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obtain control electronics that can handle signals up to 10-15 GHz. Thus, a frequency
choice of 5-10 GHz makes good sense.
For maximum flexibility, the signals that control the qubit will need to be
generated digitally. Since current high-speed digital electronics operates at frequencies
around 1 GHz, the pulses used to control the qubit should be on the order
of a few ns long. Given the best-case frequency spread of pulses of that length
(Slepian pulses), the non-linearity (i.e. difference in energy spacing between the
lowest three eigenstates) of the qubit should be at least 150-200 MHz to allow for
quick transitions (high amplitude pulses) that don’t drive undesired transitions.
The phase qubit needs at least two stable minima in its potential to allow for
the selective tunneling and decay of the | 1 〉 state into a state that is detectably
different from the | 0 〉 state.
This existence of multiple minima adds a slight
challenge to the qubit reset process as a random initial state might decay into any
one of the potential’s minima. It is possible to design the qubit such that it can
be biased to have either one or two stable minima. Such a qubit is the easiest
to reset. It is also possible to reset qubits with three or more stable minima at
a time by using a back-and-forth-tilting bias that destabilizes all minima except
for the desired one. But as the number of minima grows, this probabilistic reset
process takes longer and longer.
Therefore, potentials with maximally two or
three minima are the most useful.
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