Measuring photon-assisted shot noise in capacitively-coupled quantum point contacts

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• Dr Yodchay Jompol, Mahidol University
• Monday 15 December 2014, 14:15-15:15
• Mott Seminar Room, Cavendish Laboratory, Department of Physics.

If you have a question about this talk, please contact Teri Bartlett.

The physics of quantum shot noise becomes particularly interesting when the bias voltage causing a current noise is close to the noise measurement frequency (eV ≡ hv). However, the high-frequency noise measurements are usually limited to few GHz due to the large mismatch between high impedance of the quantum conductor and low impedance of the external circuit. To circumvent this problem, on-chip detection schemes have been developed using Quantum Dots or SIS junctions. These devices reduce shunting ground capacitance which would otherwise lower the coupling and hence the frequency bandwidth. However, the proximity (few tens of microns) between those emitter and detector still introduces enough capacitive shunting that prevents the measurements at very high-frequency noise (up to THz).

In this talk, I will present an experimental realisation of a novel on-chip shot noise detection scheme using photon-assisted effect in capacitively-coupled quantum point contacts (QPCs). This quantum effect differs from the ordinary “transport” shot noise because it does not need an application of a dc bias, but is due to probabilistic scattering of the electron-holes pairs partitioning between left and right contacts. This results in a current noise called photon-assisted shot noise (PASN). For a QPC , both the photocurrent and PASN are proportional to D(1-D), where D is the transmission of the QPC . Our experiment is realised using a set of quantum point contacts, one of which serves as a QPC emitter that generates high-frequency shot noise at another QPC , called QPC detector. This QPC detector is used to capture and convert the high- frequency current fluctuation from the QPC emitter into the measureable (low-frequency) voltage fluctuation with application of an on-chip interdigitated capacitor. We show that when the QPC emitter is dc biased, the measured current by the QPC detector is proportional to DE(1-DE), where DE/DD is the transmission of the emitter/detector. We also demonstrate that by varying both DE and DD for a given Vds the measured signal is qualitatively proportional to the product ~ DD(1-DD) DE(1-DE). This new way of on-chip detection scheme should find fundamental applications in electronic quantum physics.

This talk is part of the Semiconductor Physics Group Seminars series.

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