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as a function of the delay between the π-pulse and the measure pulse. The data
will look like Figure 8.10. Fitting the decaying part of the trace to the function
P (t) = P offs + V iz ∗ e −t/T 1
, gives the quantity of interest: T 1 . Since both the
measurement visibility (Viz) and the ∼ 5% offset (P offs ) are free parameters in
this fit, the measurement process does not affect the obtained value of T 1 . Even
an imperfect π-pulse would only affect the value if part of the state is excited into
the second excited state. Thus, this measurement yields a very robust number.
8.10 Ramsey
Unfortunately, measuring the dephasing time T 2 is less straightforward. This
is due to the fact that the phase of the qubit’s state only has meaning relative to
an external clock source like the state of another quantum system. The phase of
the qubit’s state can also be measured by interfering it with the microwave drive.
This is done by using a pulse of half the area of the π-pulse, i.e. a π -pulse, to
2
excite the qubit into the equator of the Bloch sphere. There, the qubit is allowed
to dephase for a time t Delay and finally hit with another π -pulse to complete the
2
rotation into the excited state before it is measured. As a function of t Delay , the
occupation probability of the excited state looks like Figure 8.11b. The problem
with this measurement is that the decay envelope only gives a correct measure
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