Randomized benchmarking of a high-fidelity remote CNOT gate over a meter-scale microwave interconnect

  1. Kentaro Heya,
  2. Timothy Phung,
  3. Moein Malekakhlagh,
  4. Rachel Steiner,
  5. Marco Turchetti,
  6. William Shanks,
  7. John Mamin,
  8. Wen-Sen Lu,
  9. Yadav Prasad Kandel,
  10. Neereja Sundaresan,
  11. and Jason Orcutt
In the modular superconducting quantum processor architecture, high-fidelity, meter-scale microwave interconnect between processor modules is a key technology for extending system size
beyond constraints imposed by device manufacturing equipment, yield, and signal delivery. While there have been many demonstrations of remote state transfer between modules, these relied on tomographic experiments for benchmarking, but this technique does not reliably separate State Preparation And Measurement (SPAM) error from error per state transfer. Recent developments based on randomized benchmarking provide a compatible theory for separating these two errors. In this work, we present a module-to-module interconnect based on Tunable-Coupling Qubits (TCQs) and benchmark, in a SPAM error tolerant manner, a remote state transfer fidelity of 0.988 across a 60cm long coplanar waveguide (CPW). The state transfer is implemented via superadiabatic transitionless driving method, which suppresses intermediate excitation in internal modes of CPW. We also introduce the frame tracking technique to correct unintended qubit phase rotations before and after the state transfers, which enables the SPAM-error-tolerant benchmarking of the state transfers. We further propose and construct a remote CNOT gate between modules, composed of local CZ gates in each module and remote state transfers, and report a high gate fidelity of 0.933 using randomized benchmarking method. The remote CNOT construction and benchmarking we present is a more complete metric that fully characterizes the module to module link operation going forward as it more closely represents interconnect operation in a circuit.

Reducing unitary and spectator errors in cross resonance with optimized rotary echoes

  1. Neereja Sundaresan,
  2. Isaac Lauer,
  3. Emily Pritchett,
  4. Easwar Magesan,
  5. Petar Jurcevic,
  6. and Jay M. Gambetta
We present an improvement to the cross resonance gate realized with the addition of resonant, target rotary pulses. These pulses, applied directly to the target qubit, are simultaneous
to and in phase with the echoed cross resonance pulses. Using specialized Hamiltonian error amplifying tomography, we confirm a reduction of error terms with target rotary — directly translating to improved two-qubit gate fidelity. Beyond improvement in the control-target subspace, the target rotary reduces entanglement between target and target spectators caused by residual quantum interactions. We further characterize multi-qubit performance improvement enabled by target rotary pulsing using unitarity benchmarking and quantum volume measurements, achieving a new record quantum volume for a superconducting qubit system.