Manipulating Fock states of a harmonic oscillator while preserving its linearity
Kristinn Júlíusson, Simon Bernon, Xin Zhou, Vivien Schmitt, H. le Sueur, Patrice Bertet, D. Vion, Mazyar Mirrahimi, Pierre Rouchon, Daniel Esteve · Physical Review A · 2016
We present a scheme for controlling the quantum state of a harmonic oscillator by coupling it to an anharmonic multilevel system (MLS) with first- to second-excited-state transition on resonance with the oscillator. In this scheme, which we call ef-resonant, the spurious oscillator Kerr nonlinearity inherited from the MLS is very small, while its Fock states can still be selectively addressed via an MLS transition at a frequency that depends on the number of photons. We implement this concept in a circuit-QED setup with a microwave three-dimensional cavity (the oscillator, with frequency 6.4 GHz and quality factor ${Q}_{O}=2\ifmmode\times\else\texttimes\fi{}{10}^{6}$) embedding a frequency tunable transmon qubit (the MLS). We characterize the system spectroscopically and demonstrate selective addressing of Fock states and a Kerr nonlinearity below 350 Hz. At times much longer than the transmon coherence times, a nonlinear cavity response with driving power is also observed and explained.