We employ an intermediate excited charge state of a lateral quantum dot device to increase the charge detection contrast during the qubit state readout procedure, allowing us to increase the visibility of coherent qubit oscillations. This approach am
plifies the coherent oscillation magnitude but has no effect on the detector noise resulting in an increase in the signal to noise ratio. In this letter we apply this scheme to demonstrate a significant enhancement of the fringe contrast of coherent Landau-Zener-Stuckleberg oscillations between singlet S and triplet T+ two-spin states.
Qubits based on the singlet (S) and the triplet (T0, T+) states in double quantum dots have been demonstrated in separate experiments. It has been recently proposed theoretically that under certain conditions a quantum interference could occur from t
he interplay between these two qubit species. Here we report experiments and modeling which confirm these theoretical predictions and identify the conditions under which this interference occurs. Density matrix calculations show that the interference pattern manifests primarily via the occupation of the common singlet state. The S/T0 qubit is found to have a much longer coherence time as compared to the S/T+ qubit.
This paper reports on the observation and analysis of magnetotransport phenomena in the nonlinear differential resistance $r_{xx}=dV_{xx}/dI$ of high-mobility InGaAs/InP and GaAs/AlGaAs Hall bar samples driven by direct current, $Idc$. Specifically,
it is observed that Shubnikov -de Haas (SdH) oscillations at large filling factors invert their phase at sufficiently large values of $Idc$. This phase inversion is explained as being due to an electron heating effect. In the quantum Hall effect regime the $r_{xx}$ oscillations transform into diamond-shaped patterns with different slopes corresponding to odd and even filling factors. The diamond-shaped features at odd filling factors can be used as a probe to determine spin energy gaps. A Zero Current Anomaly (ZCA) which manifests itself as a narrow dip in the $r_{xx}(Idc)$ characteristics at zero current, is also observed. The ZCA effect strongly depends upon temperature, vanishing above 1 K while the transport diamonds persist to higher temperatures. The transport diamonds and ZCA are fully reproduced in a higher mobility GaAs/AlGaAs Hall bar structure confirming that these phenomena reflect intrinsic properties of two-dimensional systems.
