We report the measurement of the fluctuations of the two quadratures of the electromagnetic field generated by a quantum conductor, a dc- and ac-biased tunnel junction placed at very low temperature. We observe that the variance of the fluctuations on one quadrature can go below that of vacuum, i.e., that the radiated field is squeezed. This demonstrates the quantum nature of the radiated electromagnetic field.

We report measurements of the low frequency current fluctuations of a tunnel junction placed at very low temperature biased by a time-dependent voltage V (t) = V (1 + cos 2πνt). We observe that the excess noise generated by the ac excitation exhibits quantum oscillations as a function of the dc bias, with a period given by hν/e with e the charge of a single electron. This is a direct consequence of the quantum nature of electricity in a normal conductor.Quantum oscillations are the most prominent manifestation of the quantum nature of a phenomenon. For instance, the macroscopic phase coherence in superconductors leads to current oscillations in a voltage biased Josephson junction [1]. These engender, in dc transport under irradiation at frequency ν, periodic anomalies vs bias voltage with a period hν/(2e), 2e being the charge of the Cooper pairs [2]. Despite the absence of macroscopic phase coherence, electrons in a normal metal are quantum objects described by a wavefunction with a phase. That phase is random from one electron to the next, yet the effect of the bias voltage is the same for all electrons: it shifts the phase of their wavefunction by the same amount φ. Thus, quantum oscillations with a period related to the voltage are expected.A time-dependent bias voltage V (t) is a uniform potential in an electrical contact. This causes all the electronic wavefunctions to acquire a phase factor e iφ with [3]:Eq. (1) captures the effect of both dc voltage and ac excitation on the transport. The dc voltage V dc leads to a linear increase of the phase with time, φ(t) = eV dc t/ , which is equivalent to an energy shift by eV dc , i.e. a shift of the chemical potential. The ac part of the voltage leads to a spreading in energy. For a linear system this is irrelevant, hence the ac voltage has no effect on dc average transport: there is no photo-assisted dc current. In contrast, the variance of current fluctuations (usually called noise) is a two-particle quantity for which the phase of the wavefunction matters. As a consequence, shot noise is sensitive to the ac voltage, a phenomenon called photoassisted noise [4,5]. This has been studied both theoretically and experimentally, for an ac excitation being a simple sine wave [6][7][8][9], as well as for more complex timedependent periodic shapes [10][11][12][13] or non-periodic pulses [14,15]. In this letter we report the observation of quantum oscillations of the low frequency excess noise of a a tunnel junction between normal metals excited by a timedependent voltage V (t) = V (1 + cos 2πνt), as first proposed in [14]. The oscillations as a function of the voltage V have a period hν/e. The experimental conditions to observe these oscillations are: i) a very low electron temperature T = 27 mK, ii) the measurement of the noise generated by the sample (a tunnel junction) at low frequency, here in the range 1-80 MHz ( k B T /h = 440 MHz), and iii) an excitation at very high frequency, here 10 or 20 GHz ( k B T /h).Experimental setup. We cooled to 10 mK in a dilution refr...

Charge transfer in a tunnel junction is studied under dc and ac voltage bias using quantum shot noise. Under dc voltage bias $V$, spectral density of noise measured within a very large bandwidth enables to deduce the current-current correlator in the time domain by Fourier transform. This correlator exhibits regular oscillations proving that electrons try to cross the junction regularly, every $h/eV$. Using harmonic and bi-harmonic ac voltage bias, we then show that quasiparticles excitations can be transferred through the junction in a controlled way. By measuring the reduction of the excess shot noise, we are able to determine the number of electron-hole pairs surrounding the injected electrons and demonstrate that bi-harmonic voltage pulses realize an on-demand electron source with a very small admixture of electron-hole pairs.Comment: 8 pages, 8 figure

We report experimental evidence that the microwave electromagnetic field generated by a normal conductor, here a tunnel junction placed at ultra-low temperature, can be non-classical. By measuring the quadratures of the electromagnetic field at one or two frequencies in the GHz range, we demonstrate the existence of squeezing as well as entanglement in such radiation. In one experiment, we observe that the variance of one quadrature of the photo-assisted noise generated by the junction goes below its vacuum level. In the second experiment, we demonstrate the existence of correlations between the quadratures taken at two frequencies, which can be stronger than allowed by classical mechanics, proving that the radiation at those two frequencies are entangled. To cite this article: J.-C. Forgues, G. Gasse, C. Lupien, B. Reulet, C. R. Physique 6 (2016). (Inspired from previous works/Inspiré de travaux antérieurs [1, 2]) Rayonnement non classiqueémis par un conducteur cohérent Nous rapportons des preuves expérimentales que le champélectromagnétique micro-ondes généré par un conducteur normal, une jonction tunnel placéeà ultra-basse température, peut avoir un comportement non-classique. Nous démontrons l'existence de compression d'état ainsi que d'enchevêtrement dans cette radiation en mesurant les quadratures du champélectromagnétiqueà une ou deux fréquences de l'ordre du GHz. Dans une expérience, nous observons que la variance d'une quadrature du bruit photo-assisté généré par la jonction descend sous son niveau du vide. Dans une deuxième expérience, nous démontrons l'existence de corrélations entre les quadratures observéesà deux fréquences, corrélations qui peuvent etre supérieuresà ce qui est permis par la mécanique classique, prouvant que la radiationà ces deux fréquences est enchevêtrée. Pour citer cet article : J.-C. Forgues, G. Gasse, C. Lupien, B. Reulet, C. R. Physique 6 (2016).