Neel Thakur

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

Error-detectable bosonic entangling gates with a noisy ancilla

  1. Takahiro Tsunoda,
  2. James D. Teoh,
  3. William D. Kalfus,
  4. Stijn J. de Graaf,
  5. Benjamin J. Chapman,
  6. Jacob C. Curtis,
  7. Neel Thakur,
  8. Steven M. Girvin,
  9. and Robert J. Schoelkopf
Bosonic quantum error correction has proven to be a successful approach for extending the coherence of quantum memories, but to execute deep quantum circuits, high-fidelity gates between
encoded qubits are needed. To that end, we present a family of error-detectable two-qubit gates for a variety of bosonic encodings. From a new geometric framework based on a „Bloch sphere“ of bosonic operators, we construct ZZL(θ) and eSWAP(θ) gates for the binomial, 4-legged cat, dual-rail and several other bosonic codes. The gate Hamiltonian is simple to engineer, requiring only a programmable beamsplitter between two bosonic qubits and an ancilla dispersively coupled to one qubit. This Hamiltonian can be realized in circuit QED hardware with ancilla transmons and microwave cavities. The proposed theoretical framework was developed for circuit QED but is generalizable to any platform that can effectively generate this Hamiltonian. Crucially, one can also detect first-order errors in the ancilla and the bosonic qubits during the gates. We show that this allows one to reach error-detected gate fidelities at the 10−4 level with today’s hardware, limited only by second-order hardware errors.
arxiv pdf