the pulses illustrated in Fig. \[fig:1\]C. As we describe below, the qubit is first prepared in its ground state using a harmonic cooling pulse with amplitude $V_{\textrm{c}}$ and frequency $\nu_{\textrm{c}}$. Quantum-state transitions are then driven using a harmonic RF pulse with amplitude $V$ and frequency $\nu$. These fields are mutually coupled to the qubit through a small antenna. This is followed by a SQUID readout current pulse using the “sample and hold” technique [@Oliver05a; @Chiorescu03a]. If the sample current exceeds the SQUID switching current, a voltage pulse will appear at the output during the hold phase. Threshold detection looks for the presence or absence of a SQUID voltage, and this constitutes a digital measurement of the qubit state. Alternatively, although not used in these experiments, we have incorporated the SQUID into a resonant circuit and realized qubit readout via the shift in resonance frequency and phase for both the linear and non-linear resonance regimes [@Lee05a; @Lee07a].

The “qubit step” is shown in Fig. \[fig:1\]D as a function of the SQUID sample current and the flux detuning. The diabatic states $|L \rangle$ and $|R \rangle$ correspond to different levels of sample current (dashed lines) located symmetrically about the degeneracy point $\delta 