4 -150 -100 -50 0 50 100 150 0 1 2 N( ν ) 2 1 0 x10 -4 2 h  ν /e V$_{ds}$(µV) 2 2 2 2 Data Predicti Prediction  on   ( ( ))  ( ( )) I sym I S S v P P v δ δ α α 1 1 1 1 / ( 0) out out out out ds P P P P S S V Δ = 0.0 0.5 1.0 1.5 0 1 2 3 N( ν ) x10 -2 x10 -2  Theory 1 1 / n n T T Δ 1 1 1 1 / ( 0) out out out out ds P P P P S S V Δ = 1 1 1 1 / ( 0) out out out out ds P P P P S S V Δ = 1 2 FIG. 3: Normalized auto-correlation spectrum of power fluc- tuations vs normalized excess noise temperature. Symbols: experimental data. Line: Theoretical prediction. The top axis gives the corresponding N ( ν ). FIG. 4: Open squares: Normalized cross-correlation spectrum of power fluctuations. Solid line: theoretical prediction, as- suming S PP ∝ S 2 $_{I}$. Dashed line theoretical prediction, as- suming S PP ∝ S 2 I, $_{sym}$. The right axis gives the corresponding N ( ν ). The dotted line represent the onset of finite frequency excess shot noise [5, 7, 11]. range show similar agreement, with a maximum system- atic deviation of roughly 3%. As shown by figure 4, the cross-correlated power fluctuations are positive, showing the bosonic character of the emitted excitations. On a quantitative level, one should observe: our observations. In conclusion, we have performed the first experi- ment probing the statistical properties of photons emit- ted by a phase coherent conductor. The data are found in perfect agreement with the predictions of Beenakker and Schomerus, showing that a biased low impedance tunnel junction emits a chaotic radiation. The cross- correlated power fluctuations are found to be propor- tional to the square of the emission electronic noise den- sity. This opens the way to the investigation of the statistical properties of photons emitted by mesoscopic conductors where electronic correlations might have a stronger impact, such as quantum point contacts, for which it is expected that the sub-Poissonian statistics of electronic shot-noise could be ’imprinted’ on the cor- responding emitted photons [2, 13]. It’s a pleasure to acknowledge precious help from Q. Le Masne, P. Bertet and D. Vion with the sample fabrica- tion. We greatly benefited from help during the measure- ments from B. Dubost. This work was supported by the ANR contract 2e-BQT, and by the C’Nano Idf contract QPC-SinPS. We would like to add a short note to the question of the symmetrization of the current correlator involved in this shot noise measurement. The treatment of Beenakker and Schomerus[2] yields power fluctuations proportional to the square of the electronic emission noise density [12] S$_{I}$ ( ν ) = 2 ∫ + ∞ −∞ 〈 I (0) I ( τ ) 〉 e i 2 $^{πντ}$dτ . Using a sym- metrized correlator S$_{I,}$$_{sym}$ = 1 $_{2}$( S$_{I}$ ( ν )+ S$_{I}$ ( − ν )) increases the emitted power per unit bandwidth by hν/ 2. As the difference doesn’t depend on bias voltage, excess noise measurements cannot distinguish between the two definitions. However, the quadratic dependence of the power fluctuations with the emitted power allows to dis- tinguish them. As shown by figure 4, assuming power fluctuations S$_{PP}$ ∝ S 2 I, sym yields a prediction Δ S$_{PP}$ ∝ [(Δ S$_{I}$ ) 2 + 4 G$_{sample}$hν Δ S$_{I}$ ], which is incompatible with Figure 4 shows both the cross-correlated power fluc- tuations, normalized to the equilibrium auto-correlated power fluctuations, and the product of the excess noise temperatures normalized by (Δ ν$_{min}$/ Δ ν$_{max}$ ) 1 / $^{2}$. They are found to coincide within 4%. As the ’photon noise’ is related to a fourth order-correlator of the electronic cur- rent [2, 4], this constitutes, to the best of our knowledge, the first measurement of such a correlator in the quan- tum regime. It shows that the emitted radiation remains chaotic, even when the occupation number of the emitted photon modes tends to zero ( eV$_{ds}$, k$_{B}$T ≪ hν ). Δ S$_{P}$ out 1 P out 2 [ S$_{P}$ out 1 P out 1 S$_{P}$ out 2 P out 2 ] 1 / 2 V$_{ds}$ =0 = Δ T$_{n}$$_{1}$ T$_{n}$$_{1}$ Δ T$_{n}$$_{2}$ T$_{n}$$_{2}$ √ Δ ν$_{min}$ Δ ν$_{max}$ , ∗ Present address: Walter Schottky Institut Technische Universit¨ at M¨ unchen Am Coulombwall 3, 85748 Garch- ing, Germany † Also at Laboratoire Pierre Aigrain, D´ epartement de Physique de l’Ecole Normale Sup´ erieure, 24 rue Lhomond, 75231 Paris Cedex 05, France ‡ Electronic address: fabien.portier@cea.fr