Overcoming the limitations in the coherent driving of quantum superconducting circuits – OCTAVES
In the steep path towards fault-tolerant quantum computation with superconducting circuits, various recent experiments point towards a same roadblock for achieving high-fidelity operations well above the error correction threshold. For yet poorly understood reasons, the quantum properties of Josephson circuits are impacted by the presence of moderately strong microwave drives. This strongly limits the rate and fidelity of qubit readouts required for error syndrome measurements and the performance of parametric Hamiltonian engineering for autonomous error correction.
On one hand, improving the readout fidelity, as well as the readout speed and quantum non-demolition property plays a crucial role in active quantum error correction and thus in the quest for a fault-tolerant quantum processor. Such qubit readouts are routinely performed in experimental labs and are based on the dispersive interaction of superconducting qubits with a driven readout resonator. One expects that increasing the number of cavity readout photons should lead to an increase of the measurement rate and therefore faster and higher-fidelity readout. In practice however, the increase in the number of circulating photons rapidly comes at the expense of undesired effects such as an enhanced relaxation of the transmon qubit or an increase in thermal excitation probability.
On the other hand, recent proposals and experiments have shown that it is possible to engineer a particular nonlinear interaction between a quantum system and its bath, in order to reroute the dissipation, and evacuate the entropy of errors leading to an autonomous quantum error correction. The multi-wave mixing property of a Josephson junction, in conjunction with microwave drives at well-chosen frequencies satisfying some matching conditions, leads to an effective nonlinear interaction between a harmonic memory mode and a cold bath. Such a so-called parametric method leads to an effective nonlinear dissipation mechanism that confines the dynamics to a degenerate manifold in which the logical qubit states are encoded. Here, the strength of confinement, or equivalently the rate at which the entropy is evacuated, can be enhanced through an increase of the parametric drive power. However, once again, this increase in the drive power comes at the expense of new error processes which we propose to study in this project.
We are convinced that in both cases, the unstable behavior of the Josephson circuit in presence of moderately strong microwave drives is reminiscent of the complex dynamics of RF current biased Josephson junction well studied in the classical regime. OCTAVES brings together a strong consortium of four groups with a broad and complementary range of theoretical and experimental expertise to thoroughly investigate these limitations and put forward new solutions with novel readout circuit designs and parametric pumping methods to engineer multi-photon dissipation. Efficient theoretical tools will be developed to analyze and quantify the complex dynamics of quantum Josephson circuits under strong driving. The two major experimental breakthroughs of this project will be a record high-fidelity quantum demolition qubit readout and a full first-order error corrected bosonic code.
Olivier Buisson (Institut Néel)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
LPT LABORATOIRE DE PHYSIQUE THEORIQUE
NEEL Institut Néel
ENSL LABORATOIRE DE PHYSIQUE DE L'ENS DE LYON
Inria de Paris Centre de Recherche Inria de Paris
Help of the ANR 534,152 euros
Beginning and duration of the scientific project: December 2021 - 48 Months