2,523 research outputs found
Spin exchange in quantum rings and wires in the Wigner-crystal limit
We present a controlled method for computing the exchange coupling in
strongly correlated one-dimensional electron systems. It is based on the
asymptotically exact relation between the exchange constant and the
pair-correlation function of spinless electrons. Explicit results are obtained
for thin quantum rings with realistic Coulomb interactions, by calculating this
function via a many-body instanton approach.Comment: 7 pages, 2 figures. Changes in the text and figures to improve
readability; added reference
Second Harmonic Coherent Driving of a Spin Qubit in a Si/SiGe Quantum Dot
We demonstrate coherent driving of a single electron spin using second
harmonic excitation in a Si/SiGe quantum dot. Our estimates suggest that the
anharmonic dot confining potential combined with a gradient in the transverse
magnetic field dominates the second harmonic response. As expected, the Rabi
frequency depends quadratically on the driving amplitude and the periodicity
with respect to the phase of the drive is twice that of the fundamental
harmonic. The maximum Rabi frequency observed for the second harmonic is just a
factor of two lower than that achieved for the first harmonic when driving at
the same power. Combined with the lower demands on microwave circuitry when
operating at half the qubit frequency, these observations indicate that second
harmonic driving can be a useful technique for future quantum computation
architectures.Comment: 9 pages, 9 figure
Pilot Model Development and Human Manual Control Considerations for Helicopter Hover Displays
Head-down hover displays and instrument panels theoretically provide all necessary 2ight data information to control low-speed helicopter manoeuvring. However, past experiments have shown that head-down displays can incur high workload, control instability, and even loss of control when used as the sole 2ight data source. This paper investigates the reasons for this instability incurred by replacing good outside visuals with a head-down hover display and an instrument panel. A pilot model based on crossover theory is developed for a linear six-degree-of-freedom Bo. helicopter model. Utilising a target trajectory based on-theory and assuming perfect information availability, the developed model can perform the required manoeuvring task with a control time-delay stability margin of . s (with SAS) or . s (without SAS). Then, the actual information availability based on human perception methods and limitations is discussed. A pilot-in-the-loop experiment in the SIMONA Research Simulator qualitatively validates the developed pilot model for good outside visuals. However, the pilot model does not capture the added diZculties of having to utilise the hover display and instrument panel instead of good outside visuals; during the experiment, the task was impossible to complete with only these displays. This is likely caused by an increase in control time-delay, which in turn is caused by the loss of peripheral and flow 1eld information, a more abstract information representation compared to good outside visuals, and the fact that the pilot now needs to scan multiple displays to acquire all necessary 2ight state information. Improving head-down hover display symbology and scaling factors might rectify some, but probably not all of these effects.Control & Simulatio
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