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arXiv:1711.02401 · Optics Letters

Engineering spectrally unentangled photon pairs from nonlinear microring resonators by pump manipulation

J. B. Christensen, J. G. Koefoed, Karsten K. Rottwitt, C. J. McKinstrie

Published Feb 12, 2018• 46 citations
arXivHTMLPDF

Abstract

The future of integrated quantum photonics relies heavily on the ability to engineer refined methods for preparing the quantum states needed to implement various quantum protocols. An important example of such states is quantum-correlated photon pairs, which can be efficiently generated using spontaneous nonlinear processes in integrated microring-resonator structures. In this work, we propose a method for generating spectrally unentangled photon pairs from a standard microring resonator. The method utilizes interference between a primary and a delayed secondary pump pulse to effectively increase the pump spectral width inside the cavity. This enables on-chip generation of heralded single photons with state purities in excess of 99% without spectral filtering.

DOI
10.1364/ol.43.000859

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The future of integrated quantum photonics relies heavily on the ability to engineer refined methods for preparing the quantum states needed to implement various quantum protocols. An important example of such states is quantum-correlated photon pairs, which can be efficiently generated using spontaneous nonlinear processes in integrated microring-resonator structures. In this work, we propose a method for generating spectrally unentangled photon pairs from a standard microring resonator. The method utilizes interference between a primary and a delayed secondary pump pulse to effectively increase the pump spectral width inside the cavity. This enables on-chip generation of heralded single photons with state purities in excess of 99% without spectral filtering.
We demonstrate that an integrated silicon microring resonator is capable of efficiently producing photon pairs that are completely unentangled; such pairs are a key component of heralded single-photon sources. A dual-channel interferometric coupling scheme can be used to independently tune the quality factors associated with the pump and signal and idler modes, yielding a biphoton wavefunction with a Schmidt number arbitrarily close to unity. This will permit the generation of heralded single-photon states with unit purity.
Considerable research work is being devoted to the integration of quantum optical components on a silicon chip [1]. Many results have been achieved on the generation of quantum states of light in silicon photonic devices, but all the sources demonstrated so far need off-chip filtering stages to clean the generated photons before detection. This necessity for off-chip components hinders usability in a completely integrated platform. In this work [2] we demonstrate for the first time on-chip generation of photon pairs via a silicon microring resonator fully integrated with spectral filters for the pump on the same chip. The sample, shown in Fig. 1 a, is a reconfigurable silicon photonic integrated circuit based on Bragg reflectors and tunable ring resonators. The combination of these devices results in pump rejection by more than 95 dB in a single chip, reducing the pump intensity to less than the flux of the photon pairs generated by spontaneous four wave mixing (Fig 1 b).
The intrinsic third-order nonlinearity in silicon has proven it to be quite useful in the field of quantum optics. Silicon is suitable for producing time-correlated photon pairs that are sources of heralded single-photon states for quantum integrated circuits. A quantum signal source in the form of single photons is an inherent requirement for the principles of quantum key distribution technology for secure communications. Here, we present numerical simulations of a silicon ring with a 6 μ m radius side-coupled with a bus waveguide as the source for the generation of single photons. The photon pairs are generated by exploring the process of degenerate spontaneous four-wave mixing (SFWM). The free spectral range (FSR) of the ring is quite large, simplifying the extraction of the signal/idler pairs. The phase-matching condition is considered by studying relevant parameters like the dispersion and nonlinearity. We optimize the ring for a high quality factor by varying the gap between the bus and the ring waveguide. This is the smallest ring studied for photon pair generation with a quality factor in the order of 10 5 . The width of the waveguides is chosen such that the phase-matching condition is satisfied, allowing for the propagation of fundamental modes only. The bus waveguide is pumped at one of the ring resonances with the minimum dispersion (1543.5 nm in our case) to satisfy the principle of energy conservation. The photon pair generation rate achieved is comparable to the state of the art. The photon pair sources exploiting nonlinear frequency conversion/generation processes is a promising alternative to atom-like single-photon emitters in the field of integrated photonics. Such miniaturized structures will benefit future on-chip architectures where multiple single-photon source devices are required on the same chip.
We experimentally demonstrate a high-spectral-purity photon source by designing a dual-Mach–Zehnder-interferometer-coupled silicon ring resonator, wherein the linewidths of pump and signal (idler) resonances can be engineered independently. A high spectral purity of 95 % ± 1.5 % is obtained via a time-integrated g ( 2 ) correlation measurement, which exceeds the theoretical 93% bound of a traditional ring’s spontaneous four-wave-mixing photon source. This source also possesses high performance in other metrics including a measured coincidence of 9599 pairs/s and a preparation heralding efficiency of 52.4% at a relatively low pump power of 61 µW as well as high drop-to-through suppression of 20.2 dB. By overcoming the trade-off between spectral purity and brightness in the post-filtering way, such a method guarantees bright pure photons and will pave the way for development of on-chip quantum information processing with improved operation fidelity and efficiency.