N. Anders Petersson

Noise-specific beats in the higher-level Ramsey curves of a transmon qubit

  1. L.A. Martinez,
  2. Z. Peng,
  3. D. Appelö,
  4. D. M. Tennant,
  5. N. Anders Petersson,
  6. J. L. DuBois,
  7. and Y. J. Rosen
In the higher levels of superconducting transmon devices, and more generally charge sensitive devices, T∗2 measurements made in the presence of low-frequency time-correlated 1/f charge
noise and quasiparticle-induced parity flips can give an underestimation of the total dephasing time. The charge variations manifest as beating patterns observed in the overlay of several Ramsey fringe curves, and are reproduced with a phenomenological Ramsey curve model which accounts for the charge variations. T∗2 dephasing times which more accurately represent the total dephasing time are obtained. The phenomenological model is compared with a Lindblad master equation model. Both models are found to be in agreement with one another and the experimental data. Finally, the phenomenological formulation enables a simple method in which the power spectral density (PSD) for the low-frequency noise can be inferred from the overlay of several Ramsey curves.
arxiv pdf

High-fidelity software-defined quantum logic on a superconducting qudit

  1. Xian Wu,
  2. S.L. Tomarken,
  3. N. Anders Petersson,
  4. L.A. Martinez,
  5. Yaniv J. Rosen,
  6. and Jonathan L DuBois
Nearly all modern solid-state quantum processors approach quantum computation with a set of discrete qubit operations (gates) that can achieve universal quantum control with only a
handful of primitive gates. In principle, this approach is highly flexible, allowing full control over the qubits‘ Hilbert space without necessitating the development of specific control protocols for each application. However, current error rates on quantum hardware place harsh limits on the number of primitive gates that can be concatenated together (with compounding error rates) and remain viable. Here, we report our efforts at implementing a software-defined 0↔2 SWAP gate that does not rely on a primitive gate set and achieves an average gate fidelity of 99.4%. Our work represents an alternative, fully generalizable route towards achieving nontrivial quantum control through the use of optimal control techniques. We describe our procedure for computing optimal control solutions, calibrating the quantum and classical hardware chain, and characterizing the fidelity of the optimal control gate.
arxiv pdf