Nonlocal dynamics in a superconducting circuit

Quantum harmonic oscillators are prized in the field of superconducting qubits for their long lifetimes, which make them attractive for encoding quantum information in multi-photon states. These bosonic encoded qubits rely on residual small nonlinearities for state preparation, manipulation, and readout. For example, an oscillator coupled to a transmon qubit inherits a small two-photon interaction Hamiltonian, enabling the creation of non-classical states of light [1], but are there others? We present the design and experimental status of an alternative species of weakly-anharmonic oscillator where the nonlinearity comprises large displacements in phase space. Physically, this oscillator is made by combining extremely small Josephson energies and extremely large quantum phase fluctuations. Such oscillators could prove essential for error syndrome measurements in concatenated cat codes.

[1] Kirchmair et al. Nature 495 (2013).