (AR-PAM) based on a new optically transparent focused polyvinylidene fluoride (PVDF) transducer with a high acoustic numerical aperture (NA) of 0.64. Owing to the improved fabrication process, the new transducer has a much higher NA (0.64) than the previously reported low-NA transducer ($\mathrm{N}\mathrm{A}=0.23$). The acoustic center frequency and (pulse-echo) bandwidth are also increased to 36 and 44 MHz, respectively, which provides a 38 µm acoustic focal spot size and 210 µm acoustic depth of focus. For demonstration, AR-PAM was conducted on a twisted wire target in water and chicken breast tissue, andin vivoon a mouse tail. The imaging results show that high acoustic resolution and sensitivity can be achieved with a simple and compact setup to resolve the target at different depths. Such capabilities can be useful for the development of new AR-PAM systems for handheld, wearable, and even endoscopic imaging applications.
4. Multi-atom quasiparticle scattering interference for superconductor energy-gap symmetry determination

   https://doi.org/10.1038/s41535-020-00303-4  

   Sharma, Rahul ; Kreisel, Andreas ; Sulangi, Miguel Antonio ; Böker, Jakob ; Kostin, Andrey ; Allan, Milan P. ; Eisaki, H. ; Böhmer, Anna E. ; Canfield, Paul C. ; Eremin, Ilya ; et al ( January 2021 , npj Quantum Materials)

   Complete theoretical understanding of the most complex superconductors requires a detailed knowledge of the symmetry of the superconducting energy-gap$${\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha$$${\Delta }_k^{\alpha }$, for all momentakon the Fermi surface of every bandα. While there are a variety of techniques for determining$$|{\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha |$$$\mid {\Delta }_k^{\alpha }\mid$, no general method existed to measure the signed values of$${\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha$$${\Delta }_k^{\alpha }$. Recently, however, a technique based on phase-resolved visualization of superconducting quasiparticle interference (QPI) patterns, centered on a single non-magnetic impurity atom, was introduced. In principle, energy-resolved and phase-resolved Fourier analysis of these images identifies wavevectors connecting allk-space regions where$${\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha$$${\Delta }_k^{\alpha }$has the same or opposite sign. But use of a single isolated impurity atom, from whose precise location the spatial phase of the scattering interference pattern must be measured, is technically difficult. Here we introduce a generalization of this approach for use with multiple impurity atoms, and demonstrate its validity by comparing the$${\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha$$${\Delta }_k^{\alpha }$it generates to the$${\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha$$${\Delta }_k^{\alpha }$determined from single-atom scattering in FeSe where s_±energy-gap symmetry is established. Finally, to exemplify utility, we use the multi-atom technique on LiFeAs and find scattering interference between the hole-like and electron-like pockets as predicted for$${\mathrm{{\Delta}}}_{\mathbf{k}}^\alpha$$${\Delta }_k^{\alpha }$of opposite sign.
5. Enhancing the sensitivity of an atom interferometer to the Heisenberg limit using increased quantum noise

   https://doi.org/10.1364/JOSAB.396358  

   Fang, Renpeng ; Sarkar, Resham ; Shahriar, Selim M. ( June 2020 , Journal of the Optical Society of America B)

   In a conventional atomic interferometer employing$N$atoms, the phase sensitivity is at the standard quantum limit:$1/\sqrt{N}$. Under usual spin squeezing, the sensitivity is increased by lowering the quantum noise. It is also possible to increase the sensitivity by leaving the quantum noise unchanged while producing phase amplification. Here we show how to increase the sensitivity, to the Heisenberg limit of$1/N$, while increasing the quantum noise by$\sqrt{N}$and amplifying the phase by a factor of$N$. Because of the enhancement of the quantum noise and the large phase magnification, the effect of excess noise is highly suppressed. The protocol uses a Schrödinger cat state representing a maximally entangled superposition of two collective states of$N$atoms. The phase magnification occurs when we use either atomic state detection or collective state detection; however, the robustness against excess noise occurs only when atomic state detection is employed. We show that for one version of the protocol, the signal amplitude is$N$when$N$is even, and is vanishingly small when$N$is odd, for both types of detection. We also show how the protocol can be modified to reverse the nature of the signal for odd versus even values of$N$. Thus, formore »a situation where the probability of$N$being even or odd is equal, the net sensitivity is within a factor of$\sqrt{2}$of the Heisenberg limit. Finally, we discuss potential experimental constraints for implementing this scheme via one-axis-twist squeezing employing the cavity feedback scheme, and show that the effects of cavity decay and spontaneous emission are highly suppressed because of the increased quantum noise and the large phase magnification inherent to the protocol. As a result, we find that the maximum improvement in sensitivity can be close to the ideal limit for as many as 10 million atoms.

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