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Register a new userExperimental Test of the High-Frequency Quantum Shot Noise Theory in a Quantum Point Contact
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Authors
Eva Zakka-Bajjani
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We report on direct measurements of the electronic shot noise of a quantum point contact at frequencies nu in the range 4-8 GHz. The very small energy scale used ensures energy independent transmissions of the few transmitted electronic modes and their accurate knowledge. Both the thermal energy and the quantum point contact drain-source voltage Vds are comparable to the photon energy hnu leading to observation of the shot noise suppression when $V_{ds}<h u/e$. Our measurements provide the first complete test of the finite frequency shot noise scattering theory without adjustable parameters.
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We have calculated the finite-frequency current noise of a superconductor-ferromagnet quantum point contact (SF QPC). This signal is qualitatively affected by the spin-dependence of interfacial phase shifts (SDIPS) acquired by electrons upon reflection on the QPC. For a weakly transparent QPC, noise steps appear at frequencies or voltages determined directly by the SDIPS. These steps can occur at experimentally accessible temperatures and frequencies. Finite frequency noise is thus a promising tool to characterize the scattering properties of a SF QPC.
We report on charge sensing measurements of a GaAs semiconductor quantum dot device using a radio frequency quantum point contact (rf-QPC). The rf-QPC is fully characterized at 4 K and milli-Kelvin temperatures and found to have a bandwidth exceeding 20 MHz. For single-shot charge sensing we achieve a charge sensitivity of 2x10^-4 e/(sqrt)Hz referred to the neighboring dots charge. The rf-QPC compares favorably with rf-SET electrometers and promises to be an extremely useful tool for characterizing and measuring semiconductor quantum systems on fast timescales.
We have operated a quantum point contact (QPC) charge detector in a radio frequency (RF) mode that allows fast charge detection in a bandwidth of tens of megahertz. We find that the charge sensitivity of the RF-QPC is limited not by the noise of a secondary amplifier, but by non-equilibrium noise f the QPC itself. We have performed frequency-resolved measurements of the noise within a 10 MHz bandwidth around our carrier wave. When averaged over our bandwidth, we find that the noise is in good agreement with the theory of photon-assisted shot noise. Our measurements also reveal strong frequency dependence of the noise, asymmetry with respect to the carrier wave, the appearance of sharp local maxima that are correlated with mechanical degrees of freedom in the sample, and noise suppression indicative of many-body physics near the 0.7 structure.
The conductance quantization and shot noise below the first conductance plateau $G_0 = 2e^2/h$ are measured in a quantum point contact fabricated in a GaAs/AlGaAs tunnel-coupled double quantum well. From the conductance measurement, we observe a clear quantized conductance plateau at $0.5G_0$ and a small minimum in the transconductance at $0.7 G_0$. Spectroscopic transconductance measurement reveals three maxima inside the first diamond, thus suggesting three minima in the dispersion relation for electric subbands. Shot noise measurement shows that the Fano factor behavior is consistent with this observation. We propose a model that relates these features to a wavenumber directional split subband due to a strong Rashba spin--orbit interaction that is induced by the center barrier potential gradient of the double-layer sample.
We study non-equilibrium differential conductance and current fluctuations in a single quantum point contact. The two-terminal electrical transport properties -- differential conductance and shot noise -- are measured at 1.5 K as a function of the drain-source voltage and the Schottky split-gate voltage. In differential conductance measurements, conductance plateaus appear at integer multiples of $2e^2/h$ when the drain-source voltage is small, and the plateaus evolve to a fractional of $2e^2/h$ as the drain-source voltage increases. Our shot noise measurements correspondingly show that the shot noise signal is highly suppressed at both the integer and the non-integer conductance plateaus. This main feature can be understood by the induced electrostatic potential model within a single electron picture. In addition, we observe the 0.7 structure in the differential conductance and the suppressed shot noise around 0.7 ($2e^2/h$); however, the previous single-electron model cannot explain the 0.7 structure and the noise suppression, suggesting that this characteristic relates to the electron-electron interactions.
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