Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru
Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru
Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru
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4.5 QPC Versus SET<br />
Semiconduc<strong>to</strong>r Few-Electron <strong>Quantum</strong> Dots as Spin Qubits 71<br />
Our measurements clearly demonstrate that a QPC can serve as a fast and<br />
sensitive charge detec<strong>to</strong>r. Compar<strong>ed</strong> <strong>to</strong> an SET, a QPC offers several practical<br />
advantages. First, a QPC requires fabrication and tuning of just a single<br />
additional gate when integrat<strong>ed</strong> with a quantum dot defin<strong>ed</strong> by metal gates,<br />
whereas an SET requires two tunnel barriers, and a gate <strong>to</strong> set the island potential.<br />
Second, QPCs are more robust and easy <strong>to</strong> use in the sense that spurious,<br />
low-frequency fluctuations of the electrostatic potential hardly change<br />
the QPC sensitivity <strong>to</strong> charges on the dot (the transition between quantiz<strong>ed</strong><br />
conductance plateaus has an almost constant slope over a wide range of electrostatic<br />
potential), but can easily spoil the SET sensitivity.<br />
With an RF-SET, a sensitivity <strong>to</strong> charges on a quantum dot of ≈2 ×<br />
10 −4 e/ √ Hz has been reach<strong>ed</strong> [58], and theoretically even a ten times better<br />
sensitivity is possible [57]. Could a QPC reach similar sensitivities?<br />
The noise level in the present measurement could be r<strong>ed</strong>uc<strong>ed</strong> by a fac<strong>to</strong>r<br />
of two or three using a JFET input-stage which better balances input voltage<br />
noise and input current noise. Further improvements can be obtain<strong>ed</strong> by lowering<br />
the capacitance of the filters in the line, or the line capacitance itself,<br />
by placing the IV-conver<strong>to</strong>r close <strong>to</strong> the sample, inside the refrigera<strong>to</strong>r.<br />
Much more significant r<strong>ed</strong>uctions in the inst<strong>ru</strong>mentation noise could be<br />
realiz<strong>ed</strong> by emb<strong>ed</strong>ding the QPC in a resonant electrical circuit and measuring<br />
the damping of the resona<strong>to</strong>r. We estimate that with an “RF-QPC” and a<br />
low-temperature HEMT amplifier, a sensitivity of 2 × 10 −4 e/ √ Hz could be<br />
achiev<strong>ed</strong> with the present sample. The noise from the amplifier circuitry is<br />
then only 2.5 times larger than the shot noise level.<br />
To what extent the signal can be increas<strong>ed</strong> is unclear, as we do not yet<br />
understand the mechanism through which the dot occupancy is disturb<strong>ed</strong> for<br />
Vi > 1mV 4 . Certainly, the capacitive coupling of the dot <strong>to</strong> the QPC channel<br />
can easily be five times larger than it is now by optimizing the gate design [60].<br />
Keeping Vi = 1 mV , the sensitivity would then be 4×10 −5 e/ √ Hz, and a single<br />
electron charge on the dot could be measur<strong>ed</strong> within a few ns.<br />
Finally, we point out that a QPC can reach the quantum limit of detection<br />
[63, 64], where the measurement induc<strong>ed</strong> decoherence takes the minimum<br />
value permitt<strong>ed</strong> by quantum mechanics. Qualitatively, this is because (1) information<br />
on the charge state of the dot is transferr<strong>ed</strong> only <strong>to</strong> the QPC current<br />
and not <strong>to</strong> degrees of fre<strong>ed</strong>om which are not observ<strong>ed</strong>, and (2) an external<br />
perturbation in the QPC current does not couple back <strong>to</strong> the charge state of<br />
the dot.<br />
4 The statistics of the RTS were alter<strong>ed</strong> for Vi > 1 mV, irrespective of (1) whether<br />
Vi was appli<strong>ed</strong> <strong>to</strong> the QPC source or drain, (2) the potential difference between<br />
the reservoir and the QPC source/drain, and (3) the QPC transmission T .