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1. Pump-efficient Josephson parametric amplifiers with high saturation power

   https://doi.org/10.1103/PhysRevA.111.022611  

   Hougland, Nicholas M; Li, Zhuan; Kaufman, Ryan; Mesits, Boris; Mong, Roger_S K; Hatridge, Michael; Pekker, David (February 2025, Physical Review A)

   Circuit quantum-electrodynamics-based quantum information processing relies on low noise amplification for signal readout. In the realm of microwave superconducting circuits, this amplification is often achieved via Josephson parametric amplifiers (JPAs). In the past, these amplifiers exhibited low power added efficiency (PAE), which is roughly the fraction of pump power that is converted to output signal power. This is increasingly relevant because recent attempts to build high saturation power amplifiers achieve this at the cost of very low PAE, which in turn puts a high heat load on the cryostat and limits the number of these devices that a dilution refrigerator can host. Here, we numerically investigate upper bounds on PAE. We focus on a class of parametric amplifiers that consists of a capacitor shunted by a nonlinear inductive block. We first set a benchmark for this class of amplifiers by considering nonlinear blocks described by an arbitrary polynomial current-phase relation. Next, we identify that it is important for amplifiers with inductive blocks composed of repeating elements to have monotonic current-phase relations for each element in order to avoid exciting high-frequency modes. Using this design rule, we propose two circuit implementations for repeating elements in JPA inductive blocks. Finally, we investigate polynomial amplifier chains. We find that while amplifiers with higher gain have a lower PAE, regardless of the gain there is considerable room to improve as compared with state-of-the-art devices. For example, for a degenerate amplifier with a power gain of 20 dB, the PAE is ≈0.1% for typical JPAs, 37.9% for our simpler circuit JPAs, 42.6% for our more complex circuit JPAs, 63.3% for our arbitrary polynomial amplifiers, and at least 98% for our amplifier chains. 

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2. Generalized Class-J Theory

   https://doi.org/10.1109/LAMC.2018.8699065  

   Roblin, Patrick; Chang, Hsiu-Chen; Gomez-Perez, Jose-Maria; Martinez-Lopez, Jose I. (December 2018, 2018 IEEE MTT-S Latin America Microwave Conference (LAMC 2018))

   Class J has introduced a new rigorous framework for designing broadband PAs. However, the design of broadband Class-J amplifiers remains challenging, as the fundamental impedance termination is non-Foster when the 2 nd harmonic is Foster. In addition the peak instantaneous voltage varies by up to 50% as the frequency changes. In this paper, a generalized Class-J theory is introduced. Then a DC tunable broadband PA with constant instantaneous peak voltage and Foster fundamental and harmonic terminations is proposed. 

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3. Broadband subwavelength imaging of flexural elastic waves in flat phononic crystal lenses

   https://doi.org/10.1038/s41598-023-34314-5  

   Danawe, Hrishikesh; Tol, Serife (December 2023, Scientific Reports)

   Abstract Subwavelength imaging of elastic/acoustic waves using phononic crystals (PCs) is limited to a narrow frequency range via the two existing mechanisms that utilize either the intense Bragg scattering in the first phonon band or negative effective properties (left-handed material) in the second (or higher) phonon band. In the first phonon band, the imaging phenomenon can only exist at frequencies closer to the first Bragg band gap where the equal frequency contours (EFCs) are convex. Whereas, for the left-handed materials, the subwavelength imaging is restricted to a narrow frequency region where wave vectors in PC and background material are close to each other, which is essential for single-point image formation. In this work, we propose a PC lens for broadband subwavelength imaging of flexural waves in plates exploiting the second phonon band and the anisotropy of a PC lattice for the first time. Using a square lattice design with square-shaped EFCs, we enable the group velocity vector to always be perpendicular to the lens interface irrespective of the frequency and incidence angle; thus, resulting in a broadband imaging capability. We numerically and experimentally demonstrate subwavelength imaging using this concept over a significantly broadband frequency range. 

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4. Discharge Directionality and Dominance of Right-Handed Modes in Helicon Plasmas due to Radial Electron Density Gradients

   Granetzny, Marcel; Zepp, Michael; Schmitz, Oliver (December 2022, arXivorg)

   Helicon waves are magnetized plasma waves, similar to whistler waves in Earth's ionosphere, that are used to create high-density laboratory plasmas. We demonstrate that the discharge direction can be reversed by changing the antenna helicity or the magnetic field direction. Simulations reproduce these findings if a radial density gradient exists. A helicon wave equation that includes such a density gradient gives rise to a modulating magnetic field that amplifies right-handed but attenuates left-handed helicon modes. This explains for the first time consistently the dominance of right-handed over left-handed modes and the discharge directionality in helicon plasmas. 

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5. Integrating planar circuits with superconducting 3D microwave cavities using tunable low-loss couplers

   https://doi.org/10.1063/5.0153936  

   Zhao, Ziyi; Gurra, Eva; Rosenthal, Eric I; Vale, Leila R; Hilton, Gene C; Lehnert, K W (July 2023, Applied Physics Letters)

   We design and test a low-loss interface between superconducting three-dimensional microwave cavities and two-dimensional circuits, where the coupling rate is highly tunable. This interface seamlessly integrates a loop antenna and a Josephson junction-based coupling element. We demonstrate that the loss added by connecting this interface to the cavity is 1.28 kHz, corresponding to an inverse quality factor of 1/(4.5×106). Furthermore, we show that the cavity's external coupling rate to a 50 Ω transmission line can be tuned from negligibly small to over 3 orders of magnitude larger than its internal loss rate in a characteristic time of 3.2 ns. This switching speed does not impose additional limits on the coupling rate because it is much faster than the coupling rate. Moreover, the coupler can be controlled by low frequency signals to avoid interference with microwave signals near the cavity or qubit frequencies. Finally, the coupling element introduces a 0.04 Hz/photon self-Kerr nonlinearity to the cavity, remaining linear in high photon number operations. 

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