Bipolar spin blockade and coherent state superpositions in a triple quantum dot

Spin qubits based on interacting spins in double quantum dots have been successfully demonstrated. Readout of the qubit state involves a conversion of spin to charge information, universally achieved by taking advantage of a spin blockade phenomenon resulting from Pauli's exclusion principle. The archetypal spin blockade transport signature in double quantum dots takes the form of a rectified current. Currently more complex spin qubit circuits including triple quantum dots are being developed. Here we show both experimentally and theoretically (a) that in a linear triple quantum dot circuit, the spin blockade becomes bipolar with current strongly suppressed in both bias directions and (b) that a new quantum coherent mechanism becomes relevant. Within this mechanism charge is transferred non-intuitively via coherent states from one end of the linear triple dot circuit to the other without involving the centre site. Our results have implications in future complex nano-spintronic circuits.Comment: 21 pages, 7 figure

Theory of electronic transport through a triple quantum dot in the presence of magnetic field

Theory of electronic transport through a triangular triple quantum dot subject to a perpendicular magnetic field is developed using a tight binding model. We show that magnetic field allows to engineer degeneracies in the triple quantum dot energy spectrum. The degeneracies lead to zero electronic transmission and sharp dips in the current whenever a pair of degenerate states lies between the chemical potential of the two leads. These dips can occur with a periodicity of one flux quantum if only two levels contribute to the current or with half flux quantum if the three levels of the triple dot contribute. The effect of strong bias voltage and different lead-to-dot connections on Aharonov-Bohm oscillations in the conductance is also discussed

The origin of switching noise in GaAs/AlGaAs lateral gated devices

We have studied the origin of switching (telegraph) noise at low temperature in lateral quantum structures defined electrostatically in GaAs/AlGaAs heterostructures by surface 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. We propose a model in which the noise originates from a leakage current of electrons that tunnel through the Schottky barrier under the gate into the doped layer. The key to reducing noise is to keep this barrier opaque under experimental conditions. Bias cooling reduces the density of ionized donors, which builds in an effective negative gate voltage. A smaller negative bias is therefore 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. Support for the model comes from our direct observation of the leakage current into a closed quantum dot, around $10^{-20} \mathrm{A}$ 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.Comment: 8 pages, 7 figure

A finite element model to improve noise reduction based attenuation measurement of earmuffs in a directional sound field

The real attenuation of hearing protection devices (HPD) can be assessed in the field using a method based on continuous field microphone-in-real-ear (F-MIRE) measurements. The two-microphone method provides an indicator called the measured noise reduction (NR∗), defined as the difference between the measured exterior (outside the protector) and interior (under the protector) sound pressure levels (SPL). The HPD's attenuation expressed in terms of the more common insertion loss (IL) can then be obtained from NR∗ using compensation factors. For earmuffs, NR∗ has been shown to vary of up to 20 dB depending on the angle of incidence of the sound source. Therefore, there is a need to use sound incidence dependent compensation factors to relate NR∗ and IL. To evaluate these factors and more generally to improve the continuous F-MIRE method, a finite-element (FE) model of an earmuff on an ATF (acoustic test fixture) exposed to a directional sound field has been developed and its predictions compared with lab measurements for several incidence angles. Regarding the external microphone SPL and the NR∗, in one-third of octave bands, the model correlates very well with measurements for frequencies below 1250 Hz whatever the sound incidence. Above 1250 Hz, the FE model captures the trends, as a function of the incidence angle, but the agreement generally decreases with increasing frequency. A better correlation between the FE model and the experimental data is achieved for the variation of NR∗ (ΔNR∗) as a function of the sound incidence. Actions, such as (i) accounting for the headband in the model, (ii) refining the modeling of the sound source, (iii) improving the cushion modeling and (iv) better describing the backplate/cushion coupling conditions, are suggested to improve the model accuracy. To illustrate the potential of the modeling to improve the continuous F-MIRE measurement method, the FE model is used to determine an optimal position of the external microphone and to obtain estimates of exposure levels using the left and right ear exterior microphones. © 2016 Elsevier Lt

Laboratory and field measurements of nail guns' noise emission

Field measurements and laboratory measurements using EN 12549 was presented. The rig was held in a small comfortable backpack and even if the sensors were wired to the acquisition card, the worker could work without obstruction. Between 8 and 12 trials of 10 impacts were recorded for each nailer/worker combination. Eight framing nailers and two roofing nailers were tested in the laboratory under controlled conditions as per the EN 12549 standard. From this standard, three operators were required to perform five trials of 10 nails each, with each trial lasting a period of 30 seconds. The measures were performed in a semi anechoic room where both the sound power and the sound pressure level at the worker?s ear were measured. Concerning the EN 12549 standard, it seems appropriate in order to perform representative workplace ranking of nailers following their sound power level values. Concerning the reduction of workers? noise exposure, the battery operated nailer stands out as the best choice as its level is at least 6 dBs lower than any other tested nailer in both lab and field measurements

Identification of noise sources using a time domain beamforming on pneumatic, gas and electric nail guns

In the construction industry, many workers are exposed daily to harmful levels of impulsive noise from nail guns. Therefore, a better knowledge of the noise generated by these tools is required in order to propose noise reduction solutions. The objective of this work is to propose an approach for source identification using a microphone array together with a source identification algorithm based on recent development in the generalized cross-correlation technique. In addition to the pneumatic nail gun, for which sources have been partially identified in the literature, the proposed approach is applied to two other types of nail guns, an electric and a gas powered one. First, the standardized acoustic power spectrum of these three nail guns is measured for global comparison purposes and result in a ranking of the three nail guns. Second, the generalized cross-correlation technique applied to nail gun noise source identification is presented. Third, acoustic maps for successive small time segments are presented, providing a fine identification of noise sources for the three nail guns and an explanation of the observed sound power level ranking. © 2019 Institute of Noise Control Engineering

Spin selective Aharonov-Bohm oscillations in a lateral triple quantum dot

We present a theory for spin selective Aharonov-Bohm oscillations in a lateral triple quantum dot. We show that to understand the Aharonov-Bohm (AB) effect in an interacting electron system within a triple quantum dot molecule (TQD) where the dots lie in a ring configuration requires one to not only consider electron charge but also spin. Using a Hubbard model supported by microscopic calculations we show that, by localizing a single electron spin in one of the dots, the current through the TQD molecule depends not only on the flux but also on the relative orientation of the spin of the incoming and localized electrons. AB oscillations are predicted only for the spin singlet electron complex resulting in a magnetic field tunable "spin valve".Comment: 4 pages, 4 figure