Ants Remm

Entanglement Stabilization using Parity Detection and Real-Time Feedback in Superconducting Circuits

  1. Christian Kraglund Andersen,
  2. Ants Remm,
  3. Stefania Balasiu,
  4. Sebastian Krinner,
  5. Johannes Heinsoo,
  6. Jean-Claude Besse,
  7. Mihai Gabureac,
  8. Andreas Wallraff,
  9. and Christopher Eichler
Fault tolerant quantum computing relies on the ability to detect and correct errors, which in quantum error correction codes is typically achieved by projectively measuring multi-qubit
parity operators and by conditioning operations on the observed error syndromes. Here, we experimentally demonstrate the use of an ancillary qubit to repeatedly measure the ZZ and XX parity operators of two data qubits and to thereby project their joint state into the respective parity subspaces. By applying feedback operations conditioned on the outcomes of individual parity measurements, we demonstrate the real-time stabilization of a Bell state with a fidelity of F≈74% in up to 12 cycles of the feedback loop. We also perform the protocol using Pauli frame updating and, in contrast to the case of real-time stabilization, observe a steady decrease in fidelity from cycle to cycle. The ability to stabilize parity over multiple feedback rounds with no reduction in fidelity provides strong evidence for the feasibility of executing stabilizer codes on timescales much longer than the intrinsic coherence times of the constituent qubits.
arxiv pdf

Rapid high-fidelity multiplexed readout of superconducting qubits

  1. Johannes Heinsoo,
  2. Christian Kraglund Andersen,
  3. Ants Remm,
  4. Sebastian Krinner,
  5. Theodore Walter,
  6. Yves Salathé,
  7. Simone Gasperinetti,
  8. Jean-Claude Besse,
  9. Anton Potočnik,
  10. Christopher Eichler,
  11. and Andreas Wallraff
The duration and fidelity of qubit readout is a critical factor for applications in quantum information processing as it limits the fidelity of algorithms which reuse qubits after measurement
or apply feedback based on the measurement result. Here we present fast multiplexed readout of five qubits in a single 1.2 GHz wide readout channel. Using a readout pulse length of 80 ns and populating readout resonators for less than 250 ns we find an average correct assignment probability for the five measured qubits to be 97%. The differences between the individual readout errors and those found when measuring the qubits simultaneously are within 1%. We employ individual Purcell filters for each readout resonator to suppress off-resonant driving, which we characterize by the dephasing imposed on unintentionally measured qubits. We expect the here presented readout scheme to become particularly useful for the selective readout of individual qubits in multi-qubit quantum processors.
arxiv pdf