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1. Non-Hermitian Nonlinear Optics without Gain and Loss

   https://doi.org/10.1364/NLO.2019.NM2B.5  

   Zhai, Yanhua; Jiang, Yue; Mei, Yefeng; Zuo, Ying; Du, Shengwang; Wen, Jianming (July 2019, Nonlinear Optics (NLO) OSA Technical Digest (Optical Society of America, 2019))

   We propose and demonstrate non-Hermitian but parity-time-symmetric four-wave mixing in cold atoms without linear gain and loss. Besides the occurrence of nontrivial phase transition, efficient nonlinear conversion without phase matching has also been observed. 

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2. Self phase-matched broadband amplification with a left-handed Josephson transmission line

   Kow, C; Podolskiy, V; Kamal, A (January 2022, ArXivorg)

   Josephson Traveling Wave Parametric Amplifiers (J-TWPAs) are promising platforms for realizing broadband quantum-limited amplification of microwave signals. However, substantial gain in such systems is attainable only when strict constraints on phase matching of the signal, idler and pump waves are satisfied -- this is rendered particularly challenging in the presence of nonlinear effects, such as self- and cross-phase modulation, which scale with the intensity of propagating signals. In this work, we present a simple J-TWPA design based on left-handed (negative-index) nonlinear Josephson metamaterial, which realizes autonomous phase matching without the need for any complicated circuit or dispersion engineering. The resultant efficiency of four-wave mixing process can implement gains in excess of 20 dB over few GHz bandwidths with much shorter lines than previous implementations. Furthermore, the autonomous nature of phase matching considerably simplifies the J-TWPA design than previous implementations based on right-handed (positive index) Josephson metamaterials, making the proposed architecture particularly appealing from a fabrication perspective. The left-handed JTL introduced here constitutes a new modality in distributed Josephson circuits, and forms a crucial piece of the unified framework that can be used to inform the optimal design and operation of broadband microwave amplifiers. 

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3. Anti-Parity-Time Symmetric Optical Four-Wave Mixing in Cold Atoms

   https://doi.org/https://doi.org/10.1103/PhysRevLett.123.193604  

   Jiang, Y.; Mei, Y.; Zuo, Y.; Zhai, Y.; Li, J.; Wen, J.; Du, S. (November 2019, Physical review letters)

   Non-Hermitian optical systems with parity-time (PT) symmetry have recently revealed many intriguing prospects that outperform conservative structures. The previous works are mostly rooted in complex arrangements with controlled gain-loss interplay. Here, we demonstrate anti-PT symmetry inherent in the nonlinear optical interaction based upon forward optical four-wave mixing in a laser-cooled atomic ensemble with negligible linear gain and loss. We observe that the pair of frequency modes undergo a nontrivial anti-PT phase transition between coherent power oscillation and optical parametric amplification in presence of a large phase mismatch. 

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4. Multidimensional four-wave mixing signals detected by quantum squeezed light

   https://doi.org/10.1073/pnas.2105601118  

   Dorfman, Konstantin; Liu, Shengshuai; Lou, Yanbo; Wei, Tianxiang; Jing, Jietai; Schlawin, Frank; Mukamel, Shaul (August 2021, Proceedings of the National Academy of Sciences)

   Four-wave mixing (FWM) of optical fields has been extensively used in quantum information processing, sensing, and memories. It also forms a basis for nonlinear spectroscopies such as transient grating, stimulated Raman, and photon echo where phase matching is used to select desired components of the third-order response of matter. Here we report an experimental study of the two-dimensional quantum noise intensity difference spectra of a pair of squeezed beams generated by FWM in hot Rb vapor. The measurement reveals details of the χ ( 3 ) susceptibility dressed by the strong pump field which induces an AC Stark shift, with higher spectral resolution compared to classical measurements of probe and conjugate beam intensities. We demonstrate how quantum correlations of squeezed light can be utilized as a spectroscopic tool which unlike their classical counterparts are robust to external noise. 

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5. Nanopatterned parametric oscillators for laser conversion beyond an octave

   https://doi.org/10.1364/OPTICA.545158  

   Brodnik, Grant M; Liu, Haixin; Carlson, David R; Black, Jennifer A; Papp, Scott B (January 2025, Optica)

   Many uses of lasers place the highest importance on access to specific wavelength bands. For example, mobilizing optical-atomic clocks for a leap in sensing requires compact lasers at frequencies spread across the visible and near-infrared. Integrated photonics enables high-performance, scalable laser platforms. However, customizing laser-gain media to support wholly new bands is challenging and often prohibitively mismatched in scalability to early quantum-based sensing and information systems. Here, we demonstrate a tantalum pentoxide microresonator optical-parametric oscillator (OPO) that converts a pump laser to an output wave within a frequency span exceeding an octave. We control phase matching for oscillation by nanopatterning the microresonator to open a photonic-crystal bandgap on the mode of the pump laser. The photonic crystal splits only the pump mode and preserves the broader mode structure of the resonator, thus affording a single parameter to control output waves across the octave span using a nearly fixed frequency pump laser. We also demonstrate tuning the oscillator in free-spectral-range steps, more finely with temperature, and minimal additive frequency noise of the laser-conversion process. Our work shows that nanophotonic structures offer control of laser conversion in microresonators, bridging phase-matching of nonlinear optics and application requirements for laser designs. 

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