Experimental investigation of microwave photons emitted by a quantum point contact*

Quantum transport investigates the dynamics of electrical circuits displaying a quantum mechanical behavior. This is achievable by patterning circuits in the nm/um scale in clean room environments, and cooling them at T∼15 mK in dilution fridges. A remarkable aspect of such quantum dynamics is that the electrical current fluctuates, even in response to a strictly DC bias. Detecting these quantum fluctuations is highly informative as it conveys information on the granularity of charge, the statistics of the carriers but also on the characteristic transport times such as the electronic scattering time or on interaction effects.

Our lab has developed several experimental schemes and technics in order to measure efficiently such quantum fluctuations in the few GHz range.  In a qualitative level, measuring at this frequency range f_det∼6 GHz gives access to the quantum optical regime hf_det>>k_BT, where one needs to provide a quantum description not only for the electrical current flowing through the conductor, but also for the electromagnetic fields exchanged with its detection scheme.

In practice, we couple a large frequency bandwidth coil to a quantum point contact (QPC) developed on a 2D electron gas. In the quantum Hall effect regime, this gives rise to a single channel potential barrier with a tunable electronic transmission. Then, we amplify the radiated RF field leaking from the resonator in a HBT geometry and sample it at room temperature, in order to compute its second order coherence function. The final goal of our work is to prove experimentally that photons radiated by the QPC inherit the fermionic nature of the scattering electrons. In other words, radiated photons are antibunched in accordance with theoretical predictions. [1-2]

[1] Beenaker & Schomerus, Phys.Rev.Lett. 93, 096801 (2004)

[2] Hassler & Otten, Phys. Rev. B 92, 195417 (2015)