A persistent-current-biased and current-actuated switch for superconducting circuits

  1. Ziyi Zhao,
  2. Eva Gurra,
  3. Michael R. Vissers,
  4. and K. W. Lehnert
Broadband and low-loss superconducting switches can facilitate large-scale quantum information processors and cryogenic detectors by dynamically reconfiguring the connectivity of their
circuits. The time dependent connectivity is enabled by the nonlinearity of lossless Josephson junctions, which are often wired into superconducting loops to be controlled by magnetic flux. However, this approach needs a power-consuming constant flux bias and dynamic flux actuation, both of which are hard to isolate from other switches or flux sensitive elements, limiting their integration density. Here, we design and characterize a microwave switch that implements a persistent current bias and direct current actuation to reduce static power consumption, actuation energy and potential crosstalk to other devices. We show that persistent current associated with tens of flux quanta is stable and long-lived, reducing the need for on-the-fly tuning. We further demonstrate that our switch has desirable performance for superconducting-circuit-based quantum information processing, including an off mode with more than 20 dB isolation comparable to commercial ferrite isolators, power handling larger than 100 pW sufficient for resonator readout tones and amplifier pumps, and modulation bandwidth broader than 600 MHz useful for multiplexing schemes.

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

  1. Ziyi Zhao,
  2. Eva Gurra,
  3. Eric I. Rosenthal,
  4. Leila R. Vale,
  5. Gene C. Hilton,
  6. and K. W. Lehnert
We design and test a low-loss interface between superconducting 3-dimensional microwave cavities and 2-dimensional circuits, where the coupling rate is highly tunable. This interface
seamlessly integrates a magnetic antenna and a Josephson junction based coupling element with a cavity, and we demonstrate that the introduced loss from this integration only limits the quality factor to 4.5 million. The cavity external coupling rate can then be tuned from negligibly small to over 3 orders of magnitude larger than the internal loss rate with 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 baseband 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.