Photon beams exhibit temporal correlations that are characteristics of their emission mechanism. For instance, photons issued from incoherent sources tend to be detected in bunches. This striking 'bunching' behaviour has been observed in the seminal experiment by Hanbury-Brown and Twiss (HBT) in the fifties, who measured the time of arrival of partially coherent photons on two separate photon-counting modules 1. Since then, HBT interferometry has become a widespread technique to study photon correlations down to only the nanosecond range, because of the detector-limited bandwidth, preventing the observation of bunching for real thermal sources. It has been suggested later that two-photon absorption (TPA) could measure the photon temporal correlations at a much shorter timescale 2,3 , as it involves an almost simultaneous absorption of two photons, within a maximum delay given by the Heisenberg principle. Here, for the first time, this prediction is experimentally demonstrated using TPA in a GaAs photon-counting module. We have observed photon bunching in the femtosecond range for real blackbody sources (an enhancement of six orders of magnitude in the time resolution of present techniques), opening the way to monitor optical quantum statistics at the ultrashort timescale. The Hanbury-Brown and Twiss (HBT) experiment actually measures the degree of second-order coherence (DSOC) g (2) (τ) = I (t) I (t + τ) / I (t) 2 of optical fields, where I (t) is the intensity of the fields, τ is the optical path time difference between the two detectors and designates statistical averaging. A straightforward calculation, in which the fields are treated in terms of classical fluctuating electromagnetic waves, shows that g (2) (0) is always larger than 1 and is equal to 2 for chaotic sources 3. The experiment was first carried out in the radiofrequency domain, using radiotelescopes, where the undulatory nature of light is unquestionable. It was later extended by the two authors to the optical domain, where they measured a value of g (2) (0) = 1.93 using photocounters and a narrowband partially coherent light source (a 546.1 nm Hg discharge lamp followed by an interference filter) 4. Although this result can still be interpreted classically in wave-like terms, its interpretation in terms of light corpuscles-more suited to photon counters-is more subtle, as it implies that photons emitted by incoherent sources are correlated: this is the so-called 'photon bunching effect'. Actually, the HBT experiment was a great surprise at that time and fostered a lot of controversy in the literature 5-8. Later, Glauber 7 developed a unifying quantum theory of optical coherence. Using a quantum approach of the LETTERS NATURE PHYSICS

One of the main challenges for future quantum information technologies is miniaturization and integration of high performance components in a single chip. In this context, electrically driven sources of non-classical states of light have a clear advantage over optically driven ones. Here we demonstrate the first electrically driven semiconductor source of photon pairs working at room temperature and telecom wavelength. The device is based on type-II intracavity Spontaneous Parametric Down-Conversion in an AlGaAs laser diode and generates pairs at 1.57 µm. Time-correlation measurements of the emitted pairs give an internal generation efficiency of 7 × 10 −11 pairs/injected electron. The capability of our platform to support generation, manipulation and detection of photons opens the way to the demonstration of massively parallel systems for complex quantum operations.PACS numbers: 42.65. Lm, 03.67.Bg, 42.55.Px, Photons have a peculiar advantage in the development of quantum information technologies [1-3], since they behave naturally as flying qubits presenting a high speed transmission over long distances and being almost immune to decoherence [4,5]. The intrinsic scalability and reliability of integrated photonic circuits has recently given rise to a new generation of devices for quantum communication, computation and metrology [6]. Nevertheless even if great progress have been made in the manipulation [7,8] and detection [9] of nonclassical state of light on chip, a complete integration of the light source in the photonic circuitry stays one of the main challenges on the way towards large scale applications; such devices would have a clear advantage over optically driven ones in terms of portability, energy consumption and integration. Semiconductor materials are ideal to achieve extremely compact and massively parallel devices: concerning photon-pair sources, the bi-exciton cascade of a quantum dot has been used to demonstrate an entangledlight-emitting diode at a wavelength of 890 nm [10]. However, even if the use of a single emitter guarantees a deterministic emission, these devices operate at cryogenic temperature, greatly limiting their potential for applications.Optical parametric conversion offers an alternative approach. Despite its non-deterministic nature, this process is the most widely used to produce photon pairs for quantum information and communications protocols. Up to now, entangled photon pairs have been generated by optical pumping in passive semiconductor waveguides by exploiting four-wave mixing in Silicon [11] or SPDC in Aluminium Gallium Arsenide (AlGaAs) [12,13]. Thanks to its direct band gap, the latter platform presents an evident interest for the electrical injection. In order to deal with the isotropic structure of this crystal, several solutions have been proposed to achieve nonlinear optical conversion in AlGaAs waveguides [14][15][16][17][18]; among these, modal phase matching, in which the phase velocity mismatch is compensated by multimode waveguide dispersion, is one of the mos...