Summary form only given. Here, we demonstrate the first scheme to generate pulsed quantum frequency combs without the need for an external laser. We constructed a laser cavity (Fig. 1a) around a 4-port integrated micro-ring resonator, featuring narrow bandpass filters to select a single resonance, an optical gain component (erbium-doped fiber amplifier), and a mode-locking element (an intensity modulator). By setting the intensity modulation frequency to the fundamental repetition rate of the cavity (determined by the external cavity length), we achieved mode-locked pulsed operation (Fig. 1b), exciting a pulsed two-photon comb. Driving the intensity modulator at multiples of the fundamental repetition rate (upwards of 4x) also resulted in stable mode-locking (Fig. 1c), significant as this enables an increase of the photon pair generation rate without sacrificing its coincidence-to-accidental ratio (CAR). We characterized the photon statistics as a function of the average ring input power (see Fig. 1d), achieving a maximum CAR of 110 that decreases with increasing power and increasing coincidence rates. Autocorrelation measurements confirmed the purity of the generated state (pure single-frequency modes, even at multiples of the repetition rate), confirming that the resonance was excited over its entire bandwidth, and obtained a heralded anti-bunching dip of 0.245<;0.5 as expected of a source operating in the single-photon regime.
Summary form only given. Here, we demonstrate the first scheme to generate pulsed quantum frequency combs without the need for an external laser. We constructed a laser cavity (Fig. 1a) around a 4-port integrated micro-ring resonator, featuring narrow bandpass filters to select a single resonance, an optical gain component (erbium-doped fiber amplifier), and a mode-locking element (an intensity modulator). By setting the intensity modulation frequency to the fundamental repetition rate of the cavity (determined by the external cavity length), we achieved mode-locked pulsed operation (Fig. 1b), exciting a pulsed two-photon comb. Driving the intensity modulator at multiples of the fundamental repetition rate (upwards of 4x) also resulted in stable mode-locking (Fig. 1c), significant as this enables an increase of the photon pair generation rate without sacrificing its coincidence-to-accidental ratio (CAR). We characterized the photon statistics as a function of the average ring input power (see Fig. 1d), achieving a maximum CAR of 110 that decreases with increasing power and increasing coincidence rates. Autocorrelation measurements confirmed the purity of the generated state (pure single-frequency modes, even at multiples of the repetition rate), confirming that the resonance was excited over its entire bandwidth, and obtained a heralded anti-bunching dip of 0.245<;0.5 as expected of a source operating in the single-photon regime.
The on-chip generation of large and complex optical quantum states will enable low-cost and accessible advances for quantum technologies, such as secure communications and quantum computation. Integrated frequency combs are on-chip light sources with a broad spectrum of evenly-spaced frequency modes, commonly generated by four-wave mixing in optically-excited nonlinear micro-cavities, whose recent use for quantum state generation has provided a solution for scalable and multi-mode quantum light sources. Pulsed quantum frequency combs are of particular interest, since they allow the generation of single-frequency-mode photons, required for scaling state complexity towards, e.g., multi-photon states, and for quantum information applications. However, generation schemes for such pulsed combs have, to date, relied on micro-cavity excitation via lasers external to the sources, being neither versatile nor power-efficient, and impractical for scalable realizations of quantum technologies. Here, we introduce an actively-modulated, nested-cavity configuration that exploits the resonance pass-band characteristic of the micro-cavity to enable a mode-locked and energy-efficient excitation. We demonstrate that the scheme allows the generation of high-purity photons at large coincidence-to-accidental ratios (CAR). Furthermore, by increasing the repetition rate of the excitation field via harmonic mode-locking (i.e. driving the cavity modulation at harmonics of the fundamental repetition rate), we managed to increase the pair production rates (i.e. source efficiency), while maintaining a high CAR and photon purity. Our approach represents a significant step towards the realization of fully on-chip, stable, and versatile sources of pulsed quantum frequency combs, crucial for the development of accessible quantum technologies.
We present a method for the generation and coherent manipulation of pulsed quantum frequency combs. Until now, methods of preparing high-dimensional states on-chip in a practical way have remained elusive due to the increasing complexity of the quantum circuitry needed to prepare and process such states. Here, we outline how high-dimensional, frequency-bin entangled, two-photon states can be generated at a stable, high generation rate by using a nested-cavity, actively mode-locked excitation of a nonlinear micro-cavity. This technique is used to produce pulsed quantum frequency combs. Moreover, we present how the quantum states can be coherently manipulated using standard telecommunications components such as programmable filters and electro-optic modulators. In particular, we show in detail how to accomplish state characterization measurements such as density matrix reconstruction, coincidence detection, and single photon spectrum determination. The presented methods form an accessible, reconfigurable, and scalable foundation for complex high-dimensional state preparation and manipulation protocols in the frequency domain.
The on-chip generation of optical quantum states will enable accessible advances for quantum technologies. We demonstrate that integrated quantum frequency combs (based on high-Q microring resonators made from a CMOS-compatible, high refractive-index doped-glass platform) can enable the generation of pure heralded single photons, cross-polarized photon pairs, as well as bi- and multi-photon entangled qubit states over a broad frequency comb covering the S, C, L telecommunications band, with photon frequencies corresponding to standard telecommunication channels spaced by 200 GHz. Exploiting a self-locked, intra-cavity excitation configuration, a highly-stable source of multiplexed heralded single photons is demonstrated, operating continuously for several weeks with less than 5% fluctuations. The photon bandwidth of 110 MHz is compatible with quantum memories, and high photon purity was confirmed through single-photon auto-correlation measurements. In turn, by simultaneously exciting two orthogonal polarization mode resonances, we demonstrate the first realization of type-II spontaneous FWM (in analogy to type-II spontaneous parametric down-conversion), allowing the direct generation of orthogonally-polarized photon pairs on a chip. By using a double-pulse excitation, we demonstrate the generation of time-bin entangled photon pairs. We measure qubit entanglement with visibilities above 90%, enabling the implementation of quantum information processing protocols. Finally, the excitation field and the generated photons are intrinsically bandwidth-matched due to the resonant characteristics of the ring cavity, enabling the multiplication of Bell states and the generation of a four-photon time-bin entangled state. We confirm the generation of this four-photon entangled state through four-photon quantum interference.
We demonstrate the generation of multiphoton quantum states of light by cavity-enhanced parametric down-conversion in the high-repetition-rate pulsed regime. An external enhancement cavity resonant with the spectral comb of modes of a mode-locked pump laser provides a coherent buildup of the pump intensity and greatly enhances the parametric gain without sacrificing its high repetition rate and comb structure. We probe the parametric gain enhancement by the conditional generation and tomographic analysis of two-photon Fock states. Besides its potential impact for efficiently generating highly nonclassical or entangled multiphoton states in many existing experimental setups, this scheme opens exciting perspectives toward the combination of quantum and comb technologies for enhanced measurements and advanced quantum computation protocols.
Integrated sources of single and entangled photons are currently one of the main topics in the field of integrated quantum optics [1]. Sources at telecom wavelength and compatible with the electronic \nintegrate circuit technology (CMOS) are particularly interesting for their compatibility with the existing fiber network and industrial fabrication processes [2]. As for classical communication, wavelength-division multiplexing (WDM,i.e. the possibility of simultaneously transmitting on the same channel different signals each encoded in a different frequency) could in principle be exploited also for quantum communication for increasing the transmission bit-rate. However, to date only few approaches have been proposed for generating wavelength multiplexing photon pairs in integrated platforms [3,4]. Moreover, until now no integrated photon source managed to achieve at the same time practical requirements such as narrow linewidths (required for quantum memories), compatibility with CMOS technology, multichannel emission, and stable long-term operation [5]. This is due to the fact that narrow linewidths are typically achieved with structures fundamentally incompatible with large-scale integration, while CMOS-compatible devices fail to achieve the high Q-factor required for exhibiting narrow linewidths. Here we report an integrated photon pair source based on a CMOS -compatible micro-ring resonator generating multiple, simultaneous, and independent photon pairs at different wavelengths distributed in a frequency comb, compatible with standard wavelength division multiplexing channels (DWDM, 200 GHz separation) and quantum memories (140 MHz linewidth), covering the full telecommunication band (L+C). We exploit a self-locked pumping scheme allowing an intrinsically stable operation even at the very low pump power levels typically used for photon pair generation. By enclosing the microring in an active external cavity[see Fig. 1(a)], the pump laser includes the microring itself, thus resulting in a passive self-locked pumping mechanism leading to several days of stable operation (compared to only few tens of minutes for standard schemes without active stabilization) [5]. The signal/idler photons are then separated according to their wavelength using a standard commercial DWDM filter, and then characterized by coincidence detection. Clear coincidence peaks [see Fig. 1(b)], with Coincidence to Accidental Ratios higher than 10, are obtained when selecting conjugate frequencies (while no coincidences are observed for non-conjugate signal/idler pair). Finally, Fig. 3(c) reports the measured heralded autocorrelation function, g (2)h, featured by a dip as low as 0.14<0.5, indicating the suitability of our device as heralded single photon source.\n