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Owing to the limitations due to the dissipative nature of the rf-SET, we have been developing an inductive read-out technique for a
superconducting SET, called the L-SET (Inductive SET). With zero DC-voltage,
a superconducting SET (SSET) behaves as an energy-storing reactive component
because of the Josephson coupling. Tuning the gate charge changes the
effective inductance of the SSET between the drain and the source. This
effect is used to tune the resonance frequency of a resonator in the L-SET
configuration.
We consider the L-SET the most promising method of sensitive and fast electrometry. In addition, since its operation is based on correlated
supercurrent, the back action noise of a L-SET is clearly less than that of
a regular SET. Energy sensitivity of hbar appears to be within reach
using rather standard rf-techniques.
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Principle of the L-SET measurement.
In the L-SET circuit, the plasma frequency in a superconducting SET (SSET)
is tuned below one GHz by a parallel LC circuit. Since the Josephson
inductance of the SSET has a substantial gate charge dependence, resonant
frequency of the whole circuit is gate-dependent (in linear regime).
Charge sensitivity 3x10^-5 e/sqrt{Hz}, limited by preamplifier, has been
achieved in an operation mode which takes advantage of the nonlinearity of
the Josephson potential. Owing to reactive
readout, our setup has more than two orders of magnitude lower dissipation
than the existing method of radio-frequency electrometry. |
