Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru

82 J.M. Elzerman et al.
We now choose the optimal value of the threshold as the one for which
the visibility 1 − α − β is maximal (dotted vertical line in Fig. 34b). For this
setting, α ≈ 0.07, β1 ≈ 0.17, β2 ≈ 0.15, so the measurement fidelity for the
spin-↑ and the spin-↓ state is ∼0.93 and ∼0.72 respectively. The measurement
visibility in a single-shot measurement is thus at present 65%.
Significant improvements in the spin measurement visibility can be made
by lowering the electron temperature (smaller α) and especially by making the
charge measurement faster (smaller β). Already, the demonstration of singleshot
spin read-out and the observation of T1 of order 1 ms are encouraging
results for the use of electron spins as quantum bits.
6 Semiconductor Few-Electron Quantum Dots
as Spin Qubits
In the previous sections we have described experiments aimed at creating
a quantum dot spin qubit according to the proposal by Loss and DiVincenzo
[2] (see also paragraph 1.3). The key ingredients for these experiments –
performed over the last two years – are a fully tunable few-electron double
quantum dot and a quantum point contact (QPC) charge detector. We have
operated the QPC in three different ways:
1. By measuring its DC conductance, changes in the average charge on the
double dot are revealed, which can be used to identify the charge configuration
of the system.
2. By measuring the conductance in real-time (with a bandwidth of ∼100 kHz),
we can detect individual electrons tunnelling on or off the dot (in less than
10 µs).
3. By measuring the QPC response to a gate voltage pulse train (with the
proper frequency) using a lock-in amplifier, we can determine the tunnel
rate between the dot and a reservoir. In addition, by using a large pulse amplitude
and measuring changes in the effective tunnel rate, we can identify
excited states of the dot.
Using these techniques, we have demonstrated that our GaAs/AlGaAs quantum
dot circuit is a promising candidate for a spin qubit. However, we do
not have a fully functional qubit yet, as coherent manipulation of a singleor
a two-spin system has so far remained elusive. In this section, we evaluate
the experimental status of the spin qubit project in terms of the DiVincenzo
spin-↑) isgivenby1− β1. The probability that this tunnel event is detected (i.e.
is not too fast) is given by 1−β2. Therefore, the probability that a spin-↓ electron
tunnels out and is detected, is (1−β1)(1−β2). In addition, there is the possibility
that the ↓-electron relaxes, with probability β1, but a step in the QPC signal is
nevertheless detected, with probability α, due to the “dark count” mechanism.
Therefore, the total probability that a spin-↓ electron is declared “spin-down” is
given by (1 − β1)(1 − β2)+(αβ1) approximately.