The fabrication of ultrasmall high-quality intrinsic Josephson junctions (IJJs) using a cuprate superconductor is critical for the realization of a qubit. We investigated the mechanism of damage induced by a Ga + beam in a Bi 2 Sr 2 CaCu 2 O 8+δ IJJ during focused ion beam (FIB) processing. On the basis of the results, we developed a process that allowed the successful fabrication of an ultrasmall and high-quality IJJ. The damage induced by the FIB is reduced by restricting the direction of the Ga + beam so that the junction area is not directly irradiated by the ion beam. The fabricated ultrasmall IJJ device has a junction area of 0.3 µm 2 and shows excellent current–voltage characteristics.
This paper presents a compact, single-ended high-temperature superconducting (HTS) helium-focused ion beam (He-FIB) Josephson junction (JJ) mixer. The planar circuit is fabricated from a thin film layer of YBa2Cu3O7-xdeposited on a 10 mm × 10 mm Lanthanum Strontium Aluminium Tantalate (LSAT) substrate. A comprehensive six-step microstrip design methodology is outlined, detailing the JJ's physical geometries, high frequency and DC grounding, impedance matching, and filtering networks. The complete microstrip topology is designed to achieve an approximate radio frequency (RF) input passband between 9.9 GHz to 10.6 GHz with high out-of-band isolation, a single local oscillator (LO) resonance at 11.54 GHz and an intermediate frequency (IF) lowpass cut-off at 2.3 GHz. The proposed RF impedance matching network is uniquely designed to mitigate the significant performance limitation associated with normal resistances ($\rm{R}_\rm{n}$) scatter observed in HTS JJs. By implementing the proposed design methodology, a stable percentage bandwidth andS11≤-10 dB is maintained between${\mathit{R}}_\rm{n} =5\Omegato10\Omega,whichcoincideswiththefabricationvarianceforHe−FIBJJs.TheHTSmixerismanufacturedanditsDCandhighfrequencybehavioursarecharacterisedat6K,10K,20K,30K,and40K.Themeasured{\mathit{R}}_\rm{n}is9.77\Omega,whichsignificantlydeviatedfromthepredictedvalueof5\Omega$, emphasising the importance of the adaptability and versatility of the implemented impedance matching network. Exposed to the lowest measured temperature, the HTS mixer exhibited a maximum conversion gain of -4.93 dB which demonstrates the potential capabilities in He-FIB induced barrier JJs on LSAT substrates as frequency mixers.
Superinductances are superconducting circuit elements that combine a large inductance with a low parasitic capacitance to ground, resulting in a characteristic impedance exceeding the resistance quantum RQ=h/(2e)2≃6.45kΩ. In recent years, these components have become key enablers for emerging quantum circuit architectures. However, achieving high characteristic impedance while maintaining scalability and fabrication robustness remains a major challenge. In this work, we present two fabrication techniques for realizing superinductances based on vertically stacked Josephson junctions. Using a multi-angle Manhattan (MAM) process and a zero-angle (ZA) evaporation technique -- in which junction stacks are connected pairwise using airbridges -- we fabricate one-dimensional chains of stacks that act as high-impedance superconducting transmission lines. Two-tone microwave spectroscopy reveals the expected n scaling of the impedance with the number of junctions per stack. The chain fabricated using the ZA process, with nine junctions per stack, achieves a characteristic impedance of ∼16kΩ, a total inductance of 5.9μH, and a maximum frequency-dependent impedance of 50kΩ at 1.4 GHz. Our results establish junction stacking as a scalable, robust, and flexible platform for next-generation quantum circuits requiring ultra-high impedance environments.