F. Battistel

Logical-qubit operations in an error-detecting surface code

  1. J. F. Marques,
  2. B. M. Varbanov,
  3. M. S. Moreira,
  4. H. Ali,
  5. N. Muthusubramanian,
  6. C. Zachariadis,
  7. F. Battistel,
  8. M. Beekman,
  9. N. Haider,
  10. W. Vlothuizen,
  11. A. Bruno,
  12. B. M. Terhal,
  13. and L. DiCarlo
We realize a suite of logical operations on a distance-two logical qubit stabilized using repeated error detection cycles. Logical operations include initialization into arbitrary states,
measurement in the cardinal bases of the Bloch sphere, and a universal set of single-qubit gates. For each type of operation, we observe higher performance for fault-tolerant variants over non-fault-tolerant variants, and quantify the difference through detailed characterization. In particular, we demonstrate process tomography of logical gates, using the notion of a logical Pauli transfer matrix. This integration of high-fidelity logical operations with a scalable scheme for repeated stabilization is a milestone on the road to quantum error correction with higher-distance superconducting surface codes.
arxiv pdf

High-fidelity controlled-Z gate with maximal intermediate leakage operating at the speed limit in a superconducting quantum processor

  1. V. Negîrneac,
  2. H. Ali,
  3. N. Muthusubramanian,
  4. F. Battistel,
  5. R. Sagastizabal,
  6. M. S. Moreira,
  7. J. F. Marques,
  8. W. Vlothuizen,
  9. M. Beekman,
  10. N. Haider,
  11. A. Bruno,
  12. and L. DiCarlo
We introduce the sudden variant (SNZ) of the Net Zero scheme realizing controlled-Z (CZ) gates by baseband flux control of transmon frequency. SNZ CZ gates operate at the speed limit
of transverse coupling between computational and non-computational states by maximizing intermediate leakage. The key advantage of SNZ is tuneup simplicity, owing to the regular structure of conditional phase and leakage as a function of two control parameters. We realize SNZ CZ gates in a multi-transmon processor, achieving 99.93±0.24% fidelity and 0.10±0.02% leakage. SNZ is compatible with scalable schemes for quantum error correction and adaptable to generalized conditional-phase gates useful in intermediate-scale applications.
arxiv pdf

Leakage detection for a transmon-based surface code

  1. B. M. Varbanov,
  2. F. Battistel,
  3. B. M. Tarasinski,
  4. V.P. Ostroukh,
  5. T. E. O'Brien,
  6. L. DiCarlo,
  7. and B. M. Terhal
Leakage outside of the qubit computational subspace, present in many leading experimental platforms, constitutes a threatening error for quantum error correction (QEC) for qubits. We
develop a leakage-detection scheme via Hidden Markov models (HMMs) for transmon-based implementations of the surface code. By performing realistic density-matrix simulations of the distance-3 surface code (Surface-17), we observe that leakage is sharply projected and leads to an increase in the surface-code defect probability of neighboring stabilizers. Together with the analog readout of the ancilla qubits, this increase enables the accurate detection of the time and location of leakage. We restore the logical error rate below the memory break-even point by post-selecting out leakage, discarding about 47% of the data. Leakage detection via HMMs opens the prospect for near-term QEC demonstrations, targeted leakage reduction and leakage-aware decoding and is applicable to other experimental platforms.
arxiv pdf