Impedance-Engineered Josephson Parametric Amplifier with Single-Step Lithography

Abstract

We present the experimental demonstration of an impedance-engineered Josephson parametric amplifier (IEJPA) fabricated in a single-step lithography process. Impedance engineering is implemented using a lumped-element series LC circuit. We use a simpler lithography process where the entire device -- impedance transformer and JPA -- are patterned in a single electron beam lithography step, followed by a double-angle Dolan bridge technique for Al-AlO$_x$-Al deposition. We observe amplification with 18 dB gain over a wide $400\,$MHz bandwidth centered around $5.3$GHz with added noise approaching the quantum limit, and a saturation power of $-114$dBm. To accurately explain our experimental results, we extend existing theories for impedance-engineered JPAs to incorporate the full sine nonlinearity of both the JPA and the transformer. Our work shows a path to simpler realization of broadband JPAs and provides a theoretical foundation for a regime of JPA operation that has been less explored in literature.

Figures (4)

• Figure 1: False-colored optical micrograph of an Impedance-engineered JPA. (a) Micrograph of the full IEJPA. From the left to right, the green colored component is the input pad, the blue colored components make up the impedance transformer, and the yellow colored elements make up the JPA. (b) Zoomed-in micrograph of the JJ array of the transformer. (c) Zoomed-in micrograph of the SQUID in the JPA.
• Figure 2: Measured (a) gain, (b) added noise, and (c) 1dB compression point for the IEJPA operated at a pump frequency and pump power of $5.342\,$GHz and $-88\,$dBm respectively. In the plot (b), the solid line represents the estimated $n_\text{add}$ while the dashed line represents the standard quantum limit of $0.5$ photons for a quantum limited amplifier.
• Figure 3: Equivalent circuit diagram of the device.
• Figure 4: Comparision of experimentally measured gain with different theoretical models of the IEJPA gain for $\omega_p = 2\pi\times 5.347\,$GHz. Here, the blue solid line shows the experimentally measured gain. The non-solid lines all represent gain obtained from different models of the pumped IEJPA. The yellow dash-dotted line shows the gain obtained by considering the circuit in Fig. \ref{['fig:eq_circuit']} without the transformer, but considering the full nonlinearity of the JPA junctions. The red dashed line represents the gain calculated for the IEJPA while considering only the quartic nonlinearities of the pumped junctions. Finally, the green dotted line shows the gain calculated for the pumped IEJPA while considering the full pumped junction nonlinearities up to the rotating wave approximation.