Volodymyr V. Sivak

Quantum Error Correction of Qudits Beyond Break-even

  1. Benjamin L. Brock,
  2. Shraddha Singh,
  3. Alec Eickbusch,
  4. Volodymyr V. Sivak,
  5. Andy Z. Ding,
  6. Luigi Frunzio,
  7. Steven M. Girvin,
  8. and Michel H. Devoret
Hilbert space dimension is a key resource for quantum information processing. A large Hilbert space is not only an essential requirement for quantum error correction, but it can also
be advantageous for realizing gates and algorithms more efficiently. There has thus been considerable experimental effort in recent years to develop quantum computing platforms using qudits (d-dimensional quantum systems with d>2) as the fundamental unit of quantum information. Just as with qubits, quantum error correction of these qudits will be necessary in the long run, but to date error correction of logical qudits has not been demonstrated experimentally. Here we report the experimental realization of error-corrected logical qutrits (d=3) and ququarts (d=4) by employing the Gottesman-Kitaev-Preskill (GKP) bosonic code in a circuit QED architecture. Using a reinforcement learning agent, we optimize the GKP qutrit (ququart) as a ternary (quaternary) quantum memory and achieve beyond break-even error correction with a gain of 1.82 +/- 0.03 (1.87 +/- 0.03). This work represents a new way of leveraging the large Hilbert space of a harmonic oscillator for hardware-efficient quantum error correction.
arxiv pdf

Fully Directional Quantum-limited Phase-Preserving Amplifier

  1. Gangqiang Liu,
  2. Andrew Lingenfelter,
  3. Vidul Joshi,
  4. Nicholas E. Frattini,
  5. Volodymyr V. Sivak,
  6. Shyam Shankar,
  7. and Michel H. Devoret
We present a way to achieve fully directional, quantum-limited phase-preserving amplification in a four-port, four-mode superconducting Josephson circuit by utilizing interference between
six parametric processes that couple all four modes. Full directionality, defined as the reverse isolation surpassing forward gain between the matched input and output ports of the amplifier, ensures its robustness against impedance mismatch that might be present at its output port during applications. Unlike existing directional phase-preserving amplifiers, both the minimal back-action and the quantum-limited added noise of this amplifier remains unaffected by noise incident on its output port. In addition, the matched input and output ports allow direct on-chip integration of these amplifiers with other circuit QED components, facilitating scaling up of superconducting quantum processors.
arxiv pdf