Investigating the performance of RPM JTWPAs by optimizing LC-resonator elements

TL;DR

This work analyzes RPM JTWPA performance by numerically optimizing parametrized resonator elements to maximize gain $G(x,\omega)=10 \log_{10}|u(x,\omega)|^2$ and quadrature squeezing $S_X(\omega)$. A quantized, strong-pump model is used, and loss is incorporated with a beam-splitter of transmittance $\sqrt{\eta}$, enabling assessment of ideal and lossy scenarios. In the lossless case, optimized $(C_c, C_r)$ significantly enhances peak gain and squeezing, with an optimal region around $C_r \approx 50$ pF and $C_c \approx 20$ fF, though bandwidth is traded off. Introducing lumped-element loss shows gain saturating with increasing loss while squeezing degrades rapidly, highlighting loss as the dominant barrier to squeezing improvements and the practical necessity of loss minimization for high-performance RPM JTWPA implementations.

Abstract

Resonant phase-matched Josephson traveling-wave parametric amplifiers (RPM JTWPAs) play a key role in quantum computing and quantum information applications due to their low-noise, broadband amplification, and quadrature squeezing capabilities. This research focuses on optimizing RPM JTWPAs through numerical optimization of parametrized resonator elements to maximize gain, bandwidth and quadrature squeezing. Our results show that optimized resonators can increase the maximum gain and squeezing by more than 5 dB in the ideal noiseless case. However, introducing the effects of loss through a lumped-element model reveals that gain saturates with increasing loss, while squeezing modes degrade rapidly, regardless of resonator optimization. These results highlight the potential of resonator design to significantly improve amplifier performance, as well as the challenges posed by current fabrication technologies and inherent losses.

Figures (7)

• Figure 1: Unit-cell diagram of an RPM JTWPA. Gray dots labeled by $\Phi_{n}$, $\Phi_{n+1}$ and $\Psi_n$ are node fluxes Devoret1995. The blue cross indicates the Josephson junction, $C_{0}$, $C_{c}$ and $C_{r}$ are the capacitances and $L_r$ is the resonator inductance. Gold color depicts the resonator's parameters which will be investigated here.
• Figure 2: Gain (a),(c) and absolute value of the quadrature squeezing (b) and (d) depending on the signal frequency.
• Figure 3: Gain (yellow line) and absolute value of the squeezing (black line with diamonds) at 5 GHz depending on: (a) the resonator capacitance for a fixed coupling capacitance $C_c=43.1\,$fF, and (b) the coupling capacitance for a fixed $C_r=11 \,$pF. These lines correspond to the same colored lines in Fig. \ref{['fig:GainOpt']}.
• Figure 4: (a) Gain and (b) squeezing absolute value at ${f=5\,}$GHz depending on $C_c$ and $C_r$.
• Figure 5: Bandwidth above 16 dB depending on $C_c$ and $C_r$ for (a) $G$ and (b) $|S_X|$.