J. Shi

Characterization of multi-level dynamics and decoherence in a high-anharmonicity capacitively shunted flux circuit

  1. M. A. Yurtalan,
  2. J. Shi,
  3. G.J.K. Flatt,
  4. and A. Lupascu
We present the design and characterization of a three-Josephson-junction superconducting loop circuit with three large shunt capacitors. The circuit used as a qubit shows long energy
relaxation times, of the order of 40 μs, and a spin-echo dephasing time of 9.4 μs. The circuit has high anharmonicity, of 2π×3.69 GHz. We extract the multi-level relaxation and dephasing rates of the circuit used as a qutrit and discuss the possible sources for the decoherence. The high anharmonicity allows for fast qubit control with 99.92% average gate fidelity, characterized by randomized benchmarking. These results demonstrate interesting potential use for fast nanosecond time scale two-qubit gates and multi-level quantum logic
arxiv pdf

Implementation of a Walsh-Hadamard gate in a superconducting qutrit

  1. M. A. Yurtalan,
  2. J. Shi,
  3. M. Kononenko,
  4. A. Lupascu,
  5. and S. Ashhab
We have implemented a Walsh-Hadamard gate, which performs a quantum Fourier transform, in a superconducting qutrit. The qutrit is encoded in the lowest three energy levels of a capacitively
shunted flux device, operated at the optimal flux-symmetry point. We use an efficient decomposition of the Walsh-Hadamard gate into two unitaries, generated by off-diagonal and diagonal Hamiltonians respectively. The gate implementation utilizes simultaneous driving of all three transitions between the three pairs of energy levels of the qutrit, one of which is implemented with a two-photon process. The gate has a duration of 35 ns and an average fidelity over a representative set of states, including preparation and tomography errors, of 99.2%, characterized with quantum state tomography. Compensation of ac-Stark and Bloch-Siegert shifts is essential for reaching high gate fidelities.
arxiv pdf