Paul Varosy

Active isolation of transmon readout with a tunable inductive bridge

  1. Chunyang Ding,
  2. Chuyao Tong,
  3. Paul Varosy,
  4. Nikitha Jain,
  5. Ziyi Zhao,
  6. Eric I. Rosenthal,
  7. Konrad W. Lehnert,
  8. and David I. Schuster
Scalable superconducting quantum computation requires fast, high-fidelity readout. Currently, this is assisted through cryogenic amplification of the readout signal. In traditional
dispersive readout schemes, the amplifiers that are used to provide high signal-to-noise ratio also generate thermal noise that can decohere the qubits, demanding additional ferrite-based isolators to suppress this noise. Here, we integrate a chip-scale microwave switch, the tunable inductive bridge, into the readout chain of a fixed-frequency transmon. Flux-pulsing the switch, such that it transmits only during the readout, did not make a discernable difference to qubit coherence, residual excited-state population, or readout fidelity metrics. By injecting broadband noise that mimics amplifier thermal photons, we show that the switch suppresses the resulting qubit dephasing by more than 30~dB, consistent with its independently measured on-off ratio, establishing it as a potential chip-based integrated replacement for readout-chain isolators.
arxiv pdf

Error-divisible two-qubit gates

  1. David Rodriguez Perez,
  2. Paul Varosy,
  3. Ziqian Li,
  4. Tanay Roy,
  5. Eliot Kapit,
  6. and David Schuster
We introduce a simple, widely applicable formalism for designing „error-divisible“ two qubit gates: a quantum gate set where fractional rotations have proportionally reduced
error compared to the full entangling gate. In current noisy intermediate-scale quantum (NISQ) algorithms, performance is largely constrained by error proliferation at high circuit depths, of which two-qubit gate error is generally the dominant contribution. Further, in many hardware implementations, arbitrary two qubit rotations must be composed from multiple two-qubit stock gates, further increasing error. This work introduces a set of criteria, and example waveforms and protocols to satisfy them, using superconducting qubits with tunable couplers for constructing continuous gate sets with significantly reduced error for small-angle rotations. If implemented at scale, NISQ algorithm performance would be significantly improved by our error-divisible gate protocols.
arxiv pdf