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Register a new userRenormalized dynamics in charge qubit measurements by a single electron transistor
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We investigate charge qubit measurements using a single electron transistor, with focus on the backaction-induced renormalization of qubit parameters. It is revealed the renormalized dynamics leads to a number of intriguing features in the detectors noise spectra, and therefore needs to be accounted for to properly understand the measurement result. Noticeably, the level renormalization gives rise to a strongly enhanced signal-to-noise ratio, which can even exceed the universal upper bound imposed quantum mechanically on linear-response detectors.
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We investigate qubit measurements using a single electron transistor (SET). Applying the Schrodinger equation to the entire system we find that an asymmetric SET is considerably more efficient than a symmetric SET. The asymmetric SET becomes close to an ideal detector in the large asymmetry limit. We also compared the SET detector with a point-contact detector. This comparison allows us to illuminate the relation between information gain in the measurement process and the decoherence generated by these measurement devices.
We calculate the charge sensitivity of a recently demonstrated device where the Josephson inductance of a single Cooper-pair transistor is measured. We find that the intrinsic limit to detector performance is set by oscillator quantum noise. Sensitivity better than $10^{-6}$e$/sqrt{mathrm{Hz}}$ is possible with a high $Q$-value $sim 10^3$, or using a SQUID amplifier. The model is compared to experiment, where charge sensitivity $3 times 10^{-5}$e$/sqrt{mathrm{Hz}}$ and bandwidth 100 MHz are achieved.
Single dopants in semiconductor nanostructures have been studied in great details recently as they are good candidates for quantum bits, provided they are coupled to a detector. Here we report coupling of a single As donor atom to a single-electron transistor (SET) in a silicon nanowire field-effect transistor. Both capacitive and tunnel coupling are achieved, the latter resulting in a dramatic increase of the conductance through the SET, by up to one order of magnitude. The experimental results are well explained by the rate equations theory developed in parallel with the experiment.
A new method to fabricate non-superconducting mesoscopic tunnel junctions by oxidation of Ti is presented. The fabrication process uses conventional electron beam lithography and shadow deposition through an organic resist mask. Superconductivity in Ti is suppressed by performing the deposition under a suitable background pressure. We demonstrate the method by making a single electron transistor which operated at $T < 0.4$ K and had a moderate charge noise of $2.5 times 10^{-3}$ e/$sqrt{mathrm{Hz}}$ at 10 Hz. Based on nonlinearities in the current-voltage characteristics at higher voltages, we deduce the oxide barrier height of approximately 110 mV.
Starting from the Kubo formula for conductance, we calculate the frequency-dependent response of a single-electron transistor (SET) driven by an ac signal. Treating tunneling processes within the lowest order approximation, valid for a wide range of parameters, we discover a finite reactive part even under Coulomb blockade due to virtual processes. At low frequencies this can be described by an effective capacitance. This effect can be probed with microwave reflection measurements in radio-frequency (rf) SET provided that the capacitance of the surroundings does not completely mask that of the SET.
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