George B. Long

Echo Cross Resonance gate error budgeting on a superconducting quantum processor

  1. Travers Ward,
  2. Russell P. Rundle,
  3. Richard Bounds,
  4. Norbert Deak,
  5. Gavin Dold,
  6. Apoorva Hegde,
  7. William Howard,
  8. Ailsa Keyser,
  9. George B. Long,
  10. Benjamin Rogers,
  11. Jonathan J. Burnett,
  12. and Bryn A. Bell
High fidelity quantum operations are key to enabling fault-tolerant quantum computation. Superconducting quantum processors have demonstrated high-fidelity operations, but on larger
devices there is commonly a broad distribution of qualities, with the low-performing tail affecting near-term performance and applications. Here we present an error budgeting procedure for the native two-qubit operation on a 32-qubit superconducting-qubit-based quantum computer, the OQC Toshiko gen-1 system. We estimate the prevalence of different forms of error such as coherent error and control qubit leakage, then apply error suppression strategies based on the most significant sources of error, making use of pulse-shaping and additional compensating gates. These techniques require no additional hardware overhead and little additional calibration, making them suitable for routine adoption. An average reduction of 3.7x in error rate for two qubit operations is shown across a chain of 16 qubits, with the median error rate improving from 4.6% to 1.2% as measured by interleaved randomized benchmarking. The largest improvements are seen on previously under-performing qubit pairs, demonstrating the importance of practical error suppression in reducing the low-performing tail of gate qualities and achieving consistently good performance across a device.
arxiv pdf

Capturing Complex Behaviour in Josephson Travelling Wave Parametric Amplifiers

  1. Tom Dixon,
  2. Jacob W. Dunstan,
  3. George B. Long,
  4. Jonathan M. Williams,
  5. Phil J. Meeson,
  6. and Connor D. Shelly
We present an analysis of wave-mixing in recently developed Josephson Travelling Wave Parametric Amplifiers (JTWPAs). Circuit simulations performed using WRspice show the full behaviour
of the JTWPA allowing propagation of all tones. The Coupled Mode Equations (CMEs) containing only pump, signal, and idler propagation are shown to be insufficient to completely capture complex mixing behaviour in the JTWPA. Extension of the CMEs through additional state vectors in the analytic solutions allows closer agreement with WRspice. We consider an ordered framework for the systematic inclusion of extended eigenmodes and make a qualitative comparison with WRspice at each step. The agreement between the two methods validates both approaches and provides insight into the operation of the JTWPA. We show that care should be taken when using the CMEs and propose that WRspice should be used as a design tool for non-linear superconducting circuits such as the JTWPA.
arxiv pdf