Increasing Spectroscopic Coherence Times of a DC SQUID Phase Qubit by Operating Deeper in the Potential Well

We present measurements of the spectroscopic coherence time, T
₂
* of a Nb-AlOx-Nb dc SQUID phase qubit. T
₂
* is calculated from the spectroscopic width of the |0rang to |1rang transition measured at 25 mK. We find that T
₂
*increases at higher transition frequencies corresponding to measurements deeper in the potential well. The measured are consistent with root mean square current noise saturating at 4 nA.