Abstract

On-chip generation of non-classical states of light is a fundamental building block for the development of quantum information [1]. A convenient method harnesses resonantly enhanced parametric processes, since they operate at room temperature. We consider here spontaneous Four-Wave-Mixing (FWM). The generation of time correlated, entangled photons pairs and heralded single photons via FWM has been demonstrated on silicon and on different III-V semiconductor ring resonators with very large efficiency [2,3]. Here, an alternative type of cavity is used : a Photonic Crystal (PhC) cavity. These periodic structures offer the possibility to reduce the mode volume V in all-dielectric structure, with a micrometer-scale footprint, which strongly increases the efficiency of the nonlinear process (∝ Q³/V²) if high quality factor Q is preserved. We achieve it with a bichromatic Photonic Crystal cavity depicted in fig.1.a). The cavity is located between two lines of holes with reduced period a′ compared to the rest of the holes with period a; while light propagates in a waveguide created with a missing line of holes ended by a reflector [4]. This design imposes an effective parabolic potential for the optical field, that ultimately leads to equispaced resonances in frequency, which is a strict condition for efficient resonant enhancement of the interaction imposed by the energy conservation of FWM. The cavity is pumped with a CW laser with pump power level of P₀ transferred to the cavity. A self thermal tuning technique is used to compensate for any residual frequency mismatch of the resonance triplet [4] after fabrication. After the chip, the generated photon pairs encounter an unbalanced Mach-Zehnder ”Franson” interferometer [5]. At the output, they are time-energy entangled. They travel through a filtering stage to suppress any residual pump photons before being detected by superconducting nanowire single-photon detectors. The efficiency of our source is measured by bypassing the Franson interferometer. Taking into account the losses between the cavity and the detectors, it is estimated to be 16 GHz/mW² and is comparable to the state-of-the-art of semiconductor based resonators for photon pair generation (20 GHz/mW², fig.1.b)) [2]; while our average Q-factor (2.1 × 10⁵) is five times smaller for this sample but could be larger (up to 7 × 10⁵). Comparable efficiency is achieved because the resonator volume is at least one order of magnitude smaller than in ring resonators. Consequently, the footprint is very small (< 70 µm², ≈600 and 300 µm² in [2] and [3]), which is an important asset for scalability. The entangled nature of the photon pair is quantified with two-photon interferences depicted in fig.1.c). For pump power of 15 µW, large net visibility is measured up to 96.6 ± 4.2 %. This work shows that PhC cavities are novel and promising sources for integrated quantum optics.

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