Protecting quantum information in superconducting circuits

Xinyuan You

Zaki Leghtas

Andrey R. Klots, R. McDermott, L. B. Ioffe

András Gyenis, Agustín Di Paolo, Jens Koch +3 more

Artificial atoms realized by superconducting circuits offer unique opportunities to store and process quantum information with high fidelity.Among them, implementations of circuits that harness intrinsic noise protection have been rapidly developed in recent years.These noise-protected devices constitute a new class of qubits in which the computational states are largely decoupled from local noise channels.The main challenges in engineering such systems are simultaneously guarding against both bit-and phase-flip errors, and also ensuring high-fidelity qubit control.Although partial noise protection is possible in superconducting circuits relying on a single quantum degree of freedom, the promise of complete protection can only be fulfilled by implementing multimode or hybrid circuits.This Perspective reviews the theoretical principles at the heart of these new qubits, describes recent experiments, and highlights the potential of robust encoding of quantum information in superconducting qubits.

Robert J. Schoelkopf

Andrea Maiani, Morten Kjærgaard, Constantin Schrade

Novel qubits with intrinsic noise protection constitute a promising route for improving the coherence of quantum information in superconducting circuits.However, many protected superconducting qubits exhibit relatively low transition frequencies, which could make their integration with conventional transmon circuits challenging.In this work, we propose and study a scheme for entangling a tunable transmon with a Cooper-pair parity-protected qubit, a paradigmatic example of a low-frequency protected qubit that stores quantum information in opposite Cooper-pair parity states on a superconducting island.By tuning the external flux on the transmon, we show that noncomputational states can mediate a two-qubit entangling gate that preserves the Cooper-pair parity independent of the detailed pulse sequence.Interestingly, the entangling gate bears similarities to a controlled-phase gate in conventional transmon devices.Hence, our results suggest that standard high-precision gate calibration protocols could be repurposed for operating heterogeneous quantum processors.