We show experimentally that a dc biased Josephson junction in series with a high-enoughimpedance microwave resonator emits antibunched photons. Our resonator is made of a simple micro-fabricated spiral coil that resonates at 4.4 GHz and reaches a 1.97 kΩ characteristic impedance. The second order correlation function of the power leaking out of the resonator drops down to 0.3 at zero delay, which demonstrates the antibunching of the photons emitted by the circuit at a rate of 6 10 7 photons per second. Results are found in quantitative agreement with our theoretical predictions. This simple scheme could offer an efficient and bright single-photon source in the microwave domain.PACS numbers: 74.50+r, 73.23Hk, 85.25Cp Single photon sources constitute a fundamental resource for many quantum information technologies, notably secure quantum state transfer using flying photons. In the microwave domain, although photon propagation is more prone to losses and thermal photons present except at extremely low temperature, applications can nevertheless be considered [1,2]. Single microwave photons were first demonstrated in [3] using the standard design of single-photon emitters: an anharmonic atom-like quantum system excited from its ground state relaxes by emitting a single photon on a well-defined transition before it can be excited again. The first and second order correlation functions of such a source [4] demonstrate a rather low photon flux limited by the excitation cycle duration, but an excellent antibunching of the emitted photons. We follow a different approach, where the tunnelling of discrete charge carriers through a quantum coherent conductor creates photons in its embedding circuit. The resulting quantum electrodynamics of this type of circuits [5][6][7][8][9][10][11] has been shown to provide e.g. masers [12][13][14][15], simple sources of non-classical radiation [16][17][18], or near quantum-limited amplifiers [19]. When the quantum conductor is a Josephson junc-tion, dc biased at voltage V in series with a linear microwave resonator, exactly one photon is created in the resonator each time a Cooper pair tunnels through the junction, provided that the Josephson frequency 2eV /h matches the resonator's frequency [20].We demonstrate here that in the strong coupling regime between the junction and the resonator, the presence of a single photon in the resonator inhibits the further tunneling of Cooper pairs, leading to the antibunching of the photons leaking out of the resonator [21,22]. Complete antibunching is expected when the characteristic impedance of the resonator reaches Z c = 2R Q /π, with R Q = h/(2e) 2 6.45 kΩ the superconducting resistance quantum. This regime, for which the analogue of the fine structure constant of the problem is of order 1, has recently attracted attention [23,24], as it allows the investigation of many-body physics with photons [25,26] or ultra-strong coupling physics [27], offering new strategies for the generation of non classical radiation [28].The simple circuit used in...