Benjamin L. Brock

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

Quantum Control of an Oscillator with a Kerr-cat Qubit

  1. Andy Z. Ding,
  2. Benjamin L. Brock,
  3. Alec Eickbusch,
  4. Akshay Koottandavida,
  5. Nicholas E. Frattini,
  6. Rodrigo G. Cortinas,
  7. Vidul R. Joshi,
  8. Stijn J. de Graaf,
  9. Benjamin J. Chapman,
  10. Suhas Ganjam,
  11. Luigi Frunzio,
  12. Robert J. Schoelkopf,
  13. and Michel H. Devoret
Bosonic codes offer a hardware-efficient strategy for quantum error correction by redundantly encoding quantum information in the large Hilbert space of a harmonic oscillator. However,
experimental realizations of these codes are often limited by ancilla errors propagating to the encoded logical qubit during syndrome measurements. The Kerr-cat qubit has been proposed as an ancilla for these codes due to its theoretically-exponential noise bias, which would enable fault-tolerant error syndrome measurements, but the coupling required to perform these syndrome measurements has not yet been demonstrated. In this work, we experimentally realize driven parametric coupling of a Kerr-cat qubit to a high-quality-factor microwave cavity and demonstrate a gate set enabling universal quantum control of the cavity. We measure the decoherence of the cavity in the presence of the Kerr-cat and discover excess dephasing due to heating of the Kerr-cat to excited states. By engineering frequency-selective dissipation to counteract this heating, we are able to eliminate this dephasing, thereby demonstrating a high on-off ratio of control. Our results pave the way toward using the Kerr-cat to fault-tolerantly measure error syndromes of bosonic codes.
arxiv pdf