Mikko Tuokkola

Optimal Quantum-Circuit Synthesis and Implementation of General Unitaries with Parallelized Bitstring-Conditioned Rotations

  1. Heikki Suominen,
  2. Suman Kundu,
  3. Heidi Kivijärvi,
  4. Sasu Tuohino,
  5. Mikko Tuokkola,
  6. Eemeli Forsbom,
  7. Madhavi Chand,
  8. Vasilii Vadimov,
  9. R. Vijay,
  10. and Mikko Möttönen
We theoretically introduce a quantum-circuit synthesis protocol to realize any n-qubit transformation U∈U(2n), up to a diagonal. The resulting circuit is directly native to and optimal
in gate count and depth for systems where bitstring-conditioned rotations with tunable angle and axis, C(n−1)R(θ,ϕ), form the set of native operations. Subsequently, we suggest how this protocol can be implemented in a few-mode superconducting circuit, and experimentally demonstrate the method for the smallest nontrivial case, i.e., for two qubits. We report an average gate fidelity of 97.6 % for Haar-random two-qubit unitaries up to phase shifts in the computational basis. Importantly, these results suggest that multimode devices, such as those in the multimon or multiunimon families may become competitive with qubit-array networks for general-unitary implementations for qubit counts as low as three.
arxiv pdf

Methods to achieve near-millisecond energy relaxation and dephasing times for a superconducting transmon qubit

  1. Mikko Tuokkola,
  2. Yoshiki Sunada,
  3. Heidi Kivijärvi,
  4. Leif Grönberg,
  5. Jukka-Pekka Kaikkonen,
  6. Visa Vesterinen,
  7. Joonas Govenius,
  8. and Mikko Möttönen
Superconducting qubits are one of the most promising physical systems for implementing a quantum computer. However, executing quantum algorithms of practical computational advantage
requires further improvements in the fidelities of qubit operations, which are currently limited by the energy relaxation and dephasing times of the qubits. Here, we report our measurement results of a high-coherence transmon qubit with energy relaxation and echo dephasing times surpassing those in the existing literature. We measure a qubit frequency of 2.890 GHz, an energy relaxation time with a median of 502 us and a maximum of (765 +/- 82.6) us, and an echo dephasing time with a median of 541 us and a maximum of (1057 +/- 138) us. We report details of our design, fabrication process, and measurement setup to facilitate the reproduction and wide adoption of high-coherence transmon qubits in the academia and industry.
arxiv pdf