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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Comparing the linewidths from single-pass SPDC and singly resonant cavity SPDC

Oliver Slattery, Lijun Ma, Paulina S. Kuo, Xiao Hong Tang

Published Sep 3, 2015• 3 citations
Publisher

Abstract

Spontaneous parametric down-conversion (SPDC) is a common method to generate entangled photon pairs for use in quantum communications. The generated single photon linewidth is a critical issue for photon-atom interactions in quantum memory applications. We compare the linewidths of greatly non-degenerate single photon pairs from SPDC generated in the single-pass case and the singly-resonant cavity case. For a 6 mm periodically poled lithium niobate (PPLN) crystal, the linewidth of the generated signal photons is reduced from 1 THz in the single pass case to tens of MHz in the singly-resonant cavity case, while the brightness within the modal lineiwdth is increased by a factor of the cavity finesse, though the overall SPDC generation rate remains unchanged.

DOI
10.1117/12.2187059

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Spontaneous parametric down-conversion (SPDC) is a common method to generate entangled photon pairs for use in quantum communications. The generated single photon linewidth is a critical issue for photon-atom interactions in quantum memory applications. We compare the linewidths of greatly non-degenerate single photon pairs from SPDC generated in the single-pass case and the singly-resonant cavity case. For a 6 mm periodically poled lithium niobate (PPLN) crystal, the linewidth of the generated signal photons is reduced from 1 THz in the single pass case to tens of MHz in the singly-resonant cavity case, while the brightness within the modal lineiwdth is increased by a factor of the cavity finesse, though the overall SPDC generation rate remains unchanged.
In this Letter, we theoretically study spontaneous parametric downconversion (SPDC) in a periodically poled structure composed of two linearly uncoupled resonators that are nonlinearly coupled via a Mach-Zehnder interferometer. The device does not require dispersion engineering to achieve efficient doubly resonant SPDC, and, unlike the case of a single resonator, one can reconfigure the system to generate photon pairs over a bandwidth of hundreds of nm. We consider the case of SPDC pumped at 775 nm in a periodically poled lithium niobate (PPLN) device compatible with up-to-date technological platforms. We calculated pair generation rates of up to 250 MHz/mW pump power for a single resonance and integrated pair generation rates of up to 100 THz/mW pump power over 170 nm. When properly reconfigured, a single device can efficiently generate over a bandwidth of some 300 nm, covering the S, C, L, and U infrared bands.
Cavity-enhanced spontaneous parametric down-conversion (SPDC) can be used to implement heralded single-photon sources with narrow bandwidth down to tens of megahertz, thus compatible with atom-based quantum memories. In this work, we propose and study the cavities that are either singly resonant at the frequency of only one of the generated photons or doubly resonant at the frequency of one generated photon and for the pump field. We derive analytical expressions for the generation rate and the spectral brightness as a function of the main structure parameters. Our analysis shows that by exploiting counterpropagating SPDC inside a cavity with properly designed distributed Bragg reflectors, pure narrow-band heralded single photons can be generated. In particular, when a pulsed pump illuminates a 1-mm-long singly resonant cavity, the bandwidth of the resonant and the nonresonant photons will be 120 MHz and 5.7 GHz, respectively, with heralded single-photon purity as high as 0.993. Due to its small absorption probability inside the cavity, the heralding efficiency can approach unity if the nonresonant partner is used as a heralded single photon. Finally, we also show that continuous-wave excitation of a 1-cm-long cavity resonant for the pump and copropagating fields can provide photon pairs in a narrow bandwidth of 10 MHz with spectral brightness as high as $2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ $({\text{s}\phantom{\rule{4.pt}{0ex}}\text{mW}\phantom{\rule{4.pt}{0ex}}\text{MHz})}^{\ensuremath{-}1}$. This source can efficiently couple with quantum memories and can find applications in quantum information processing and communication.
Spontaneous parametric downconversion (SPDC) has been crucial for producing non-classical light, such as correlated photon pairs and squeezed states, essential for optical quantum technologies. Recently, thin-film lithium niobate (TFLN)-based devices have enabled high-performance SPDC in on-chip integrated photonic platforms. Here, we developed an X-cut periodically poled lithium niobate (PPLN) racetrack micro-resonator with a compact footprint and demonstrated bright SPDC photon pair generation characterized by distinct resonances and high extraction efficiency. We separately assessed the internal and loaded photon pair generation rates, measuring them at 4.525 MHz/µW and 62.73 kHz/µW, respectively. Our platform is integrated with maturing electro-/thermo-optic TFLN circuit elements and has meaningful potential for advancing active quantum photonic applications.
Spontaneous parametric down-conversion (SPDC) in a nonlinear crystal has been a workhorse for the generation of entangled and correlated single-photon pairs used for quantum communications applications for nearly three decades. However, as a naturally broadband process, the ability of SPDC to interface with the very narrow energy transitions in atomic ensembles for implementing quantum memories, which are needed for quantum repeaters to extend the reach of quantum communications, was initially limited. To overcome this limitation, the process was enhanced by placing the nonlinear crystal inside a resonating cavity. This modified process has some important advantages, including narrowing the spectral linewidth of generated photons into brighter resonant modes of the cavity, and the ability to lock the desired mode of the cavity to the targeted transition frequency of the atomic ensemble. This paper presents an overview of the principle of cavity-enhanced SPDC, a review of works to date using this technique, and an example of one of these implementations.
Spontaneous parametric down-conversion (SPDC) in nonlinear crystals is the most commonly used means to generate single photon pairs for quantum key distribution systems [1]. By using waveguides instead of bulk crystals, highly efficient devices can be obtained. Both, type-I and type-II phase-matched integrated devices have been demonstrated in periodically poled lithium niobate (PPLN) [2–4]. Although the type-II process is less efficient, it has two favourable properties: the spectrum of down-converted photons is narrower compared to type-I and orthogonally polarised photons are emitted.