Heikki Suominen

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

Multiplexed readout of ultrasensitive bolometers

  1. Priyank Singh,
  2. András Gunyhó,
  3. Heikki Suominen,
  4. Giacomo Catto,
  5. Florian Blanchet,
  6. Qi-Ming Chen,
  7. Arman Alizadeh,
  8. Aarne Keränen,
  9. Jian Ma,
  10. Timm Mörstedt,
  11. Wei Liu,
  12. and Mikko Möttonen
Recently, ultrasensitive calorimeters have been proposed as a resource-efficient solution for multiplexed qubit readout in superconducting large-scale quantum processors. However, experiments
demonstrating frequency multiplexing of these superconductor-normal conductor-superconductor (SNS) sensors are coarse. To this end, we present the design, fabrication, and operation of three SNS sensors with frequency-multiplexed input and probe circuits, all on a single chip. These devices have their probe frequencies in the range \SI{150}{\mega\hertz} — \SI{200}{\mega\hertz}, which is well detuned from the heater frequencies of \SI{4.4}{\giga\hertz} — \SI{7.6}{\giga\hertz} compatible with typical readout frequencies of superconducting qubits. Importantly, we show on-demand triggering of both individual and multiple low-noise SNS bolometers with very low cross talk. These experiments pave the way for multiplexed bolometric characterization and calorimetric readout of multiple qubits, a promising step in minimizing related resources such as the number of readout lines and microwave isolators in large-scale superconducting quantum computers.
arxiv pdf