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Home page > Asymmetry in the quantum noise of a mesoscopic system

P.-M. Billangeon (PhD student), R. Deblock and H. Bouchiat, collaboration : F. Pierre (LPN, Marcoussis)

Non equilibrium current fluctuations detection is a powerful tool to get information not accessible by transport experiments on mesoscopic systems. They can provide information on the statistic of carrier (bosons or fermions), their charge and correlations related to interaction effect. Whereas there are now a number of current noise measurements at low frequencies on different systems, there have been only a few measurements in the frequency range 1–100 GHz, which is experimentally more challenging. Such frequencies correspond to the typical energy scales and inverse propagation times involved in most mesoscopic phenomena.

To measure noise at high-frequency we couple on the same chip the system under study (the source) to a detector, in our case a small superconductor-insulator-superconductor (SIS) junction. The detection consists in measuring the quasiparticle photo-assisted current due to high-frequency current fluctuations of the system under study. This imposes to fabricate an on-chip circuitry to couple at high frequency the source and the detector (see Fig. 1).

Fig. 1 : Left : Optical image of a sample used to detect high-frequency current fluctuations of mesoscopic devices. Right : Spectral density of quasiparticle noise of a Josephson junction, with a superconducting gap Δ, at different bias voltage VS.

An important issue in this type of detection, where a quantum detector is used, is to know what quantity is exactly measured. Indeed as the detector is itself a mesoscopic device, the device under study and the detector need to be considered on the same level. We have shown that this type of detection allows to discriminate between emission processes (where energy goes from the source to the detector) and absorption processes (where energy is emitted by the detector and absorbed by the source). A "classical" detector is not able to distinguish between this two types of processes. We have measured with this detection scheme a strong asymmetry between emission and absorption noise in a Josephson junction [1]. This experiment demonstrates that, in the quantum regime, when frequency is higher than the energy scale associated with temperature, one has to take into account the difference between emission and absorption processes.

Reference :

[1] Emission and absorption asymmetry in the quantum noise of a Josephson junction, P.-M. Billangeon, F. Pierre, H. Bouchiat and R. Deblock, Phys. Rev. Lett. 96, 136804 (2006).