Wireless Josephson parametric amplifier above 20 GHz

Abstract

Operating superconducting qubits at elevated temperatures offers increased cooling power and thus system scalability, but requires suppression of thermal photons to preserve coherence and readout fidelity. This motivates migration to higher operation frequencies, which demands high-frequency amplification with near-quantum-limited noise characteristics for qubit readout. Here, we report the design and experimental realization of a wireless Josephson parametric amplifier (WJPA) operating above 20~GHz. The wireless design eliminates losses and impedance mismatches that become problematic at high frequencies. The WJPA achieves more than 20~dB of gain across a tunable frequency range of 21--23.5~GHz, with a typical dynamic bandwidth of 3~MHz. Through Y-factor measurements and a qubit-based photon number calibration, we show that the amplifier exhibits an added noise of approximately two photons.

Figures (3)

• Figure 1: High-frequency WJPA device and package. (a) Photograph of the copper waveguide package with the sapphire chip mounted in one half of the rectangular cavity. (b) Close-up view of the assembled chip inside the copper package. (c) Optical image of the resonator region showing a lumped-element resonator consisting of an Al-AlO$_x$ JJ SQUID shunted by interdigitated finger capacitors, also coupled to the slotline ground planes (tantalum) via capacitors. (d) Electron micrograph showing the central SQUID loop. Arrows indicate the junctions. (e) Simplified circuit diagram of the WJPA.
• Figure 2: WJPA linear response and gain. (a) Reflection coefficient phase as a function of external flux $\varphi_\text{ext}$. (b) Extracted resonant frequencies ($f_\text{res}$, black), external and internal coupling rates ($\kappa_\text{ext}$ in blue and $\kappa_\text{int}$ in orange) versus external flux. (c) Reflection coefficient when operated as a parametric amplifier with the pump applied (red), compared to the un-pumped response (blue). (d) Gain as a function of probe power for various pump powers, with extracted $P_\text{1dB}$ points (circles). A linear fit of $P_\text{1dB}$ versus pump power in dBm is shown by the gray dashed line whose slope is about 0.6. (e) Example gain profiles at various fluxes demonstrating frequency tunability from 21 GHz to 23.5 GHz.
• Figure 3: WJPA noise performance. (a) Measured output noise power $P_{N,\mathrm{out}}$ as a function of spectrum analyzer (SA) frequency for various temperatures of the VTS (colorbar). The peak near the JPA center frequency corresponds to the pump tone seen on the spectrum analyzer. (b) WJPA gain measured by the VNA at corresponding temperatures of the VTS in part (a). (c) Extracted JPA-added noise $N_{\mathrm{add}}$ at the reference plane of the VTS. A blue bar marks the amplifier bandwidth of $7$ MHz. A dip near the JPA center frequency (gray area) corresponds to the pump tone. (d) Power spectrum referred to the input of the WJPA. The gray trace corresponds to the unpumped case (WJPA OFF). The blue trace shows the spectrum with the WJPA pump turned on. The central peak is the pump tone, and the adjacent upper side peak is the probe tone injected by a separate microwave generator while the lower side peak of the blue trace is the idler tone. The red dashed line indicates the standard quantum limit (SQL) at 22 GHz. Both traces were acquired with a spectrum analyzer resolution bandwidth of 4.7 kHz.