Harshvardhan K. Babla

Dual-rail encoding with superconducting cavities

  1. James D. Teoh,
  2. Patrick Winkel,
  3. Harshvardhan K. Babla,
  4. Benjamin J. Chapman,
  5. Jahan Claes,
  6. Stijn J. de Graaf,
  7. John W.O. Garmon,
  8. William D. Kalfus,
  9. Yao Lu,
  10. Aniket Maiti,
  11. Kaavya Sahay,
  12. Neel Thakur,
  13. Takahiro Tsunoda,
  14. Sophia H. Xue,
  15. Luigi Frunzio,
  16. Steven M. Girvin,
  17. Shruti Puri,
  18. and Robert J. Schoelkopf
The design of quantum hardware that reduces and mitigates errors is essential for practical quantum error correction (QEC) and useful quantum computations. To this end, we introduce
the circuit-QED dual-rail qubit in which our physical qubit is encoded in the single-photon subspace of two superconducting cavities. The dominant photon loss errors can be detected and converted into erasure errors, which are much easier to correct. In contrast to linear optics, a circuit-QED implementation of the dual-rail code offers completely new capabilities. Using a single transmon ancilla, we describe a universal gate set that includes state preparation, logical readout, and parametrizable single and two-qubit gates. Moreover, first-order hardware errors due to the cavity and transmon in all of these operations can be detected and converted to erasure errors, leaving background Pauli errors that are orders of magnitude smaller. Hence, the dual-rail cavity qubit delivers an optimal hierarchy of errors and rates, and is expected to be well below the relevant QEC thresholds with today’s devices.
arxiv pdf

New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds

  1. Alex P. M. Place,
  2. Lila V. H. Rodgers,
  3. Pranav Mundada,
  4. Basil M. Smitham,
  5. Mattias Fitzpatrick,
  6. Zhaoqi Leng,
  7. Anjali Premkumar,
  8. Jacob Bryon,
  9. Sara Sussman,
  10. Guangming Cheng,
  11. Trisha Madhavan,
  12. Harshvardhan K. Babla,
  13. Berthold Jäck,
  14. Andras Gyenis,
  15. Nan Yao,
  16. Robert J. Cava,
  17. Nathalie P. de Leon,
  18. and Andrew A. Houck
The superconducting transmon qubit is a leading platform for quantum computing and quantum science. Building large, useful quantum systems based on transmon qubits will require significant
improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits imposed by bulk properties of the constituent materials. This indicates that relaxation likely originates from uncontrolled surfaces, interfaces, and contaminants. Previous efforts to improve qubit lifetimes have focused primarily on designs that minimize contributions from surfaces. However, significant improvements in the lifetime of two-dimensional transmon qubits have remained elusive for several years. Here, we fabricate two-dimensional transmon qubits that have both lifetimes and coherence times with dynamical decoupling exceeding 0.3 milliseconds by replacing niobium with tantalum in the device. We have observed increased lifetimes for seventeen devices, indicating that these material improvements are robust, paving the way for higher gate fidelities in multi-qubit processors.
arxiv pdf