Shaping a time-dependent excitation to control the electron distribution function: noise minimization in a tunnel junction

We report measurements of shot noise in a tunnel junction under bi-harmonic illumination, Vac(t) = Vac1 cos(2\pi ft) + Vac2 cos(4\pi ft+ \phi). The experiment is performed in the quantum regime, hf >> k_BT at low temperature T = 70 mK and high frequency f = 10 GHz. From the measurement of noise at low frequency, we show that we can infer and control the non-equilibrium electronic distribution function by adjusting the amplitudes and phase of the excitation, thus modeling its shape. In particular, we observe that the noise depends not only on the amplitude of the two sine waves but also on their relative phase, due to coherent emission/absorption of photons at different frequencies. By shaping the excitation we can minimize the noise of the junction, which no longer reaches its minimum at zero dc bias. We show that adding an excitation at frequency 2f with the proper amplitude and phase can reduce the noise of the junction excited at frequency f only.