Optimal control in large open quantum systems: the case of transmon readout and reset

  1. Ronan Gautier,
  2. Élie Genois,
  3. and Alexandre Blais
We present a framework that combines the adjoint state method together with reverse-time back-propagation to solve otherwise prohibitively large open-system quantum control problems.
Our approach enables the optimization of arbitrary cost functions with fully general controls applied on large open quantum systems described by a Lindblad master equation. It is scalable, computationally efficient, and has a low memory footprint. We apply this framework to optimize two inherently dissipative operations in superconducting qubits which lag behind in terms of fidelity and duration compared to other unitary operations: the dispersive readout and all-microwave reset of a transmon qubit. Our results show that, given a fixed set of system parameters, shaping the control pulses can yield 2x improvements in the fidelity and duration for both of these operations compared to standard strategies. Our approach can readily be applied to optimize quantum controls in a vast range of applications such as reservoir engineering, autonomous quantum error correction, and leakage-reduction units.

Quantum control of a cat-qubit with bit-flip times exceeding ten seconds

  1. Ulysse Réglade,
  2. Adrien Bocquet,
  3. Ronan Gautier,
  4. Antoine Marquet,
  5. Emanuele Albertinale,
  6. Natalia Pankratova,
  7. Mattis Hallén,
  8. Felix Rautschke,
  9. Lev-Arcady Sellem,
  10. Pierre Rouchon,
  11. Alain Sarlette,
  12. Mazyar Mirrahimi,
  13. Philippe Campagne-Ibarcq,
  14. Raphaël Lescanne,
  15. Sébastien Jezouin,
  16. and Zaki Leghtas
Binary classical information is routinely encoded in the two metastable states of a dynamical system. Since these states may exhibit macroscopic lifetimes, the encoded information inherits
a strong protection against bit-flips. A recent qubit – the cat-qubit – is encoded in the manifold of metastable states of a quantum dynamical system, thereby acquiring bit-flip protection. An outstanding challenge is to gain quantum control over such a system without breaking its protection. If this challenge is met, significant shortcuts in hardware overhead are forecast for quantum computing. In this experiment, we implement a cat-qubit with bit-flip times exceeding ten seconds. This is a four order of magnitude improvement over previous cat-qubit implementations, and six orders of magnitude enhancement over the single photon lifetime that compose this dynamical qubit. This was achieved by introducing a quantum tomography protocol that does not break bit-flip protection. We prepare and image quantum superposition states, and measure phase-flip times above 490 nanoseconds. Most importantly, we control the phase of these superpositions while maintaining the bit-flip time above ten seconds. This work demonstrates quantum operations that preserve macroscopic bit-flip times, a necessary step to scale these dynamical qubits into fully protected hardware-efficient architectures.

Combined Dissipative and Hamiltonian Confinement of Cat Qubits

  1. Ronan Gautier,
  2. Alain Sarlette,
  3. and Mazyar Mirrahimi
Quantum error correction with biased-noised qubits can drastically reduce the hardware overhead for universal and fault-tolerant quantum computation. Cat qubits are a promising realization
of biased-noised qubits as they feature an exponential error bias inherited from their non-local encoding in the phase space of a quantum harmonic oscillator. To confine the state of an oscillator to the cat qubit manifold, two main approaches have been considered so far: a Kerr-based Hamiltonian confinement with high gate performances, and a dissipative confinement with robust protection against a broad range of noise mechanisms. We introduce a new combined dissipative and Hamiltonian confinement scheme based on two-photon dissipation together with a Two-Photon Exchange (TPE) Hamiltonian. The TPE Hamiltonian is similar to Kerr nonlinearity, but unlike the Kerr it only induces a bounded distinction between even- and odd-photon eigenstates, a highly beneficial feature for protecting the cat qubits with dissipative mechanisms. Using this combined confinement scheme, we demonstrate fast and bias-preserving gates with drastically improved performance compared to dissipative or Hamiltonian schemes. In addition, this combined scheme can be implemented experimentally with only minor modifications of existing dissipative cat qubit experiments.