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Wavelength-multiplexed heralded single photon source on a chip

Lucia Caspani, Christian Reimer, Matteo Clerici, Marcello Ferrera, Michael Kues, Marco Peccianti, Alessia Pasquazi, Luca Razzari, Brent E. Little, Sai Tak Chu, David Moss, Roberto Morandotti

Published Jan 1, 2015• 0 citations
OpenAlex

Abstract

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

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•Sep 1, 2014•73 citations
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
We report an integrated photon pair source based on a CMOS-compatible microring resonator that generates multiple, simultaneous, and independent photon pairs at different wavelengths in a frequency comb compatible with fiber communication wavelength division multiplexing channels (200 GHz channel separation) and with a linewidth that is compatible with quantum memories (110 MHz). It operates in a self-locked pump configuration, avoiding the need for active stabilization, making it extremely robust even at very low power levels.
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 present a pulsed and integrated, highly non-degenerate parametric down-conversion (PDC) source of heralded single photons at telecom wavelengths, paired with heralding photons around 800 nm. The active PDC section is combined with a passive, integrated wavelength division demultiplexer on-chip, which allows for the spatial separation of signal and idler photons with efficiencies of more than 96.5%, as well as with multi-band reflection and anti-reflection coatings which facilitate low incoupling losses and a pump suppression at the output of the device of more than 99%. Our device is capable of preparing single photons with efficiencies of 60% with a coincidences-to-accidentals ratio exceeding 7400. Likewise, it shows practically no significant background noise compared to continuous wave realizations. For low pump powers, we measure a conditioned second-order correlation function of g (2) = 3.8 × 10 −3 , which proves almost pure single-photon generation. In addition, our source can feature a high brightness of 〈 n pulse 〉 = 0.24 generated photon pairs per pump pulse at pump power levels below 100 μ W. The high quality of the pulsed PDC process in conjunction with the integration of highly efficient passive elements makes our device a promising candidate for future quantum networking applications, where an efficient miniaturization plays a crucial role.
Summary form only given. Recent progress in quantum information processing has highlighted the benefits of integrated optic devices for quantum applications (e.g.[l]). The miniaturization of systems with increased complexity drastically reduces the required space and paves the way for future commercialization.In the past five years, several groups have demonstrated promising approaches to achieve quantum interference and photonic gates on-chip (e.g.[2]). However, in all of these experiments the preparation of the photon pairs has been performed outside the integrated devices employing bulk crystal parametric down-conversion (PDC) sources, while the efficient coupling between these sources and the integrated circuit remains as a bottleneck for the design of systems with increasing complexity. On the other hand remarkable efforts have been devoted to the development of integrated PDC sources for photon pair generation inside channel waveguides [3]. The main benefits of guided-wave PDC processes include high conversion efficiencies and spatial mode control. We combine the prospects of an efficient type-I PDC source based on Titanium-indiffused periodically poled Z-cut Lithium Niobate waveguides (Ti:PPLN) with a passive wavelength division demultiplexing coupler on-chip, shown in Figure 1. The coupler allows for excellent spatial separation generated photon pairs. The non-degenerate decay of pump photons at 532 nm into signal and idler photons provides us with a photon pair at wavelengths of 1575 nm and 800 nm, respectively. Thus low-loss fiber transmission of the prepared states at 1575 nm can be accomplished on the one hand, while on the other hand efficient heralding of conditioned states can be implemented with off-the-shelf silicon-based detectors. The quality and efficiency of our source was improved by the deposition of endface-coatings. This allows for the efficient launching of pump light into the waveguide and it simultaneously provides a close-toperfect transmission of signal and idler at the output of our source, reflecting the pump light by more than 99%. In conditioned coincidence measurements in the picosecond pulsed regime we achieved heralding of single photons with almost no influence of background radiation, expressed by high coincidences-toaccidentals ratios up to CAR > 7400. The preparation exceeded 60% of efficiency in the low pump power regime, accompanied with ultra-low second-order correlation functions of g(2) (0) <; 4.10-3. These figures of merit in conjunction with the integration of active and passive elements make our source a candidate for a basic building block in more complex and miniaturized quantum network realizations in the future.
We present a pulsed and integrated, highly non-degenerate parametric downconversion (PDC) source of heralded single photons at telecom wavelengths, paired with heralding photons around 800 nm. The active PDC section is combined with a passive, integrated wavelength division demultiplexer on-chip, which allows for the spatial separation of signal and idler photons with efficiencies of more than 96.5 %, as well as with multi-band reflection and anti-reflection coatings which facilitate low incoupling losses and a pump suppression at the output of the device of more than 99 %. Our device is capable of preparing single photons with efficiencies of 60 % with a coincidences-to-accidentals ratio exceeding 7400. Likewise it shows practically no significant background noise compared to continuous wave realizations. For low pump powers, we measure a conditioned second-order correlation function of g^(2)(0)=0.0038, which proves almost pure single photon generation. In addition, our source can feature a high brightness of <n_pulse>=0.24 generated photon pairs per pump pulse at pump power levels below 100 uW. The high quality of the pulsed PDC process in conjunction with the integration of highly efficient passive elements makes our device a promising candidate for future quantum networking applications, where an efficient miniaturization plays a crucial role.
We demonstrate the generation of quantum-correlated photon pairs combined with the spectral filtering of the pump field by more than 95 dB on a single silicon chip using electrically tunable ring resonators and passive Bragg reflectors. Moreover, we perform the demultiplexing and routing of signal and idler photons after transferring them via an optical fiber to a second identical chip. Nonclassical two-photon temporal correlations with a coincidence-to-accidental ratio of 50 are measured without further off-chip filtering. Our system, fabricated with high yield and reproducibility in a CMOS-compatible process, paves the way toward large-scale quantum photonic circuits by allowing sources and detectors of single photons to be integrated on the same chip.