Origin of switching noise inGaAs∕AlxGa1−xAslateral gated devices

Abstract

We have studied switching (telegraph) noise at low temperature in GaAs/AlGaAs heterostructures with lateral gates and introduced a model for its origin, which explains why noise can be suppressed by cooling samples with a positive bias on the gates. The noise was measured by monitoring the conductance fluctuations around e 2 /h on the first step of a quantum point contact at around 1.2 K. Cooling with a positive bias on the gates dramatically reduces this noise, while an asymmetric bias exacerbates it. Our model is that the noise originates from a leakage current of electrons that tunnel through the Schottky barrier under the gate into the conduction band and become trapped near the active region of the device. The key to reducing noise is to keep the barrier opaque under experimental conditions. Cooling with a positive bias on the gates reduces the density of ionized donors. This builds in an effective negative gate voltage so that a smaller negative bias is needed to reach the desired operating point. This suppresses tunnelling from the gate and hence the noise. The reduction in the density of ionized donors also strengthens the barrier to tunneling at a given applied voltage. Further support for the model comes from our direct observation of the leakage current into a closed quantum dot, around 10 −20 A for this device. The current was detected by a neighboring quantum point contact, which showed monotonic steps in time associated with the tunneling of single electrons into the dot. If asymmetric gate voltages are applied, our model suggests that the noise will increase as a consequence of the more negative gate voltage applied to one of the gates to maintain the same device conductance. We observe exactly this behaviour in our experiments.

Screening of charged impurities with multielectron singlet-triplet spin qubits in quantum dots

“…This is, of course, a familiar conundrum in all proposals of quantum computation. The outlook is nevertheless quite promising, since random fluctuations in the electrostatic potential can be suppressed by bias cooling 15 or by adding a negatively biased insulated top gate 16 in order to suppress leakage of electrons from the gates into the two-dimensional electron gas. Furthermore, intentional changes in the electrostatic potential for the purposes of two-qubit operations can be enhanced by a floating interdot capacitive gate.…”

“…[10][11][12][13] This makes it difficult to find samples suitable for spin qubit realization. Furthermore, typically the impurities do introduce some switching noise, [14][15][16][17] so that even in samples in which the impurities are all far enough from the DQD that a two-electron singlettriplet qubit can be accessed, the interdot exchange energy is still subject to random fluctuation, leading to gate errors and decoherence. [18][19][20] This necessitates operating in a parameter regime such that the sensitivity of the exchange energy to the charge noise is minimized, a so-called "sweet spot".…”

Charged-impurity-induced dephasing of a voltage-controlled coded qubit based on electron spin in a triple quantum dot

“…In the electron-spin-based qubits described here electron spins are spatially localized in the plane of the GaAs/GaAlAs heterojunction using voltages applied to metallic gates at the GaAs surface. Due to random charge fluctuations of remote impurities 7 the confining potential may fluctuate and affect the electronic states of qubits. Hu and Das Sarma 8 studied the effect of charged impurity on two-qubit gate defined by exchange interaction between two electron-spin qubits localized in a double quantum dot.…”

Field-effect-induced two-dimensional electron gas utilizing modulation-doped ohmic contacts

“…However, the presence of ionized dopants inherent to modulationdoping can have adverse effects on the behavior of nanostructures and are a possible source of decoherence for spin-qubits [9,10]. Ionized dopants can act as active trapping sites for electrons injected from the metal surface gate through the Schottky barrier [11], giving rise to random switching of the charge state of the impurities. These fluctuations cause instability through a time-dependent potential landscape [10][11][12][13].…”

“…These fluctuations cause instability through a time-dependent potential landscape [10][11][12][13]. Methods such as 'bias cooling' in which nanostructures are cooled while a positive bias applied to the gates aid in reducing fluctuations [11], but charge noise is still believed to a dominant mechanism limiting gate fidelities in spin qubits [9]. …”