Andras Gyenis

Experimental realization of an intrinsically error-protected superconducting qubit

  1. Andras Gyenis,
  2. Pranav S. Mundada,
  3. Agustin Di Paolo,
  4. Thomas M. Hazard,
  5. Xinyuan You,
  6. David I. Schuster,
  7. Jens Koch,
  8. Alexandre Blais,
  9. and Andrew A. Houck
Encoding a qubit in logical quantum states with wavefunctions characterized by disjoint support and robust energies can offer simultaneous protection against relaxation and pure dephasing.
Using a circuit-quantum-electrodynamics architecture, we experimentally realize a superconducting 0−π qubit, which hosts protected states suitable for quantum-information processing. Multi-tone spectroscopy measurements reveal the energy level structure of the system, which can be precisely described by a simple two-mode Hamiltonian. We find that the parity symmetry of the qubit results in charge-insensitive levels connecting the protected states, allowing for logical operations. The measured relaxation (1.6 ms) and dephasing times (25 μs) demonstrate that our implementation of the 0−π circuit not only broadens the family of superconducting qubits, but also represents a promising candidate for the building block of a fault-tolerant quantum processor.
arxiv pdf

Universal gates for protected superconducting qubits using optimal control

  1. Mohamed Abdelhafez,
  2. Brian Baker,
  3. Andras Gyenis,
  4. Pranav Mundada,
  5. Andrew A. Houck,
  6. David Schuster,
  7. and Jens Koch
We employ quantum optimal control theory to realize quantum gates for two protected superconducting circuits: the heavy-fluxonium qubit and the 0-π qubit. Utilizing automatic differentiation
facilitates the simultaneous inclusion of multiple optimization targets, allowing one to obtain high-fidelity gates with realistic pulse shapes. For both qubits, disjoint support of low-lying wave functions prevents direct population transfer between the computational-basis states. Instead, optimal control favors dynamics involving higher-lying levels, effectively lifting the protection for a fraction of the gate duration. For the 0-π qubit, offset-charge dependence of matrix elements among higher levels poses an additional challenge for gate protocols. To mitigate this issue, we randomize the offset charge during the optimization process, steering the system towards pulse shapes insensitive to charge variations. Closed-system fidelities obtained are 99% or higher, and show slight reductions in open-system simulations.
arxiv pdf