Entanglement is a fundamental resource in quantum information processing. Several studies have explored the integration of sources of entangled states on a silicon chip, but the devices demonstrated so far require millimeter lengths and pump powers of the order of hundreds of milliwatts to produce an appreciable photon flux, hindering their scalability and dense integration. Microring resonators have been shown to be efficient sources of photon pairs, but entangled state emission has never been proven in these devices. Here we report the first demonstration, to the best of our knowledge, of a microring resonator capable of emitting time-energy entangled photons. We use a Franson experiment to show a violation of Bell’s inequality by more than seven standard deviations with an internal pair generation exceeding 10 7 Hz . The source is integrated on a silicon chip, operates at milliwatt and submilliwatt pump power, emits in the telecom band, and outputs into a photonic waveguide. These are all essential features of an entangled state emitter for a quantum photonic network.

Figures (4)

Fig. 1. Sample structure and characterization. (a) Sketch of the sample together with the input/output light coupling mechanism. The R = 10 μm ring resonator is evanescently coupled to a silicon nanowire waveguide via a deep-etched 150 nm gap point coupler. An optical microscope image of the ring is shown in the inset. The waveguide ends at both sides with spot-size converters: 300 μm long silicon inverse tapers ending in a 20 nm tip width covered by 1.5 μm × 2.0 μm polymer waveguides. Light injection from a collimated pump laser is achieved by the use of an aspheric lens with numerical aperture NA = 0.5 . The output from the sample is collected with a PM lensed fiber with a working distance of 3 μm. (b) Transmission spectrum of the resonator. The pump resonance is highlighted in green, and the signal and idler resonances employed in the experiment are indicated in blue and red, respectively. Download Full Size | PPT Slide | PDF

Fig. 2. SFWM and coincidences. (a) Spectra of the SFWM experiment for five different coupled pump powers; signal and idler intensities are divided by the corresponding pump power to underline the superlinear growth of the intensity. The slight difference in intensity between the two peaks is due to slightly different coupling to the input/output bus of the signal and idler modes. For coupled pump powers above 1 mW, the pump wavelength was retuned to compensate the red shift of the resonance due to the thermo-optic effect [ 22 ]. The horizontal scale is expanded around the signal and idler resonances, while the complete absence of detected photons at the pump resonance confirms the excellent rejection of the transmitted pump intensity in the setup. (b) Scaling of the internal generation rates of signal (blue squares) and idler (red circles) photons in SFWM, varying the coupled pump power. The black dashed line is a guide to the eye proportional to the square of the pump power. The left axis indicates the photon flux measured at the sample output. (c) Measured coincidence histogram for a coupled pump power P p = 1 mW . The time resolution is 75 ps, and it is driven by the response time of the detectors. Download Full Size | PPT Slide | PDF

Fig. 3. Correlations at the output of a double interferomenter. (a) Sketch of the signal and idler Michelson interferometers. The arm length difference of the two interferometers is the same to well within the coherence length of the generated photons ( Supplement 1 ). The movable mirrors on the short arms are connected to a piezo actuator and are used to control the relative phase between the short and long arms. At the outputs of the interferometers are two superconducting single-photon detectors (SSPDs). (b) Instance of coincidence histogram measured at the output of the interferometers, taken for a coupled pump power of 1.5 mW. The integration time is 120 s. The error bars indicate the error on the counts. The inset shows the absolute intensity at the output of each interferometer while varying the respective phase: the complete absence of interference confirms that the arm length difference is much larger than the coherence time on the generated photons. Download Full Size | PPT Slide | PDF