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presses the Al superconductivity. The two sides of the tunnel junction are separately connected to 50 Ω coaxial transmission lines via two quarter wave length impedance adapters, raising the effective input impedance of the de- tection lines to Z$_{eff}$ = 200Ω over a one octave bandwidth centered at 6 GHz. Two rf-circulators, thermalized at mixing chamber temperature ensure a circuit environ- ment at base temperature. 

We note δI$_{1}$$_{,}$$_{2}$ the fluctuating currents in either detec- tion branches resulting from the fluctuations of the cur- rent through the tunnel junction. S$_{I}$ 1 , S$_{I}$ 2 , S$_{I}$ 1 ,I 2 stand for the autocorrelated and cross-correlated spectral den- sities. From the equivalent circuit represented in figure 1, one easily sees that: 

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where the two first equalities result from current conser- vation. The noise power detected in each detection line in a frequency range Δ ν reads 

where P$_{em}$ = R$_{t}$S$_{I}$ ( ν, T, V$_{ds}$ )Δ ν/ 4 is the emitted power. This can be expressed as an excess noise temperature Δ T$_{n}$$_{1}$$_{,}$$_{2}$ = P$_{1}$$_{,}$$_{2}$/ [ k$_{B}$ Δ ν$_{1}$$_{,}$$_{2}$ ]. 

The two emitted signals are then amplified by two cryo- genic Low Noise Amplifiers. Up to a calculable gain fac- tor, the detected noise power contains the weak excess noise Δ T$_{n}$$_{1}$$_{,}$$_{2}$ on top of a large additional noise gener- ated by the cryogenic amplifiers T$_{n}$$_{1}$$_{,}$$_{2}$ ≃ 5K. After fur- 

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ther room t emperature amplification and eventually nar- row bandp ass filtering, current fluctuations are detected using three alternative techniques. First (fig. 1.a), we implement ed the measurement scheme described in [7], using two calibrated quadratic detectors whose output voltage is proportional to noise power. Secondly (fig. 1.b), curre nt fluctuations are digitized, after down con- version ach ieved by mixing with a suited local microwave signal, usin g an AP240 Acqiris Acquisition Card able to sample at 1Gsample/s. A quantitative comparison with the well es tablished first method has qualified this new method. The third method, dedicated to the study of photon no ise, is a hybridization of the two previous ones: the output s of the two quadratic detectors are digitized to perform the photon HBT cross and auto correlations fluctuation s of P$_{1}$$_{,}$$_{2}$ . 

First ex periment: mean photon occupation number. We measur e the increase in noise temperature due do the photon emission by shot noise, as a function of V$_{ds}$ and the measuring frequency ν , using the quadratic detec- tors. In order to remove the background noise of the am- plifiers, we measure the excess noise, Δ S$_{I}$ 1 , 2 ( ν, T, V$_{ds}$ ) = S$_{I}$ 1 , 2 ( ν, T, V$_{ds}$ ) − S$_{I}$ 1 , 2 ( ν, T, 0). Practically, this is done by applying a 93Hz 0- V$_{ds}$ square-wave bias voltage on the sample through the DC input of a bias-Tee, and detect- ing the first harmonic of the square-wave noise response of the detectors using lock-in techniques. The results are quite similar to the one reported in ref.[7], and lead to an electron temperature T$_{e}$ ∼ 70 MA. Although T$_{e}$ is significantly higher than the mixing chamber tempera- ture, it is low enough to make the thermal population of photons negligible in the 4-8 GHz frequency range, where all our measurements are done. In the high bias limit ( eV$_{ds}$ ≫ k$_{B}$T, hν ), Δ S$_{I}$ ∼ 2 eI . Equation 2 then yields an excess noise temperature Δ T$_{n}$$_{1}$$_{,}$$_{2}$ = eV$_{ds}$/ 4 k$_{B}$ in both detection branches in the case of ideal coupling R$_{t}$ = 2 Z$_{eff}$ . In practice, we get excess noise temperatures ∼ 2dB lower than expected from the independently mea- sured attenuation of the various microwave components connecting the sample to both amplifiers. A ∼ 100fF capacitance for the junction, shunting part of the mi- crowave signal, accounts for this discrepancy. This value is quite reasonable given the area of our tunnel junction (1.4 µ m $^{2}$). 

Second experiment: auto and cross-correlated elec- tronic noise. We record the current fluctuations with the acquisition card using a 5 ns sampling time, chosen large enough to avoid any correlation between successive points, thus maximizing the effective bandwidth. Here again, we eliminate background noise and parasitic cor- relation between the two inputs of the acquisition card by measuring excess fluctuations . Δ δV 2 1 and Δ δV 2 are proportional to the excess noise power: 

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